Global Aerosol Distribution Research Articles

Spatial Statistical Data Fusion for Remote Sensing Applications

Aerosols are tiny solid or liquid particles suspended in the atmosphere; examples of aerosols include windblown dust, sea salts, volcanic ash, smoke from wildfires, and pollution from factories. The global distribution of aerosols is a topic of great interest in climate studies since aerosols can either cool or warm the atmosphere depending on their location, type, and interaction with clouds. Aerosol concentrations are important input components of global climate models, and it is crucial to accurately estimate aerosol concentrations from remote sensing instruments so as to minimize errors “downstream” in climate models. Currently, space-based observations of aerosols are available from two remote sensing instruments on board NASA's Terra spacecraft: the Multiangle Imaging SpectroRadiometer (MISR), and the MODerate-resolution Imaging Spectrometer (MODIS). These two instruments have complementary coverage, spatial support, and retrieval characteristics, making it advantageous to combine information from both sources to make optimal inferences about global aerosol distributions. In this article, we predict the true aerosol process from two noisy and possibly biased datasets, and we also estimate the uncertainties of these estimates. Our data-fusion methodology scales linearly and bears some resemblance to Fixed Rank Kriging (FRK), a variant of kriging that is designed for spatial interpolation of a single, massive dataset. Our spatial statistical approach does not require assumptions of stationarity or isotropy and, crucially, allows for change of spatial support. We compare our methodology to FRK and Bayesian melding, and we show that ours has superior prediction standard errors compared to FRK and much faster computational speed compared to Bayesian melding.

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.

The global middle-atmosphere aerosol model MAECHAM5-SAM2: comparison with satellite and in-situ observations

Abstract. In this paper we investigate results from a three-dimensional middle-atmosphere aerosol-climate model which has been developed to study the evolution of stratospheric aerosols. Here we focus on the stratospheric background period and evaluate several key quantities of the global distribution of stratospheric aerosols and their precursors with observations and other model studies. It is shown that the model fairly well reproduces in situ observations of the aerosol size and number concentrations in the upper troposphere and lower stratosphere (UT/LS). Compared to measurements from the limb-sounding SAGE II satellite instrument, modelled integrated aerosol quantities are more biased the lower the moment of the aerosol population is. Both findings are consistent with earlier work analysing the quality of SAGE II retrieved e.g. aerosol surface area densities in the volcanically unperturbed stratosphere (SPARC/ASAP, 2006; Thomason et al., 2008; Wurl et al., 2010). The model suggests that new particles are formed over large areas of the LS, albeit nucleation rates in the upper troposphere are at least one order of magnitude larger than those in the stratosphere. Hence, we suggest that both, tropospheric sulphate aerosols and particles formed in situ in the LS are maintaining the stability of the stratospheric aerosol layer in the absence of direct stratospheric emissions from volcanoes. Particle size distributions are clearly bimodal, except in the upper branches of the stratospheric aerosol layer where aerosols evaporate. Modelled concentrations of condensation nuclei (CN) are smaller than measured in regions of the aerosol layer where aerosol mixing ratios are largest. This points to an overestimated particle growth by coagulation. Transport regimes of tropical stratospheric aerosol have been identified from modelled aerosol mixing ratios and correspond to those deduced from satellite extinction measurements. We found that convective updraft in the Asian Monsoon region significantly contributes to both stratospheric aerosol load and size. The timing of formation and descend of layers of fine mode particles in the winter and spring polar stratosphere (CN layer) are well reproduced by the model. Where temperatures in the stratosphere increase with altitude, nucleation is unlikely to occur. Nevertheless, in these regions we find a significant concentration of fine mode aerosols. The place of origin of these particles is in the polar stratosphere. They are mixed into the mid-latitudes by planetary waves. There enhanced condensation rates of sulphuric acid vapour counteract evaporation and extend aerosol lifetime in the upper branches of the stratospheric aerosol layer. Measured aerosol precursors concentrations, SO2 and sulphuric acid vapour, are fairly well reproduced by the model throughout the stratosphere.

지구관측 위성자료를 이용한 주요 대기 에어러솔 성분의 공간분포 분석

대기 에어러솔은 대기중에 존재하는 미세입자로 인체 유해성을 지니고 있으며 기후변화에 있어서도 중요한 역할을 하는 변수로 알려져 있다. 게다가 대기 에어러솔의 주요 성분에 대한 정확한 평가는 환경에 미치는 영향에 있어서도 매우 중요하다. 이 때문에 지구관측 위성자료는 대기질 감시측면에서도 효과적인 수단으로 이용되고 있다. 따라서 본 연구에서는 인공위성 관측 자료를 이용하여 전구적인 규모의 대기 에어러솔의 주요 성분별 분포변화를 알아보고자 MODIS 에어러솔 광학두께(AOT; Aerosol Optical Thickness)와 미세입자분율(FMF; Fine Mode Fraction), 그리고 TOMS 자외선 흡수성 에어러솔 지수(AAI; UV Absorbing Aerosol Index)를 이용하여 네 가지의 주요 에어러솔의 성분(먼지, 탄소성, 황산화물, 해염)을 구분할 수 있는 방법을 제시하였다. 여기서 얻어진 결과물을 검증하기 위하여 에어러솔 예측 모델자료와 각 격자별 자료로 선형회귀분석을 수행한 결과, 본 연구에서 산출된 결과물이 에어러솔의 성분별 전반적인 패턴을 잘 표현하는 것을 확인 할 수 있었다. 이렇게 주요 에어러솔 타입으로 구분된 위성자료의 사용은 대기질 감시뿐만 아니라 기후변화연구에 있어서도 도움을 줄 것으로 사료된다. Atmospheric aerosols, small particles in the atmosphere, are one of the important parameters in climate change and human health. Additionally, accurate estimates of aerosol species are increasingly important in environmental impact assessment studies. Recent advances in global satellite remote sensing provide powerful tool for air quality monitoring. This study explores the potential usage of satellite derived data such as atmospheric aerosols for air quality monitoring as well as climate change study. The objectives of this study is to understand the general features of the global distribution of type dependent aerosols. A detailed spatio-temporal variability of the each different satellite dataset shows the variation of the global zonal average and specific geographical regions where the strong emission sources are located. Especially, significantly large aerosol amounts are observed in Asia and Africa because of the desert dust storm, anthropogenic and biomass burning emissions.

Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations

Abstract. We combine in situ measurements of sea salt aerosols (SS) from open ocean cruises and ground-based stations together with aerosol optical depth (AOD) observations from MODIS and AERONET, and the GEOS-Chem global chemical transport model to provide new constraints on SS emissions over the world's oceans. We find that the GEOS-Chem model using the Gong (2003) source function overestimates cruise observations of coarse mode SS mass concentrations by factors of 2–3 at high wind speeds over the cold waters of the Southern, North Pacific and North Atlantic Oceans. Furthermore, the model systematically underestimates SS over the warm tropical waters of the Central Pacific, Atlantic, and Indian Oceans. This pattern is confirmed by SS measurements from a global network of 15 island and coastal stations. The model discrepancy at high wind speeds (>6 m s −1) has a clear dependence on sea surface temperature (SST). We use the cruise observations to derive an empirical SS source function depending on both wind speed and SST. Implementing this new source function in GEOS-Chem results in improved agreement with in situ observations, with a decrease in the model bias from +64% to +33% for the cruises and from +32% to −5% for the ground-based sites. We also show that the wind speed-SST source function significantly improves agreement with MODIS and AERONET AOD, and provides an explanation for the high AOD observed over the tropical oceans. With the wind speed-SST formulation, global SS emissions show a small decrease from 5200 Mg yr−1 to 4600 Mg yr−1, while the SS burden decreases from 9.1 to 8.5 mg m−2. The spatial distribution of SS, however, is greatly affected, with the SS burden increasing by 50% in the tropics and decreasing by 40% at mid- and high-latitudes. Our results imply a stronger than expected halogen source from SS in the tropical marine boundary layer. They also imply stronger radiative forcing of SS in the tropics and a larger response of SS emissions to climate change than previously thought.

The CALIPSO Mission

Aerosols and clouds have important effects on Earth's climate through their effects on the radiation budget and the cycling of water between the atmosphere and Earth's surface. Limitations in our understanding of the global distribution and properties of aerosols and clouds are partly responsible for the current uncertainties in modeling the global climate system and predicting climate change. The CALIPSO satellite was developed as a joint project between NASA and the French space agency CNES to provide needed capabilities to observe aerosols and clouds from space. CALIPSO carries CALIOP, a two-wavelength, polarization-sensitive lidar, along with two passive sensors operating in the visible and thermal infrared spectral regions. CALIOP is the first lidar to provide long-term atmospheric measurements from Earth's orbit. Its profiling and polarization capabilities offer unique measurement capabilities. Launched together with the CloudSat satellite in April 2006 and now flying in formation with the A-train satellite constellation, CALIPSO is now providing information on the distribution and properties of aerosols and clouds, which is fundamental to advancing our understanding and prediction of climate. This paper provides an overview of the CALIPSO mission and instruments, the data produced, and early results.

Online simulations of global aerosol distributions in the NASA GEOS‐4 model and comparisons to satellite and ground‐based aerosol optical depth

We have implemented a module for tropospheric aerosols (GOCART) online in the NASA Goddard Earth Observing System version 4 model and simulated global aerosol distributions for the period 2000–2006. The new online system offers several advantages over the previous offline version, providing a platform for aerosol data assimilation, aerosol‐chemistry‐climate interaction studies, and short‐range chemical weather forecasting and climate prediction. We introduce as well a methodology for sampling model output consistently with satellite aerosol optical thickness (AOT) retrievals to facilitate model‐satellite comparison. Our results are similar to the offline GOCART model and to the models participating in the AeroCom intercomparison. The simulated AOT has similar seasonal and regional variability and magnitude to Aerosol Robotic Network (AERONET), Moderate Resolution Imaging Spectroradiometer, and Multiangle Imaging Spectroradiometer observations. The model AOT and Angstrom parameter are consistently low relative to AERONET in biomass‐burning‐dominated regions, where emissions appear to be underestimated, consistent with the results of the offline GOCART model. In contrast, the model AOT is biased high in sulfate‐dominated regions of North America and Europe. Our model‐satellite comparison methodology shows that diurnal variability in aerosol loading is unimportant compared to sampling the model where the satellite has cloud‐free observations, particularly in sulfate‐dominated regions. Simulated sea salt burden and optical thickness are high by a factor of 2–3 relative to other models, and agreement between model and satellite over‐ocean AOT is improved by reducing the model sea salt burden by a factor of 2. The best agreement in both AOT magnitude and variability occurs immediately downwind of the Saharan dust plume.

Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility

Abstract. Processes occurring in the tropical upper troposphere (UT), the Tropical Transition Layer (TTL), and the lower stratosphere (LS) are of importance for the global climate, for stratospheric dynamics and air chemistry, and for their influence on the global distribution of water vapour, trace gases and aerosols. In this contribution we present aerosol and trace gas (in-situ) measurements from the tropical UT/LS over Southern Brazil, Northern Australia, and West Africa. The instruments were operated on board of the Russian high altitude research aircraft M-55 "Geophysica" and the DLR Falcon-20 during the campaigns TROCCINOX (Araçatuba, Brazil, February 2005), SCOUT-O3 (Darwin, Australia, December 2005), and SCOUT-AMMA (Ouagadougou, Burkina Faso, August 2006). The data cover submicron particle number densities and volatility from the COndensation PArticle counting System (COPAS), as well as relevant trace gases like N2O, ozone, and CO. We use these trace gas measurements to place the aerosol data into a broader atmospheric context. Also a juxtaposition of the submicron particle data with previous measurements over Costa Rica and other tropical locations between 1999 and 2007 (NASA DC-8 and NASA WB-57F) is provided. The submicron particle number densities, as a function of altitude, were found to be remarkably constant in the tropical UT/LS altitude band for the two decades after 1987. Thus, a parameterisation suitable for models can be extracted from these measurements. Compared to the average levels in the period between 1987 and 2007 a slight increase of particle abundances was found for 2005/2006 at altitudes with potential temperatures, Θ, above 430 K. The origins of this increase are unknown except for increases measured during SCOUT-AMMA. Here the eruption of the Soufrière Hills volcano in the Caribbean caused elevated particle mixing ratios. The vertical profiles from Northern hemispheric mid-latitudes between 1999 and 2006 also are compact enough to derive a parameterisation. The tropical profiles all show a broad maximum of particle mixing ratios (between Θ≈340 K and 390 K) which extends from below the TTL to above the thermal tropopause. Thus these particles are a "reservoir" for vertical transport into the stratosphere. The ratio of non-volatile particle number density to total particle number density was also measured by COPAS. The vertical profiles of this ratio have a maximum of 50% above 370 K over Australia and West Africa and a pronounced minimum directly below. Without detailed chemical composition measurements a reason for the increase of non-volatile particle fractions cannot yet be given. However, half of the particles from the tropical "reservoir" contain compounds other than sulphuric acid and water. Correlations of the measured aerosol mixing ratios with N2O and ozone exhibit compact relationships for the tropical data from SCOUT-AMMA, TROCCINOX, and SCOUT-O3. Correlations with CO are more scattered probably because of the connection to different pollution source regions. We provide additional data from the long distance transfer flights to the campaign sites in Brazil, Australia, and West-Africa. These were executed during a time window of 17 months within a period of relative volcanic quiescence. Thus the data represent a "snapshot picture" documenting the status of a significant part of the global UT/LS fine aerosol at low concentration levels 15 years after the last major (i.e., the 1991 Mount Pinatubo) eruption. The corresponding latitudinal distributions of the measured particle number densities are presented in this paper to provide data of the UT/LS background aerosol for modelling purposes.

Transport of dusts from East Asian and non-East Asian sources to Hong Kong during dust storm related events 1996–2007

Over a twelve year period from 1996 to 2007, 76 dust storm related events (as days) in Hong Kong were selected for study, based on Aluminium and Calcium concentrations in PM 10. Four of the 76 events reach episodic levels with exceedances of the Hong Kong air quality standards. The purpose of the study is to identify and characterize dust sources impacting Hong Kong. Global distribution of aerosols in NASA’s daily aerosol index images from TOMS and OMI, are compared to plots generated by NRL(US)’s Navy Aerosol Analysis and Prediction System. Possible source areas are assigned by computing air parcel backward trajectories to Hong Kong using the NOAA HYSPLIT model. PM 10 and elemental data are analyzed for crustal mass concentrations and element mass ratios. Our analysis reveals that 73 out of the 76 dust events (96%) involve non-East Asian sources-the Thar, Central/West Asian, Arabian and Sahara deserts (Saharan influence is found in 63 events), which are previously not known to affect Hong Kong. The Gobi desert is the most frequent origin of dust, affecting 68 dust events while the Taklamakan desert impacts only 30 of the dust events. The impact of the Gobi desert in March and December is apparently associated with the northeast monsoon in East Asia. Our results also show a seasonal pattern in dust impact from both East Asian and more remote sources, with a maximum in March. Dust event occurrences are conspicuously absent from summer. Dust transport to Hong Kong is commonly associated with the passage of frontal low-pressure systems. The coarse size fraction of PM 10 concentrations were, as indicated by Al, Ca and Fe concentrations, about 4–8 times higher during dust events. The mean Ca/Al ratios of sources involving the Taklamakan desert are notably higher than those for non-East Asian sources owing to a higher Ca content of most of the East Asian deserts. The Fe/Al ratios follow a similar trend. Contributions from the desert sources are grossly estimated where possible, by using the average Al abundance of 8% in the upper continental crust to convert the Al mass in the PM 10 to dust concentrations. This is done for the six events identified with air mass purely of non-East Asian origin and the two events related only to the Thar/Arabian/Sahara deserts. Results reveal that the average contribution from the non-East Asian sources (including C/W Asia) is approximately 10% and, that from the Thar/Arabian/Sahara deserts is about 8%.

Global Aerosol Distribution Derived from Multi-directional Data by ADEOS/POLDER

Global Aerosol Distribution Derived from Multi-directional Data by ADEOS/POLDER

A simulation of the global distribution and radiative forcing of soil dust aerosols at the Last Glacial Maximum

Abstract. In this study an integrated simulation of the global distribution and the radiative forcing of soil dust aerosols at the Last Glacial Maximum (LGM) is performed with an aerosol climate model, SPRINTARS. It is compared with another simulation for the present climate condition. The global total emission flux of soil dust aerosols at the LGM is simulated to be about 2.4 times as large as that in the present climate, and the simulated deposition flux is in general agreement with estimations from ice core and marine sediment samplings though it appears to be underestimated over the Antarctic. The calculated direct radiative forcings of soil dust aerosols at the LGM is close to zero at the tropopause and −0.4 W m−2 at the surface. These radiative forcings are about twice as large as those in the present climate. SPRINTARS also includes the microphysical parameterizations of the cloud-aerosol interaction both for liquid water and ice crystals, which affect the radiation budget. The positive radiative forcing from the indirect effect of soil dust aerosols is mainly caused by their properties to act as ice nuclei. This effect is simulated to be smaller (−0.9 W m−2) at the LGM than in the present. It is suggested that atmospheric dust might contribute to the cold climate during the glacial periods both through the direct and indirect effects, relative to the interglacial periods.

Aerosol optical properties at Nam Co, a remote site in central Tibetan Plateau

To reduce uncertainties in the assessment of aerosol effects on regional climate and validate the satellite products, aerosol optical measurements were made at a remote site, Nam Co Station (AERONET) in central Tibetan Plateau, since August 2006. Very low aerosol optical values were observed with annual mean Aerosol Optical Depth (AOD) of 0.05 at 500 nm during August 2006 to July 2007. Angstrom parameters were also low with an average of 0.42 ± 0.27. Angstrom parameter varies greatly over a narrow range of AOD, indicating the occurrence of different types of aerosol particles. The seasonal variation of the monthly average AOD shows maximum values in the spring (April, May) and remains high values in summer monsoon season (June, July, August, and September). The former corresponds to the prevalence of soil dust particle in spring and the later reflects the enhanced human activities in summer. The seasonal variation of columnar size distribution is also consistent with this result. Distinct seasonal variation of water vapor content was observed, with high that occurred in summer monsoon season. Nam Co appears to represent a clean continental background site for atmospheric composition investigation. This work could fill the gap which exists in our knowledge on the global aerosol distribution over the huge high elevation area (i.e. Tibetan Plateau).

A height resolved global view of dust aerosols from the first year CALIPSO lidar measurements

Based on the first year of CALIPSO lidar measurements under cloud‐free conditions, a height‐resolved global distribution of dust aerosols is presented for the first time. Results indicate that spring is the most active dust season, during which ∼20% and ∼12% of areas between 0 and 60°N are influenced by dust at least 10% and 50% of the time, respectively. In summer within 3–6 km, ∼8.3% of area between 0 and 60°N is impacted by dust at least 50% of the time. Strong seasonal cycles of dust layer vertical extent are observed in major source regions, which are similar to the seasonal variation of the thermally driven boundary layer depth. The arid and semiarid areas in North Africa and the Arabian Peninsula are the most persistent and prolific dust sources. African dust is transported across the Atlantic all yearlong with strong seasonal variation in the transport pathways mainly in the free troposphere in summer and at the low altitudes in winter. However, the trans‐Atlantic dust is transported at the low altitudes is important for all seasons, especially transported further cross the ocean. The crossing Atlantic dusty zones are shifted southward from summer to winter, which is accompanied by a similar southward shift of dust‐generating areas over North Africa. The Taklimakan and Gobi deserts are two major dust sources in East Asia with long‐range transport mainly occurring in spring. The large horizontal and vertical coverage of dust aerosols indicate their importance in the climate system through both direct and indirect aerosol effects.

Global distribution of solid and aqueous sulfate aerosols: Effect of the hysteresis of particle phase transitions

The partitioning between solid and aqueous phases of tropospheric sulfate‐ammonium particles is simulated with a global 3‐D chemical transport model (CTM). The simulation explicitly accounts for the hysteresis of particle phase transitions by transporting aqueous sulfate and three solid sulfate forms (namely, ammonium sulfate, letovicite, and ammonium bisulfate). Composition‐dependent deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) are based on recent laboratory data. We find that the solids mass fraction on a sulfate basis is 0.34, partitioned as 93% ammonium sulfate, 6% letovicite, and 1% ammonium bisulfate. The fraction increases with altitude from 0.10 to 0.30 in the boundary layer to 0.60–0.80 in the upper troposphere. The dominance of solids in the upper troposphere arises in part from high sulfate neutralization, reflecting in our simulation a low retention efficiency of NH3 upon cloud freezing. High sulfate neutralization is consistent with the few available observations in the upper troposphere. High acidity with a dominant aqueous phase, however, can occur following volcanic eruptions. Seasonal variation of the solids mass fraction in both the lower and upper troposphere is modulated by emissions of NH3 from the terrestrial biosphere and biomass burning as well as by emissions of dimethylsulfide from the ocean biosphere. The timescale of phase transitions, as driven by changes in relative humidity, varies from 10 to 50 h in the boundary layer to 150–400 h in the upper troposphere. Omission of the hysteresis effect in the CTM by assuming that particle phase follows the lower side of the hysteresis loop increases the solids mass fraction from 0.34 to 0.56. An upper side assumption decreases the fraction to 0.17. Lower and upper side assumptions better approximate particle phase for high and low altitudes, respectively. Fluctuations in the CRH, which can be induced by other constituents in sulfate particles such as minerals or organic molecules, strongly affect the solids mass fraction in the boundary layer but not at higher altitudes. Further studies are needed to determine the effects of large solids mass fraction on heterogeneous chemistry and cirrus cloud formation.

A system for operational aerosol optical depth data assimilation over global oceans

In this study, we present an aerosol data assimilation system destined for operational use at the Fleet Numerical Meteorological and Oceanographic Center (FNMOC). The system is an aerosol physics version of the Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS) that is already operational. The purpose of this new system, NAVDAS‐Aerosol Optical Depth (NAVDAS‐AOD) is to improve the NRL Aerosol Analysis and Prediction System (NAAPS)'s forecasting capability by assimilating observational data sources with NAAPS forecast fields. This will allow for not only improved aerosol forecasting but also for dramatically enhanced global scale research capabilities for the study of aerosol‐meteorology interaction. NAVDAS‐AOD assimilates a newly developed over‐water Moderate‐Resolution Imaging Spectroradiometers (MODIS) level 3 aerosol product with NAAPS. This paper is the second in a series which describes NRL's program to realistically monitor global aerosol distributions. Here we explain the reasons and procedures for constructing the over‐water level 3 MODIS aerosol product, describe the theoretical basis for NAVDAS‐AOD, and provide a thorough statistical error analysis for both the MODIS observations and the NAAPS model background fields that are critical to aerosol data assimilation. Using 5 months of analysis, our study shows that by carefully screening over‐water satellite observations to ensure only the best quality data are used in the aerosol assimilation process, the NAVDAS‐AOD can significantly improve the NAAPS global aerosol optical depth analysis as well as improve the aerosol forecast skill.

Trace gas and aerosol interactions in the fully coupled model of aerosol‐chemistry‐climate ECHAM5‐HAMMOZ: 2. Impact of heterogeneous chemistry on the global aerosol distributions

We use the ECHAM5‐HAMMOZ aerosol‐chemistry‐climate model to quantify the influence of trace gas–aerosol interactions on the regional and global distributions and optical properties of aerosols for present‐day conditions. The model includes fully interactive simulations of gas phase and aerosol chemistry including a comprehensive set of heterogeneous reactions. We find that as a whole, the heterogeneous reactions have only a small effect on the SO2 and sulfate burden because of competing effects. The uptake of SO2 on dust and sea salt decreases the SO2 concentrations while the decrease in OH (that results from the uptake of HO2, N2O5, and O3) tends to increase SO2 (because of reduced oxidation). The sulfate formed in sea salt aerosols from SO2 uptake accounts for 3.7 Tg(S) a−1 (5%) of the total sulfate production. Uptake and subsequent reaction of SO2 on mineral dust contributes to a small formation of sulfate (0.55 Tg(S) a−1, <1%), but is responsible for the coating of mineral dust particles, resulting in an extra 300 Tg a−1 of dust being transferred from the insoluble to the soluble mixed modes. The burden of dust in the insoluble modes is reduced by 44%, while the total burden is reduced by 5% as a result of enhanced wet deposition efficiency. Changes in the sulfur cycle affect the H2SO4 concentrations and the condensation of H2SO4 on black carbon. Accounting for heterogeneous reactions enhances the global mean burden of hydrophobic black carbon particles by 4%. The changes in aerosol mixing state result only in a small change in the global and annual aerosol optical depth (AOD) and absorption optical depth (ABS), but have significant implications on regional and seasonal scale. For example, in the main polluted regions of the Northern Hemisphere, AOD and ABS increase by 10–30% and up to 15%, respectively, in winter.

Retrieving global aerosol sources from satellites using inverse modeling

Abstract. Understanding aerosol effects on global climate requires knowing the global distribution of tropospheric aerosols. By accounting for aerosol sources, transports, and removal processes, chemical transport models simulate the global aerosol distribution using archived meteorological fields. We develop an algorithm for retrieving global aerosol sources from satellite observations of aerosol distribution by inverting the GOCART aerosol transport model. The inversion is based on a generalized, multi-term least-squares-type fitting, allowing flexible selection and refinement of a priori algorithm constraints. For example, limitations can be placed on retrieved quantity partial derivatives, to constrain global aerosol emission space and time variability in the results. Similarities and differences between commonly used inverse modeling and remote sensing techniques are analyzed. To retain the high space and time resolution of long-period, global observational records, the algorithm is expressed using adjoint operators. Successful global aerosol emission retrievals at 2°×2.5 resolution were obtained by inverting GOCART aerosol transport model output, assuming constant emissions over the diurnal cycle, and neglecting aerosol compositional differences. In addition, fine and coarse mode aerosol emission sources were inverted separately from MODIS fine and coarse mode aerosol optical thickness data, respectively. These assumptions are justified, based on observational coverage and accuracy limitations, producing valuable aerosol source locations and emission strengths. From two weeks of daily MODIS observations during August 2000, the global placement of fine mode aerosol sources agreed with available independent knowledge, even though the inverse method did not use any a priori information about aerosol sources, and was initialized with a "zero aerosol emission" assumption. Retrieving coarse mode aerosol emissions was less successful, mainly because MODIS aerosol data over highly reflecting desert dust sources is lacking. The broader implications of applying our approach are also discussed.

Global statistical analysis of MISR aerosol data: a massive data product from NASA's Terra satellite

AbstractIn climate models, aerosol forcing is the major source of uncertainty in climate forcing, over the industrial period. To reduce this uncertainty, instruments on satellites have been put in place to collect global data. However, missing and noisy observations impose considerable difficulties for scientists researching the global distribution of aerosols, aerosol transportation, and comparisons between satellite observations and global‐climate‐model outputs. In this paper, we fit a Spatial Mixed Effects (SME) statistical model to predict the missing values, denoise the observed values, and quantify the spatial‐prediction uncertainties. The computations associated with the SME model are linear scalable to the number of data points, which makes it feasible to process massive global satellite data. We apply the methodology, which is called Fixed Rank Kriging (FRK), to the level‐3 Aerosol Optical Depth (AOD) dataset collected by NASA's Multi‐angle Imaging SpectroRadiometer (MISR) instrument flying on the Terra satellite. Overall, our results were superior to those from non‐statistical methods and, importantly, FRK has an uncertainty measure associated with it that can be used for comparisons over different regions or at different time points. Copyright © 2007 John Wiley & Sons, Ltd.

Assimilation of POLDER aerosol optical thickness into the LMDz‐INCA model: Implications for the Arctic aerosol burden

The large spatial and temporal variability of atmospheric aerosol load makes it a challenge to quantify aerosol effect on climate. This study is one of the first attempts to apply data assimilation for the analysis of global aerosol distribution. Aerosol optical thickness (AOT) observed from the Polarization and Directionality of the Earth Reflectances (POLDER) spaceborne instrument are assimilated into a three‐dimensional chemistry model. POLDER capabilities to distinguish between fine and coarse AOT are used to constrain them separately in the model. Observation and model errors are a key component of such a system and are carefully estimated on a regional basis using some of the high‐quality surface observations from the Aerosol Robotic Network (AERONET). Other AERONET data provide an independent evaluation of the a posteriori fields. Results for the fine mode show improvements, in terms of reduction of root‐mean‐square errors, in most regions with the largest improvements found in the Mediterranean Sea and Eurasia. We emphasize the results for the Arctic, where there is growing evidence of a strong aerosol impact on climate, but a lack of regional and continuous aerosol monitoring. The a posteriori fields noticeably well reproduce the winter‐spring “Arctic Haze” peak measured in Longyearbyen (15°E, 78°N) and typical seasonal variations in the Arctic region, where AOT increase by up to a factor of three between a posteriori and a priori. Enhanced AOT are found over a longer period in spring 2003 than in 1997, suggesting that the large Russian fires in 2003 have influenced the Arctic aerosol load.

Role of ammonia chemistry and coarse mode aerosols in global climatological inorganic aerosol distributions

We use an inorganic aerosol thermodynamic equilibrium model in a three-dimensional chemical transport model to understand the roles of ammonia chemistry and natural aerosols on the global distribution of aerosols. The thermodynamic equilibrium model partitions gas-phase precursors among modeled aerosol species self-consistently with ambient relative humidity and natural and anthropogenic aerosol emissions during the 1990s. Model simulations show that accounting for aerosol inorganic thermodynamic equilibrium, ammonia chemistry and dust and sea-salt aerosols improve agreement with observed SO 4, NO 3, and NH 4 aerosols especially at North American sites. This study shows that the presence of sea salt, dust aerosol and ammonia chemistry significantly increases sulfate over polluted continental regions. In all regions and seasons, representation of ammonia chemistry is required to obtain reasonable agreement between modeled and observed sulfate and nitrate concentrations. Observed and modeled correlations of sulfate and nitrate with ammonium confirm that the sulfate and nitrate are strongly coupled with ammonium. SO 4 concentrations over East China peak in winter, while North American SO 4 peaks in summer. Seasonal variations of NO 3 and SO 4 are the same in East China. In North America, the seasonal variation is much stronger for NO 3 than SO 4 and peaks in winter. Natural sea salt and dust aerosol significantly alter the regional distributions of other aerosols in three main ways. First, they increase sulfate formation by 10–70% in polluted areas. Second, they increase modeled nitrate over oceans and reduce nitrate over Northern hemisphere continents. Third, they reduce ammonium formation over oceans and increase ammonium over Northern Hemisphere continents. Comparisons of SO 4, NO 3 and NH 4 deposition between pre-industrial, present, and year 2100 scenarios show that the present NO 3 and NH 4 deposition are twice pre-industrial deposition and present SO 4 deposition is almost five times pre-industrial deposition.