Abstract: The global impact of shipping on atmospheric chemistry and radiative forcing, as well as the associated uncertainties, have been quantified using an ensemble of ten state-of-the-art atmospheric chemistry models and a predefined set of emission data. The analysis is performed for present-day conditions ( year 2000) and for two future ship emission scenarios. In one scenario ship emissions stabilize at 2000 levels; in the other ship emissions increase with a constant annual growth rate of 2.2% up to 2030 ( termed the "Constant Growth Scenario" (CGS)). Most other anthropogenic emissions follow the IPCC ( Intergovernmental Panel on Climate Change) SRES ( Special Report on Emission Scenarios) A2 scenario, while biomass burning and natural emissions remain at year 2000 levels. An intercomparison of the model results with observations over the Northern Hemisphere (25 degrees - 60 degrees N) oceanic regions in the lower troposphere showed that the models are capable to reproduce ozone (O-3) and nitrogen oxides (NOx= NO+ NO2) reasonably well, whereas sulphur dioxide (SO2) in the marine boundary layer is significantly underestimated. The most pronounced changes in annual mean tropospheric NO2 and sulphate columns are simulated over the Baltic and North Seas. Other significant changes occur over the North Atlantic, the Gulf of Mexico and along the main shipping lane from Europe to Asia, across the Red and Arabian Seas. Maximum contributions from shipping to annual mean near-surface O-3 are found over the North Atlantic ( 5 - 6 ppbv in 2000; up to 8 ppbv in 2030). Ship contributions to tropospheric O3 columns over the North Atlantic and Indian Oceans reach 1 DU in 2000 and up to 1.8 DU in 2030. Tropospheric O-3 forcings due to shipping are 9.8 +/- 2.0 mW/m(2) in 2000 and 13.6 +/- 2.3 mW/m(2) in 2030. Whilst increasing O-3, ship NOx simultaneously enhances hydroxyl radicals over the remote ocean, reducing the global methane lifetime by 0.13 yr in 2000, and by up to 0.17 yr in 2030, introducing a negative radiative forcing. The models show future increases in NOx and O-3 burden which scale almost linearly with increases in NOx emission totals. Increasing emissions from shipping would significantly counteract the benefits derived from reducing SO2 emissions from all other anthropogenic sources under the A2 scenario over the continents, for example in Europe. Globally, shipping contributes 3% to increases in O-3 burden between 2000 and 2030, and 4.5% to increases in sulphate under A2/CGS. However, if future ground based emissions follow a more stringent scenario, the relative importance of ship emissions will increase. Inter-model differences in the simulated O-3 contributions from ships are significantly smaller than estimated uncertainties stemming from the ship emission inventory, mainly the ship emission totals, the distribution of the emissions over the globe, and the neglect of ship plume dispersion.
Abstract: Air quality, ecosystem exposure to nitrogen deposition, and climate change are intimately coupled problems: we assess changes in the global atmospheric environment between 2000 and 2030 using 26 state-of-the-art global atmospheric chemistry models and three different emissions scenarios. The first (CLE) scenario reflects implementation of current air quality legislation around the world, while the second (MFR) represents a more optimistic case in which all currently feasible technologies are applied to achieve maximum emission reductions. We contrast these scenarios with the more pessimistic IPCC SRES A2 scenario. Ensemble simulations for the year 2000 are consistent among models and show a reasonable agreement with surface ozone, wet deposition, and NO2 satellite observations. Large parts of the world are currently exposed to high ozone concentrations and high deposition of nitrogen to ecosystems. By 2030, global surface ozone is calculated to increase globally by 1.5 +/- 1.2 ppb (CLE) and 4.3 +/- 2.2 ppb (A2), using the ensemble mean model results and associated +/- 1 sigma standard deviations. Only the progressive MFR scenario will reduce ozone, by -2.3 +/- 1.1 ppb. Climate change is expected to modify surface ozone by -0.8 +/- 0.6 ppb, with larger decreases over sea than over land. Radiative forcing by ozone increases by 63 +/- 15 and 155 +/- 37 mW m(-2) for CLE and A2, respectively, and decreases by -45 +/- 15 mW m(-2) for MFR. We compute that at present 10.1% of the global natural terrestrial ecosystems are exposed to nitrogen deposition above a critical load of 1 g N m(-2) yr(-1). These percentages increase by 2030 to 15.8% (CLE), 10.5% (MFR), and 25% (A2). This study shows the importance of enforcing current worldwide air quality legislation and the major benefits of going further. Nonattainment of these air quality policy objectives, such as expressed by the SRES-A2 scenario, would further degrade the global atmospheric environment.
Abstract: A global 3-D chemistry-transport model STOCHEM has been employed to study trends in the mole fractions of methane, carbon monoxide and ozone in baseline air masses entering Europe from the Atlantic Ocean over the period from 1990 to 2030. With a range of emission scenarios for man-made ozone precursor emission sources available, a wide range of model trends were predicted for the period up to 2030. In the scenario based on current planned air pollution controls, IIASA CLE, methane shows a strong upward trend, ozone shows a weaker upward trend, and carbon monoxide is approximately flat in baseline air masses. In one of the more pessimistic IPCC SRES scenarios, A2, all three gases show future increases. However, in the scenario based on maximum feasible emission reductions, IIASA MFR all three trace gases decline. By 2030, projected climate change reduces the growth in CH4, but has insignificant effects on baseline CO and O-3 in these simulations. Global or hemispheric ozone precursor emissions and their controls exert a potentially large external influence on Europe's air quality. This influence is currently not taken into account in future European air quality policy formulation. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Changes in atmospheric ozone have occurred since the preindustrial era as a result of increasing anthropogenic emissions. Within ACCENT, a European Network of Excellence, ozone changes between 1850 and 2000 are assessed for the troposphere and the lower stratosphere ( up to 30 km) by a variety of seven chemistry-climate models and three chemical transport models. The modeled ozone changes are taken as input for detailed calculations of radiative forcing. When only changes in chemistry are considered ( constant climate) the modeled global-mean tropospheric ozone column increase since preindustrial times ranges from 7.9 DU to 13.8 DU among the ten participating models, while the stratospheric column reduction lies between 14.1 DU and 28.6 DU in the models considering stratospheric chemistry. The resulting radiative forcing is strongly dependent on the location and altitude of the modeled ozone change and varies between 0.25 Wm(-2) and 0.45 Wm(-2) due to ozone change in the troposphere and - 0.123 Wm(-2) and + 0.066 Wm(-2) due to the stratospheric ozone change. Changes in ozone and other greenhouse gases since preindustrial times have altered climate. Six out of the ten participating models have performed an additional calculation taking into account both chemical and climate change. In most models the isolated effect of climate change is an enhancement of the tropospheric ozone column increase, while the stratospheric reduction becomes slightly less severe. In the three climate-chemistry models with detailed tropospheric and stratospheric chemistry the inclusion of climate change increases the resulting radiative forcing due to tropospheric ozone change by up to 0.10 Wm(-2), while the radiative forcing due to stratospheric ozone change is reduced by up to 0.034 Wm(-2). Considering tropospheric and stratospheric change combined, the total ozone column change is negative while the resulting net radiative forcing is positive.
Abstract: [ 1] We investigate how El Nino Southern Oscillation (ENSO) influences tropospheric ozone and its precursors in a coupled climate-chemistry model. As shown in similar studies, tropospheric column ozone (TCO) decreases in the central and east Pacific and increases in the west Pacific/Indonesia in response to circulation and convective changes during El Nino conditions. Simulated changes in TCO for "peak'' El Nino events in the central and east Pacific are in good agreement but are underestimated in the west Pacific compared to previous observational and modeling studies for October 1997. Tropospheric column-average NOx decreases over Indonesia and generally over South America as a result of suppressed convection and lightning over these land regions. NOx and HOx changes during El Nino modify ozone chemical production and destruction. When we include annually varying biomass burning emissions in our model simulations we find that these emissions peak over Indonesia 1 - 2 months in advance of the peak elevated sea-surface temperatures (SSTs) and hence the "meteorological'' El Nino. We underestimate the strength of the TCO increase due to El Nino - related dry conditions over Indonesia in October 1997 compared to observations. We also examine how future mean and variability changes in ENSO, as simulated in the HadCM3 climate model, impacts tropospheric ozone. A mean future El Nino - like state is simulated in the tropical Pacific in HadCM3, but this has no discernable impact on the future TCO trend in this region. However, we do simulate increased variability in precipitation and TCO related to ENSO in the future.
Abstract: We analyze present-day and future carbon monoxide (CO) simulations in 26 state-of-the-art atmospheric chemistry models run to study future air quality and climate change. In comparison with near-global satellite observations from the MOPITT instrument and local surface measurements, the models show large underestimates of Northern Hemisphere (NH) extratropical CO, while typically performing reasonably well elsewhere. The results suggest that year-round emissions, probably from fossil fuel burning in east Asia and seasonal biomass burning emissions in south-central Africa, are greatly underestimated in current inventories such as IIASA and EDGAR3.2. Variability among models is large, likely resulting primarily from intermodel differences in representations and emissions of nonmethane volatile organic compounds (NMVOCs) and in hydrologic cycles, which affect OH and soluble hydrocarbon intermediates. Global mean projections of the 2030 CO response to emissions changes are quite robust. Global mean midtropospheric (500 hPa) CO increases by 12.6 +/- 3.5 ppbv (16%) for the high-emissions (A2) scenario, by 1.7 +/- 1.8 ppbv (2%) for the midrange (CLE) scenario, and decreases by 8.1 +/- 2.3 ppbv (11%) for the low-emissions (MFR) scenario. Projected 2030 climate changes decrease global 500 hPa CO by 1.4 +/- 1.4 ppbv. Local changes can be much larger. In response to climate change, substantial effects are seen in the tropics, but intermodel variability is quite large. The regional CO responses to emissions changes are robust across models, however. These range from decreases of 10-20 ppbv over much of the industrialized NH for the CLE scenario to CO increases worldwide and year-round under A2, with the largest changes over central Africa (20-30 ppbv), southern Brazil (20-35 ppbv) and south and east Asia (30-70 ppbv). The trajectory of future emissions thus has the potential to profoundly affect air quality over most of the world's populated areas.
Abstract: [1] We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: ( 1) current air quality legislation (CLE); ( 2) an optimistic case of the maximum emissions reductions currently technologically feasible ( MFR); and ( 3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60 - 70% of the model-calculated wet deposition rates agree to within +/- 50% with quality-controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to +/- 1 sigma variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36 - 51% of all NOy, NHx, and SOx is deposited over the ocean, and 50 - 80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the "critical load'' threshold of 1000 mg(N) m(-2) yr(-1). The regions most affected are the United States (20% of vegetation), western Europe ( 30%), eastern Europe ( 80%), South Asia (60%), East Asia 40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m(-2) yr(-1) increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.
Abstract: We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997 - 2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10: 30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30 - 50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions ( between - 25% and + 10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81 - 0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10: 30 local time ( by 2.5 - 4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals ( 10 - 50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.
Abstract: Global tropospheric ozone distributions, budgets, and radiative forcings from an ensemble of 26 state-of-the-art atmospheric chemistry models have been intercompared and synthesized as part of a wider study into both the air quality and climate roles of ozone. Results from three 2030 emissions scenarios, broadly representing "optimistic,'' "likely,'' and "pessimistic'' options, are compared to a base year 2000 simulation. This base case realistically represents the current global distribution of tropospheric ozone. A further set of simulations considers the influence of climate change over the same time period by forcing the central emissions scenario with a surface warming of around 0.7K. The use of a large multimodel ensemble allows us to identify key areas of uncertainty and improves the robustness of the results. Ensemble mean changes in tropospheric ozone burden between 2000 and 2030 for the 3 scenarios range from a 5% decrease, through a 6% increase, to a 15% increase. The intermodel uncertainty (+/-1 standard deviation) associated with these values is about +/-25%. Model outliers have no significant influence on the ensemble mean results. Combining ozone and methane changes, the three scenarios produce radiative forcings of -50, 180, and 300 mW m(-2), compared to a CO2 forcing over the same time period of 800-1100 mW m(-2). These values indicate the importance of air pollution emissions in short-to medium-term climate forcing and the potential for stringent/lax control measures to improve/worsen future climate forcing. The model sensitivity of ozone to imposed climate change varies between models but modulates zonal mean mixing ratios by +/-5 ppbv via a variety of feedback mechanisms, in particular those involving water vapor and stratosphere-troposphere exchange. This level of climate change also reduces the methane lifetime by around 4%. The ensemble mean year 2000 tropospheric ozone budget indicates chemical production, chemical destruction, dry deposition and stratospheric input fluxes of 5100, 4650, 1000, and 550 Tg(O-3) yr(-1), respectively. These values are significantly different to the mean budget documented by the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). The mean ozone burden (340 Tg(O-3)) is 10% larger than the IPCC TAR estimate, while the mean ozone lifetime (22 days) is 10% shorter. Results from individual models show a correlation between ozone burden and lifetime, and each model's ozone burden and lifetime respond in similar ways across the emissions scenarios. The response to climate change is much less consistent. Models show more variability in the tropics compared to midlatitudes. Some of the most uncertain areas of the models include treatments of deep tropical convection, including lightning NOx production; isoprene emissions from vegetation and isoprene's degradation chemistry; stratosphere-troposphere exchange; biomass burning; and water vapor concentrations.
Abstract: Model-predicted atmospheric concentrations of Rn-222 based on two different Rn-222 source terms have been compared with observations in the lower troposphere. One simulation used a globally uniform Rn-222 source term from ice-free, the other assumed a northwards-decreasing source term (linear decrease from I atom land surfaces of I atom cm(-2) s(-1). cm(-2) s(-1) at 30 degrees N to 0.2 atom cm(-2) s(-1) at 70 degrees N). Zero emissions were assigned to oceans. The northwards-decreasing source term improved predictions at four out of six stations north of 50 degrees N, reducing the mean prediction/observation ratio from 2.8 to 0.87. In the latitudinal band between 30 degrees N and 50 degrees N, the northwards-decreasing source term resulted in systematic under-prediction of atmospheric Rn-222, whereas the uniform source term provided predictions close to observations. Predictions based on the northwards-decreasing source term were significantly (p < 0.01) better than those based on the uniform source term for an averaged vertical Rn-222 profile around 44 degrees N. but were not for one around 38 degrees N. The results indicate that a northwards-decreasing source term could be a more realistic representation of actual Rn-222 emissions than a uniform 1 atom cm(-2) s(-1) source term. However, the decrease in Rn-222 source strength with increasing latitude might not begin at 30 degrees N but somewhat further north. This hypothesis should be investigated through model-independent means.
Abstract: Two coupled climate-chemistry model experiments for the period 1990-2030 were conducted: one with a fixed climate and the other with a varying climate forced by the is92a scenario. By comparing results from these experiments we have attempted to identify changes and variations in physical climate that may have important influences upon tropospheric chemical composition. Climate variables considered include: temperature, humidity, convective mass fluxes, precipitation, and the large-scale circulation. Increases in humidity, directly related to increases in temperature, exert a major influence on the budgets of ozone and the hydroxyl radical: decreasing O-3 and increasing OH. Warming enhances decomposition of PAN, releasing NOx, and increases the rate of methane oxidation. Surface warming enhances vegetation emissions of isoprene, an important ozone precursor. In the changed climate, tropical convection generally reduces, but penetrates to higher levels. Over northern continents, convection tends to increase. These changes in convection affect both vertical mixing and lightning NOx emissions. We find no global trend in lightning emissions, but significant changes in its distribution. Changes in precipitation and the large-scale circulation are less important for composition, at least in these experiments. Higher levels of the oxidants OH and H2O2 lead to increases in aerosol formation and concentrations. These results indicate that climate-chemistry feedbacks are dominantly negative (less O-3, a shorter CH4 lifetime, and more aerosol). The major mode of inter-annual variability in the is92a climate experiment is ENSO. This strongly modulates isoprene emissions from vegetation via tropical land surface temperatures. ENSO is also clearly the dominant source of variability in tropical column ozone, mainly through changes in the distribution of convection. The magnitude of inter-annual variability in ozone is comparable to the changes brought about by emissions and climate changes between the 1990s and 2020s, suggesting that it will be difficult to disentangle the different components of near-future changes.
Abstract: The impact of convection on tropospheric O-3 and its precursors has been examined in a coupled chemistry-climate model. There are two ways that convection affects O-3. First, convection affects O-3 by vertical mixing of O-3 itself. Convection lifts lower tropospheric air to regions where the O-3 lifetime is longer, whilst mass-balance subsidence mixes O-3-rich upper tropospheric (UT) air downwards to regions where the O-3 lifetime is shorter. This tends to decrease UT O-3 and the overall tropospheric column of O-3. Secondly, convection affects O-3 by vertical mixing of O-3 precursors. This affects O-3 chemical production and destruction. Convection transports isoprene and its degradation products to the UT where they interact with lightning NOx to produce PAN, at the expense of NOx. In our model, we find that convection reduces UT NOx through this mechanism; convective down-mixing also flattens our imposed profile of lightning emissions, further reducing UT NOx. Over tropical land, which has large lightning NOx emissions in the UT, we find convective lofting of NOx from surface sources appears relatively unimportant. Despite UT NOx decreases, UT O-3 production increases as a result of UT HOx increases driven by isoprene oxidation chemistry. However, UT O-3 tends to decrease, as the effect of convective overturning of O-3 itself dominates over changes in O-3 chemistry. Convective transport also reduces UT O-3 in the mid-latitudes resulting in a 13% decrease in the global tropospheric O-3 burden. These results contrast with an earlier study that uses a model of similar chemical complexity. Differences in convection schemes as well as chemistry schemes - in particular isoprene-driven changes are the most likely causes of such discrepancies. Further modelling studies are needed to constrain this uncertainty range.
Abstract: To explore the relationship between tropospheric ozone and radiative forcing with changing emissions, we compiled two sets of global scenarios for the emissions of the ozone precursors methane (CH4), carbon monoxide (CO), non-methane volatile organic compounds (NMVOC) and nitrogen oxides (NOx) up to the year 2030 and implemented them in two global Chemistry Transport Models. The "Current Legislation" (CLE) scenario reflects the current perspectives of individual countries on future economic development and takes the anticipated effects of presently decided emission control legislation in the individual countries into account. In addition, we developed a "Maximum technically Feasible Reduction" (MFR) scenario that outlines the scope for emission reductions offered by full implementation of the presently available emission control technologies, while maintaining the projected levels of anthropogenic activities. Whereas the resulting projections of methane emissions lie within the range suggested by other greenhouse gas projections, the recent pollution control legislation of many Asian countries, requiring introduction of catalytic converters for vehicles, leads to significantly lower growth in emissions of the air pollutants NOx, NMVOC and CO than was suggested by the widely used and more pessimistic IPCC (Intergovernmental Panel on Climate Change) SRES (Special Report on Emission Scenarios) scenarios (Nakicenovic et al., 2000), which made Business-as-Usual assumptions regarding emission control technology. With the TM3 and STOCHEM models we performed several long-term integrations (1990 2030) to assess global, hemispheric and regional changes in CH4, CO, hydroxyl radicals, ozone and the radiative climate forcings resulting from these two emission scenarios. Both models reproduce broadly the observed trends in CO, and CH4 concentrations from 1990 to 2002. For the "current legislation" case, both models indicate an increase of the annual average ozone levels in the Northern Hemisphere by 5 ppbv, and up to 15 ppbv over the Indian sub-continent, comparing the 2020s (2020-2030) with the 1990s (1990 2000). The corresponding higher ozone and methane burdens in the atmosphere increase radiative forcing by approximately 0.2 Wm(-2). Full application of today's emissions control technologies, however, would bring down ozone below the levels experienced in the 1990s and would reduce the radiative forcing of ozone and methane to approximately -0.1 Wm(-2). This can be compared to the 0.14-0.47 Wm(-2) increase of methane and ozone radiative forcings associated with the SRES scenarios. While methane reductions lead to lower ozone burdens and to less radiative forcing, further reductions of the air pollutants NOx and NMVOC result in lower ozone, but at the same time increase the lifetime of methane. Control of methane emissions appears an efficient option to reduce tropospheric ozone as well as radiative forcing.
Abstract: A global three-dimensional Lagrangian chemistry-transport model STOCHEM is used to describe the European regional acid deposition and ozone air quality impacts along the Atlantic Ocean seaboard of Europe, from the SO2, NOx, VOCs and CO emissions from international shipping under conditions appropriate to the year 2000. Model-derived total sulfur deposition from international shipping reaches over 200 mg S M-2 yr(-1) over the southwestern approaches to the British Isles and Brittany. The contribution from international shipping to surface ozone concentrations during the summertime, peaks at about 6 ppb over Ireland, Brittany and Portugal. Shipping emissions act as an external influence on acid deposition and ozone air quality within Europe and may require control actions in the future if strict deposition and air quality targets are to be met.
Abstract: A global three-dimensional Lagrangian chemistry-transport model is used to describe the formation, transport and destruction of tropospheric ozone using 1990s global emissions and 1998 meteorological archives. Using a labelling technique, the geographical origins of the ozone formed within the troposphere have been revealed, showing whether the ozone found at the surface in Europe has had its origins above the continents of North America, Europe or Asia or elsewhere in the world. In this way, contributions to the ozone found at 21 surface monitoring sites across Europe can be attributed to production over North America and Asia, demonstrating that intercontinental ozone transport is an efficient process. Sensitivity tests to the global man-made sources of NO, and carbon monoxide indicate that global ozone precursor emission controls may contribute towards reaching regional air quality policy goals for ozone in Europe. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: [1] A chemistry-climate model has been applied to study the radiative forcings generated by aircraft NOx emissions through changes in ozone and methane. Four numerical experiments, where an extra pulse of aircraft NOx was emitted into the model atmosphere for a single month ( January, April, July, or October), were compared to a control experiment, allowing the aircraft impact to be isolated. The extra NOx produces a short-lived ( few months) pulse of ozone that generates a positive radiative forcing. However, the NOx and O-3 both generate OH, which leads to a reduction in CH4. A detailed analysis of the OH budget reveals the spatial structure and chemical reactions responsible for the generation of the OH perturbation. Methane's long lifetime means that the CH4 anomaly decays slowly ( perturbation lifetime of 11.1 years). The negative CH4 anomaly also has an associated negative O-3 anomaly, and both of these introduce a negative radiative forcing. There are important seasonal differences in the response of O-3 and CH4 to aircraft NOx, related to the annual cycle in photochemistry; the O-3 radiative forcing calculations also have a seasonal dependence. The long-term globally integrated annual mean net forcing calculated here is approximately zero, although earlier work suggests a small net positive forcing. The model design (e.g., upper tropospheric chemistry, convection parameterization) and experimental setup ( pulse magnitude and duration) may somewhat influence the results: further work with a range of models is required to confirm these results quantitatively.
Abstract: The long 1783-1784 eruption of Laki in southern Iceland, was one of the first eruptions to have been linked to an observed climate anomaly, having been held responsible for cold temperatures over much of the Northern Hemisphere in the period 1783-1785. Results from the first climate model simulation of the impact of a similar eruption to that of 1783-1784 are presented. Using sulphate aerosol fields produced in a companion chemical transport model simulation by Stevenson et al. (2003), the radiative forcing and climate response due to the aerosol are calculated here using the Reading Intermediate General Circulation Model (IGCM). The peak Northern Hemisphere mean direct radiative forcing is -5.5 Wm(-2) in August 1783. The radiative forcing dies away quickly as the emissions from the volcano decrease; however, a small forcing remains over the Mediterranean until March 1784. There is little forcing in the Southern Hemisphere. There is shown to be an uncertainty of at least 50% in the direct radiative forcing due to assumptions concerning relative humidity and the sophistication of the radiative transfer code used. The indirect effects of the Laki aerosol are potentially large but essentially unquantifiable at the present time. In the IGCM at least, the aerosol from the eruption produces a climate response that is spatially very variable. The Northern Hemisphere mean temperature anomaly averaged over the whole of the calendar year containing most of the eruption is -0.21 K, statistically significant at the 95% level and in reasonable agreement with the available observations of the temperature during 1783.
Abstract: Ozone is an air quality problem today for much of the world's population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.
Abstract: In this study we examine the anthropogenically forced climate response over the historical period, 1860 to present, and projected response to 2100, using updated emissions scenarios and an improved coupled model (HadCM3) that does not use flux adjustments. We concentrate on four new Special Report on Emission Scenarios (SRES) namely (A1FI, A2, B2, B1) prepared for the Intergovernmental Panel on Climate Change Third Assessment Report, considered more self-consistent in their socio-economic and emissions structure, and therefore more policy relevant, than older scenarios like IS92a. We include an interactive model representation of the anthropogenic sulfur cycle and both direct and indirect forcings from Sulfate aerosols, but omit the second indirect forcing effect through cloud lifetimes. The modelled first indirect forcing effect through cloud droplet size is near the centre of the IPCC uncertainty range. We also model variations in tropospheric and stratospheric ozone. Greenhouse gas-forced climate change response in B2 resembles patterns in IS92a but is smaller. Sulfate aerosol and ozone forcing substantially modulates the response, cooling the land, particularly northern mid-latitudes, and altering the monsoon structure. By 2100, global mean warming in SRES scenarios ranges from 2.6 to 5.3 K above 1900 and precipitation rises by 1%/K through the twenty first century (1.4%/K omitting aerosol changes). Large-scale patterns of response broadly resemble those in an earlier model (HadCM2). but with important regional differences, particularly in the tropics. Some divergence in future response occurs across scenarios for the regions considered, but marked drying in the mid-USA and southern Europe and significantly wetter conditions for South Asia, in June-July-August, are robust and significant.
Abstract: Results from the first chemistry-transport model study of the impact of the 1783 - 1784 Laki fissure eruption (Iceland: 64 degreesN, 17 degreesW) upon atmospheric composition are presented. The eruption released an estimated 61 Tg(S) as SO2 into the troposphere and lower stratosphere. The model has a high resolution tropopause region, and detailed sulphur chemistry. The simulated SO2 plume spreads over much of the Northern Hemisphere, polewards of similar to40 degreesN. About 70% of the SO2 gas is directly deposited to the surface before it can be oxidised to sulphuric acid aerosol. The main SO2 oxidants, OH and H2O2, are depleted by up to 40% zonally, and the lifetime of SO2 consequently increases. Zonally averaged tropospheric SO2 concentrations over the first three months of the eruption exceed 20 ppbv, and sulphuric acid aerosol reaches similar to2 ppbv. These compare to modelled preindustrial/ present-day values of 0.1/0.5 ppbv SO2 and 0.1/1.0 ppbv sulphate. A total sulphuric acid aerosol yield of 17 - 22 Tg( S) is produced. The mean aerosol lifetime is 6 - 10 days, and the peak aerosol loading of the atmosphere is 1.4-1.7 Tg(S) ( equivalent to 5.9-7.1 Tg of hydrated sulphuric acid aerosol). These compare to modelled pre-industrial/presentday sulphate burdens of 0.28/0.81 Tg( S), and lifetimes of 6/5 days, respectively. Due to the relatively short atmospheric residence times of both SO2 and sulphate, the aerosol loading approximately mirrors the temporal evolution of emissions associated with the eruption. The model produces a reasonable simulation of the acid deposition found in Greenland ice cores. These results appear to be relatively insensitive to the vertical profile of emissions assumed, although if more of the emissions reached higher levels (> 12 km), this would give longer lifetimes and larger aerosol yields. Introducing the emissions in episodes generates similar results to using monthly mean emissions, because the atmospheric lifetimes are similar to the repose periods between episodes. Most previous estimates of the global aerosol loading associated with Laki did not use atmospheric models; this study suggests that these earlier estimates have been generally too large in magnitude, and too long-lived. Environmental effects following the Laki eruption may have been dominated by the widespread deposition of SO2 gas rather than sulphuric acid aerosol.
Abstract: Radiative forcing due to changes in ozone is expected for the 21st century. An assessment on changes in the tropospheric oxidative state through a model intercomparison ("OxComp'') was conducted for the IPCC Third Assessment Report (IPCC-TAR). OxComp estimated tropospheric changes in ozone and other oxidants during the 21st century based on the "SRES'' A2p emission scenario. In this study we analyze the results of 11 chemical transport models (CTMs) that participated in OxComp and use them as input for detailed radiative forcing calculations. We also address future ozone recovery in the lower stratosphere and its impact on radiative forcing by applying two models that calculate both tropospheric and stratospheric changes. The results of OxComp suggest an increase in global-mean tropospheric ozone between 11.4 and 20.5 DU for the 21st century, representing the model uncertainty range for the A2p scenario. As the A2p scenario constitutes the worst case proposed in IPCC-TAR we consider these results as an upper estimate. The radiative transfer model yields a positive radiative forcing ranging from 0.40 to 0.78 W m(-2) on a global and annual average. The lower stratosphere contributes an additional 7.5-9.3 DU to the calculated increase in the ozone column, increasing radiative forcing by 0.15-0.17 W m(-2). The modeled radiative forcing depends on the height distribution and geographical pattern of predicted ozone changes and shows a distinct seasonal variation. Despite the large variations between the 11 participating models, the calculated range for normalized radiative forcing is within 25%, indicating the ability to scale radiative forcing to global-mean ozone column change.
Abstract: A global 3-D Lagrangian chemistry-transport model STOCHEM is used to describe the tropospheric distributions of four components of the secondary atmospheric aerosol: nitrate, sulphate, ammonium and organic compounds. The model describes the detailed chemistry of the formation of the acid precursors from the oxidation of SO2, DMS, NOx, NH3 and terpenes and their uptake into the aerosol. Model results are compared in some detail with the available surface observations. Comparisons are made between the global budgets and burdens found in other modelling studies. The global distributions of the total mass of secondary aerosols have been estimated for the pre-industrial, present day and 2030 emissions and large changes have been estimated in the mass fractions of the different secondary aerosol components.
Abstract: [1] This paper investigates the impact of circulation changes in a changed climate on the exchange of ozone between the stratosphere and the troposphere. We have identified an increase in the net transport of ozone into the troposphere in the future climate of 37%, although a decreased ozone lifetime means that the overall tropospheric burden decreases. There are regions in the midlatitudes to high latitudes where surface ozone is predicted to increase in the spring. However, these increases are not significant. Significant ozone increases are predicted in regions of the upper troposphere. The general increase in the stratospheric contribution (O(3)s tracer) to tropospheric ozone in the climate changed scenario indicates that the stratosphere will play an even more significant role in the future.
Abstract: Oxidation by hydroxyl radicals is the main removal process for organic compounds in the troposphere. This oxidation acts as a source of ozone and as a removal process for hydroxyl and peroxy radicals, thereby reducing the efficiency of methane oxidation and promoting the build-up of methane. Emissions of organic compounds may therefore lead to the build-up of two important radiatively-active trace gases: methane and ozone. Emission pulses of 10 organic compounds were followed in a global 3-D Lagrangian chemistry-transport model to quantify their indirect greenhouse gas impacts through changes induced in the tropospheric distributions of methane and ozone. The main factors influencing the global warming potentials of the 10 organic compounds were found to be their spatial emission patterns, chemical reactivity and transport, molecular complexity and oxidation products formed. The indirect radiative forcing impacts of organic compounds may be large enough that ozone precursors should be considered in the basket of trace gases through which policy-makers aim to combat global climate change.
Abstract: [1] We assess the contribution made to the interannual variability of the global methane accumulation rate from its atmospheric sink using the STOCHEM tropospheric chemistry model coupled to the HadCM3 climate model. For both control and climate change scenarios, the standard deviation of the detrended accumulation rate was 1.4 ppbv/yr for the period 1990-2009, compared with the measured standard deviation of 3.1 ppbv/yr for the period 1984-1999. As the model emissions have no variability, the methane sink processes in the model are responsible for all the simulated variability of the methane accumulation rate. This appears to explain a significant fraction of the observed variability and was well correlated with simulated water vapour. The largest component of the model interannual variability is derived from the El-Nino Southern Oscillation cycle in the coupled Ocean-Atmosphere model, and this mode of variation is shown to be present in the methane accumulation rate.
Abstract: We have developed a detailed parametrization scheme to represent the effects of subgrid-scale convective transport in a three-dimensional chemistry-tran sport model (CTM). The CTM utilize.,, the meteorological fields generated by a general-circulation model (GCM) to redistribute over 70 chemical species. The convective transport is implemented using the convective mass fluxes. entrainment rates and detrainment rates from the GCM. We compare the modelled distributions of Rn-222 with observations. This shows that the vertical profile of this species is affected by the choice of convective-transport parametrization, The neck parametrization is found to improve significantly the simulation of Rn-222 over the summertime continents.
Abstract: We present results from two experiments carried out with a coupled ocean-atmosphere-tropospheric chemistry model run continously over the period 1990 to 2100. In the control experiment, climate is unforced, but emissions of trace gases to the chemical model increase in line with an illustrative scenario for future trace gas emissions with medium high growth. In the climate change experiment trace gas emissions are identical to the control, but climate is also forced using greenhouse gas concentrations and SO2 emissions from the same scenario. Global average methane in the climate change experiment increased from 1670 ppbv in 1990 to 3230 ppbv Ly 2100, compared to 3650 ppbv by 2100 in the control. The methane increase in the control experiment is therefore 27 % more than in the control. This difference is due to both temperature and OH changes which increase the rate of methane oxidation and act in the opposite direction to the negative feedback of methane on itself through OH. Mid-latitude northern hemisphere ozone concentrations in July for the mid-troposphere rose from 39 ppbv in 1990s to 64 ppbv in the 2090s in the control experiment and to 49 ppbv in the climate change experiment. The direct role of climate change is therefore predicted to be a negative feedback on the radiative forcing from the change to tropospheric ozone and methane concentrations.
Abstract: The global three-dimensional Lagrangian chemistry-transport model STOCHEM has been used to follow the changes in the tropospheric distributions of the two major radiatively-active trace gases, methane and tropospheric ozone, following the emission of pulses of the short-lived tropospheric ozone precursor species, methane, carbon monoxide, NOx and hydrogen. The radiative impacts of NOx emissions were dependent on the location chosen for the emission pulse, whether at the surface or in the upper troposphere or whether in the northern or southern hemispheres. Global warming potentials were derived for each of the short-lived tropospheric ozone precursor species by integrating the methane and tropospheric ozone responses over a 100 year time horizon. Indirect radiative forcing due to methane and tropospheric ozone changes appear to be significant for all of the tropospheric ozone precursor species studied. Whereas the radiative forcing from methane changes is likely to be dominated by methane emissions, that from tropospheric ozone changes is controlled by all the tropospheric ozone precursor gases, particularly NOx emissions. The indirect radiative forcing impacts of tropospheric ozone changes may be large enough such that ozone precursors should be considered in the basket of trace gases through which policy-makers aim to combat global climate change.
Abstract: We present a range of estimates for future radiative forcings due to changes in tropospheric ozone (O-3T) Ozone distributions were generated by the UKMO 3-D chemistry-transport model for 1990, 2030, 2060, and 2100, using four sets of boundary conditions. Anthropogenic emissions evolved following either the IPCC SRES "high" (A2) or "central" (B2) case. Each scenario was run with both a fixed (1990) climate, and with a changing climate, as generated by a coupled ocean-atmosphere GCM, forced with IS92a emissions. Calculated global mean O-3T radiative forcings for the A2 (B2) cases for 1990-2100 were +0.43 (+0.22) W m(-2) when climate change was ignored; these fell to +0.27 (+0.09) W m(-2) when climate change was included. Without climate change, CH4 lifetimes (tau(CH4)) lengthened by 7-12 % between 1990 and 2100; however, when climate change was included, tau(CH4), fell by 0-5 %. Hence climate warming exerts a negative feedback on itself by enhancing O-3T and CH4 destruction.
Abstract: Because of global-scale increases in trace gas emissions, ozone concentrations in northern hemisphere may increase over the next decade, driving up ozone concentrations within Europe.:Over this same period, policy actions are anticipated which will reduce the internal European regional-scale ozone Production capacity. The overall success of these regional policies will be determined by the resultant of these global- and regional-scale influences. A global three-dimensional Lagrangian chemistry model STOCHEM has been used to look at the relative magnitudes of these two influences on the European regional ozone distribution under some illustrative emission scenarios up to the year 2015. Substantial reductions in European NOx emissions should bring a significant improvement in ozone air quality, but they may not be enough to keep future peak ozone levels below internationally accepted environmental criteria without action: on the global scale to control emissions of tropospheric ozone precursors: methane, carbon monoxide, NOx and VOCs. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: The spatial and temporal distribution of tropospheric ozone is controlled by transport processes, including advection, convection and dispersion and by stratosphere-troposphere exchange, surface deposition to the earth's surface and by photochemical production and destruction within the troposphere itself. These processes have been represented in some detail in a global three-dimensional Lagrangian chemistry (STOCHEM) model which has been used to construct global ozone budgets and identify the main features in the spatial distribution of daily ozone tendencies. The annually and spatially integrated net chemical production of ozone is about twice as large as the stratosphere-troposphere exchange flux, so that the concentration-dependent surface deposition balancing term is about three times larger than the stratosphere-troposphere exchange term. The total production and loss terms for ozone by tropospheric photochemistry are much greater than the net chemical production, with total chemical production about five times larger than the net term. The ozone turnover time is therefore around 30 days, around one tenth of the turnover time due to stratosphere-troposphere exchange alone. Human activities may influence future tropospheric ozone levels through at least two distinct mechanisms: first, increasing emissions of tropospheric ozone precursor gases: methane, oxides of nitrogen, carbon monoxide and hydrocarbons, leading to increased ozone levels; second, human-induced climate change may lead to increased temperatures and water vapour concentrations, and hence increased ozone destruction and decreased ozone concentrations.
Abstract: We report on results from a World Climate Research Program workshop on representations of scavenging and deposition processes in global transport models of the atmosphere. 15 models were evaluated by comparing simulations of radon, lead, sulfur dioxide, and sulfate against each other, and against observations of these constituents. This paper provides a survey on the simulation differences between models. It identifies circumstances where models are consistent with observations or with each other, and where they differ from observations or with each other. The comparison shows that most models are able to simulate seasonal species concentrations near the surface over continental sites to within a factor of 2 over many regions of the globe. Models tend to agree more closely over source (continental) regions than for remote (polar and oceanic) regions. Model simulations differ most strongly in the upper troposphere for species undergoing wet scavenging processes. There are not a sufficient number of observations to characterize the climatology (long-term average) of species undergoing wet scavenging in the upper troposphere. This highlights the need for either a different strategy for model evaluation (e.g., comparisons on an event by event basis) or many more observations of a few carefully chosen constituents.
Abstract: A Lagrangian chemistry-transport model (STOCHEM) was driven with meteorology derived from a slab ocean general circulation model for conditions appropriate to the present-day and at double CO2, and with emission scenarios appropriate for present day conditions and for the year 2075. The results show conclusively that the effect of including the predicted chang eu to future climate is to reduce the simulated tropospheric ozone concentrations. The response of global tropospheric ozone in the period 1990-2075 was an increase of 6.4 ppb when both climate and emissions changes were included, compared to an increase of 10.3 ppb when only emissions changes were considered. This difference is mainly due to water vapor and temperature increases, together with some dynamical effects. There are considerable changes to other tropospheric oxidants, with OH, HO2, and H2O2 all increasing considerably in response to climate changes. In contrast, OH decreases when only the emissions are allowed to change. A replicate run of the control scenario with STOCHEM using; a different year of meteorology showed considerable interannual variability in local monthly mean ozone concentrations.
Abstract: This paper presents a model study of the changes in upper tropospheric HOx (= OH + HO2) due to upward convective transport of surface pollutants. The model used is a three-dimensional global Lagrangian tropospheric chemistry transport model of 70 chemical species and 150 reactions including nonmethane hydrocarbon chemistry. It is driven by meteorological data from the U.K. Meterological Office with a 6 hour time resolution. We find that the effect of convection is to increase upper tropospheric (300-200 hPa) HOx globally by over 50%. The effect is greatest over the tropical continents where convection and VOC emissions from vegetation are colocated. The convection of isoprene, and hydroperoxides has the greatest effect. Convecting formaldehyde and acetone has a lesser effect. The contribution from isoprene depends more on the convection of its degradation products than the convection of isoprene itself. The upper tropospheric HOx budget is shown to be very sensitive to the model implementation of convective wet deposition.
Abstract: The tall, aerodynamically rough surfaces of forests provide for the efficient exchange of heat and momentum between terrestrial surfaces and the atmosphere. The same properties of forests also provide for large potential rates of deposition of pollutant gases, aerosols and cloud droplets. For some reactive pollutant gases, including SO2, HNO3 and NH3, rates of deposition may be large and substantially larger than onto shorter vegetation and is the cause of the so called "filtering effect" of forest canopies. Pollutant inputs to moorland and forest have been compared using measured ambient concentrations from an unpolluted site in southern Scotland and a more polluted site in south eastern Germany. The inputs of S and N to forest at the Scottish site exceed moorland by 16% and 31% respectively with inputs of 7.3 kg S ha(-1) y and 10.6 kg N ha(-1) y(-1). At the continental site inputs to the forest were 43% and 48% larger than over moorland for S and N deposition with totals of 53.6 kg S ha(-1) y(-1) and 69.5 kg N ha(-)1 y(-)1 respectively. The inputs of acidity to global forests show that in 1985 most of the areas receiving > 1 kg H+ ha(-1) y(-1) as S are in the temperate latitudes, with 8% of total global forest exceeding this threshold. By 2050, 17% of global forest will be receiving > 1 kg H-1 ha(-1) as S and most of the increase is in tropical and sub-tropical countries. Forests throughout the world are also exposed to elevated concentrations of ozone. Taking 60 ppb O-3 as a concentration likely to be phytotoxic to sensitive forest species, a global model has been used to simulate the global exposure of forests to potentially phytotoxic O-3 concentrations for the years 1860, 1950, 1970, 1990 and 2100. The model shows no exposure to concentrations in excess of 60 ppb in 1860, and of the 6% of global forest exposed to concentrations > 60 ppb in 1950, 75% were in temperate latitudes and 25% in the tropics. By 1990 24% of global forest is exposed to O-3 concentrates > 60 ppb, and this increases to almost 50% of global forest by 2100. While the uncertainty in the future pollution climate of global forest is considerable, the likely impact of O-3 and acid deposition is even more difficult to assess because of interactions between these pollutants and substantial changes in ambient CO2 concentration, N deposition and climate over the same period, but the effects are unlikely to be beneficial overall.
Abstract: We present the first estimate of the evolution of tropospheric ozone (O-3(T)) radiative forcing since 1860 and into the future. The UKMO 3-D chemistry-transport model (STOCHEM) was used to simulate the tropospheric composition in 1860, 1950, 1970, 1990 and 2100, by changing trace gas emissions. The future scenario used a doubled CO2 climate. STOCHEM includes extensive non-methane hydrocarbon (NMHC) chemistry, and produces a reasonable simulation of present-day O-3(T) Radiative forcings caused by the modelled changes in O-3(T) since 1860 were calculated using the UKMO radiation code, and included clouds and stratospheric temperature adjustment. Calculated changes in the global annual mean forcing since 1860 were 0.13, 0.22, 0.29 and 0.48 W m(-2) for the four years. Up to 1990 this forcing scales linearly with the change in total NOx emissions since 1860; this linearity breaks down in 2100. The 1990 forcing is at the lower end of the range from previous modelling studies (0.28 - 0.51 W m(-2)), but is still significant, enhancing the well-mixed greenhouse gas forcing by over 10%.
Abstract: The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO2-saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission fate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power. and inversely proportional to the degassed mag ma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this damp ens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the similar to 700kg s(-1) magmatic water flux can be modelled with a 4-10 m radius conduit, degassing 20-100% of the available water and all of the 1 to 4 vol.% CO2 chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the similar to 500 kg s(-1) magmatic water flux can be modelled with a 22-60 m radius conduit, degassing similar to 2-90% of the available water and all of the 0.1 to 3 vol.% CO2 chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes.
Abstract: Transport of the short-lived (half-life 3.83 days) isotope Rn-222, which is emitted from unfrozen soils, is used to compare transport in several versions of the UK Meteorological Office Lagrangian chemistry-transport model and in the Unified Model, the UK Meteorological Office general circulation model. The same Rn-222 experiment is repeated for all the model versions, illustrating the impact on global transport of various model improvements: adding boundary-layer schemes, including sub-grid scale convection, increasing model spatial resolution, and increasing the temporal resolution of the meteorological fields used for driving the off-line model. Results from all model versions are compared with a limited observational data set, and also with results from the same Rn-222 simulations carried out with several global atmospheric transport models as part of the World Climate Research Program in December 1993 (Jacob et al. 1997). Versions of the Lagrangian chemistry-transport model that include sub-grid scale convection, transport Rn-222 in a manner that is similar to the Unified Model and most general circulation models, supporting the simple and computationally inexpensive Lagrangian approach taken.
Abstract: For volcanoes characterised by activity from open degassing conduits and fumarole fields, thermal data from remote sensing instruments can provide an integrated data set capable of measuring various volcanic system parameters (e.g., magma depth, thermal flux and vent areas) and of mapping the distribution of vents and other thermal features. At Stromboli such thermal data define a persistent vent system aligned along a SE-NE tectonic line, fed by a shallow (<1 km deep) magma chamber. Total thermal losses from the conduit between the magma surface and vent are similar to 4 MW for high-temperature degassing vents, and similar to 14 MW for all vents within the crater terrace. At Vulcano, heat from a <4 lan deep magma body drives a hydrothermal system feeding a 400-470 m(2) exhalative area. Consistent thermal flux measurements at Vulcano (38.6+/-2 W m(-2)) between 1985 and 1995 are validated by detailed ground measurements. Time series constructed from repeated satellite and aircraft over-passes are capable of monitoring fluctuations in activity. At Stromboli, a time-series constructed for 1985 to 1995 distinguishes phases of intense explosive, lava pond and effusive activity. At Vulcano, a steady level of activity is identified over the same period. The good agreement with ground data suggests that the techniques presented here could be used at other volcanoes characterised by open conduit or fumarolic activity, either when no other data sources are available, or as a reliable supplement to more traditional data sources (e.g., seismic and chemical analyses).
Abstract: The passive transport of aircraft emissions of nitrogen oxides (NOx = NO + NO2) has been studied with a hierarchy of models ranging from two-dimensional and three-dimensional chemistry transport models up to three-dimensional models of the general circulation. The sink of NOx was parameterized by an exponential decay process with a globally constant half-lifetime of 10 days. By performing a simple experiment the importance of the various transport processes has been studied. The three-dimensional models show that the monthly mean volume mixing ratio of NOx varies by a factor of three in the longitudinal direction and the temporal variability is of the order of 30%. In view of the nonlinearity of the chemical processes leading to ozone formation in the presence of NOx this implies that the assessment of the effects of subsonic aircraft emissions of NOx should be done with three-dimensional models. Vertical redistribution by convection strongly affects the maximum NOx mixing ratio at cruise altitudes, but due to the limited lifetime of NOx of the order of ten days the most important contribution to the mixing ratio at a certain level usually stems from emissions around that level. The strong static stability in the stratosphere hampers significant dispersion of the subsonic aircraft emissions above the height where the emissions take place for the lifetimes considered here. Some model deficiencies and biases have been identified and discussed. Examples are the oscillatory signature of NOx distributions obtained with a spectral advection scheme, the strong diffusion of one of the GCMs into the polar regions, and the too intense interhemispheric exchange of one of the two-dimensional CTMs. For the vertical redistribution of the emissions it may be necessary to include not only updrafts but also downdrafts in the convective parametrization of the transport model. (C) 1997 Elsevier Science Ltd.
Abstract: A three-dimensional Lagrangian tropospheric chemistry model is used to investigate the impact of human activities on the tropospheric distribution of ozone and hydroxyl radicals. The model describes the behaviour of 50 species including methane, carbon monoxide, oxides of nitrogen, sulphur dioxide and nine organic compounds emitted from human activities and a range of other sources. The chemical mechanism involves about 100 chemical reactions of which 16 are photochemical reactions whose diurnal dependence is treated in full. The model utilises a five minute chemistry time step and a three hour advection time step for the 50,000 air parcels. Meteorological data for the winds, temperatures, clouds and so on are taken from the UK Meteorological Office global model for 1994 onwards. The impacts of a 50% reduction in European NOX emissions on global ozone concentrations are assessed. Surface ozone concentrations decrease in summertime and rise in wintertime, but to different extents.
Abstract: We present three models for magma budget during steady-state volcanic activity, by which non-erupted magma is emplaced as dykes or cumulates or crystallises in place. Using gas and thermal data we apply our models at Vulcano to calculate degassing, cooling and crystallisation of magma at a rate of 40-375 kg s(-1) within a magma body with an upper surface at a similar to 2 km depth. At Stromboli we calculate a steady magma supply of 300-1300 kg s(-1) to shallow (<1 km) depths.
Abstract: A three-dimensional Lagrangian tropospheric chemistry-transport model is used to investigate the impact of aircraft NOx emissions upon the concentrations of ozone and hydroxyl radicals. The model has a five minute chemistry time-step, and a three-hour advection time step, and hence resolves diurnal variations in chemistry,The model contains a representation of tropospheric inorganic chemistry, as well as that of methane and nine emitted hydrocarbons. Aircraft NOx emissions were found to contribute up to 20-60% towards background upper troposphere NOx levels, and to be responsible for up to 5-10% of upper tropospheric O-3, with a maximum contribution in April. The magnitude of the peak ozone change in July is 13 ppb and 8.5 ppb in January, and its location shifts with incident solar radiation. Calculated globally averaged radiative forcing exerted by the extra ozone is 0.05 W m(-2), mainly concentrated in the Northern hemisphere. (C) 1997 Elsevier Science Ltd.
Abstract: Transport of gas from subsurface magma to the atmosphere is modelled as flow through a narrow, rough pipe, and through a wider conduit filled with spheres, representing flow in a porous pipe. Macroscopic mass, momentum and energy balances are written for a control volume representing a short vertical section of the pipe. The balances are solved for a perfect gas with the approximate thermodynamic properties of steam, losing heat by steady-state radial conduction through the pipe walls, with boundary conditions of magmatic temperature at the base of the pipe, atmospheric pressure at the surface, and an ambient geotherm at a specified distance from the conduit. Results yield the variation of exit temperature with magmatic source depth, conduit radius, mass flow rate per unit area, distance to ambient temperatures, vent porosity, and porous particle size. Two ambient geotherms are used: (1) a linear gradient from atmospheric to magmatic temperatures; and (2) a 'hydrothermal' geotherm, based on water saturation temperatures assuming a hydrostatic pressure gradient. Because individual vents commonly cluster, the porous pipe model is considered to be more realistic than the rough pipe model, mainly because the conductive cooling approximation is more likely to remain valid. Exit temperatures fall as conductive losses increase, i.e. as the source depth increases, the conduit narrows, the mass flow decreases, the distance to ambient temperatures reduces, or the ambient geotherm cools. The porous pipe model is applied to data from two fumarole fields, and indicates magmatic source depths of < 100 m for the approximately 900-degrees-C fumaroles present at Poas in 1982, and < 1 500 m for the approximately 300-degrees-C field at Vulcano during 1983-87. Because additional cooling mechanisms, such as hydrothermal circulation in the country rock and addition of cool hydrothermal fluid to the gas, are likely to occur, these calculated depths to magma are maxima. Calculated pressures at the base of conduits are typically near atmospheric, indicating that the majority of decompression from magmatic pressures occurs immediately as the gas escapes. Escape is thought to be best approximated as a Joule-Kelvin expansion, and has negligible affect on gas temperature. Low pressures throughout the conduit may have important implications for equilibrium pressures calculated from exit chemical compositions, and will encourage invasion of hydrothermal fluids with time. Small changes in atmospheric pressure will affect the pressure differential across the magma boundary, and hence may influence the degassing rate, mass flow, and exit temperature.
Abstract: LAVA lakes and active strombolian vents have persisted at some volcanoes for periods exceeding the historic record. They liberate prodigious amounts of volatiles and thermal energy but erupt little lava, a paradox that raises questions about how volcanoes grow. Although long-lasting surface manifestations can be sustained by convective exchange of magma with deeper reservoirs, residence times of magmas beneath several basaltic volcanoes are approximately 10-100 years1,2, indicating that where surface activity continues for more than 100-1,000 years, the reservoirs are replenished by new magma. Endogenous growth of Kilauea volcano (Hawaii) through dyke intrusion and cumulate formation is a well-understood consequence of the steady supply of mantle-derived magma3,4. As we show here, inferred heat losses from the Halemaumau lava lake indicate a period of dominantly endogenous growth of Kilauea volcano during the nineteenth century. Moreover, heat losses and degassing rates for several other volcanoes, including Stromboli, also indicate cryptic influxes of magma that far exceed visible effluxes of lavas. We propose that persistent activity at Stromboli, and at other volcanoes in different tectonic settings, is evidence of endogenous growth, involving processes similar to those at Kilauea.
Abstract: Microgravity monitoring of active volcanoes can provide evidence of sub-surface mass and/or density changes that precede eruptions. If, in addition, precursory increases in thermal emissions are observed, an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions. Crater-lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calorimetry. Here we summarize 10 years of gravity and thermal data from Poas volcano, Costa Rica. Mass/energy balance calculations demonstrate that, in the steady-state, the large thermal inertia of the crater lake acts as a buffer to short-term changes in the energy input from the cooling magma feeder pipe. Since February 1986, it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface. This follows from unambiguous, gravity increases, constrained by elevation control, that are coincident in time with a period of long-term increased energy input to the crater lake. Progressive reduction of the lake volume by evaporation/seepage culminated in an (April 1989) ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity.
