Papers by William Collins
The impact of convection on tropospheric O 3 and its precursors has been examined in a coupled ch... more 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 ozone 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 ozone and the overall tropospheric column of O 3 . Secondly, convection affects O 3 by vertical mixing of ozone precursors. This affects O 3 chemical production and destruction. Convection transports isoprene and its degradation products to the UT where they interact with lightning NO x to produce PAN, at the expense of NO x . The combined effect of NO x to PAN conversions and downward transport of lightning NO x results in UT NO x decreases. Convective lofting of NO x from surface sources appears relatively unimportant. Despite UT NO x decreases, UT O 3 production increases as a result of UT HO x 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. The changes in tropical UT O 3 are transported polewards resulting in a 15% 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 chemistry schemes -in particular isoprene-driven changes, as well as differences in convection schemes themselves, are the most likely causes of such discrepancies. Further modelling studies are needed to constrain this uncertainty range.
For short-lived climate forcers such as black carbon (BC), the atmospheric concentrations, radiat... more For short-lived climate forcers such as black carbon (BC), the atmospheric concentrations, radiative forcing (RF) and, ultimately, the subsequent effects on climate, depend on the location and timing of the emissions. Here, we employ the NorESM1-Happi version of Norwegian Earth System Model to systematically study how the RF associated with BC emissions depends on the latitude, longitude and seasonality of the emissions. The model aerosol scheme is run in an offline mode, to allow for an essentially noise-free evaluation of the RF associated with even minor changes in emissions. Both the BC direct RF (dirRF) and the RF associated with BC in snow/ice (snowRF) are calculated for emissions in 192 latitude-longitude boxes covering the globe, both for seasonally uniform emissions and for emissions in each of the four seasons separately. We also calculate a rough estimate of the global temperature response to regional emissions, and provide a fortran-based tool to facilitate the further use of our results. Overall, the results demonstrate that the BC RFs strongly depend on the latitude, longitude and season of the emissions. In particular, the global-mean dirRF normalized by emissions (direct specific forcing; dirSF) depends much more strongly on the emission location than suggested by previous studies that have considered emissions from continental/subcontinental-scale regions. Even for seasonally uniform emissions, dirSF varies by more than a factor of ten depending on emission location. These variations correlate strongly with BC lifetime, which varies from less than 2 days to 11 days. BC dirSF is largest for emissions in tropical convective regions and in subtropical and midlatitude continents in summer, both due to the abundant solar radiation and strong convective transport, which increases BC lifetime and the amount of BC above clouds. The dirSF is also relatively large for emissions in high-albedo high-latitude regions such as Antarctica and Greenland. The dependence of snow specific forcing (snowSF) on the emission location is even larger. While BC emissions originating from most low-latitude regions result in negligible snowSF, the maxima of snowSF for emissions in polar regions greatly exceed the largest values of dirSF for low-latitude emissions. The large magnitude of snowSF for high-latitude BC emissions suggests that, for a given mass of BC emitted, also the climate impacts are largest for high-latitude emissions.
We would like to thank the two anonymous referees for their useful and supportive comments. Their... more We would like to thank the two anonymous referees for their useful and supportive comments. Their comments are repeated below reviewer 1 in black, reviewer 2 in blue, with our responses in red. The paper is an interesting summary of the magnitude of chemistry and aerosol feedbacks in available CMIP6 climate models. The paper is generally well-written, however in its current form the manuscript is somewhat fragmented and some important discussion about is missing. Some aspects of the methodology are described concisely, yet some important details are missing entirely, or are described only briefly. The chemical and aerosol forcing agents are considered independently which helps compartmentalise the results and some of these sections include important insights. However, other sections have not been crafted with the same care. Thornhill et al. analyse a set of Earth System Model simulations with atmospheric chemistry and aerosol parameterisations to quantify climate feedbacks associated with aerosol and chemistry processes. The methodology allows to attribute the climate feedback to different chemical and aerosol processes and thereby provides in some cases important insights. The paper is highly relevant and fits well to the scope of ACP. The paper is generally well written, but the quality of the individual sections varies considerably. We thank the reviewers for their positive comments. The comments regarding the fragmentation, missing discussion and individual sections are addressed in responses to specific comments below. The choice of the authors to rely on 4xCO2 experiments to diagnose climate feedback implies that some of the feedbacks considered are less climate change related, but mediated by the effect of CO2 on vegetation productivity and cover. This is an important caveat that should be explained in the Methods section for those processes that do respond to CO2 as well as climate. Also, this needs to be reflected critically in the Conclusions section/Abstract. This is discussed to some extent in the text, but we agree it could be brought out in the Conclusions and Abstract. All figures require subfigure labels as per ACP guidelines, to match references in the captions and main text. These labels will be added. Line 34: "with warmer temperatures" needs a fuller description. 4Xco2 induced warming L35: define warmer temperatures Accepted: "warmer" will be defined.
Journal of Geophysical Research, 2003
This paper presents one of the first extensive intercomparisons of models and methods used for es... more This paper presents one of the first extensive intercomparisons of models and methods used for estimating stratosphere-troposphere exchange (STE). The study is part of the European Union project Influence of Stratosphere Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO). Nine different models and methods, including three trajectory methods, one Eulerian method, two Lagrangian and one Eulerian transport model, and two general circulation models applied the same initialization. Stratospheric and tropospheric tracers have been simulated, and the tracer mass fluxes have been calculated through the tropopause and the 700 hPa surface. For a 12-day case study over Europe and the northeast Atlantic the simulated tracer mass fluxes have been intercompared. For this case the STE simulations show the same temporal evolution and the same geographical pattern of STE for most models and methods, but with generally different amplitudes (up to a factor of 4). On the other hand, for some simulations also the amplitudes are very similar.
Journal of Geophysical Research, 2003
During the EU-project Influence of Stratosphere-Troposphere exchange in a Changing Climate on Atm... more During the EU-project Influence of Stratosphere-Troposphere exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), a combined approach of a measurement network and numerical simulations was used to estimate the strength and frequency of stratosphere-to-troposphere transport (STT) events and their influence on tropospheric chemistry. Measurements of surface ozone, beryllium-7, and beryllium-10 concentrations and meteorological parameters at four European high mountain stations, as well as atmospheric profiles obtained by ozone soundings and a high-resolution lidar, were carried out. In order to simulate STT events, seven different models have been applied by the STACCATO partners. These are two trajectory models (LAGRANTO and FLEXTRA), a Lagrangian transport model (FLEXPART), a Lagrangian chemistry-transport model (STOCHEM), a Eulerian transport model (TM3), and two general circulation models (ECHAM4 and MA-ECHAM4). In order to investigate the strengths and weaknesses of each of these models and to identify the reasons for their discrepancies, a detailed comparison with measured data is presented in this paper. These models provided fluxes and concentrations of a stratospheric tracer, as well as the vertical profiles of ozone and radionuclides for a stratospheric intrusion case study that occurred over Europe in the year 1996. The comparison of the model results with the measurement data and the satellite observations revealed that all the models captured the general behavior of the event. However, great differences were found in the intensity and spatial development of the simulated intrusion event.
HAL (Le Centre pour la Communication Scientifique Directe), Feb 6, 2003
Results from the first chemistry-transport model study of the impact of the 1783-1784 Laki fissur... more Results from the first chemistry-transport model study of the impact of the 1783-1784 Laki fissure eruption (Iceland: 64 • N, 17 • W) upon atmospheric composition are presented. The eruption released an estimated 122 Tg(SO 2 ) into the troposphere and lower stratosphere. The model has a high resolution tropopause region, and detailed sulphur chemistry. The simulated SO 2 plume spreads over much of the Northern Hemisphere, polewards of ∼40 • N. About 70% of the SO 2 gas is directly deposited to the surface before it can be oxidised to sulphuric acid aerosol. The main SO 2 oxidants, OH and H 2 O 2 , are depleted by up to 40% zonally, and the lifetime of SO 2 consequently increases. Zonally averaged tropospheric SO 2 concentrations over the first three months of the eruption exceed 20 ppbv, and sulphuric acid aerosol reaches ∼2 ppbv. A total aerosol yield of 51-66 Tg(H 2 SO 4 ) is produced. The mean aerosol lifetime is only 6-9 days, and the peak aerosol loading of the atmosphere is only ∼7 Tg(H 2 SO 4 .2H 2 O). Due to the relatively short atmospheric residence times of both the SO 2 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. This study suggests that most previous estimates of the global aerosol loading associated with Laki 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 SO 2 gas rather than sulphuric acid aerosol.
Carolina Digital Repository (University of North Carolina at Chapel Hill), 2012
Ozone (O 3 ) precursor emissions influence regional and global climate and air quality through ch... more Ozone (O 3 ) precursor emissions influence regional and global climate and air quality through changes in tropospheric O 3 and oxidants, which also influence methane (CH 4 ) and sulfate aerosols (SO 4 2À ). We examine changes in the tropospheric composition of O 3 , CH 4 , SO 4 2À and global net radiative forcing (RF) for 20% reductions in global CH 4 burden and in anthropogenic O 3 precursor emissions (NO x , NMVOC, and CO) from four regions (East Asia, Europe and Northern Africa, North America, and South Asia) using the Task Force on Hemispheric Transport of Air Pollution Source-Receptor global chemical transport model (CTM) simulations, assessing uncertainty (mean AE 1 standard deviation) across multiple CTMs. We evaluate steady state O 3 responses, including long-term feedbacks via CH 4 . With a radiative transfer model that includes greenhouse gases and the aerosol direct effect, we find that regional NO x reductions produce global, annually averaged positive net RFs (0.2 AE 0.6 to 1.7 AE 2 mWm À2 /Tg N yr À1 ), with some variation among models. Negative net RFs result from reductions in global CH 4 (À162.6 AE 2 mWm À2 for a change from 1760 to 1408 ppbv CH 4 ) and regional NMVOC (À0.4 AE 0.2 to À0.7 AE 0.2 mWm À2 /Tg C yr À1 ) and CO emissions (À0.13 AE 0.02 to À0.15 AE 0.02 mWm À2 /Tg CO yr À1 ). Including the effect of O 3 on CO 2 uptake by vegetation likely makes these net RFs more negative by À1.9 to À5.2 mWm À2 /Tg N yr À1 , À0.2 to À0.7 mWm À2 /Tg C yr À1 , and À0.02 to À0.05 mWm À2 / Tg CO yr À1 . Net RF impacts reflect the distribution of concentration changes, where RF is affected locally by changes in SO 4 2À , regionally to hemispherically by O 3 , and globally by CH 4 . Global annual average SO 4 2À responses to oxidant changes range from 0.4 AE 2.6 to À1.9 AE 1.3 Gg for NO x reductions, 0.1 AE 1.2 to À0.9 AE 0.8 Gg for NMVOC reductions, and À0.09 AE 0.5 to À0.9 AE 0.8 Gg for CO reductions, suggesting additional research is needed. The 100-year global warming potentials (GWP 100 ) are calculated for the global CH 4 reduction (20.9 AE 3.7 without stratospheric O 3 or water vapor, 24.2 AE 4.2 including those components), and for the regional NO x , NMVOC, and CO reductions (À18.7 AE 25.9 to À1.9 AE 8.7 for NO x , 4.8 AE 1.7 to 8.3 AE 1.9 for NMVOC, and 1.5 AE 0.4 to 1.7 AE 0.5 for CO). Variation in GWP 100 for NO x , NMVOC, and CO suggests that regionally specific GWPs may be necessary and could support the inclusion
Atmospheric Chemistry and Physics, Dec 5, 2005
The impact of convection on tropospheric O 3 and its precursors has been examined in a coupled ch... more The impact of convection on tropospheric O 3 and its precursors has been examined in a coupled chemistryclimate 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 NO x to produce PAN, at the expense of NO x . In our model, we find that convection reduces UT NO x through this mechanism; convective down-mixing also flattens our imposed profile of lightning emissions, further reducing UT NO x . Over tropical land, which has large lightning NO x emissions in the UT, we find convective lofting of NO x from surface sources appears relatively unimportant. Despite UT NO x decreases, UT O 3 production increases as a result of UT HO x 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.
Journal of Geophysical Research, Aug 1, 1999
A Laõranõian chemistry-transport model (STOCHEM) was driven with meteorology derived from a slab ... more A Laõranõian chemistry-transport model (STOCHEM) was driven with meteorology derived from a slab ocean õeneral 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 changes 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 H202 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. 1. Introduction It is widely accepted that tropospheric ozone is one of the most important radiatively active trace gases along with carbon dioxide, methane, and nitrous oxide. However, estimates of radiative forcing due to tropospheric ozone are inherently more uncertain than those for the other trace gases for several reasons. There are no equivalent ice core records of historical concentrations for ozone which are comparable with those of other trace gases, and, owing to the shorter lifetime of ozone, the available measurements are inadequate to assess its recent changes. Consequently, ozone changes have to be estimated with atmospheric chemistry models and important questions remain about the adequacy with which chemical processes are represented in current chemistry-transport models [Prather et al., 1995]. The importance of tropospheric ozone as a radiatively active trace gas through its absorption and emission of terrestrial infi'ared radiation in the 8-10 /•rn region and absorption in the visible and ultraviolet was first recognized two decades ago [Fishman et al., 1979; Ramanathan and Dickinson, 1979]. The extent of human influences on tropospheric ozone has been documented Published in 1999 by the American Geophysical Union. Paper number 1999JD900204. with •nodels of progressively increasing complexity from global models [Crutzen, 1974], one-dimensional (l-D) models [Thompson and Cicervne, 1986], twodimensional (2-D) models [
As part of the Atmospheric Chemistry and Climate Model Intercomparison Project (AC-CMIP), we eval... more As part of the Atmospheric Chemistry and Climate Model Intercomparison Project (AC-CMIP), we evaluate the historical black carbon (BC) aerosols simulated by 8 ACCMIP models against observations including 12 ice core records, long-term surface mass concentrations and recent Arctic BC snowpack measurements. We also estimate BC albedo forcing by performing additional simulations using offline models with prescribed meteorology from 1996-2000. We evaluated the vertical profile of BC snow concentrations from these offline simulations using the recent BC snowpack measurements. Despite using the same BC emissions, the global BC burden differs by approximately a factor of 3 among models due to differences in aerosol removal parameterizations and simulated meteorology: 34 Gg to 103 Gg in 1850 and 82 Gg to 315 Gg in 2000. However, the global BC burden from preindustrial to present-day increases by 2.5-3 times with little variation among models, roughly matching the 2.5-fold increase in total BC emissions during the same period. We find a large divergence among models at both Northern Hemisphere (NH) and Southern Hemisphere (SH) high latitude regions for BC burden and at SH high latitude regions for deposition fluxes. The ACCMIP simulations match the observed BC surface mass concentrations well in Europe and North America except at Jungfraujoch and Ispra. However, the models fail to predict the Arctic BC seasonality due to severe underestimations during winter and spring. The simulated vertically resolved BC snow concentrations are, on average, within a factor of 2-3 of the BC snowpack measurements except for Greenland and the Arctic Ocean. For the ice core evaluation, models tend to capture both the observed temporal trends and the magnitudes well at Greenland sites. However, models fail to predict the decreasing trend of BC depositions/ice-core concentrations from the 1950s to the 1970s in most Tibetan Plateau ice cores. The distinct temporal trend at the Tibetan Plateau ice cores indicates a strong influence from Western Europe, but the modeled BC increases in that period are consistent with the emission changes in Eastern Europe, the Middle East, South and East Asia. At the Alps site, the simulated BC suggests 21715
We examine the climate effects of the emissions of near-term climate forcers (NTCFs) from 4 conti... more We examine the climate effects of the emissions of near-term climate forcers (NTCFs) from 4 continental regions (East Asia, Europe, North America and South Asia) using radiative forcing from the task force on hemispheric transport of air pollution sourcereceptor global chemical transport model simulations. These simulations model the transport of 3 aerosol species (sulphate, particulate organic matter and black carbon) and 4 ozone precursors (methane, nitric oxides (NO x ), volatile organic compounds and carbon monoxide). From the equilibrium radiative forcing results we calculate global climate metrics, global warming potentials (GWPs) and global temperature change potentials (GTPs) and show how these depend on emission region, and can vary as functions of time. For the aerosol species, the GWP(100) values are -37 ± 12, -46 ± 20, and 350 ± 200 for SO 2 , POM and BC respectively for the direct effects only. The corresponding GTP(100) values are -5.2 ± 2.4, -6.5 ± 3.5, and 50 ± 33. This analysis is further extended by examining the temperature-change impacts in 4 latitude bands. This shows that the latitudinal pattern of the temperature response to emissions of the NTCFs does not directly follow the pattern of the diagnosed radiative forcing. For instance temperatures in the Arctic latitudes are particularly sensitive to NTCF emissions in the northern mid-latitudes. At the 100-yr time horizon the ARTPs show NO x emissions can have a warming effect in the northern mid and high latitudes, but cooling in the tropics and Southern Hemisphere. The northern mid-latitude temperature response to northern mid-latitude emissions of most NTCFs is approximately twice as large as would be implied by the global average.
Research Square (Research Square), Mar 17, 2021
Synoptic weather and larger scale circulation patterns are closely coupled and have a major influ... more Synoptic weather and larger scale circulation patterns are closely coupled and have a major influence on regional weather and extreme events. This study examines the role of regional circulations on meteorology and extreme events for the present and future years over Asia with the WRF model driven by HadGEM2 global model boundary conditions that includes RCP4.5 scenarios based bicentennial transient simulation. The regional scale analysis was based on boundary conditions derived from 40 years of global model outputs spanning periods
This article has been accepted for publication and undergone full peer review but has not been th... more This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Journal of Geophysical Research: Atmospheres, 2003
This paper provides a review of stratosphere‐troposphere exchange (STE), with a focus on processe... more This paper provides a review of stratosphere‐troposphere exchange (STE), with a focus on processes in the extratropics. It also addresses the relevance of STE for tropospheric chemistry, particularly its influence on the oxidative capacity of the troposphere. After summarizing the current state of knowledge, the objectives of the project Influence of Stratosphere‐Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), recently funded by the European Union, are outlined. Several papers in this Journal of Geophysical Research–Atmospheres special section present the results of this project, of which this paper gives an overview. STACCATO developed a new concept of STE in the extratropics, explored the capacities of different types of methods and models to diagnose STE, and identified their various strengths and shortcomings. Extensive measurements were made in central Europe, including the first monitoring over an extended period of time o...
Geophysical Research Letters, 1998
We present the first estimate of the evolution of tropospheric ozone (O3(T)) radiative forcing si... more We present the first estimate of the evolution of tropospheric ozone (O3(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 O3(T). Radiative forcings caused by the modelled changes in O3(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 %.
Geophysical Research Letters, 2002
We assess the contribution made to the interannual variability of the global methane accumulation... more 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‐Niño Southern Oscillation cycle in the coupled Ocean‐Atmosphere model, and this mode of variation is shown to be present in the methane accumulation rate.
Chemosphere - Global Change Science, 1999
The current generation of 3-D global models used for chemistry and climate global simulations hav... more The current generation of 3-D global models used for chemistry and climate global simulations have been compared in the frame of the Tropospheric Ozone (O 3 ) Global Model Intercomparison Exercise performed in 1997. The objective was to systematically evaluate their capabilities to simulate tropospheric ozone and their precursor gases, including carbon monoxide, and to identify key areas of uncertainty in our understanding of the tropospheric O 3 budget. This exercise was organised by Global Integration Modelling (GIM) Activity of the International Global Atmospheric Chemistry (IGAC) Project.
Atmospheric Chemistry and Physics, 2003
Results from the first chemistry-transport model study of the impact of the 1783-1784 Laki fissur... more Results from the first chemistry-transport model study of the impact of the 1783-1784 Laki fissure eruption (Iceland: 64 • N, 17 • W) upon atmospheric composition are presented. The eruption released an estimated 61 Tg(S) as SO 2 into the troposphere and lower stratosphere. The model has a high resolution tropopause region, and detailed sulphur chemistry. The simulated SO 2 plume spreads over much of the Northern Hemisphere, polewards of ∼40 • N. About 70% of the SO 2 gas is directly deposited to the surface before it can be oxidised to sulphuric acid aerosol. The main SO 2 oxidants, OH and H 2 O 2 , are depleted by up to 40% zonally, and the lifetime of SO 2 consequently increases. Zonally averaged tropospheric SO 2 concentrations over the first three months of the eruption exceed 20 ppbv, and sulphuric acid aerosol reaches ∼2 ppbv. These compare to modelled preindustrial/present-day values of 0.1/0.5 ppbv SO 2 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 SO 2 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
Atmospheric Chemistry and Physics, 2008
We present the chemistry-climate model UM CAM in which a relatively detailed tropospheric chemica... more We present the chemistry-climate model UM CAM in which a relatively detailed tropospheric chemical module has been incorporated into the UK Met Office's Unified Model version 4.5. We obtain good agreements between the modelled ozone/nitrogen species and a range of observations including surface ozone measurements, ozone sonde data, and some aircraft campaigns. Four 2100 calculations assess model responses to projected changes of anthropogenic emissions (SRES A2), climate change (due to doubling CO 2 ), and idealised climate change-associated changes in biogenic emissions (i.e. 50% increase of isoprene emission and doubling emissions of soil-NO x ). The global tropospheric ozone burden increases significantly for all the 2100 A2 simulations, with the largest response caused by the increase of anthropogenic emissions. Climate change has diverse impacts on O 3 and its budgets through changes in circulation and meteorological variables. Increased water vapour causes a substantial ozone reduction especially in the tropical lower troposphere (>10 ppbv reduction over the tropical ocean). On the other hand, an enhanced stratosphere-troposphere exchange of ozone, which increases by 80% due to doubling CO 2 , contributes to ozone increases in the extratropical free troposphere which subsequently propagate to the surface. Projected higher temperatures favour ozone chemical production and PAN decomposition which lead to high surface ozone levels in certain regions. Enhanced convection transports ozone precursors more rapidly out of the boundary layer resulting in an increase of ozone production in the free troposphere. Lightning-produced NO x increases by about 22% in the doubled CO 2 climate and contributes to ozone production. The response to the increase of isoprene emissions shows that the change of ozone is largely determined by background NO x levels: high NO x environment increases ozone produc-
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Papers by William Collins