Global insights on absorption characteristics of aerosols

Abstract

A comprehensive analysis of aerosol absorption parameters—single scattering albedo (SSA) and absorption aerosol optical depth (AAOD)—using high-quality AErosol RObotic NETwork (AERONET) observations, and space-time collocated validation of Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2) simulations over the globe, is conducted on a seasonal scale, for the first time. AERONET SSA is lower in Central Asia (0.86) and South Asia (0.90) than East Asia (0.93) and Southeast Asia (0.94). Over Asia, the annual mean AAOD is higher over South Asia (~0.07) and it is ~50 % lower over Southeast Asia and East Asia. Globally, AAOD (>0.1) is highest over central Africa in winter because of high aerosol optical depth (AOD) and lower SSA which arise due to intense biomass burning and dust aerosols. Seasonal variabilities in spectral SSA and AAOD over North America, South America, Europe, and Middle East are not statistically significant which reveals no significant variations in aerosol composition and size distribution throughout the year over these regions. Bias in MERRA-2 SSA is lower with high Global Climate Observing System (GCOS) fraction (>50 %) for moderately absorbing aerosols (0.90 ≤ SSA < 0.95) as the distribution of simulated SSA is narrow in this SSA range as compared to less absorbing (SSA ≥ 0.95) and more absorbing (SSA < 0.90) aerosols (GCOS fraction: <50 %). Bias in MERRA-2 AAOD is less (0.003) for low AAOD (≤0.025) conditions, and higher (−0.05) for high AAOD (≥0.075) conditions. During peak fire seasons over biomass-burning dominated sites, MERRA-2 overestimates SSA (mean bias error (MBE): ~0.04) with low GCOS fraction (<50 %), and underestimates AAOD (MBE > 0.03, in magnitude) with <50 % data points lying within the expected error. Our findings provide critical global insights for better understanding the characteristics and variabilities of aerosol absorption, and for improving model simulated aerosol absorption on seasonal and global scales, which are useful to substantially reduce the uncertainties in global assessment of radiative and climate impact of aerosols.