A mesoscale network of 14 AERONET Sun photometers was established in the UAE and adjacent Arabian Gulf from August through September 2004 as a component of the United Arab Emirates Unified Aerosol ...Experiment (UAE2). These measurements allowed for spatial, temporal and spectral characterization of the complex aerosol mixtures present in this environment where coarse mode desert dust aerosols often mix with fine mode pollution aerosols largely produced by the petroleum industry. Aerosol loading was relatively high with 2‐month averages of aerosol optical depth (AOD) at 500 nm (τa500) ranging from 0.40 to 0.53. A higher fine mode fraction of AOD was observed over Arabian Gulf island sites with Angstrom exponent at 440–870 nm (α440–870) of 0.77 as compared to an average of 0.64 over coastal sites and 0.50–0.57 at inland desert sites. During pollution events with α440–870 > 1 the retrieved fine mode radius was larger over an island site than a desert site probably because of hygroscopic growth over the humid marine environment. For these same pollution cases, single scattering albedo (ωo) at all wavelengths was ∼0.03 higher (less absorption) over the marine environment than over the desert, also consistent with aerosol humidification growth. At an inland desert location, the ωo at 440 nm remained relatively constant as Angstrom exponent varied since the fine mode pollution and coarse mode dust were both strong absorbers at short wavelengths. However, at longer wavelengths (675–1020 nm) the dust was much less absorbing than the pollution resulting in dynamic ωo as a function of α440–870.
Diurnal variability of aerosol optical depth is important for various applications, including satellite aerosol data validation, radiative forcing computations, studies of aerosol interaction with ...humidity and clouds, and also public health. Aerosol optical depth measurements acquired through the ground‐based Aerosol Robotic Network are analyzed. Analysis of the diurnal cycle over major urban/industrial areas within the network showed a prevailing pattern of the optical depth increase by 10–40% during the day at most sites. Diurnal variability range is <10% over various sites where dust aerosol is a major contributor to optical depth. Sites in southern Africa influenced by distant sources of biomass burning aerosol show no diurnal cycle, while the presence of local sources causes a diurnal trend with a maximum aerosol loading observed in the afternoon hours. Over oceans, because of the very low optical depth, even 20% departure from the daily average is practically within the measurement uncertainty.
Measurements of the column‐integrated aerosol optical properties in the southern African region were made by Aerosol Robotic Network (AERONET) Sun‐sky radiometers at several sites in August–September ...2000 as a part of the Southern African Regional Science Initiative (SAFARI) 2000 dry season field campaign. Fine mode biomass burning aerosols dominated in the northern part of the study region (Zambia), which is an active burning region, and other aerosols including fossil fuel burning, industrial, and aeolian coarse mode types also contributed to the aerosol mixture in other regions (South Africa and Mozambique), which were not as strongly dominated by local burning. The large amount of smoke produced in the north lead to a north‐south gradient in aerosol optical depth (τa ) in September, with biomass burning aerosol concentrations reduced by dispersion and deposition during transport. Large average diurnal variations of τa (typical diurnal range of 25%) were observed at all sites in Zambia as a result of large diurnal trends in fire counts in that region that peak in midafternoon. However, for all sites located downwind to the south, there was relatively little (∼5–10%) average diurnal trend observed as the aerosol transport is not strongly influenced by diurnal cycles. AERONET radiometer retrievals of aerosol single scattering albedo (ω0) in Zambia showed relatively constant values as a function of τa for τa440 ranging from 0.4 to ∼2.5. The wavelength dependence of ω0varied significantly over the region, with greater decreases for increasing wavelength at smoke‐dominated sites than for sites influenced by a significant coarse mode aerosol component. Retrievals of midvisible ω0 based on the fitting of Photosynthetically Active Radiation (PAR; 400–700 nm) flux measurements to modeled fluxes for smoke in Mongu, Zambia yielded an average value of 0.84. This is in close agreement with the estimated average of 0.85 derived from interpolation of the AERONET retrievals made at 440 and 675 nm for August–September 2000. The spectral dependence of ω0 independently retrieved with the AERONET measurements and with diffuse fraction measurements in Mongu, Zambia was similar for both techniques, as a result of both methods retrieving the imaginary index of refraction (∼0.030–0.035 on one day) with very little wavelength dependence.
Aerosol particle size is one of the fundamental quantities needed to determine the role of aerosols in forcing climate, modifying the hydrological cycle, and affecting human health and to separate ...natural from man‐made aerosol components. Aerosol size information can be retrieved from remote‐sensing instruments including satellite sensors such as Moderate Resolution Imaging Spectroradiometer (MODIS) and ground‐based radiometers such as Aerosol Robotic Network (AERONET). Both satellite and ground‐based instruments measure the total column ambient aerosol characteristics. Aerosol size can be characterized by a variety of parameters. Here we compare remote‐sensing retrievals of aerosol fine mode fraction over ocean. AERONET retrieves fine mode fraction using two methods: the Dubovik inversion of sky radiances and the O'Neill inversion of spectral Sun measurements. Relative to the Dubovik inversion of AERONET sky measurements, MODIS slightly overestimates fine fraction for dust‐dominated aerosols and underestimates in smoke‐ and pollution‐dominated aerosol conditions. Both MODIS and the Dubovik inversion overestimate fine fraction for dust aerosols by 0.1–0.2 relative to the O'Neill method of inverting AERONET aerosol optical depth spectra. Differences between the two AERONET methods are principally the result of the different definitions of fine and coarse mode employed in their computational methodologies. These two methods should come into better agreement as a dynamic radius cutoff for fine and coarse mode is implemented for the Dubovik inversion. MODIS overestimation in dust‐dominated aerosol conditions should decrease significantly with the inclusion of a nonspherical model.
The optical properties of aerosols such as smoke from biomass burning vary due to aging processes and these particles reach larger sizes at high concentrations. We compare the spectra of aerosol ...optical depth (τa), column‐integrated volume size distributions, refractive indices, and single scattering albedo retrieved from AERONET observations for four selected events of very high smoke optical depth (τa ∼ 2 at 500 nm). Two case studies are from tropical biomass burning regions (Brazil and Zambia) and two are cases of boreal forest and peat fire smoke transported long distances to sites in the US and Moldova. Smoke properties for these extreme events can be significantly different from those reported in more typical plumes. In particular, large differences in smoke fine mode particle radius (∼0.17 to 0.25 μm) and single scattering albedo (∼0.88 to 0.99 at 440 nm) were observed as a result of differences in fuels burned, combustion phase, and aging.
AErosol RObotic NETwork (AERONET) data are the primary benchmark for evaluating satellite-retrieved aerosol properties. However, despite its extensive coverage, the representativeness of the AERONET ...data is rarely discussed. Indeed, many studies have shown that satellite retrieval biases have a significant degree of spatial correlation that may be problematic for higher-level processes or inverse-emissions-modeling studies. To consider these issues and evaluate relative performance in regions of few surface observations, cross-comparisons between the Aerosol Optical Depth (AOD) products of operational MODIS Collection 5.1 Dark Target (DT) and operational MODIS Collection 5.1 Deep Blue (DB) with MISR version 22 were conducted. Through such comparisons, we can observe coherent spatial features of the AOD bias while sidestepping the full analysis required for determining when or where either retrieval is more correct. We identify regions where MODIS to MISR AOD ratios were found to be above 1.4 and below 0.7. Regions where lower boundary condition uncertainty is likely to be a dominant factor include portions of Western North America, the Andes mountains, Saharan Africa, the Arabian Peninsula, and Central Asia. Similarly, microphysical biases may be an issue in South America, and specific parts of Southern Africa, India Asia, East Asia, and Indonesia. These results help identify high-priority locations for possible future deployments of both in situ and ground based remote sensing measurements. The Supplement includes a kml file.
The variation of the aerosol optical depth and its first and second spectral derivatives (α and α′) can be largely described in terms of the spectral interaction between the individual optical ...components of a bimodal size distribution. Simple analytical expressions involving the separate optical components of each mode explain virtually all the features seen in spectra of the aerosol optical depth and its derivatives. Illustrations are given for a variety of measured optical depth spectra; these include comparative simulations of the diurnal behavior of α and α′ spectra as well as the diurnal and general statistical behavior of α and α′ as a function of optical depth (optical depth space). Each mode acts as a fixed “basis vector” from which much of the behavior in spectral and optical depth space can be generated by varying the extensive (number density dependent) contributions of fine and coarse mode optical depths. Departures from these basis vectors are caused by changes in aerosol type (average size and refractive index) and thus are associated with differing synoptical air masses, source trajectories or humidity conditions. Spectral parameters are very sensitive to interband errors in measured optical depth data. Third‐order polynomial fits within the visible‐NIR spectral region effectively filter such errors while representing the limit of useful extractable information.
The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass ...Burning Emissions Experiment (ZIBBEE) during August‐September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun‐sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (τa), aerosol size distributions, and single‐scattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured τa and the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single‐scattering albedo (ω0) of the aerosols were retrieved using several approaches, yielding average values of ω0 at ∼550 nm during ZIBBEE varying from ∼0.82 to ∼0.85, thus showing good agreement within the retrieval uncertainty of ∼0.03 of these methods. In general, ω0 was relatively constant as a function of aerosol loading, with very little change occurring for τa at 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of ω0 with increasing wavelength. Variations in the spectral change of the Angström wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.
We present a synthesis of single scattering albedo for biomass burning aerosol from the SAFARI 2000 field campaign. Values at 550 nm were derived from three methods: airborne in situ measurements of ...aerosol scattering and absorption; airborne flux radiometry; and ground‐based sun‐photometer/radiometer retrievals from AERONET. Collocated comparisons indicate that uncertainties are well understood for all three methods. The new (Version 2) AERONET retrieval gives substantially lower single scattering albedo over bright surfaces, and the comparisons herein represent the first independent check of this retrieval. Combined in situ and AERONET data yield a regional value of 0.85 ± 0.02 (mean and total uncertainty), which we propose is representative of single scattering albedo for the Southern African region during the biomass burning season. This value agrees with the “highly absorbing smoke” model used in MODIS aerosol retrievals, but indicates that many of the AeroCom models overestimate single scattering albedo for this region and season.
Aerosol optical depth data representative of various types of aerosols was employed to empirically demonstrate that the lognormal probability distribution is a better reference for reporting optical ...depth statistics than a normal probability distribution.
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