AERONET-based microphysical and optical properties of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

Abstract

Smoke aerosols from biomass burning are an important component of the global aerosol cycle. Analysis of Aerosol Robotic Network (AERONET) retrievals of size distribution and refractive index reveals variety between biomass burning aerosols in different global source regions, in terms of aerosol particle size and single scatter albedo (SSA). Case studies of smoke transported to coastal/island AERONET sites also mostly lie within the range of variability at near-source sites. Two broad ''families'' of aerosol properties are found, corresponding to sites dominated by boreal forest burning (larger, broader fine mode, with midvisible SSA ∼0.95), and those influenced by grass, shrub, or crop burning with additional forest contributions (smaller, narrower particles with SSA ∼0.88–0.9 in the midvisible). The strongest absorption is seen in southern African savannah at Mongu (Zambia), with average SSA ∼0.85 in the midvisible. These can serve as candidate sets of aerosol microphysical/optical properties for use in satellite aerosol optical depth (AOD) retrieval algorithms. The models presently adopted by these algorithms over ocean are often insufficiently absorbing to represent these biomass burning aerosols. A corollary of this is an underestimate of AOD in smoke outflow regions, which has important consequences for applications of these satellite datasets

Evaluating the impact of aerosol particles above cloud on cloud optical depth retrievals from MODIS

“…The Gulf of Tonkin is frequently subject to smoke particle outflow as a result of a significant fire activity through the central and northern regions of the Indochinese Peninsula of Southeast Asia. This annual phenomenon has been detected and documented previously using MODIS, most active during the February to May timeframe [e.g., Sayer et al ., ; Reid et al ., ].…”

“…Above-cloud aerosol events are also found over the Persian Gulf, Mediterranean Sea, western Pacific coastlines and Gulf of Tonkin. We choose the Gulf of Tonkin (17°N-25°N, 105°E-120°E) as the secondary focus region in our study, as smoke particles are frequently observed over dense marine layer stratocumulus clouds in this region during early spring [e.g., Reid et al, 2013;Sayer et al, 2013]. Further, this demonstrates the significance of the problem over a limited, though hardly inconsequential, regional domain.…”