Aerosol Burden Research Articles

The effect of aerosol representation on cloud microphysical properties in Northeast Asia

This study performed a three-dimensional regional-scale simulation of aerosol and cloud fields using a meso-scale non-hydrostatic model with a bin-based cloud microphysics. The representation of aerosols in the model has been improved to account for more realistic multi-modal size distribution and multiple chemical compositions. Two case studies for shallow stratocumulus over Northeast Asia in March 2005 were conducted with different aerosol conditions to evaluate model performance. Improved condensation nuclei (CN) and cloud condensation nuclei (CCN) are attributable to the newly constructed aerosol size distribution. The simulated results of cloud microphysical properties (cloud droplet effective radius, liquid water path, and optical thickness) with improved CN/CCN number are close to the retrievals from satellite-based observation. The effects of aerosol on the microphysical properties of shallow stratocumulus are investigated by model simulation, in terms of columnar aerosol number concentration. Enhanced aerosol number concentration results in increased liquid water path in humid case, but invariant liquid water path in dry case primarily due to precipitation occurrence. The changes of cloud microphysical properties are more predominant for small aerosol burden than for large aerosol burden with the retarded changes in cloud mass and size due to inactive condensation and collision-coalescence processes. Quantitative evaluation of sensitivity factor between aerosol and cloud microphysical properties indicates a strong aerosol-cloud interaction in Northeast Asian region.

Ultraviolet radiation in the background region over Northeast China

Measurements of broadband global solar and ultraviolet radiation (UV-KZ) by a Kipp & Zonen radiometer from 2005 to 2011 at three sites in the background region over Northeast China were used to investigate the characteristics of the temporal variability of UV-KZ radiation in Northeast China. The highest annual mean daily values of UV-KZ (0.53 MJ m−2 d−1) were observed at Sanjiang and Changbai Mountain. The lowest value (0.49 MJ m−2 d−1) was measured at Hailun due to the high aerosol burden and the long path of solar radiation. The diurnal variation in UV-KZ radiation on four clear days near the equinoxes and solstices exhibited bell-shaped curves, with a maximum at approximately noon. There was little difference in UV-KZ radiation between representative spring and autumn days except that the amount of UV-KZ radiation during the spring was higher than that during the autumn. The relationship between the maximum values of UV-KZ radiation and clearness index can be accurately demonstrated with three polynomial equations. An empirical estimation model suited for all weather conditions was developed using measurements collected at Hailun. The slope of the linear regression between the measured and modeled UV-KZ radiation was approximately 1, the intercept of the linear regression equation was near zero, and the relative error of the equation was less than 8.5 %. These validation results suggest that this model can accurately estimate the UV-KZ radiation based on more conventional measured radiation data. The empirical estimation model can also serve as a valuable method for the study of ecological processes in other regions.

The influence of anthropogenic aerosol on multi-decadal variations of historical global climate

Analysis of single forcing runs from CMIP5 (the fifth Coupled Model Intercomparison Project) simulations shows that the mid-twentieth century temperature hiatus, and the coincident decrease in precipitation, is likely to have been influenced strongly by anthropogenic aerosol forcing. Models that include a representation of the indirect effect of aerosol better reproduce inter-decadal variability in historical global-mean near-surface temperatures, particularly the cooling in the 1950s and 1960s, compared to models with representation of the aerosol direct effect only. Models with the indirect effect also show a more pronounced decrease in precipitation during this period, which is in better agreement with observations, and greater inter-decadal variability in the inter-hemispheric temperature difference. This study demonstrates the importance of representing aerosols, and their indirect effects, in general circulation models, and suggests that inter-model diversity in aerosol burden and representation of aerosol–cloud interaction can produce substantial variation in simulations of climate variability on multi-decadal timescales.

Volcanic sulfate deposition to Greenland and Antarctica: A modeling sensitivity study

AbstractReconstructions of the atmospheric sulfate aerosol burdens resulting from past volcanic eruptions are based on ice core‐derived estimates of volcanic sulfate deposition and the assumption that the two quantities are directly proportional. We test this assumption within simulations of tropical volcanic stratospheric sulfur injections with the MAECHAM5‐HAM aerosol‐climate model. An ensemble of 70 simulations is analyzed, with SO2 injections ranging from 8.5 to 700 Tg, with eruptions in January and July. Modeled sulfate deposition flux to Antarctica shows excellent spatial correlation with ice core‐derived estimates for Pinatubo and Tambora, although the comparison suggests the modeled flux to the ice sheets is 4–5 times too large. We find that Greenland and Antarctic deposition efficiencies (the ratio of sulfate flux to each ice sheet to the maximum hemispheric stratospheric sulfate aerosol burden) vary as a function of the magnitude and season of stratospheric sulfur injection. Changes in simulated sulfate deposition for large SO2 injections are connected to increases in aerosol particle size, which impact aerosol sedimentation velocity and radiative properties, the latter leading to strong dynamical changes including strengthening of the winter polar vortices, which inhibits the transport of stratospheric aerosols to high latitudes. The resulting relationship between Antarctic and Greenland volcanic sulfate deposition is nonlinear for very large eruptions, with significantly less sulfate deposition to Antarctica than to Greenland. These model results suggest that variability of deposition efficiency may be an important consideration in the interpretation of ice core sulfate signals for eruptions of Tambora‐magnitude and larger.

Organosulfates in cloud water above the Ozarks' isoprene source region

Secondary organic aerosol formation via aqueous processing, particularly from the oxidation of biogenic volatile organic compounds, is hypothesized to contribute significantly to the global aerosol burden. In this study, electrospray ionization coupled with mass spectrometry (ESI-MS) was utilized to detect organosulfates and oligomers in cloud water collected in July above the Missouri Ozarks, an environment significantly influenced by isoprene oxidation. Community Multiscale Air Quality (CMAQ) modeling suggested that the aerosol at cloud height was characterized by high water, sulfate, and biogenic secondary organic aerosol content, conducive to aqueous-phase processing and organosulfate formation. CMAQ modeling also suggested the presence of gas-phase organic peroxides and nitrates, which can partition into the particle-phase and form organosulfates. Several potential organosulfates from isoprene, monoterpene, and sesquiterpene oxidation were detected in the cloud water. In particular, the ubiquitous organosulfate C5H12O7S (detected by ESI-MS at m/z −215), derived from isoprene epoxydiols, was detected. These results highlight the role of aqueous-phase reactions in biogenic SOA formation and cloud processes in isoprene oxidation-influenced regions.

Fifteen‐year aerosol optical depth climatology for Salt Lake City

Aerosol optical depth (AOD) and its wavelength dependence have been measured for the past 15 years in the Salt Lake City metropolitan area using a multifilter rotating shadowband radiometer. The instrument has not experienced a major hardware failure. It has been continuously field calibrated for extraterrestrial responses in its five aerosol channels. The instrument's cosine response was measured in 1996 and again in 2012. In our analysis of this 15 year data set, linear interpolation of these two cosine responses was used to approximate the angular response between the two characterizations. The Salt Lake City aerosol burden increased through the mid‐2000s, but has dropped to its lowest level of the record since that time despite a population increase of approximately 25%. Annually, the aerosol burden is highest in midspring and midsummer with relatively coarse aerosols during the spring peak and fine aerosols during the summer peak. There is no indication of a diurnal cycle in AOD. There is a significant, but low, correlation between PM2.5 and 500 nm AOD, and a slightly lower correlation between PM10 and 500 nm AOD. The correlations between the surface‐based measurements and total column AOD explain only 13% and 9% of the variance, respectively. Measurements are continuing to track future trends.

An empirical model of global climate – Part 1: A critical evaluation of volcanic cooling

Abstract. Observed reductions in Earth's surface temperature following explosive volcanic eruptions have been used as a proxy for geoengineering of climate by the artificial enhancement of stratospheric sulfate. Earth cools following major eruptions due to an increase in the reflection of sunlight caused by a dramatic enhancement of the stratospheric sulfate aerosol burden. Significant global cooling has been observed following the four major eruptions since 1900: Santa María, Mount Agung, El Chichón and Mt. Pinatubo, leading IPCC (2007) to state "major volcanic eruptions can, thus, cause a drop in global mean surface temperature of about half a degree Celsius that can last for months and even years". We use a multiple linear regression model applied to the global surface temperature anomaly to suggest that exchange of heat between the atmosphere and ocean, driven by variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC), has been a factor in the decline of global temperature following these eruptions. The veracity of this suggestion depends on whether sea surface temperature (SST) in the North Atlantic, sometimes called the Atlantic Multidecadal Oscillation, but here referred to as Atlantic Multidecadal Variability (AMV), truly represents a proxy for the strength of the AMOC. Also, precise quantification of global cooling due to volcanoes depends on how the AMV index is detrended. If the AMV index is detrended using anthropogenic radiative forcing of climate, we find that surface cooling attributed to Mt. Pinatubo, using the Hadley Centre/University of East Anglia surface temperature record, maximises at 0.14 °C globally and 0.32 °C over land. These values are about a factor of 2 less than found when the AMV index is neglected in the model and quite a bit lower than the canonical 0.5 °C cooling usually attributed to Pinatubo. This result is driven by the high amplitude, low frequency component of the AMV index, demonstrating that reduced impact of volcanic cooling upon consideration of the AMV index is driven by variations in North Atlantic SST that occur over time periods much longer than those commonly associated with major volcanic eruptions. The satellite record of atmospheric temperature from 1978 to present and other century-long surface temperature records are also consistent with the suggestion that volcanic cooling may have been over estimated by about a factor of 2 due to prior neglect of ocean circulation. Our study suggests a recalibration may be needed for the proper use of Mt. Pinatubo as a proxy for geoengineering of climate. Finally, we highlight possible shortcomings in simulations of volcanic cooling by general circulation models, which are also being used to assess the impact of geoengineering of climate via stratospheric sulfate injection.

Epoxide as a precursor to secondary organic aerosol formation from isoprene photooxidation in the presence of nitrogen oxides.

Isoprene is a substantial contributor to the global secondary organic aerosol (SOA) burden, with implications for public health and the climate system. The mechanism by which isoprene-derived SOA is formed and the influence of environmental conditions, however, remain unclear. We present evidence from controlled smog chamber experiments and field measurements that in the presence of high levels of nitrogen oxides (NO(x) = NO + NO2) typical of urban atmospheres, 2-methyloxirane-2-carboxylic acid (methacrylic acid epoxide, MAE) is a precursor to known isoprene-derived SOA tracers, and ultimately to SOA. We propose that MAE arises from decomposition of the OH adduct of methacryloylperoxynitrate (MPAN). This hypothesis is supported by the similarity of SOA constituents derived from MAE to those from photooxidation of isoprene, methacrolein, and MPAN under high-NOx conditions. Strong support is further derived from computational chemistry calculations and Community Multiscale Air Quality model simulations, yielding predictions consistent with field observations. Field measurements taken in Chapel Hill, North Carolina, considered along with the modeling results indicate the atmospheric significance and relevance of MAE chemistry across the United States, especially in urban areas heavily impacted by isoprene emissions. Identification of MAE implies a major role of atmospheric epoxides in forming SOA from isoprene photooxidation. Updating current atmospheric modeling frameworks with MAE chemistry could improve the way that SOA has been attributed to isoprene based on ambient tracer measurements, and lead to SOA parameterizations that better capture the dependency of yield on NO(x).

Modeling nitrate aerosol distributions and its direct radiative forcing in East Asia with RAMS-CMAQ

The geographical and seasonal characteristics in nitrate aerosol and its direct radiative forcing over East Asia are analyzed by using the air quality modeling system RAMS-CMAQ coupled with an aerosol optical properties/radiative transfer module. For evaluating the model performance, nitrate ion concentration in precipitation, and mixing ratios of PM10, and some gas precursors of aerosol during the whole year of 2007 are compared against surface observations at 17 stations located in Japan, Korea, and China, and the satellite retrieved NO2 columns. The comparison shows that the simulated values are generally in good agreement with the observed ones. Simulated monthly averaged values are mostly within a factor of 2 of the measurements at the observation stations. The distribution patterns of NO2 from simulation and satellite measurement are also similar with each other. Analysis of the distribution features of monthly and yearly averaged mass concentration and direct radiative forcing (DRF) of nitrate indicates that the nitrate aerosol could reach about 25–30% of the total aerosol mass concentration and DRF in Sichuan Basin, Southeast China, and East China where the high mass burden of all major aerosols concentrated. The highest mass concentration and strongest DRF of nitrate could exceed 40μg/m3 and −5W/m2, respectively. It also indicates that other aerosol species, such as carbonaceous and mineral particles, could obviously influence the nitrate DRF for they are often internally mixed with each other.

Determination of isoprene-derived secondary organic aerosol tracers (2-methyltetrols) by HPAEC-PAD: Results from size-resolved aerosols in a tropical rainforest

Secondary organic aerosol (SOA) formed from oxidation of isoprene, the most abundant nonmethane hydrocarbon in the atmosphere, has been estimated to contribute significantly to the global aerosol burden. Measurement of isoprene-derived SOA molecular markers has become an effective method for the investigation of biogenic aerosol contributions in the atmosphere. The primary goals of this work are to present a new method based on high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to quantify diastereoisomeric 2-methyltetrols (2-methylerythritol and 2-methylthreitol), marker compounds of isoprene-derived SOA, and thus to obtain better understanding regarding their abundance and size distribution specifically in a rainforest area. The 2-methyltetrol data, along with water-soluble inorganic ion concentrations, were obtained from size-segregated samples collected at a tropical rainforest site in South China during the period from May to June, 2010. The concentrations of 2-methyltetrols from selected samples measured by HPAEC-PAD showed good agreement with those measured by GC/MS. Overall, the HPAEC-PAD method provides a simple and fast, yet selective and sensitive, alternative to GC/MS for 2-methyltetrol determination, allowing for more efficient analysis of large sample numbers. The size distributions of 2-methylerythritol and 2-methylthreitol both exhibited a unimodal pattern, peaking in the particle size range of 0.44–1.0 μm, where their average concentrations were 11.7 and 4.2 ng m−3, respectively. A strong correlation between 2-methylerythritol and 2-methylthreitol was observed among the entire set of size-segregated samples, indicating their photochemical origin and similar formation mechanism regardless of particle sizes. Compared to the results obtained from previous chamber studies, the similar isomeric fraction of 2-methyltetrols obtained in this study and other field studies confirms their formation through photooxidation of isoprene.

Climate‐induced changes in sea salt aerosol number emissions: 1870 to 2100

AbstractGlobal climate model output is combined with an emission parameterization to estimate the change in the global and regional sea salt aerosol number emission from 1870 to 2100. Global average results suggest a general increase in sea salt aerosol number emission due to increasing surface wind speed. However, the emission changes are not uniform over the aerosol size spectrum due to an increase in sea surface temperature. From 1870 to 2100 the emission of coarse mode particles (dry diameter DP>655 nm) increase by approximately 10 % (global average), whereas no significant change in the emission of ultrafine mode aerosols (dry diameter Dp<76 nm) was found over the same period. Significant regional differences in the number emission trends were also found. Based on CAM‐Oslo global climate model output, no straight‐forward relationship was found between the change in the number emissions and changes in the sea salt aerosol burden or optical thickness. This is attributed to a change in the simulated residence time of the sea salt aerosol. For the 21st century, a decrease in the residence time leads to a weaker sea salt aerosol‐climate feedback that what would be inferred based on changes in number emissions alone. Finally, quantifying any potential impact on marine stratocumulus cloud microphysical and radiative properties due to changes in sea salt aerosol number emissions is likely to be complicated by commensurate changes in anthropogenic aerosol emissions and changes in meteorology.

From BASE-ASIA toward 7-SEAS: A satellite-surface perspective of boreal spring biomass-burning aerosols and clouds in Southeast Asia

In this paper, we present recent field studies conducted by NASA's SMART-COMMIT (and ACHIEVE, to be operated in 2013) mobile laboratories, jointly with distributed ground-based networks (e.g., AERONET, http://aeronet.gsfc.nasa.gov/ and MPLNET, http://mplnet.gsfc.nasa.gov/) and other contributing instruments over northern Southeast Asia. These three mobile laboratories, collectively called SMARTLabs (cf. http://smartlabs.gsfc.nasa.gov/, Surface-based Mobile Atmospheric Research &Testbed Laboratories) comprise a suite of surface remote sensing and in-situ instruments that are pivotal in providing high spectral and temporal measurements, complementing the collocated spatial observations from various Earth Observing System (EOS) satellites. A satellite-surface perspective and scientific findings, drawn from the BASE-ASIA (2006) field deployment as well as a series of ongoing 7-SEAS (2010–13) field activities over northern Southeast Asia are summarized, concerning (i) regional properties of aerosols from satellite and in-situ measurements, (ii) cloud properties from remote sensing and surface observations, (iii) vertical distribution of aerosols and clouds, and (iv) regional aerosol radiative effects and impact assessment. The aerosol burden over Southeast Asia in boreal spring, attributed to biomass burning, exhibits highly consistent spatial and temporal distribution patterns, with major variability arising from changes in the magnitude of the aerosol loading mediated by processes ranging from large-scale climate factors to diurnal meteorological events. Downwind from the source regions, the tightly coupled-aerosol–cloud system provides a unique, natural laboratory for further exploring the micro- and macro-scale relationships of the complex interactions. The climatic significance is presented through large-scale anti-correlations between aerosol and precipitation anomalies, showing spatial and seasonal variability, but their precise cause-and-effect relationships remain an open-ended question. To facilitate an improved understanding of the regional aerosol radiative effects, which continue to be one of the largest uncertainties in climate forcing, a joint international effort is required and anticipated to commence in springtime 2013 in northern Southeast Asia.

Source categories and contribution of biomass smoke to organic aerosol over the southeastern Tibetan Plateau

An intensive measurement campaign was conducted at a mountain and suburban site at the edge of the southeastern Tibetan Plateau during spring. Concentrations of PM2.5, carbonaceous species (OC and EC) and anhydrosugars (levoglucosan, mannosan and galactosan) as well as their ratios were utilized to identify possible biomass burning categories and contributions of biomass burning smoke to ambient aerosols. The average concentrations of levoglucosan and mannosan were 193.8 and 12.4 ng m−3, respectively at the mountain site, and 713 and 61.5 ng m−3 respectively at the suburban site. According to characteristic levoglucosan/mannosan (Lev/Man) and mannosan/galactosan (Man/Gal) ratios, we identified for the first time that mixed smoke particles (18.5, 72.3 and 9.1% for crop residues, softwood and hard wood respectively) derived from the study region and Southeast Asia contributed to the aerosol burden in the southeastern Tibetan Plateau. The biomass smoke contributions to organic carbon and organic matter were estimated to be 28.4% and 18.9–25.7% respectively at the mountain site and 38.3% and 33.5–45.4% respectively at the suburban site. The large contribution estimates indicate that biomass burning was an important anthropogenic/natural source of aerosol particles which impact regional atmospheric chemistry and climate in the southeastern Tibetan Plateau.

Simulation on the spatio-temporal distribution and emission flux of dust aerosol over East Asia in 2000-2009

A regional climate model (RegCM3), coupled with an online dust module, is used to simulate the spatio-temporal distribution and emission flux of dust aerosol (smaller than 20 μm in diameter) over East Asia in the period from 2000 to 2009. The model performance is firstly evaluated against available observations. Simulation results show that the model can capture the characteristics of spatio-temporal distribution of dust aerosol very well over East Asia. There always exist two extremes of dust aerosol optical depth (AOD) and column burden (CB), one is in the Taklimakan Desert of Xinjiang Uygur Autonomous Region, China, and the other is in the Badain Jaran Desert of Inner Mongolian Autonomous Region, China. The maximum value of CB appears in spring, secondary maximum in winter and minimum in autumn. To the east of 110°E, dust is transported eastward from a maximum center at a height of 700 hPa over the East Asian continent. Dust emission sources are mainly located in the Taklimakan Desert, Badain Jaran Desert, North Qinghai-Tibetan Plateau and Southwest Mongolia. There is also an obvious seasonal variation of dust emission flux (EF). Annual mean dust EF is 1,015.34 mg/(m 2 ·d), of which 62.4% and 2.3% are re-deposited onto the East Asian continent through a dry and wet deposition process, respectively, and the remaining 35.3% is injected into the atmosphere or subject to long-range transport.

Model analysis of long-term trends of aerosol concentrations and direct radiative forcings over East Asia

Long-term trends of aerosol concentrations and direct radiative forcings in East Asia have been investigated by adopting an online-coupled regional climate-chemistry-aerosol model (RIEMS-Chemaero) and the IPCC AR5 (the Intergovernmental Panel on Climate Change Fifth Assessment Report) emission inventory with the focus on eastern China. Model evaluation against surface observations of aerosol components demonstrates that the model can reproduce the spatial distribution and temporal variation of aerosol components generally well but tends to under-predict aerosol magnitudes especially for carbonaceous aerosols. Model comparison with aerosol optical depth (AOD) from both satellite retrieval and surface measurement shows that the model is capable of reproducing the distribution feature of AOD reasonably well but tends to under-predict magnitude. North of eastern China and the Sichuan basin are high concentration centres for near-surface anthropogenic aerosols. The domain averaged column burden of inorganic aerosol over East Asia increased from 1850 to 2010 with the peak in 1990 (18.7mg m−2), whereas that in eastern China peaked in 2010 (32.9 mg m−2). The burden of carbonaceous aerosol peaked in 1980 (5.2 mg m−2) in East Asia, but it peaked in 1990 (7.1 mg m−2) in eastern China and then decreased thereafter. Both inorganic and carbonaceous aerosol burdens showed little change during 1990–2000 due to effective emission control in China. AOD values over eastern China increased almost continuously from 1850 to 2010 with the Sichuan basin the high AOD centre. All-sky aerosol direct radiative forcing at the surface (ADRFSRF) is negative over all of East Asia, while radiative forcing at the top of the atmosphere (ADRFTOA) is negative over eastern China but positive over west China. Domain and annual mean ADRFSRF in East Asia peaked in 2000 (−11.9 W m−2), but that in eastern China peaked in 2010 (−17.5 W m−2). Mean ADRFTOA became negative since 1970 and showed the strongest forcing in 1990 (−2.9 W m−2) for East Asia and in 2010 (−4.8 W m−2) for eastern China. Sensitivity simulation shows that anthropogenic emission change plays the dominant role in the variation of aerosol concentration and direct radiative forcing in this region.

Impact of very short-lived halogens on stratospheric ozone abundance and UV radiation in a geo-engineered atmosphere

Abstract. The impact of very short-lived (VSL) halogenated source species on the ozone layer and surface erythemal ultraviolet radiation (UVERY) is investigated in the context of geo-engineering of climate by stratospheric sulfur injection. For a projected 2040 model atmosphere, consideration of VSL halogens at their upper limit results in lower ozone columns and higher UVERY due to geo-engineering for nearly all seasons and latitudes, with UVERY rising by 12% and 6% in southern and northern high latitudes, respectively. When VSL halogen sources are neglected, future UVERY increases due to declines in ozone column are nearly balanced by reductions of UVERY due to scattering by the higher stratospheric aerosol burden in mid-latitudes. Consideration of VSL sources at their upper limit tips the balance, resulting in annual average increases in UVERY of up to 5% in mid and high latitudes. Therefore, VSL halogens should be considered in models that assess the impact of stratospheric sulfur injections on the ozone layer.

The changing radiative forcing of fires: global model estimates for past, present and future

Abstract. Fires are a global phenomenon that impact climate and biogeochemical cycles, and interact with the biosphere, atmosphere and cryosphere. These impacts occur on a range of temporal and spatial scales and are difficult to quantify globally based solely on observations. Here we assess the role of fires in the climate system using model estimates of radiative forcing (RF) from global fires in pre-industrial, present day, and future time periods. Fire emissions of trace gases and aerosols are derived from Community Land Model simulations and then used in a series of Community Atmosphere Model simulations with representative emissions from the years 1850, 2000, and 2100. Additional simulations are carried out with fire emissions from the Global Fire Emission Database for a present-day comparison. These results are compared against the results of simulations with no fire emissions to compute the contribution from fires. We consider the impacts of fire on greenhouse gas concentrations, aerosol effects (including aerosol effects on biogeochemical cycles), and land and snow surface albedo. Overall, we estimate that pre-industrial fires were responsible for a RF of −1 W m−2 with respect to a pre-industrial climate without fires. The largest magnitude pre-industrial forcing from fires was the indirect aerosol effect on clouds (−1.6 W m−2). This was balanced in part by an increase in carbon dioxide concentrations due to fires (+0.83 W m−2). The RF of fires increases by 0.5 W m−2 from 1850 to 2000 and 0.2 W m−2 from 1850 to 2100 in the model representation from a combination of changes in fire activity and changes in the background environment in which fires occur, especially increases and decreases in the anthropogenic aerosol burden. Thus, fires play an important role in both the natural equilibrium climate and the climate perturbed by anthropogenic activity and need to be considered in future climate projections.

Aerosol forcing based on CAM5 and AM3 meteorological fields

Abstract. We use a single aerosol model to explore the effects of the differing meteorological fields from the NCAR CAM5 and GFDL AM3 models. We simulate the global distributions of sulfate, black carbon, organic matter, dust and sea salt using the University of Michigan IMPACT model and use these fields to calculate aerosol direct and indirect forcing, thereby isolating the impacts of the differing meteorological fields. Over all, the IMPACT-AM3 model predicts larger burdens and longer aerosol lifetimes than the IMPACT-CAM5 model. However, the IMPACT-CAM5 simulations transport more black carbon to the polar regions and more dust from Asia towards North America. These differences can mainly be attributed to differences in: (1) the vertical cloud mass flux and large-scale precipitation fields which determine the wet deposition of aerosols; (2) the in-cloud liquid water content and the cloud coverage which determine the wet aqueous phase production of sulfate. The burden, lifetime and global distribution, especially black carbon in polar regions, are strongly affected by choice of the parameters used for wet deposition. The total annual mean aerosol optical depth (AOD) at 550 nm ranges from 0.087 to 0.122 for the IMPACT-AM3 model and from 0.138 to 0.186 for the IMPACT-CAM5 model (range is due to different parameters used for wet deposition). Even though IMPACT-CAM5 has smaller aerosol burdens, its AOD is larger due to the much higher relative humidity in CAM5 which leads to more hygroscopic growth. The corresponding global annual average anthropogenic and all-sky aerosol direct forcing at the top of the atmosphere ranges from −0.25 W m−2 to −0.30 W m−2 for IMPACT-AM3 and from −0.48 W m−2 to −0.64 W m−2 for IMPACT-CAM5. The global annual average anthropogenic 1st aerosol indirect effect at the top of the atmosphere ranges from −1.26 W m−2 to −1.44 W m−2 for IMPACT-AM3 and from −1.74 W m−2 to −1.77 W m−2 for IMPACT-CAM5.

Aerosol and ozone changes as forcing for climate evolution between 1850 and 2100

Global aerosol and ozone distributions and their associated radiative forcings were simulated between 1850 and 2100 following a recent historical emission dataset and under the representative concentration pathways (RCP) for the future. These simulations were used in an Earth System Model to account for the changes in both radiatively and chemically active compounds, when simulating the climate evolution. The past negative stratospheric ozone trends result in a negative climate forcing culminating at −0.15 W m−2 in the 1990s. In the meantime, the tropospheric ozone burden increase generates a positive climate forcing peaking at 0.41 W m−2. The future evolution of ozone strongly depends on the RCP scenario considered. In RCP4.5 and RCP6.0, the evolution of both stratospheric and tropospheric ozone generate relatively weak radiative forcing changes until 2060–2070 followed by a relative 30 % decrease in radiative forcing by 2100. In contrast, RCP8.5 and RCP2.6 model projections exhibit strongly different ozone radiative forcing trajectories. In the RCP2.6 scenario, both effects (stratospheric ozone, a negative forcing, and tropospheric ozone, a positive forcing) decline towards 1950s values while they both get stronger in the RCP8.5 scenario. Over the twentieth century, the evolution of the total aerosol burden is characterized by a strong increase after World War II until the middle of the 1980s followed by a stabilization during the last decade due to the strong decrease in sulfates in OECD countries since the 1970s. The cooling effects reach their maximal values in 1980, with −0.34 and −0.28 W m−2 respectively for direct and indirect total radiative forcings. According to the RCP scenarios, the aerosol content, after peaking around 2010, is projected to decline strongly and monotonically during the twenty-first century for the RCP8.5, 4.5 and 2.6 scenarios. While for RCP6.0 the decline occurs later, after peaking around 2050. As a consequence the relative importance of the total cooling effect of aerosols becomes weaker throughout the twenty-first century compared with the positive forcing of greenhouse gases. Nevertheless, both surface ozone and aerosol content show very different regional features depending on the future scenario considered. Hence, in 2050, surface ozone changes vary between −12 and +12 ppbv over Asia depending on the RCP projection, whereas the regional direct aerosol radiative forcing can locally exceed −3 W m−2.

Improving aerosol distributions below clouds by assimilating satellite-retrieved cloud droplet number

Limitations in current capabilities to constrain aerosols adversely impact atmospheric simulations. Typically, aerosol burdens within models are constrained employing satellite aerosol optical properties, which are not available under cloudy conditions. Here we set the first steps to overcome the long-standing limitation that aerosols cannot be constrained using satellite remote sensing under cloudy conditions. We introduce a unique data assimilation method that uses cloud droplet number (N(d)) retrievals to improve predicted below-cloud aerosol mass and number concentrations. The assimilation, which uses an adjoint aerosol activation parameterization, improves agreement with independent N(d) observations and with in situ aerosol measurements below shallow cumulus clouds. The impacts of a single assimilation on aerosol and cloud forecasts extend beyond 24 h. Unlike previous methods, this technique can directly improve predictions of near-surface fine mode aerosols responsible for human health impacts and low-cloud radiative forcing. Better constrained aerosol distributions will help improve health effects studies, atmospheric emissions estimates, and air-quality, weather, and climate predictions.