Long-term trends in aerosol properties derived from AERONET measurements

Abstract. Over the past two decades, remarkable changes in aerosol compositions have been observed worldwide, especially over developing countries, potentially resulting in considerable changes in aerosol properties. The Aerosol Robotic Network (AERONET) offers high precision measurements of aerosol optical parameters over about 1700 stations globally, many of which have long-term measurements for one or more decades. Here we use AERONET Level 2.0 quality assured measurements to investigate long-term trends for aerosol optical depth (AOD) and Ångström exponent (AE) trends, and quality-controlled Level 1.5 inversion products to analyze trends of absorption aerosol optical depth (AAOD) and single scattering albedo (SSA) at stations with long-term records. We also classify the aerosol properties in these sites into 6 types, and analyze the trends of each type. Results reveal decreases in AOD over the majority of the stations, except for North India and the Arabian Peninsula, where AOD increased. AE also decreased in Europe, eastern North America, and the Middle East, but increased over South Asia and East Asia. The decreased AE over Europe and eastern North America is likely due to decreased fine-mode anthropogenic aerosols, whereas that over the Arabian Peninsula is attributed to increased dust activities. Conversely, increased AE over North India is probably attributed to increased anthropogenic emissions and decreased dust loading. Most stations in Europe, North America, East Asia, and South Asia exhibit negative trends in AAOD, whereas Solar_Village in the Arabian Peninsula has positive trends. SSA at most stations increases and exhibits opposite trends to AAOD, but with several stations in central Europe and North America showing decreased SSA values. Trend analysis of different aerosol types further reveals the changes of different aerosol components that are related to AOD, AE, AAOD, and SSA trends. Stronger reductions in fine-mode absorbing species than that of non-absorbing aerosols are found over Europe and East Asia, whereas in eastern North America the reductions of aerosols are dominated by non-absorbing species. Increased aerosols in Kanpur over North India should be mainly comprised of scattering species, whereas those in Solar_Village over the Arabian Peninsula are mainly dust. Weak seasonality is found in the trends of all aerosol parameters analyzed in this work.