N. T. O'Neill

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign

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 Angstrom wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.

Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space

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.

Atmospheric Aerosol Optical Properties in the Persian Gulf

Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network (AERONET) are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March-July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (;20%-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 mm (std dev 5 0.02) and 2.57 mm (std dev 5 0.27), respectively. The geometric standard deviation of each fraction is 0.41 and 0.73, respectively. In dust-free conditions the single scattering albedo (SSA) decreases with wavelength, while in the presence of dust the SSA either stays neutral or increases slightly with wavelength. The changes in the Angstrom parameter derived from a ground-based nephelometer and a collocated sun photometer during the initial checkout period were quite similar.

High aerosol optical depth biomass burning events: A comparison of optical properties for different source regions

Received 29 May 2003; revised 8 September 2003; accepted 17 September 2003; published 21 October 2003. [1] 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 (ta), columnintegrated volume size distributions, refractive indices, and single scattering albedo retrieved from AERONET observations for four selected events of very high smoke optical depth (ta � 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 mm) 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. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation. Citation: Eck, T. F., B. N. Holben, J. S. Reid, N. T. O’Neill, J. S. Schafer, O. Dubovik, A. Smirnov, M. A. Yamasoe, and P. Artaxo, High aerosol optical depth biomass burning events: A comparison of optical properties for different source regions, Geophys. Res. Lett., 30(20), 2035, doi:10.1029/ 2003GL017861, 2003.

The lognormal distribution as a reference for reporting aerosol optical depth statistics; Empirical tests using multi‐year, multi‐site AERONET Sunphotometer data

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.

Aerosol optical depth over the oceans: Analysis in terms of synoptic air mass types

The results of spectral aerosol optical depth measurements in the Pacific Ocean, Baltic Sea, and North Atlantic are considered with regard to air mass types. It is found that the optical properties of continental and maritime air mass types differ significantly for the data employed in this study. A synoptical air mass context was also employed in demonstrating the correlation between near infrared aerosol optical depth τa(1640 nm) and wind speed as well as for investigations into the relationship between deck level relative humidity and the aerosol optical depth at 550 nm. Simulations, employing well-known aerosol parameterization models, of the aerosol optical depth spectra for various air mass types show good agreement with the experimental results in the visible and near infrared range.

A study of the link between synoptic air mass type and atmospheric optical parameters

The paper deals with air mass influence on atmospheric spectral transmittance in a rural, pollution-free area (Sherbrooke, Quebec, in eastern Canada). A statistical analysis of aerosol optical depths (0.04 ≤ τa≤0.60) and Angstrom parameters (0.20 ≤ α ≤ 2.1) derived from measurements of direct spectral solar radiation in the period from January 1989 up to August 1991 is presented. The analysis incorporates investigations into correlations with air mass type (obtained from synoptic maps) and source type (deduced from back trajectory analysis). Mean monthly values of aerosol optical depth and corresponding information about frequency of air mass occurrence are also presented. The temporal comparison of these two ensembles of data helps to explain the similarities and differences observed in the month to month variations of aerosol optical depth during the 3-year measurement period. The data arrays are then partitioned in such a way as to facilitate the interpretation in terms of determining the optical mechanisms which influence the aerosol optical depths. For air masses of arctic origin the discrimination of seasonal variations of aerosol optical depth is consistent with independent measurements of turbidity made in Alaska. The role of air mass source in defining aerosol optical depth is evaluated in terms of its being potentially a more fundamental influence than air mass type.

Measurement of atmospheric optical parameters on U.S. Atlantic coast sites, ships, and Bermuda during TARFOX

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on U.S. Atlantic coast sites, ships, and Bermuda in 1996 during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Spatial and temporal analysis of Sun photometry data was supported by synoptic analysis of air mass evolution. The spatial distribution of aerosol optical depth is presented. In several cases the aerosol size distributions deduced from sky almucantar measurements and solar disk attenuation measurements at the various coastal sites yielded similar results within the same air masses. Ship-based measurements in the Atlantic Ocean showed significant maritime aerosol optical property variations which for the most part could be attributed to the influence of continental sources and Saharan dust events. The Bermuda data (optical depths and Angstrom parameter values) illustrated changes in atmospheric optical properties for various air masses and trajectories. Almost no correlation was observed between aerosol optical depth and water vapor content when the data from all stations and ship measurements were considered together. In the case of individual stations or ship transects, different degrees of correlation could be observed. In continental conditions on the east coast, optical depth and water vapor are well correlated, while in a maritime environment, optical depth can be relatively small despite high water vapor contents.

Equivalence of pyrheliometric and monochromatic aerosol optical depths at a single key wavelength

The atmospheric aerosol optical depth (AOD) weighted over the solar spectrum is equal to the monochromatic AOD at a certain wavelength. This key wavelength is ~0.7 mum, which is only slightly influenced by air mass and aerosol content. On the basis of this result, simple relations are proposed to predict monochromatic AOD from pyrheliometric data and vice versa. The accuracy achieved is close to ?0.01 units of AOD at ~0.7 mum, estimated from simultaneous sunphotometer data. The precision required for the estimation of the precipitable water-vapor content is approximately ?0.5 cm.