A33E-1635
A Model Study of the Sulfur Cycle at the Last Glacial Maximum
The mid and high-southern latitudes are still marginally affected by anthropogenic sulfur emissions. Sulfur aerosols are well-known for their radiative impact, and thus interact with climate. Climate can in turn affect atmospheric sulfur sources, distribution and chemistry. Antarctic ice cores provide information on the evolution of climate and sulfur deposition at the surface of the ice sheet at glacial-interglacial time scales. In this sudy, an Atmospheric General Circulation Model (AGCM) was developed and used towards a better understanding of the atmospheric sulfur cycle in antarctic and sub-antarctic regions. Ice core data are used to validate model results under glacial climate conditions. The AGCM has been coupled to a sulfur chemistry module: the LMD-ZTSulfur model, version 4. An update of both the physical and chemical parts of the model was first performed. The impact of the changes on modelled sulfur cycle are evaluated for modern climate. Boundary conditions were adapted to simulate the atmospheric circulation and sulfur cycle at the Last Glacial Maximum, approximately 20,000 years ago. In the model, sulfur is found to be highly sensitive to antarctic sea-ice coverage, which is still poorly known during the ice age. An original dataset of ice-age sea-ice coverage was developed. Its impact on the oceanic emissions of dimethyl sulfide, the main precursor of sulfur aerosols at high- southern latitudes, is discussed. Using the same oceanic sulfur reservoirs as for present day climate, the model broadly reproduces the glacial deposits of sulfur aerosols on the Antarctic plateau, suggesting little impact of climate on oceanic sulfur production in the Antarctic region. Sensitivity tests were carried out in order to discuss major uncertainties and difficulties in understanding the natural atmospheric sulfur budget and its past variations with climate.
A33E-1637
Investigations of Dust Radiative Heating Over the Indo-Gangetic Plains During the Pre- Monsoon Season
Satellite and ground observations show maximum column aerosol loading over the Indo-Gangetic Plains (IGP) during the pre-monsoon period (April-May-June) caused by wind-blown dust storms that originate in deserts around the Arabian Peninsula. High dust loading significantly affects aerosol optical properties and the radiative fluxes at the top of atmosphere and surface. The frequency of dust storms over IGP peaks during May and long- term satellite data show strong positive trend in the aerosol loading over desert regions around the IGP. We use multi-satellite and ground observations obtained from MODIS, CERES, AIRS and AERONET data along with radiative transfer simulations to calculate the radiative forcing due to dust aerosols and analyze changes in temperature profiles caused by the heating associated with dust aerosols over the IGP during the pre-monsoon period. Long-term analysis of the mid-tropospheric temperature obtained from the Microwave Sounding Unit (MSU) data indicates a significant upward trend over the IGP. This positive trend is found to be highest in May with an increase of ~2.0º C in the last 25 years. In addition, higher values of temperature were found in the AIRS profile data on heavy dust loading days compared to low dust environment. This effect was particularly observed in the mid-troposphere from AIRS data. Radiative transfer model simulations combined with inputs from ground measurements of aerosol optical properties from AERONET data and CERES fluxes in conjunction with Optical Properties of Aerosols and Clouds (OPAC) modeled values of aerosol properties are used to estimate the radiative heating associated with dust aerosols.
A33E-1638
Observation Analysis Of Atmospheric Aerosol Optical Properties Over Semi-arid Loess Plateau During PACDEX
The aerosol optical depth (AOD) and Angstrom exponent are analyzed based on the CIMEL CE-318 observed data at semi-arid climate and environment observatory of Lanzhou University (SACOL) for the period of August 2006 to July 2007, especially for the period of PACDEX. The result shows that the seasonal variation of AOD is significant. The value of AOD is the highest in Spring and the lowest in Fall, respectively. The Angstrom exponent is the lowest in Spring, indicating that the ratio of larger particles increased due to the dust aerosol. The variability of Angstrom exponent is less in Winter. The surface observed AOD are also compared with the AOD derived from MODIS. http://climate.lzu.edu.cn/
A33E-1639
Aerosol single scattering albedo and its contribution to radiative forcing dung EAST- AIRE
Quantification of aerosol single scattering albedo (SSA) can improve determining aerosol radiative property. Combination technique using MODIS and ground-based Hazemeter measurement data by the East Asian Study of Tropospheric Aerosols: an International Regional Experiment (EAST-AIRE) over China is proposed to retrieve SSA. The accuracy of the retrieval of SSA increases with the aerosol loading and the uncertainties in the SSA retrieval are 0.02~0.03 (AOT=1.0) and up to 0.03~0.05 (AOT=0.5) at 0.47¥ìm, respectively. The comparison of one- year data of retrieved SSA values with those from AERONET inversion product are ~0.03 (RMSD) and ~0.02 (mean bias), respectively. Estimated SSA values were range from 0.89 to 0.93 over the study area. Since SSA is an important factor of aerosol radiative forcing, these will help to understood the study of aerosol climate effects.
A33E-1640
Biomass Burning Aerosols Intensify El Nino-Induced Drought in Equatorial Southeast Asia
Although the influence of El~Niño on inducing drought in equatorial Southeast Asia is relatively well understood, less is known about feedbacks between El Niño, biomass burning and aerosol forcing in this region Fires associated with El~Niño are a new phenomenon, arising from recent increases in human population and agriculture. We examined the impact of El Niño-induced fire aerosols on regional climate using the Community Climate System Model (CCSM), and the Global Fire Emissions Dataset (GFED) for 1997--2006. In our first experiment, we forced the CCSM and a slab ocean model (SOM) with two single-year emissions datasets, one representing a high fire year (1997) and the other a low fire year (2000). In our second experiment the CCSM, coupled with a data ocean model (DOM) of fixed sea surface temperatures (SSTs), was forced with multi-year emissions, consisting of the 10 year GFED record plus an additional 'buffer' year of 'mean' fire emissions. Climate responses to fire aerosols in the two experiments were determined by analyzing ensembles of forty (SOM) and five (DOM) simulations, respectively.Our results show links between fire, aerosols, and decreases in regional precipitation. The inclusion of fire emissions and their climate effects generally improves agreement between simulations and several satellite datasets, including MODIS, MISR and ISCCP. In the first experiment, fires produce aerosol optical depth (AOD) anomalies of 1.0--1.4 over the areas with the most fire in 1997. In the areas with the highest aerosol concentration, solar absorption by these carbonaceous aerosols increases solar heating rates between the surface and 100 hPa upwards of 0.2--0.3 K day-1 during the peak fire season (August--October). This intense heating stabilizes the troposphere, suppresses convection, and ultimately decreases precipitation the region. In peak fire season, fire-emitted aerosols reduced precipitation by more than 30% in the areas of highest AOD. This reduces soil moisture by 1--2% nearly uniformly in the region. These results suggest that recent increases in deforestation and fire emissions in equatorial Southeast Asia may intensify El~Niño-induced drought.
A33E-1641
The Correlation Between Aerosol Index (Dust Emission) And Climatic Indices
Heavy dust storms interrupt human activities, and it has become obvious that the long-range transport of dust relates the biogeochemical cycles of land, atmosphere and ocean over the past few decades (Martin and Gordon, 1988; Bergametti, 1998), probably influencing the global carbon cycle (Ridgwell, 2002), and having a significant effects on regional radiative balances (Kinne and Pueschel, 2001; Sokolik and Toon, 1996; Sokolik et al., 2001). Aerosol Index(AI) observed from Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) from 1979 to 2006 enables us to analyze the possible correlation between Asian dust aerosol and natural oscillations. The purpose of this study is to find relationships of climate indices such as ENSO-MEI, PDO, and AO with the inter-annual variability of Asian dust aerosol. In order to analyze the correlation between Asian dust aerosol and climatic indices, we concentrate on the dust source regions and divide into 5 regions over the major sources of Asian dust (R1: deserts and sands in Kazakhstan, R2: Mongolia, R3: Taklimakan desert, R4: The desert in Tsaidam and the Kumutage Desert, R5: Onqin Daga sandy land and Horqin sandy land). The variations of Asian dust aerosol show weakly, negative correlation with the ENSO-MEI and PDO, whereas show weakly positive correlation with AO. Especially, there are comparatively distinct trend in the region of Southern Tibetan source (R2). This indicates a diminished frequency of Asian dust emission in El Niño years (positive ENSO-MEI).
A33E-1642
Optically thick aerosol layers over clouds in the South-East Atlantic Ocean
Aerosols have important impacts on the earth's radiation budget and the general circulation of the atmosphere. The magnitude of the solar absorption and scattering by aerosols is poorly constrained observationally and model estimates of the direct climate forcing (DCF) differ widely as a consequence. In order to better understand the role of aerosols and clouds on Earth's radiative budget, a space borne observing system, ‘Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations' (CALIPSO) was launched in June 2006. CALIPSO is part of the A-Train multi-satellite observing system and provides high temporal and spatial resolution lidar data that is designed to shed light on both aerosol and cloud physical properties and the interactions between them. We evaluate an existing method, and present a new method, to use CALIPSO Level-1 and Level-2 data to determine the optical thickness and Angstrom exponent of tropospheric aerosol layers overlying low clouds over the tropical South-East Atlantic Ocean. This region is frequently characterized by optically thick aerosol layers from southern African biomass burning advecting over extensive marine boundary layer clouds. In such a situation the effect of the aerosols on the top-of-atmosphere radiation is unclear and is sensitive to aerosol optical properties (especially aerosol optical depth and single scattering albedo) and the underlying cloud/surface albedo. We attempt to place observational constraints on the possible magnitude of the radiative effects of these aerosol layers using a combination of CALIPSO and MODIS satellite observations together with in-situ and surface-based observations of the single scattering albedo of the aerosol layers.
A33E-1643
Hurricane Alley SST Variability in 2005 and 2006
The North Atlantic hurricane seasons of 2005 and 2006 were dramatically different for the Gulf Coast and eastern seaboard of the United States. The 2005 hurricane season was one of the most destructive seasons in history, while there was limited impact in 2006. Hurricane activity had been forecast to be above normal in 2006 but was not. One of the conspicuous differences in environmental conditions between these two years was sea surface temperature anomalies (SSTA) over a region of the Western Atlantic and Caribbean (70W-40W, 15N-30N) important for hurricane formation and intensification. SSTA was more than 1.5 standard deviations warm during the 2005 hurricane season, but was much less in 2006 through most of its hurricane season. Recent studies have highlighted the potentially important role of atmospheric dust-shielding of solar radiation in causing this SSTA difference. The intent of this study is to determine the mechanisms responsible for this SSTA difference. It is shown that the difference can be reproduced using a simple 1-dimensional ocean mixed layer model forced with surface fluxes from the NCEP/NCAR Reanalysis Project. It is found that there are two causes of SSTA difference over this region during July through September; one is latent heat flux variability caused by wind speed effects, and the second is non-linear ocean warming caused by synoptic scale atmospheric variability. Solar forcing is found to damp rather than force the year-to-year model SSTA difference, contrary to previous hypotheses based on aerosol concentration.
A33E-1644
Physical and Radiative Properties of Aerosol Particles in the Caribbean: Influence of African Dust and Soufriere Volcanic Ash
Atmospheric particles such as dust and volcanic ash have the potential of influencing the earth's radiative budget directly by scattering or absorbing solar radiation in the atmosphere and indirectly by affecting cloud condensation nuclei (CCN) concentrations and, therefore, cloud albedo. The radiatively-important properties of atmospheric particles are determined at the most fundamental level by their chemical composition and size distributions; therefore, the importance of studying the chemical, physical, and optical aerosol properties. Over the summer months, the island of Puerto Rico receives African dust incursions that reduce visibility and have an impact on public health, ecosystem, and climate. Visibility is also negatively affected when the island receives south-east winds and the Soufriere volcano (Montserrat Island) has been active. Here we present preliminary results of measurements performed during 2006 and 2007 at Cape San Juan, a ground-based station located at the northeastern tip of Puerto Rico. The cases investigated showed three possible types of air masses: clean (C), with African Dust (AD), and with volcanic ash (VA) from the Soufriere. We used a condensation particle counter to determine the particle number concentration, a sunphotometer (part of the AERONET) to determine volume size distributions and aerosol optical thickness (AOT), a 3-wavelength nephelometer to determine the scattering coefficients, and a 3-wavelength particle/soot absorption photometer (PSAP) for the absorption coefficients. The particle number concentrations were higher for AD and VA periods (up to about 700 cm-3 on average for both cases) in contrast to ~400 cm-3 for the C period. Volume size distributions showed bimodal distributions for the three cases with a greater influence of the coarse fraction for the C and VA periods and an increase in the fine particles for the AD period. The total scattering coefficient showed higher values for the AD (30 Mm-1) and the VA (26 Mm-1) cases than for the C case (11 Mm-1). The highest AOT values (at 500 nm) were observed for the AD and VA cases (up to ~0.35 and 0.3, respectively). C samples had much lower AOT values (~ 0.07). Our preliminary results clearly showed an impact of African dust and volcanic ash on the physical and radiative properties of aerosol particles. Results on the chemical composition of particles representative of the three cases will also be presented at the meeting.
A33E-1645
Chemical Characterization of Carbonaceous Aerosols in the Caribbean: Results From 2003- 2007
Carbonaceous aerosols may play a significant role in radiative forcing, with both OC and EC affecting the extinction of solar radiation. Despite the potential importance of carbonaceous aerosols in radiative and climate forcing relatively little information is available in terms of its worldwide concentrations, sources, mechanisms of formation, and radiative and nucleative properties. Over the last four years, 2003-2007, our laboratory has focused part of the investigation in the study of the carbonaceous fraction of aerosols in the trade winds. Samples have been collected in Cape San Juan (CSJ), Puerto Rico, using stacked-filter units (SFUs). 5-day back trajectories calculated with the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model from NOAA Air Resources Laboratory along with daily satellite images of aerosol optical thickness were used to have a better understanding of the air masses origin. Thermal-optical analysis (EC/OC analyzer) was used to determine the concentrations of the carbonaceous fraction, i.e., total carbon (TC), organic carbon (OC), and elemental carbon (EC). Results showed that the concentrations of OC range from 9 to 380 ng/m3, with an average value of ~200 ng/m3. The average value for the positive artifact at CSJ was 50%, clearly showing that if the correction of the positive artifact is not performed OC concentrations could be significantly overestimated. The particulate organic matter fraction represents from 2 to 50% of the total aerosol mass and up to about 90% of the non-sea-salt mass. We further present a complete dataset showing the relationship between the origin of the air masses and the concentrations of the carbonaceous fraction.
A33E-1646
Size-resolved Chemical Composition of African Dust Particles Over the Caribbean: How About Carbonaceous Aerosols?
Every year during the summer months, dust particles from the Sahara and Sahel regions of North Africa are transported by the trade winds over vast areas of the North Atlantic Ocean and the Caribbean. These dust particles may increase primary productivity in ocean and terrestrial ecosystems, and may also have negative impacts on public health, coral reefs, amphibian populations, visibility, and climate. To have a better understanding of the impact these particles have on the environment, information is needed with respect to their sources, chemical composition, and physical properties. As parts of this study, we performed chemical characterization of size-resolved atmospheric particles present in African dust events focusing on the carbonaceous fraction (organic and elemental carbon (OC and EC) and water-soluble organic carbon (WSOC)), and on the water-soluble nitrogen (WSN). Aerosol samples were collected at Cape San Juan, a marine station located at the most northeastern tip of Puerto Rico, using a 13-stage Dekati low-pressure impactor with quartz filters. Analyses were performed using the EC/OC and the total organic carbon and nitrogen analyzers. The presence of African dust was supported with satellite images of aerosol optical thickness, with the results from the air masses backward trajectories calculated with the NOAA HYSPLIT model, and with the color of the filters after sampling. Preliminary results show that OC size distributions during the summer period present three modes, two in the fine fraction (Dp = 0.40 μm and 1.65 μm) and one in the coarse fraction (Dp = 6.70 μm). The OC concentrations in the fine and coarse fractions were ~ 0.25 μg/m3 and 0.15 μg/m3, respectively. Additional results regarding WSOC, WSN, and elemental composition obtained with the scanning electron microscope/energy dispersive spectrometer will be presented.
A33E-1647
A regional climate model study of how biomass burning aerosol impacts the land- atmosphere interactions over the Amazon
A regional climate model is applied to examine the smoke aerosol direct and semi-direct effects during a dry to wet transition season in South America. By modified the soil and plants root parameters and by adding soil water to mitigate a dry bias of soil moisture, a realistic diurnal cycle of the surface sensibles and latent fluxes are obtained. Model results suggest that the decrease of cloudiness in early afternoon partially compensates the direct effects of smoke aerosols, so that the strongest changes of surface flux and PBL occur in later morning instead of in early afternoon. cloudiness decrease occurs right above the daytime PBL in the smoke area, while outside of the smoke center in equatorial Amazonia, cloudiness increases with its maximum occurring within the daytime PBL. An increase of lower-level moisture convergence in this region appears to be responsible for the increase of both specific and relative humidity in the PBL. Smoke aerosols, probably through their surface cooling, cause an increase of low-level moisture divergence in the smoke center, and compensational moisture convergence in the equatorial Amazonia. Such regional circulation changes can weaken the normal circulation transition from dry season to monsoon onset.
A33E-1648
Investigating smoke's influence on primary production throughout the Amazon
Smoke from annual burning in the Amazon causes large reduction in surface insolation and increases the diffuse fraction of photosynthetically-active radiation (PAR). These effects have competing influence on gross primary production (GPP). Recent studies indicate that the sign of net influence depends on aerosol optical depth, but the magnitude of smoke's effect on continental-scale carbon cycling is very poorly constrained and may constitute an important term of fire's net impact on carbon storage. To investigate widespread effects of Amazon smoke on surface radiation properties, we apply a version of the NCAR Community Atmosphere Model with prognostic aerosol transport, driven with re-analysis winds. Carbon aerosol emissions are derived from the Global Fire Emissions Database (GFED). We use AERONET observations to identify model biases in aerosol optical depth, single-scatter albedo, and surface radiative forcing, and prescribe new aerosol optical properties based on field observations to improve model agreement with AERONET data. Finally, we quantify a potential range of smoke-induced change in large-scale GPP based on: 1) ground measurements of GPP in the Amazon as a function of aerosol optical depth and diffuse fraction of PAR, and 2) empirical functions of ecosystem-scale photosynthesis rates currently employed in models such as the Community Land Model (CLM).
A33E-1649
Atmospheric Dust Impacts on Marine Phytoplankton
Atmospheric dust deposition is an important source of nutrients and trace metals to the ocean. It likely enhances ocean productivity and carbon sequestration, thus influencing atmospheric carbon dioxide concentrations and climate. We used well-characterized aerosol samples in incubation experiments to examine the effect of aerosol on phytoplankton growth and species distribution. Not all dust stimulates growth. The response of phytoplankton to aerosol additions depends on specific aerosol chemistry. Moreover, different species within the phytoplankton community respond differently to dust additions. The variability in the response to dust deposition may account for change in predicted distribution of oceanic primary production. To more accurately predict the impacts of expected future changes in dust deposition on climate global climate models must include these variable and complex interactions between aerosols and marine phytoplankton.
A33E-1650
Temporal And Spatial Variation Of The Aerosol Characteristics In The Urban Area
Aerosols have been recognized as one of the important factors in understanding climate changes. Due to the persistent dust storms with growing air pollutants, the air mass in Asia includes natural aerosols mixed with variable sources of anthropogenic aerosols, in particular. To date, numerous studies have examined both physical and chemical characteristics of aerosol from ground-based and satellite platform. Extensive ground- based network and satellite missions have been in operation for a long time, and produced global dataset. In this regard, we can scrutinize the temporal and spatial characteristics of aerosol using these data, in urban area compared to rural area, in particular. Seasonal variations have been observed in optical properties of aerosols, due to the emission of fine mode particle and hygroscopic effect. Weekly cycle is also one of the apparent signals in the urban area due to the anthropogenic activities. The trends of aerosol characteristics are obtained using the long-term data. Severe aerosol loading usually occurred in the East and South Asia while the loading is relatively low in Europe and North America. The temporal variations show different regional characteristics.
A33E-1651
Analyzing Surface Solar Flux Data in Oregon for Changes Due to Aerosols
The radiative impact of anthropogenic aerosol concentrations could be an important forcing to the earth's surface energy budget. That impact is uncertain, however, in large part because of a lack of aerosol measurements. The Solar Radiation Monitoring Lab has collected over 25 years of five minute resolution surface shortwave flux measurements at three sites in Oregon. Cloud-free measurements from these time-series are used to characterize the direct effect of aerosols on surface radiation in Oregon. This dataset is particularly valuable for aerosol studies because direct normal measurements are available in addition to total shortwave fluxes. Cloud- free days have been determined using the clear-sky identification method of Long and Ackerman (2000). The clear sky direct fluxes are then examined for the impact of aerosol changes over time. Measurements from low solar elevation angles are analyzed because of the higher sensitivity to scattering and absorbing by aerosols along the longer atmospheric path lengths at these angles. The impacts of events which are known to have high aerosol concentrations can be clearly seen. For example, the volcanic eruptions of El Chichón and Mt. Pinatubo reduce monthly averages of low solar elevation direct normal fluxes by 100-200 W/m2, a 12-25% reduction from typical values. Very little change is seen in the average background aerosol since 1980 when the measurements began. Limits are set on the possible amount of that change within the uncertainty of the measurements used. This analysis is strengthened by retrieving aerosol optical depths from the fluxes using radiative transfer calculations.
A33E-1652
Space Observations Reveal Biogenic Aerosols Dominate in Southeastern US
Space-based remote sensing of aerosols over North America show a pattern indicative of emission sources in addition to known industrial emissions. In particular, the spatial and temporal distribution of aerosols over the southeastern United States matches the spatial and seasonal distribution of biogenic volatile organic compound (BVOC) emissions, varies with surface temperature consistent with origins from oxidation of BVOCs, and dominates over summertime anthropogenic aerosol sources in the region. The observed aerosol optical thickness implies that this secondary aerosol source is climatically relevant with significant potential for a negative climate feedback as BVOC emissions increase with temperature.
A33E-1653
Development of Yellow Sand Image Products Using Infrared Brightness Temperature Difference Method
A technique for detection of airborne yellow sand dust using meteorological satellite has been developed from various bands from ultraviolet to infrared channels. Among them, Infrared (IR) channels have an advantage of detecting aerosols over high reflecting surface as well as during nighttime. There had been suggestion of using brightness temperature difference (BTD) between 11 and 12¥ìm. We have found that the technique is highly depends on surface temperature, emissivity, and zenith angle, which results in changing the threshold of BTD. In order to overcome these problems, we have constructed the background brightness temperature threshold of BTD and then aerosol index (AI) has been determined from subtracting the background threshold from BTD of our interested scene. Along with this, we utilized high temporal coverage of geostationary satellite, MTSAT, to improve the reliability of the determined AI signal. The products have been evaluated by comparing the forecasted wind field with the movement fiend of AI. The statistical score test illustrates that this newly developed algorithm produces a promising result for detecting mineral dust by reducing the errors with respect to the current BTD method.
A33E-1654
Analysis of dust and anthropogenic aerosol forcing over the global oceans from Terra
Using 6 years of combined Terra CERES radiance and MODIS aerosol data, we compute the global, ocean-only, shortwave (SW) and longwave (LW) radiative effect (SWRE, LWRE) for dust and anthropogenic aerosols. We will first use MODIS derived aerosol size properties to broadly classify aerosols into either sea salt, dust, or anthropogenic categories. We then use the quasi-linear relationship between SW flux and aerosol optical thickness (AOT) to derive a clear sky, aerosol free SW background. Since this relationship does not exist for LW, only pixels with low AOT are used to derive an aerosol-free LW background. Total aerosol SWRE and LWRE are calculated by subtracting the observed flux values from the clear sky background. SWRE and LWRE from individual aerosol components are then calculated using applying the ratio of the AOT from an individual aerosol species to the total AOT on a pixel-by-pixel basis. The uncertainties in the aerosol classifications will be applied to the individual SWRE and LWRE statistics to determine the overall uncertainty of individual aerosol radiative effect and the significance of differences in this effect from one aerosol species to another. Preliminary results indicate that globally averaged values for individual aerosol SWRE may not be representative of their true importance. Averaged over the entire ocean-only global, SWRE from anthropogenic sources exceeds dust SWRE by a factor of nearly 2. However, dust and anthropogenic SWRE are often maximized in certain regions during certain times of the year, are negligible elsewhere. As a result, we quantify SWRE on smaller regional and temporal scales to better examine the relationship between each. Similarly, LWRE does not appear to be significant on a globally averaged basis, but can offset SWRE up to 20 percent in high AOT, dust regions.
A33E-1655
GACP climatology of aerosol properties: an update
The Global Aerosol Climatology Project (GACP) product is the longest uninterrupted satellite record of aerosol optical thickness (AOT) and size dating back to 1981. The GACP data based on analyses of channel 1 and 2 AVHRR radiances show significant regional changes in the retrieved optical thickness of tropospheric aerosols which had occurred between the volcano-free periods 1988-91 and 2002-05. These trends appear to be generally plausible, are consistent with extensive sets of long-term ground-based observations throughout the world, and may increase the trustworthiness of the recently identified downward trend in the global and hemispheric AOT averages. To the extent that radiance calibration remains to be a potentially significant source of uncertainty, our regional results cannot prove unequivocally the existence of the relatively weak global and hemispheric downward trend. However, their plausibility appears to increase the trustworthiness of the overall tendency. An additional, albeit indirect, confirmation of the overall decreasing tendency in AOT comes from the widespread contemporaneous reversal from global solar dimming to global solar brightening. We thus believe that the totality of our results demonstrates the potential of satellite remote sensing to identify long-term aerosol trends. Nevertheless, more work still needs to be done in terms of both verification and potential improvement of the AVHRR radiance calibration and comparisons of GACP aerosols retrievals with potentially more accurate retrievals afforded by the newer satellite instruments.