A. Rap

University of Leeds, Leeds, England, United Kingdom

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Publications (26)109.48 Total impact

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    ABSTRACT: We use a global aerosol microphysics model in combination with an offline radiative transfer model to quantify the radiative effect of biogenic secondary organic aerosol (SOA) in the present-day atmosphere. Through its role in particle growth and ageing, the presence of biogenic SOA increases the global annual mean concentration of cloud condensation nuclei (CCN; at 0.2% supersaturation) by 3.6–21.1 %, depending upon the yield of SOA production from biogenic volatile organic compounds (BVOCs), and the nature and treatment of concurrent primary carbonaceous emissions. This increase in CCN causes a rise in global annual mean cloud droplet number concentration (CDNC) of 1.9–5.2%, and a global mean first aerosol indirect effect (AIE) of between +0.01Wm−2 and −0.12Wm−2. The radiative impact of biogenic SOA is far greater when biogenic oxidation products also contribute to the very early stages of new particle formation; using two organically mediated mechanisms for new particle formation, we simulate global annual mean first AIEs of −0.22Wm−2 and −0.77Wm−2. The inclusion of biogenic SOA substantially improves the simulated seasonal cycle in the concentration of CCN-sized particles observed at three forested sites. The best correlation is found when the organically mediated nucleation mechanisms are applied, suggesting that the first AIE of biogenic SOA could be as large as −0.77Wm−2. The radiative impact of SOA is sensitive to the presence of anthropogenic emissions. Lower background aerosol concentrations simulated with anthropogenic emissions from 1750 give rise to a greater fractional CCN increase and a more substantial first AIE from biogenic SOA. Consequently, the anthropogenic indirect radiative forcing between 1750 and the present day is sensitive to assumptions about the amount and role of biogenic SOA. We also calculate an annual global mean direct radiative effect of between −0.08Wm−2 and −0.78Wm−2 in the present day, with uncertainty in the amount of SOA produced from the oxidation of BVOCs accounting for most of this range.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2014; 14:447-470. · 5.51 Impact Factor
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    ABSTRACT: The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates.
    Nature 11/2013; 503(7474):67-71. · 38.60 Impact Factor
  • D. V. Spracklen, A. Rap
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    ABSTRACT: natural environment is an important source of atmospheric aerosol such as dust, sea spray, and wildfire smoke. Climate controls many of these natural aerosol sources, which, in turn, can alter climate through changing the properties of clouds and the Earth's radiative balance. However, the Earth's atmosphere is now heavily modified by anthropogenic pollution aerosol, but how this pollution may alter these natural aerosol-climate feedbacks has not been previously explored. Here we use a global aerosol microphysics model to analyze how anthropogenic aerosol alters one link within these feedbacks, namely, the sensitivity of cloud albedo to changes in natural aerosol. We demonstrate that anthropogenic aerosol in the Northern Hemisphere has halved the hemispheric mean cloud albedo radiative effect that occurs due to changes in natural aerosol emissions. Such a suppression has not occurred in the more pristine Southern Hemisphere.
    Geophysical Research Letters 10/2013; 40(19):5316-5319. · 3.98 Impact Factor
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    ABSTRACT: stratospheric ozone depletion has acted to cool the Earth's surface. As the result of the phase-out of anthropogenic halogenated compounds emissions, stratospheric ozone is projected to recover and its radiative forcing (RF-O3 ~ -0.05 W/m2 presently) might therefore be expected to decay in line with ozone recovery itself. Using results from chemistry-climate models, we find that, although model projections using a standard greenhouse gas scenario broadly agree on the future evolution of global ozone, they strongly disagree on RF-O3 because of a large model spread in ozone changes in a narrow (several km thick) layer, in the northern lowermost stratosphere. Clearly, future changes in global stratospheric ozone cannot be considered an indicator of its overall RF. The multi-model mean RF-O3 estimate for 2100 is +0.06 W/m2 but with a range such that it could remain negative throughout this century or change sign and reach up to ~0.25 W/m2.
    Geophysical Research Letters 06/2013; 40(11):2796-2800. · 3.98 Impact Factor
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    ABSTRACT: Thirty years of balloon-borne measurements over Boulder (40°N, 105°W) are used to investigate the water vapor trend in the tropopause region. This analysis extends previously published trends, usually focusing on altitudes greater than 16 km, to lower altitudes. Two new concepts are applied: (1) Trends are presented in a thermal tropopause (TP) relative coordinate system from –2 km below to 10 km above the TP, and (2) sonde profiles are selected according to TP height. Tropical (TPz > 14 km), extratropical (TPz < 12 km), and transitional air mass types (12 km < TPz < 14 km) reveal three different water vapor reservoirs. The analysis based on these concepts reduces the dynamically induced water vapor variability at the TP and principally favors refined water vapor trend studies in the upper troposphere and lower stratosphere. Nonetheless, this study shows how uncertain trends are at altitudes –2 to +4 km around the TP. This uncertainty in turn has an influence on the uncertainty and interpretation of water vapor radiative effects at the TP, which are locally estimated for the 30 year period to be of uncertain sign. The much discussed decrease in water vapor at the beginning of 2001 is not detectable between –2 and 2 km around the TP. On lower stratospheric isentropes, the water vapor change at the beginning of 2001 is more intense for extratropical than for tropical air mass types. This suggests a possible link with changing dynamics above the jet stream such as changes in the shallow branch of the Brewer-Dobson circulation.
    01/2013; 118:11269-11284.
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    ABSTRACT: Natural aerosol plays a significant role in the Earth's system due to its ability to alter the radiative balance of the Earth. Here we use a global aerosol microphysics model together with a radiative transfer model to estimate radiative effects for five natural aerosol sources in the present‐day atmosphere: dimethyl sulfide (DMS), sea‐salt, volcanoes, monoterpenes, and wildfires. We calculate large annual global mean aerosol direct and cloud albedo effects especially for DMS‐derived sulfate (–0.23 Wm–2 and –0.76 Wm–2, respectively), volcanic sulfate (–0.21 Wm–2 and –0.61 Wm–2) and sea‐salt (–0.44 Wm–2 and –0.04 Wm–2). The cloud albedo effect responds nonlinearly to changes in emission source strengths. The natural sources have both markedly different radiative efficiencies and indirect/direct radiative effect ratios. Aerosol sources that contribute a large number of small particles (DMS‐derived and volcanic sulfate) are highly effective at influencing cloud albedo per unit of aerosol mass burden.
    Geophysical Research Letters. 01/2013;
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    ABSTRACT: The Mediterranean troposphere exhibits a marked and localised summertime ozone maximum, which has the potential to strongly impact regional air quality and radiative forcing. The Mediterranean region can be perturbed by long-range pollution import from Northern Europe, North America and Asia, in addition to local emissions, which may all contribute to regional ozone enhancements. We exploit ozone profile observations from the Tropospheric Emission Spectrometer (TES) and the Global Ozone Monitoring Experiment-2 (GOME-2) satellite instruments, and an offline 3-D global chemical transport model (TOMCAT) to investigate the geographical and vertical structure of the summertime tropospheric ozone maximum over the Mediterranean region. We show that both TES and GOME-2 are able to detect enhanced levels of ozone in the lower troposphere over the region during the summer. These observations, together with surface measurements, are used to evaluate the TOMCAT model's ability to capture the observed ozone enhancement. The model is used to quantify sensitivities of the ozone maximum to anthropogenic and natural volatile organic compound (VOC) emissions, anthropogenic NOx emissions, wildfire emissions and long-range import of ozone and precursors. Our results show a dominant sensitivity to natural VOC emissions in the Mediterranean basin over anthropogenic VOC emissions. However, local anthropogenic NOx emissions are result in the overall largest sensitivity in near-surface ozone. We also show that in the lower troposphere, global VOC emissions account for 40% of the ozone sensitivity to VOC emissions in the region, whereas, for NOx the ozone sensitivity to local sources is 9 times greater than that for global emissions at these altitudes. However, in the mid and upper troposphere ozone is most sensitive to non-local emission sources. In terms of radiative effects on regional climate, ozone contributions from non-local emission sources are more important, as these have a larger impact on ozone in the upper troposphere where its radiative effects are larger, with Asian monsoon outflow having the greatest impact. Our results allow improved understanding of the large-scale processes controlling air quality and climate in the region of the Mediterranean basin.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2013; 13(5):2331-2345. · 5.51 Impact Factor
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    ABSTRACT: Contrails and especially their evolution into cirrus-like clouds are thought to have very important effects on local and global radiation budgets, though are generally not well represented in global climate models. Lack of contrail parameterisations is due to the limited availability of in situ contrail measurements which are difficult to obtain. Here we present a methodology for successful sampling and interpretation of contrail microphysical and radiative data using both in situ and remote sensing instrumentation on board the FAAM BAe146 UK research aircraft as part of the COntrails Spreading Into Cirrus (COSIC) study. Forecast models were utilised to determine flight regions suitable for contrail formation and sampling; regions that were both free of cloud but showed a high probability of occurrence of air mass being supersaturated with respect to ice. The FAAM research aircraft, fitted with cloud microphysics probes and remote sensing instruments, formed a distinctive spiral-shaped contrail in the predicted area by flying in an orbit over the same ground position as the wind advected the contrails to the east. Parts of these contrails were sampled during the completion of four orbits, with sampled contrail regions being between 7 and 30 min old. Lidar measurements were useful for in-flight determination of the location and spatial extent of the contrails, and also to report extinction values that agreed well with those calculated from the microphysical data. A shortwave spectrometer was also able to detect the contrails, though the signal was weak due to the dispersion and evaporation of the contrails. Post-flight the UK Met Office NAME III dispersion model was successfully used as a tool for modelling the dispersion of the persistent contrail; determining its location and age, and determining when there was interference from other measured aircraft contrails or when cirrus encroached on the area later in the flight. The persistent contrails were found to consist of small (~10 μm) plate-like crystals where growth of ice crystals to larger sizes (~100 μm) was typically detected when higher water vapour levels were present. Using the cloud microphysics data, extinction co-efficient values were calculated and found to be 0.01-1 km-1. The contrails formed during the flight (referred to as B587) were found to have a visible lifetime of ~40 min, and limited water vapour supply was thought to have suppressed ice crystal growth.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 09/2012; 12(17):8157-8175. · 5.51 Impact Factor
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    ABSTRACT: The upper troposphere/lower stratosphere (UTLS) region plays an important role in the climate system. Changes in the structure and chemical composition of this region result in particularly large changes in radiative forcings of the atmosphere. Quantifying the processes that control UTLS composition (e.g., stratosphere-troposphere exchange) therefore represents a crucial task. We assess the influence of uncertainties in the atmospheric mixing strength on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. Mixing, the irreversible part of transport, is controlled by the local horizontal strain and vertical shear of the atmospheric flow. We find that simulated radiative effects of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2for water vapor and ozone, respectively. For ozone, the largest impact of mixing uncertainties is observed in the extra-tropical lower stratosphere.
    Journal of Geophysical Research 08/2012; 117(D16305). · 3.17 Impact Factor
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    ABSTRACT: Observations and models have shown that con-tinuously degassing volcanoes have a potentially large ef-fect on the natural background aerosol loading and the ra-diative state of the atmosphere. We use a global aerosol microphysics model to quantify the impact of these vol-canic emissions on the cloud albedo radiative forcing under pre-industrial (PI) and present-day (PD) conditions. We find that volcanic degassing increases global annual mean cloud droplet number concentrations by 40 % under PI conditions, but by only 10 % under PD conditions. Consequently, vol-canic degassing causes a global annual mean cloud albedo effect of −1.06 W m −2 in the PI era but only −0.56 W m −2 in the PD era. This non-equal effect is explained partly by the lower background aerosol concentrations in the PI era, but also because more aerosol particles are produced per unit of volcanic sulphur emission in the PI atmosphere. The higher sensitivity of the PI atmosphere to volcanic emissions has an important consequence for the anthropogenic cloud radiative forcing because the large uncertainty in volcanic emissions translates into an uncertainty in the PI baseline cloud radia-tive state. Assuming a −50/+100 % uncertainty range in the volcanic sulphur flux, we estimate the annual mean anthro-pogenic cloud albedo forcing to lie between −1.16 W m −2 and −0.86 W m −2 . Therefore, the volcanically induced un-certainty in the PI baseline cloud radiative state substantially adds to the already large uncertainty in the magnitude of the indirect radiative forcing of climate.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2012; 12:7321-7339. · 5.51 Impact Factor
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    ABSTRACT: The composition and dynamic structure of the upper troposphere/lower stratosphere (UTLS) have a significant impact on surface climate and its variability, through radiative and dynamic coupling. Changes in the chemical composition of this region result in particularly large changes in the radiative forcing of the atmosphere. In addition, there is growing evidence that dynamic coupling between the troposphere and stratosphere has a significant impact on regional weather and climate. Improvements of forecasts by chemistry-climate models (CCMs) therefore rely on a quantitative representation of radiative and dynamic couplings and the underlying physical and chemical processes. We will give a brief overview on physical and chemical processes, determining UTLS composition (e.g., stratosphere-troposphere exchange) and influencing the dynamical coupling with the troposphere (e.g., gravity waves). In the second part, we present results of an analysis of the influence of uncertainties of one particular process, mixing of air masses, on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multi-annual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. It is shown that radiative effects of of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2 for water vapor and ozone, respectively.
    07/2012;
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    ABSTRACT: Contrails and especially their evolution into cirrus-like clouds are thought to have very important effects on local and global radiation budgets, though are generally not well represented in global climate models. Lack of contrail parameterisations is due to the limited availability of in situ contrail measurements which are difficult to obtain. Here we present a methodology for successful sampling and interpretation of contrail microphysical and radiative data using both in situ and remote sensing instrumentation on board the FAAM BAe146 UK research aircraft as part of the COntrails Spreading Into Cirrus (COSIC) study. Forecast models were utilised to determine flight regions suitable for contrail formation and sampling; regions that were both free of cloud but showed a high probability of occurrence of air mass being supersaturated with respect to ice. The FAAM research aircraft, fitted with cloud microphysics probes and remote sensing instruments, formed a distinctive spiral-shaped contrail in the predicted area by flying in an orbit over the same ground position as the wind advected the contrails to the east. Parts of these contrails were sampled during the completion of four orbits, with sampled contrail regions being between 7 and 30 min old. Lidar measurements were useful for in-flight determination of the location and spatial extent of the contrails, and also to report extinction values that agreed well with those calculated from the microphysical data. A shortwave spectrometer was also able to detect the contrails, though the signal was weak due to the dispersion and evaporation of the contrails. Post-flight the UK Met Office NAME III dispersion model was successfully used as a tool for modelling the dispersion of the persistent contrail; determining its location and age, and determining when there was interference from other measured other aircraft contrails or when cirrus encroached on the area later in the flight. The persistent contrails were found to consist of small (~10 μm) plate-like crystals where growth of ice crystals to larger sizes (~100 μm) was detected when higher water vapour levels were present. Using the cloud microphysics data, extinction co-efficient values were calculated and found to be 0.01-1 km-1. The contrails formed during the flight (referred to as B587) were found to have a visible lifetime of ~40 min, and limited water vapour supply was thought to have suppressed ice crystal growth.
    Atmospheric Chemistry and Physics 03/2012; 12(3):7829-7877. · 4.88 Impact Factor
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    ABSTRACT: A new linear parameterisation for stratospheric methane (CoMeCAT) has been developed and tested. The scheme is derived from a 3-D full chemistry transport model (CTM) and tested within the same chemistry model itself, as well as in an independent general circulation model (GCM). The new CH4/H2O scheme is suitable for any global model and here is shown to provide realistic profiles in the 3-D TOMCAT/SLIMCAT CTM and in the ECMWF (European Centre for Medium-Range Weather Forecasts) GCM. Simulation results from the new stratospheric scheme are in good agreement with the full-chemistry CTM CH4 field and with observations from the Halogen Occultation Experiment (HALOE). The CH4 scheme has also been used to derive a source for stratospheric water. Stratospheric water increments obtained in this way within the CTM produce vertical and latitudinal H2O variation in fair agreement with satellite observations. Stratospheric H2O distributions in the ECMWF GCM present realistic overall features although concentrations are lower than in the CTM run (up to 0.5 ppmv lower above 10 hPa). The potential of the new CoMeCAT scheme for evaluating long-term transport within the ECMWF model is exploited to assess the impacts of nudging the free running GCM to ERA-40 and ERA-Interim reanalyses. In this case, the nudged GCM shows similar transport patterns to the CTM forced by the corresponding reanalysis data, ERA-Interim producing better results than ERA-40. The impact that the new methane description has in the GCM radiation scheme is also explored. Compared to the default CH4 climatology used by the ECMWF model, CoMeCAT produces up to 2 K cooling in the tropical lower stratosphere. The effect of using the CoMeCAT scheme for radiative forcing (RF) calculations has been investigated using the off-line Edwards-Slingo (E-S) radiative transfer model. Compared to the use of a tropospheric global 3-D CH4 value, the CoMeCAT distributions produce an overall decrease in the annual mean net RF, with the largest decrease found over the Southern Hemisphere high latitudes. The effect of the new CH4 stratospheric distribution on these RF calculations is of up to 30 mW m-2, i.e. the same order of magnitude, and opposite sign, as the inclusion of aircraft contrails formation in the radiative model.
    Atmospheric Chemistry and Physics 01/2012; 12(1):479-523. · 4.88 Impact Factor
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    ABSTRACT: Air travel and its associated emissions are growing faster than other sectors and they are predicted to contribute a significant warming of climate over the coming century. According to current best estimates, the largest single radiative forcing component associated with aviation is due to aviation-induced cloudiness (AIC), which includes contrail cirrus and changes in the natural cirrus caused by air traffic. However, there is still a high level of uncertainty associated with these, and limited estimates for the forcing of the total effect of aviation induced cloudiness exist. This study, as part of the Contrails Spreading into Cirrus (COSIC) project, aimed to build a physically based parameterization of contrails spreading into cirrus within the UK Met Office Unified Model (UM) and thus to give an independent estimate of the climate impact of AIC. In-situ observations of contrails properties and their spreading have been performed during a series of flights with the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft. These observations were used in the development of the parameterization, which simulates contrail formation and ageing interactively with the natural cirrus module within the UM. Based on this new parameterisation, estimates of global contrail cirrus coverage, optical depth, and radiative forcing are given, investigating also the contrail effect on the natural cirrus cloud and contrail saturation regional effects of future air traffic growth.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: The budget of atmospheric secondary organic aerosol (SOA) is very uncertain, with recent estimates suggesting a global source of between 12 and 1820 Tg (SOA) a−1. We used a dataset of aerosol mass spectrometer (AMS) observations and a global chemical transport model including aerosol microphysics to produce top-down constraints on the SOA budget. We treated SOA formation from biogenic (monoterpenes and isoprene), lumped anthropogenic and lumped biomass burning volatile organic compounds (VOCs) and varied the SOA yield from each precursor source to produce the best overall match between model and observations. Organic aerosol observations from the IMPROVE network were used as an independent check of our optimised sources. The optimised model has a global SOA source of 140 ± 90 Tg (SOA) a−1 comprised of 13 ± 8 Tg (SOA) a−1 from biogenic, 100 ± 60 Tg (SOA) a−1 from anthropogenically controlled SOA, 23 ± 15 Tg (SOA) a−1 from conversion of primary organic aerosol (mostly from biomass burning) to SOA and an additional 3 ± 3 Tg (SOA) a−1 from biomass burning VOCs. Compared with previous estimates, our optimized model has a substantially larger SOA source in the Northern Hemisphere mid-latitudes. We used a dataset of 14C observations from rural locations to estimate that 10 Tg (SOA) a−1 (10%) of our anthropogenically controlled SOA is of urban/industrial origin, with 90 Tg (SOA) a−1 (90%) most likely due to an anthropogenic pollution enhancement of SOA from biogenic VOCs, almost an order-of-magnitude beyond what can be explained by current understanding. The urban/industrial SOA source is consistent with the 13 Tg a−1 estimated by de Gouw and Jimenez (2009), which was much larger than estimates from previous studies. The anthropogenically controlled SOA source results in a global mean aerosol direct effect of −0.26 ± 0.15 Wm−2 and global mean indirect (cloud albedo) effect of −0.6+0.24−0.14 Wm−2. The biogenic and biomass SOA sources are not well constrained due to the limited number of OA observations in regions and periods strongly impacted by these sources. To further improve the constraints by this method, additional observations are needed in the tropics and the Southern Hemisphere.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; · 5.51 Impact Factor
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    ABSTRACT: The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH(4)) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O(3)). These changes lead to a 70% increase in the atmospheric lifetime of methane, and an 18% decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the long-wave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH(4)-O(3) and CH(4)-OHaerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11(14):6961-6969. · 5.51 Impact Factor
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    ABSTRACT: Black carbon in carbonaceous combustion aerosol warms the climate by absorbing solar radiation, meaning reductions in black carbon emissions are often perceived as an attractive global warming mitigation option. However, carbonaceous combustion aerosol can also act as cloud condensation nuclei (CCN) so they also cool the climate by increasing cloud albedo. The net radiative effect of carbonaceous combustion aerosol is uncertain because their contribution to CCN has not been evaluated on the global scale. By combining extensive observations of CCN concentrations with the GLOMAP global aerosol model, we find that the model is biased low (normalised mean bias = -77%) unless carbonaceous combustion aerosol act as CCN. We show that carbonaceous combustion aerosol accounts for more than half (52-64%) of global CCN with the range due to uncertainty in the emitted size distribution of carbonaceous combustion particles. The model predicts that wildfire and pollution (fossil fuel and biofuel) carbonaceous combustion aerosol causes a global mean cloud albedo aerosol indirect effect of -0.34 W m(-2), with stronger cooling if we assume smaller particle emission size. We calculate that carbonaceous combustion aerosol from pollution sources cause a global mean aerosol indirect effect of -0.23 W m(-2). The small size of carbonaceous combustion particles from fossil fuel sources means that whilst pollution sources account for only one-third of the emitted mass they cause two-thirds of the cloud albedo aerosol indirect effect that is due to carbonaceous combustion aerosol. This cooling effect must be accounted for, along with other cloud effects not studied here, to ensure that black carbon emissions controls that reduce the high number concentrations of fossil fuel particles have the desired net effect on climate.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11(17):9067-9087. · 5.51 Impact Factor
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    ABSTRACT: The budget of atmospheric secondary organic aerosol (SOA) is very uncertain with recent estimates suggesting a source as large as 1820 Tg (SOA)/yr. We use a dataset of aerosol mass spectrometer (AMS) observations and a global chemical transport model including aerosol microphysics to produce new top-down constraints on the SOA budget. The model includes SOA sources from monoterpenes, isoprene, and lumped anthropogenic and biomass burning volatile organic compounds (VOCs). We tune the SOA yield from each source to produce the best overall match between model and observation. Our optimised model has a global SOA source of 144±90 Tg (SOA)/yr. Our analyses produce as a robust result that (a) a substantial fraction of the SOA is anthropogenic in origin and/or (b) anthropogenic pollution greatly enhances the production of SOA from biogenic VOCs, far beyond our current understanding. Our best estimates are 104±14, 13±6, and 27±13 Tg (SOA)/yr for anthropogenically-controlled, biogenic, and biomass burning SOA respectively. We calculate a global mean aerosol indirect (cloud albedo) forcing due to this anthropogenically-controlled SOA of -0.57 W/m2. The biogenic and biomass sources are unsufficiently constrained due to the limited number of observations in regions and periods that strongly impacted by these sources but remote from anthropogenic pollution. To further improve the constraints by this method, additional observations are needed in the tropics and the Southern Hemisphere.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Persistent contrails are believed to currently have a relatively small but significant positive radiative forcing on climate. With air travel predicted to continue its rapid growth over the coming years, the contrail warming effect on climate is expected to increase. Nevertheless, there remains a high level of uncertainty in the current estimates of contrail radiative forcing. Contrail formation depends mostly on the aircraft flying in cold and moist enough air masses. Most studies to date have relied on simple parameterizations using averaged meteorological conditions. In this paper we take into account the short-term variability in background cloudiness by developing an on-line contrail parameterization for the UK Met Office climate model. With this parameterization, we estimate that for the air traffic of year 2002 the global mean annual linear contrail coverage was approximately 0.11%. Assuming a global mean contrail optical depth of 0.2 or smaller and assuming hexagonal ice crystals, the corresponding contrail radiative forcing was calculated to be less than 10 mW m-2 in all-sky conditions. We find that the natural cloud masking effect on contrails may be significantly higher than previously believed. This new result is explained by the fact that contrails seem to preferentially form in cloudy conditions, which ameliorates their overall climate impact by approximately 40%.
    Journal of Geophysical Research 01/2010; 115. · 3.17 Impact Factor
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    ABSTRACT: Seven groups have participated in an intercomparison study of calculations of radiative forcing (RF) due to stratospheric water vapour (SWV) and contrails. A combination of detailed radiative transfer schemes and codes for global-scale calculations have been used, as well as a combination of idealized simulations and more realistic global-scale changes in stratospheric water vapour and contrails. Detailed line-by-line codes agree within about 15 % for longwave (LW) and shortwave (SW) RF, except in one case where the difference is 30 %. Since the LW and SW RF due to contrails and SWV changes are of opposite sign, the differences between the models seen in the individual LW and SW components can be either compensated or strengthened in the net RF, and thus in relative terms uncertainties are much larger for the net RF. Some of the models used for global-scale simulations of changes in SWV and contrails differ substantially in RF from the more detailed radiative transfer schemes. For the global-scale calculations we use a method of weighting the results to calculate a best estimate based on their performance compared to the more detailed radiative transfer schemes in the idealized simulations. Zusammenfassung Sieben Forschungsgruppen haben an einem Ergebnisvergleich hinsichtlich des Strahlungsantriebes von Kondensstreifen und stratosphärischem Wasserdampf teilgenommen. Dies umfasst einen Vergleich sowohl von aufwändigen Strahlungsübertragungsmodellen mit Parametrisierungsschemata aus globalen Klimamod-ellen als auch einen Vergleich von idealisierten und realistischen Mustern von Kondensstreifen und stratosphärischem Wasserdampf. Aufwändige Linienmodelle weichen für den langwelligen (LW) und kurzwelligen (SW) Strahlungsantrieb um etwa 15 % voneinander ab, in einem Fall beträgt die Abweichung 30 %. Da die LW und SW Komponenten sowohl für Kondensstreifen als auch für stratosphärischen Wasser-dampf ein unterschiedliches Vorzeichen haben, können sich die Differenzen in den Komponenten bzgl. des Netto-Strahlungsantriebes kompensieren und verstärken. Daher sind relativ gesehen die Modellunsicher-heiten beim Netto-Strahlungsantrieb am größten. Einige der Modelle, die in globalen Klimasimulationen zur Anwendung kommen, weichen für die Strahlungsantriebe von Kondensstreifen un Anderungen des stratosphärischen Wasserdampfes deutlich von den Ergebnissen aufwändigerer Strahlungsmodelle ab. Um eine beste Abschätzung für die Berechnungen des globalen Strahlungsantriebes geben zu können, wichten wir die Ergebnisse der einzelnen Modelle unter Berücksichtigung der Abweichungen, die in den idealisierten Vergleichsrechnungen im Vergleich zu den Ergebnisse der detailliertesten Modelle festgestellt wurden.
    Meteorologische Zeitschrift 01/2010; 18(16):585-596. · 1.08 Impact Factor