Estimation Of Aerosol Optical Thickness Research Articles

Improved hourly estimate of aerosol optical thickness over Asian land by fusing geostationary satellites Fengyun-4B and Himawari-9

Asian over-land aerosols are complexities due to a mixture of anthropogenic air pollutants and natural dust. The accuracy of the aerosol optical thickness (AOT) retrieved from the satellite is crucial to their application in the aerosol data assimilation system. Fusion of AOTs with high spatiotemporal resolution from next-generation geostationary satellites such as Fengyun-4B (FY-4B) and Himawari-9, provides a new high-quality dataset capturing the aerosol spatiotemporal variability for data assimilation. This study develops a complete fusion algorithm to estimate the optimal AOT over-land in Asia from September 2022 to August 2023 at 10 km × 10 km resolution with high efficiency. The data fusion involves four steps: (1) investigating the spatiotemporal variability of FY-4B AOT within the past 1 h and 12 km radius calculation domain; (2) utilizing the aerosol spatiotemporal variability characteristics to estimate FY-4B pure and hourly merged AOTs; (3) performing bias corrections for FY-4B and Himwari-9 hourly merged AOT for different observation times and seasons considering pixel-level errors for each satellite; (4) fusing the bias-corrected FY-4B and Himawari-9 hourly merged AOT based on maximum-likelihood estimation (MLE) method. Compared to the original FY-4B AOT, validation with AERONET observation confirms that the root mean square error (RMSE) of hourly merged FY-4B AOT decreases by around 40.6 % and the correlation coefficient (CORR) increases by about 27.8 %. Compared to FY-4B and Himawari-9 merged AOT, the fused AOT significantly decreases (increases) RMSE (CORR) by around 24.7 % (7.3 %) and 20.2 % (5.6 %). In addition, fused AOT is double the number of single-sensor merged AOT. Fusion aerosol map accurately describes the spatial and temporal variations in Asian regions controlled by air pollution and dust storms. Further studies are required for other landscapes with different satellite combinations to promote the application in the data assimilation system.

Improved Hourly Estimates of Aerosol Optical Thickness Using Spatiotemporal Variability Derived From Himawari-8 Geostationary Satellite

We developed a scheme to improve hourly estimates of aerosol optical thickness (AOT) from the Advanced Himawari Imager (AHI) onboard Himawari-8, the Japan Meteorological Agency’s latest geostationary satellite. Taking advantage of the sampling characteristics of the AHI (10-min temporal and subkilometer spatial resolution), we quantify temporal and spatial variability of AOT from the observations (AOToriginal) and utilize this information to develop an hourly algorithm that produces two sets of derived AOTs: AOTpure, derived by the application of strict cloud screening to AOToriginal, and AOTmerged, derived from spatial and temporal interpolations of AOTpure. The AOTs thus obtained from the hourly algorithm were validated against measurements from the aerosol robotic network. The root-mean-square errors (RMSEs) of the AOTpure and AOTmerged products were 0.14 and 0.11, respectively, providing improvement over an RMSE of 0.20 for AOToriginal.

Particulate emissions from large North American wildfires estimated using a new top-down method

Abstract. Particulate matter emissions from wildfires affect climate, weather and air quality. However, existing global and regional aerosol emission estimates differ by a factor of up to 4 between different methods. Using a novel approach, we estimate daily total particulate matter (TPM) emissions from large wildfires in North American boreal and temperate regions. Moderate Resolution Imaging Spectroradiometer (MODIS) fire location and aerosol optical thickness (AOT) data sets are coupled with HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) atmospheric dispersion simulations, attributing identified smoke plumes to sources. Unlike previous approaches, the method (i) combines information from both satellite and AERONET (AErosol RObotic NETwork) observations to take into account aerosol water uptake and plume specific mass extinction efficiency when converting smoke AOT to TPM, and (ii) does not depend on instantaneous emission rates observed during individual satellite overpasses, which do not sample night-time emissions. The method also allows multiple independent estimates for the same emission period from imagery taken on consecutive days. Repeated fire-emitted AOT estimates for the same emission period over 2 to 3 days of plume evolution show increases in plume optical thickness by approximately 10 % for boreal events and by 40 % for temperate emissions. Inferred median water volume fractions for aged boreal and temperate smoke observations are 0.15 and 0.47 respectively, indicating that the increased AOT is partly explained by aerosol water uptake. TPM emission estimates for boreal events, which predominantly burn during daytime, agree closely with bottom-up Global Fire Emission Database (GFEDv4) and Global Fire Assimilation System (GFASv1.0) inventories, but are lower by approximately 30 % compared to Quick Fire Emission Dataset (QFEDv2) PM2. 5, and are higher by approximately a factor of 2 compared to Fire Energetics and Emissions Research (FEERv1) TPM estimates. The discrepancies are larger for temperate fires, which are characterized by lower median fire radiative power values and more significant night-time combustion. The TPM estimates for this study for the biome are lower than QFED PM2. 5 by 35 %, and are larger by factors of 2.4, 3.2 and 4 compared with FEER, GFED and GFAS inventories respectively. A large underestimation of TPM emission by bottom-up GFED and GFAS indicates low biases in emission factors or consumed biomass estimates for temperate fires.

Dust detection and AOT estimation using combined VIR and TIR satellite images in urban areas of Iran

This study introduces an empirical equation for estimation of Aerosol Optical Thickness (AOT) and visibility reduction based on three main dust indices of Normalized Di erence Dust Index (NDDI), Brightness Temperature Di erence (BTD), and Thermal infrared Dust Index (TDI). The implementation of NDDI, BTD, and TDI in dust enhancement over bright to dark background was evaluated. The thresholds of BTD over dust and cloud pixels revealed the capability of Moderate Resolution Imaging Spectroradiometer (MODIS) images in separating dust from underlying bright surfaces and clouds. The results indicated that solar reffective bands were insucient to precisely separate dust from clouds, but combination of solar reffective bands and thermal infrared bands synergistically improved the accuracy. The evaluation of results revealed the remarkable correlation of AOT with dust enhancement indices over 11 synoptic stations: BTD (R2 = 0:73), NDDI (R2 = 0:67), and TDI (R2 = 0:71). Hence, AOT and visibility reduction were obtained using multi-regression equation using NDDI and BTD as variables. The accuracy assessment indicated good correlation (R2 = 0:74) between both estimated AOT and the AOT reported by Air Quality Control stations. Moreover, the results con rmed the advantage of proposed AOT as a consistent index for dust enhancement over bright surfaces and dust classi cation as well.

Aerosol Optical Depth Trends over Different Regions of Nigeria: Thirteen Years Analysis

Recurrent challenges of the theoretical estimation of aerosol optical thickness (AOT) are traceable to bogus assumption in its mathematical model. The objective of this paper is to propound a model that could describe the aerosol distribution sizes per time. The physics of the salient properties of aerosol within the Stokes regime was discussed. The mathematical model was applied to six locations within 335 ˟ 230 Km2 area of a selected portion of south-west, Nigeria. Though the climatic change is evident via thirteen years ground data set assimilation, the sensitivity of the proposed model was proven to increase by 0.1%. The research affirmed the use of some parameters (e.g. minimum temperature, cloud cover, relative humidity and rainfall) to estimate aerosol optical thickness and by extension the aerosol distribution size over an area.

Model‐based estimation of sampling‐caused uncertainty in aerosol remote sensing for climate research applications

To evaluate the effect of sampling frequency on the global monthly mean aerosol optical thickness (AOT), we use 6 years of geographical coordinates of Moderate Resolution Imaging Spectroradiometer (MODIS) L2 aerosol data, daily global aerosol fields generated by the Goddard Institute for Space Studies General Circulation Model and the chemical transport models Global Ozone Chemistry Aerosol Radiation and Transport, Spectral Radiation‐transport Model for Aerosol Species and Transport Model 5, at a spatial resolution between 1.125°× 1.125° and 2°× 3°: the analysis is restricted to 60°S–60°N geographical latitude. We found that, in general, the MODIS coverage causes an underestimate of the global mean AOT over the ocean. The long‐term mean absolute monthly difference between all and dark target (DT) pixels was 0.01–0.02 over the ocean and 0.03–0.09 over the land, depending on the model dataset. Negative DT biases peak during boreal summers, reaching 0.07–0.12 (30–45% of the global long‐term mean AOT). Addition of the Deep Blue pixels tempers the seasonal dependence of the DT biases and reduces the mean AOT difference over land by 0.01–0.02. These results provide a quantitative measure of the effect the pixel exclusion due to cloud contamination, ocean sun‐glint and land type has on the MODIS estimates of the global monthly mean AOT. We also simulate global monthly mean AOT estimates from measurements provided by pixel‐wide along‐track instruments such as the Aerosol Polarimetry Sensor and the Cloud–Aerosol LiDAR with Orthogonal Polarization. We estimate the probable range of the global AOT standard error for an along‐track sensor to be 0.0005–0.0015 (ocean) and 0.0029–0.01 (land) or 0.5–1.2% and 1.1–4% of the corresponding global means. These estimates represent errors due to sampling only and do not include potential retrieval errors. They are smaller than or comparable to the published estimate of 0.01 as being a climatologically significant change in the global mean AOT, suggesting that sampling density is unlikely to limit the use of such instruments for climate applications at least on a global, monthly scale.

Spatiotemporal Spectral Variations of AOT in India’s EEZ over Arabian Sea: Validation of OCM-II

We report the results of sun-photometric measurements of Aerosol Optical Thickness (AOT) in India’s Exclusive Economic Zone (EEZ) over the Arabian Sea along with synchronous Ocean Color Monitor (OCM-II) derived AOT estimates during December 12, 2009–January 10, 2010. Relatively higher values of Angstrom exponent (α) around 1.2 near coast and 0.2–0.8 in the India’s EEZ, observed during the cruise period, indicate the presence of smaller particles near the coast due to anthropogenic activities; and larger particles in the India’s EEZ due to advection of pollutants from Indian subcontinent via long-range transport. Results related to α and its derivative reveal four different aerosol types (urban-industrial, desert-dust, clean-marine, and mixed-type) with varying fraction during the study period. Surface radiative forcing due to aerosols is found to be 20 W/m2 over India’s EEZ. OCM-derived AOTs showed good corroboration with in situ measurements with a correlation coefficient of about 0.95. A reasonably good correlation was also observed between AOT and wind speed (R = 0.6); AOT and relative humidity (R = 0.58). The concurrent MODIS AOT data also agree well with those observed by the OCEANSAT (OCM-II) satellite during the campaign period.

Comparison of contrast reduction based MODIS AOT estimates with AERONET measurements

Current satellite aerosol retrieval products could be complemented by contrast reduction methods to overcome limitations regarding highly reflective or heterogeneous surfaces such as urban, desert or snow covered areas. Algorithms based on the contrast reduction principle, define contrast loss in an image, inside a pre-determined window size, as an exponential function of the Aerosol Optical Thickness (AOT) difference between two images (a reference and a polluted) acquired under similar observation geometry conditions. This paper presents a contrast reduction algorithm designed for the MODIS sensor, based on the Differential Texture Analysis (DTA) approach. It focuses on algorithm optimization by: a) determining an optimal AOT spatial resolution; b) constraining the relative observation geometry differences between polluted and reference images; and c) assessing the influence of several land cover classes on the accuracy of the retrievals. A comparison of the results obtained for 192 images acquired for the year 2005 with data from five European AERONET stations is performed to assess overall algorithm accuracy as well as the impact of the proposed improvements. Comparative analysis of the results for the various sites showed an optimal algorithm performance for MODIS images using a 39pixel distance window, composed of only forest and urban pixels. Comparison with AERONET AOT data showed a good agreement with a correlation coefficient of 0.78. A similar correlation is found when comparing AERONET measurements and MODIS aerosol standard product. This research supports the establishment of contrast reduction methods as a potential complement to other aerosol retrieval methodologies. Future work will aim at removing the residual aerosol influence from reference images, including BRDFs to better reproduce surface heterogeneity and observation geometry influences and expanding the scope of this study to other AERONET sites so as to further test the algorithm at a global scale.

Simulation of aerosol radiative effects over West Africa during DABEX and AMMA SOP-0

The regional climate model RegCM3 has been used to assess optical properties and clear-sky direct radiative forcing (DRF) of mineral dust and carbonaceous aerosols over West Africa for the period October 2005 to April 2006. Our results display a significant seasonal variation of the aerosol single scattering albedo (SSA) due to varying contributions from biomass burning (BB) and dust. During December-January, simulated SSA values dropped to around 0.81-0.83 at 440 nm and to 0.80-0.85 at 675 nm when absorbing aerosols from biomass burning dominate the mixture. During March and April, when mineral dust dominates, simulated SSA values increased reaching around 0.90-0.92 at 440 nm and 0.94-0.96 at 675 nm. The simulated aerosol optical thickness (AOT) was maximum over central Africa where it far exceeded estimates of AOT from satellite which showed the greatest AOT in the gulf of Guinea. This discrepancy was linked to an overestimation of BB emissions in central Africa and a possible underestimation of AOT over central Africa due a high occurrence of cloud and associated difficulties in cloud screening. The DRF calculations were extremely sensitive to aerosol optical properties and underlying surface albedo. Over dark surfaces, the sum of shortwave (SW) and longwave (LW) top of the atmosphere (TOA) direct radiative forcing averaged from December to February was negative (−5.25 to −4.0 W/m2) while over bright surfaces it was close to zero (−0.15 W/m2). Large differences between SW surface and SW TOA direct radiative forcing indicated that SW absorption had an important influence on the radiative budget. The SW radiative heating rate associated with the aerosol reached 1.2 K/d at local noon (diurnal mean of 0.40 K/d) over Niamey (∼13.5°N, 2°E) and peaked at altitudes of 2-4 km, corresponding to the BB aerosol layer.

A geostatistical data fusion technique for merging remote sensing and ground‐based observations of aerosol optical thickness

The Multiangle Imaging Spectroradiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the NASA Earth Observation System's Terra satellite have been measuring aerosol optical thickness (AOT) since early 2000. These remote‐sensing platforms complement the ground‐based Aerosol Robotic Network (AERONET) in better understanding the role of aerosols in climate and atmospheric chemistry. To date, however, there have been only limited attempts to exploit the complementary multiangle (MISR) and multispectral (MODIS) capabilities of these sensors along with the ground‐based observations in an integrated analysis. This paper describes a geostatistical data fusion technique that can take advantage of the spatial autocorrelation of the AOT distribution, while making optimal use of all available data sets. Using Level 2.0 AERONET, MISR, and MODIS AOT data for the contiguous United States, we demonstrate that this approach can successfully incorporate information from multiple sensors and provide accurate estimates of AOT with rigorous uncertainty bounds. Cross‐validation results show that the resulting AOT product is closer to the ground‐based AOT observations than either of the individual satellite measurements.

Daily estimates of aerosol optical thickness over land surface based on a directional and temporal analysis of SEVIRI MSG visible observations

This paper presents an innovative method for obtaining a daily estimate of a quality‐controlled aerosol optical thickness (AOT) of a vertical column of the atmosphere over the continents. Because properties of land surface are more stationary than the atmosphere, the temporal dimension is exploited for simultaneous retrieval of the surface and aerosol bidirectional reflectance distribution function (BRDF) coming from a kernel‐driven reflectance model. Off‐zenith geometry of illumination enhances the forward scattering peak of the aerosol, which improves the retrieval of AOT from the aerosol BRDF. The solution is obtained through an unconstrained linear inversion procedure and perpetuated in time using a Kalman filter. On the basis of numerical experiments using the 6S atmospheric code, the validity of the BRDF model is demonstrated. The application is carried out with data from the Spinning Enhanced Visible and Infra Red Imager (SEVIRI) instrument on board the geostationary Meteosat Second Generation (MSG) satellite from June 2005 to August 2007 for midlatitude regions and from March 2006 to June 2006 over desert sites. The satellite‐derived SEVIRI AOT compares favorably with Aerosol Robotic Network (AERONET) measurements for a number of contrasted stations and also similar Moderate Resolution Imaging Spectroradiometer (MODIS) products, within 20% of relative accuracy. The method appears competitive for tracking anthropogenic aerosol emissions in the troposphere and shows a potential for the challenging estimate of dust events over bright targets. Moreover, a high‐frequency distribution of AOT provides hints as to the variability of pollutants according to town density and, potentially, motor vehicle traffic. The outcomes of the present study are expected to promote a monitoring of the global distributions of natural and anthropogenic sources and sinks of aerosol, which are receiving increased attention because of their climatic implications.

An alternative method for aerosol optical thickness retrieval in the UV range

Atmospheric aerosol particle size distribution data derived from almucantar scans performed by the CIMEL sunphotometer at Belsk, Poland, in 2005 are used for the estimation of aerosol optical thickness (AOT) in the UV range by applying the Mie theory. The results obtained are compared with the direct Sun measurement data from the CIMEL sunphotometer and collocated Brewer spectrophotometer No. 064, as well as with AOT obtained by Angstrom extrapolation from the direct Sun measurements performed by the CIMEL sunphotometer in the visible range of wavelengths. Mean differences between calculated and measured values of AOT are up to about 8% in the UV-B range, which is close to the measurement uncertainty of the Brewer spectrophotometer and much less than that obtained by means of Angstrom extrapolation (over 24% in the UV-B range).

Spatio-temporal variability of satellite-derived aerosol optical thickness over Northeast Asia in 2004

In this study a modified Bremen aerosol retrieval (BAER) method was used to retrieve aerosol optical thickness (AOT) over both land and ocean using moderate resolution imaging spectro-radiometer (MODIS) data over Northeast Asia for a full year during 2004. Retrieved MODIS AOT data were in good agreement with data obtained from a ground-based AERONET sunphotometer ( r=0.90, linear slope=0.89). Seasonal variation analysis of AOT revealed maximum values in summer (∼0.41) and minimum values in winter (∼0.25). The contribution of each aerosol type to total AOT was estimated for each pixel. A spectral shape fitting procedure was used to select the optimum aerosol model for AOT retrieval among six aerosol types: urban, rural, maritime, tropospheric, Asian dust, and biomass burning. The spatio-temporal distribution of average AOT was analyzed for the following five sectors in Northeast Asia: (I) East China, (II) Yellow Sea, (III) Korea, (IV) East Sea, and (V) South Sea plus a part of Japan. Maximum AOT values of 0.75±0.18 were measured over sector (I) in summer, while minimum values of 0.10±0.02 were recorded over sector (IV) in winter. AOT estimates over sector (I) were much higher than those of other sectors due to an increased contribution to the total AOT by fine urban aerosol, which contributed up to 56.5% of the total AOT.

Impact of 3-D Clouds on Clear-Sky Reflectance and Aerosol Retrieval in a Biomass Burning Region of Brazil

Three-dimensional (3-D) cloud radiative effects on clear-sky reflectances and associated aerosol optical depth retrievals are quantified for a cumulus cloud field in a biomass burning region in Brazil through a Monte Carlo simulation. In this study the 1-km Moderate Resolution Imaging Spectroradiometer cloud optical depth and surface reflectance datasets are used to compute the 3-D radiation fields with ambient aerosol optical thickness of 0.1 at a wavelength of 0.66 /spl mu/m. The 3-D radiative effects range from -0.015 to 0.018 with an average of 0.004 and standard deviation of 0.006. The 3-D effects are most pronounced and variable for cloud neighboring pixels, where both large negative effects over shadows and positive effects near sunlit cloud edges are found. The clear next-to-cloud pixels, that contain /spl sim/83% of the clear pixel population, are affected in the most complex way and not reliable for aerosol retrieval. In the area 2 km away from clouds, the 3-D effects enhance the reflectance in clear patches. The average and variability of enhancements gradually decrease as a function of the cloud-free distance, resulting in a systematically higher aerosol optical depth estimates for pixels closer to clouds in one-dimensional (1-D) retrieval. At a distance of 3 km away from clouds, the 3-D effect is still appreciable with the average enhancement slightly less than 0.004. This enhancement will lead to an over estimate of aerosol optical thickness of /spl sim/0.04 in 1-D retrieval, which is significant for an ambient atmosphere with aerosol optical thickness of 0.1.

Analysis of shipboard aerosol optical thickness measurements from multiple sunphotometers aboard the R/V Ronald H Brown during the Aerosol Characterization Experiment—Asia

Marine sunphotometer measurements collected aboard the R/V Ronald H. Brown during the Aerosol Characterization Experiment-Asia (ACE-Asia) are used to evaluate the ability of complementary instrumentation to obtain the best possible estimates of aerosol optical thickness and Angstrom exponent from ships at sea. A wide range of aerosol conditions, including clean maritime conditions and highly polluted coastal environments, were encountered during the ACE-Asia cruise. The results of this study suggest that shipboard hand-held sunphotometers and fast-rotating shadow-band radiometers (FRSRs) yield similar measurements and uncertainties if proper measurement protocols are used and if the instruments are properly calibrated. The automated FRSR has significantly better temporal resolution (2 min) than the hand-held sunphotometers when standard measurement protocols are used, so it more faithfully represents the variability of the local aerosol structure in polluted regions. Conversely, results suggest that the hand-held sunphotometers may perform better in clean, maritime air masses for unknown reasons. Results also show that the statistical distribution of the Angstrom exponent measurements is different when the distributions from hand-held sunphotometers are compared with those from the FRSR and that the differences may arise from a combination of factors.

Estimation of aerosol optical thickness over northeast Asia from Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS) data during the 2001 ACE‐Asia intensive observation period

Radiative forcing by aerosol is one of the greatest sources of uncertainty in climate change modeling. One of the main objectives of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) was to assess the spatial and temporal variability of aerosol properties in northeast Asia. Aerosol optical parameters are retrieved from the Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS) satellite data to study aerosol characteristics over northeast Asia during the ACE‐Asia intensive observation period 2001. The aerosol optical thickness (AOT) and Ångström exponent are retrieved over land and ocean at a spatial scale of 1 × 1 km2 by using the Bremen aerosol retrieval (BAER) technique. SeaWiFS‐derived AOT data were compared with those ground‐measured AERONET AOT. The validation between SeaWiFS and Sun photometer data showed good correlation coefficients (r > 0.89) during the ACE‐Asia IOP. The SeaWiFS‐retrieved AOT showed a high AOT value of around 0.8 and an Ångström exponent of 0.51 during an Asian dust case of 13 April 2001 over Gosan. Aerosol parameters retrieved from SeaWiFS data by using the BAER technique showed promising results to monitor atmospheric aerosol loading and estimate its optical properties.

April 1998 Asian dust event over the Columbia Plateau

Surface‐based radiometers can be used to assess the atmospheric aerosol burden. During 1998, two multifilter rotating shadow‐band radiometers (MFRSRs), operated by Washington State University (WSU) and by the USDA UV‐B program, were used to collect data on the Columbia Plateau atmosphere. Analysis of these data by an automated Langley algorithm provided retrievals for total optical thickness, allowing for calculation of aerosol optical thickness (AOT) and the top‐of‐atmosphere (TOA) instrument signal. Statistical evaluation of the TOA signal permitted recalculation of optical thickness using the Bouguer‐Lambert‐Beer law and resulted in improved estimates of aerosol optical thickness (AOT). Results for AOT and the associated Ångström parameters are presented here for an April 1998 dust event for two colocated Columbia Plateau sites. AOT at 500 nm went from background levels (seasonally dominated by regional windblown dust) of ∼0.2 to more than 0.4 during the event maximum on April 27, not returning to normal levels until April 30. Comparison of 500‐nm AOT between the two MFRSR showed a root‐mean‐square (RMS) difference of 0.016. The Ångström exponent α reached a minimum of ∼0.2, and the β coefficient reached a maximum of ∼0.35, both on April 27, coincident with the AOT maximum. Contemporaneous aerosol sampling in Spokane, Washington, provided (1) elemental data that strongly support our interpretation of this event as an influx of Asian dust without significant sulfur enrichment and (2) event maximum PM10 measurements ∼80 μg/m3 consistent with Pullman event maximum AOT results, assuming a 3–4 km thick dust layer.

Comparison of aerosol optical properties and water vapor among ground and airborne lidars and Sun photometers during TARFOX

We compare aerosol optical thickness (AOT) and precipitable water vapor (PWV) measurements derived from ground and airborne lidars and Sun photometers during the Tropospheric Aerosol Radiative Forcing Observational Experiment. Such comparisons are important to verify the consistency between various remote sensing measurements before employing them in any assessment of the impact of aerosols on the global radiation balance. Total scattering ratio and extinction profiles measured by the ground‐based NASA Goddard Space Flight Center scanning Raman lidar system, which operated from Wallops Island, Virginia (37.86°N, 75.51°W), are compared with those measured by the Lidar Atmospheric Sensing Experiment (LASE) airborne lidar system aboard the NASA ER‐2 aircraft. Bias and root‐mean‐square differences indicate that these measurements generally agreed within about 10%. Aerosol extinction profiles and estimates of AOT are derived from both lidar measurements using a value for the aerosol extinction/backscattering ratio Sa = 60 sr for the aerosol extinction/backscattering ratio, which was determined from the Raman lidar measurements. The lidar measurements of AOT are found to be generally within 25% of the AOT measured by the NASA Ames Airborne Tracking Sun Photometer (AATS‐6). However, during certain periods the lidar and Sun photometer measurements of AOT differed significantly, possibly because of variations in the aerosol physical characteristics (e.g., size, composition) which affect Sa. Estimates of PWV, derived from water vapor mixing ratio profiles measured by LASE, are within 5–10% of PWV derived from the airborne Sun photometer. Aerosol extinction profiles measured by both lidars show that aerosols were generally concentrated in the lowest 2–3 km.

Satellite‐observed patterns in stratus microphysics, aerosol optical thickness, and shortwave radiative forcing

A long‐term global daily data set of satellite‐derived radiances has been applied over oceans to investigate the effect of aerosol particles on marine stratus cloud physical parameters and cloud shortwave radiative forcing. Tropospheric aerosol can indirectly influence cloud reflectance and radiative forcing by acting as cloud condensation nuclei, thus modifying the droplet size distribution and cloud optical properties. The NOAA polar‐orbiting satellite advanced very high resolution radiometer Pathfinder Atmosphere (PATMOS) data provide estimates of aerosol optical thickness (AOT) which were utilized to represent the amount of aerosol available to modify marine stratus. Periods of stratospheric aerosol loading were avoided for this analysis. PATMOS multichannel radiances for cloudy pixels were combined with radiative transfer modeling to estimate cloud droplet effective radius and cloud optical thickness for several months and years in (110 km)2 grid cells between 50°S and 50°N. The results indicate an inverse relationship between aerosol burden and cloud droplet size, including seasonal and regional variability that compares well with known aerosol sources. Analyses suggest an AOT value above which the stratus properties do not change; that is, the indirect effect reaches a “microphysical saturation limit.” Radiative flux modeling was used to evaluate the impact of observed cloud characteristics on shortwave forcing. Decreases in droplet size and increases in cloud albedo associated with enhanced AOT produce significantly increased shortwave radiative forcing estimates as compared with “clean background” cloud and aerosol conditions. These results provide observational evidence for the global influence of aerosol on the shortwave radiative forcing by stratus.

Determination of aerosol optical thickness from measurements of spectral sky radiance

Simultaneous measurements of spectral optical thickness and path radiance are used to test the relationship between path radiance and aerosol optical thickness for a given geometry of illumination and observation. The selected geometry is representative of satellite remote sensing conditions. The measurements have been accomplished in an urban location with a slightly polluted atmosphere. Ground-based measurements were made with a device that combines sun-photometer and sky radiometer functions. The measurement sequence includes measurements of transmission of the direct sunlight in order to retrieve the aerosol optical thickness in nine spectral bands between 440 and 1030 nm, followed by measurements of the sky radiance in the solar almucantar on both sides of the sun in the same spectral bands. For this purpose we have used a set of radiances acquired around 60° solar zenith angle in the antisolar direction of the solar almucantar. The results obtained show that aerosol optical thickness estimates are possible from path radiance measurements. On the other hand, it seems feasible to derive the atmospheric correction in a given spectral band from information acquired at another wavelength. The aerosol optical thickness (radiance) in one spectral band allows appropriate estimates of aerosol optical thickness (radiance) in another spectral band. In this sense, we also obtain a relationship between the spectral dependencies of aerosol optical thickness and path radiance.