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

Abstract

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.