AERONET Retrievals Research Articles

Evaluation of aerosol properties over ocean from Moderate Resolution Imaging Spectroradiometer (MODIS) during ACE‐Asia

The Aerosol Characterization Experiment‐Asia (ACE‐Asia) was conducted in March–May 2001 in the western North Pacific in order to characterize the complex mix of dust, smoke, urban/industrial pollution, and background marine aerosol that is observed in that region in springtime. The Moderate Resolution Imaging Spectroradiometer (MODIS) provides a large‐scale regional view of the aerosol during the ACE‐Asia time period. Focusing only on aerosol retrievals over ocean, MODIS data show latitudinal and longitudinal variation in the aerosol characteristics. Typically, aerosol optical depth (τa) values at 0.55 μm are highest in the 30°–50° latitude band associated with dust outbreaks. Monthly mean τa in this band ranges ∼0.40–70, although large differences between monthly mean and median values indicate the periodic nature of these dust outbreaks. The size parameters, fine mode fraction (η), and effective radius (reff) vary between monthly mean values of η = 0.47 and reff = 0.75 μm in the cleanest regions far offshore to approximately η = 0.85 and reff = 0.30 μm in near‐shore regions dominated by biomass burning smoke. The collocated MODIS retrievals with airborne, ship‐based, and ground‐based radiometers measurements suggest that MODIS retrievals of spectral optical depth fall well within expected error (Δτa = ±0.03 ± 0.05τa) except in situations dominated by dust, in which cases MODIS overestimate both the aerosol loading and the aerosol spectral dependence. Such behavior is consistent with issues related to particle nonsphericity. Comparisons of MODIS‐derived reff with AERONET retrievals at the few occurrences of collocations show MODIS systematically underestimates particle size by 0.2 μm. Multiple‐year analysis of MODIS aerosol size parameters suggests systematic differences between the year 2001 and the years 2000 and 2002, which are traced to instrumental electronic cross talk. Sensitivity studies show that such calibration errors are negligible in τa retrievals but are more pronounced in size parameter retrievals, especially for dust and sea salt.

Microphysical and optical properties of aerosol particles in urban zone during ESCOMPTE

Microphysical and optical properties of the main aerosol species on a peri-urban site have been investigated during the ESCOMPTE experiment. Ammonium sulfate (AS), nitrate (N), black carbon (BC), particulate organic matter (POM), sea salt (SS) and mineral aerosol (D) size distributions have been used, associated with their refractive index, to compute, from the Mie theory, the key radiative aerosol properties as the extinction coefficient K ext, the mass extinction efficiencies σ ext, the single scattering albedo ω 0 and the asymmetry parameter g at the wavelength of 550 nm. Optical computations show that 90% of the light extinction is due to anthropogenic aerosol and only 10% is due to natural aerosol (SS and D). 44±6% of the extinction is due to (AS) and 40±6% to carbonaceous particles (20±4% to BC and 21±4% to POM). Nitrate aerosol has a weak contribution of 5±2%. Computations of the mass extinction efficiencies σ ext, single scattering albedo ω 0 and asymmetry parameter g indicate that the optical properties of the anthropogenic aerosol are often quite different from those yet published and generally used in global models. For example, the (AS) mean specific mass extinction presents a large difference with the value classically adopted at low relative humidity ( h<60%) (2.6±0.5 instead of 6 m 2 g −1 at 550 nm). The optical properties of the total aerosol layer, including all the aerosol species, indicate a mean observed single-scattering albedo ω 0=0.85±0.05, leading to an important absorption of the solar radiation and an asymmetry parameter g=0.59±0.05 which are in a reasonably good agreements with the AERONET retrieval of ω 0 (=0.86±0.05) and g (=0.64±0.05) at this wavelength.

Airborne Sun photometer measurements of aerosol optical depth and columnar water vapor during the Puerto Rico Dust Experiment and comparison with land, aircraft, and satellite measurements

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements obtained with the six‐channel NASA Ames Airborne Tracking Sunphotometer (AATS‐6) mounted on a twin‐engine aircraft during the summer 2000 Puerto Rico Dust Experiment are presented. In general, aerosol extinction values calculated from AATS‐6 AOD measurements acquired during aircraft profiles up to 5 km above sea level (asl) reproduce the vertical structure measured by coincident aircraft in situ measurements of total aerosol number concentration. AATS‐6 extinction retrievals also agree with corresponding values derived from ground‐based lidar measurements for altitudes above the trade inversion. The spectral behavior of AOD within specific layers beneath the top of the aircraft profile is consistent with attenuation of incoming solar radiation by large dust particles or by dust plus sea salt, with mean Ångström wavelength exponents of ∼0.20. Values of CWV calculated from profile measurements by AATS‐6 at 941.9 nm and from aircraft in situ measurements agree to within ∼4% (0.13 g/cm2). AATS‐6 AOD values measured on the ground at Roosevelt Roads Naval Air Station and during low‐altitude aircraft runs over the adjacent Cabras Island aerosol/radiation ground site agree to within 0.004–0.030 with coincident data obtained with an AERONET Sun/sky radiometer located on Cabras Island. For the same observation times, AERONET retrievals of CWV exceed AATS‐6 values by ∼21%. AATS‐6 AOD values measured during low‐altitude aircraft traverses over the ocean are compared with corresponding AOD values retrieved over water from upwelling radiance measurements by the Moderate‐Resolution Imaging Spectroradiometer (MODIS), Total Ozone Mapping Spectrometer (TOMS), and GOES 8 Imager satellite sensors, with mixed results.