Long-term (1995–2018) aerosol optical depth derived using ground based AERONET and SKYNET measurements from aerosol aged-background sites

Özet

We examined long-term aerosol optical depth (AOD) trends over 53 sites across the globe which comprise 49sites from the Aerosol Robotic Network (AERONET) and 4 sites from the Sky radiometer Network (SKYNET)during 1995–2018. Most of these sites are located in remote and isolated aged-background regions, and few arein urban/semi-urban sites having averaged AOD ∼0.1 at 500 nm. These selected sites have a global distributionincluding tropical, mid-latitudes, high-latitudes and Polar regions. Among them, there are 14 high-altitudestations (∼1028–5050 m amsl), including Himalayan and Polar regions. The main objective of the present workis to evaluate the AOD trends over the aged-background sites across the globe. We found that significant numberof sites located in North-South America, Europe, Arctic and Australia have statistically significant and negativetrends varied from −6.3x$10^{-3}$ to −1.0x$10^{-3}$ AOD $year^{−1}$. The negative trends over these sites could be attributedto reduction in anthropogenic emission. Furthermore, there are mixed trends of positive as well as negative overAsian and southern oceanic regions including Antarctica. Some of the trends are weak and statistically nonsignificant, probably due to non-availability of long-term ground based data. However, the AOD trends overthese regions show increasing tendency with statistically significant trends of 8.0x$10^{-4}$ to 4.7x$10^{-3}$ AOD $year^{−1}$.The present study has also many important aspects on global and regional climate change at high-mountain andaged-background sites in particular, where the satellite based measurements are inaccurate and biased due toextremely low AOD.