The climatology of aerosol optical thickness and radiative effects in Southeast Asia from 18-years of ground-based observations

The present study utilizes 18 years of long-term (2001–2018) data collected from six active AERONET sites over the Indo-Gangetic Plain (IGP) and the North China Plain (NCP) areas in Southeast Asia. The annual mean (±SD) aerosol optical thickness at 440 nm (AOT440) was found high at XiangHe (0.92 ± 0.69) and Taihu (0.90 ± 0.51) followed by Beijing (0.81 ± 0.69), Lahore (0.81 ± 0.43), and Kanpur (0.73 ± 0.35) and low at Karachi (0.52 ± 0.23). Seasonally, high AOT440 with corresponding high Ångström exponent (ANG440-870) noticed during JJA for all sites, except Kanpur, suggesting the dominance of fine-mode particles, generally associated with large anthropogenic emissions. Climatologically, an increasing (decreasing) trend was observed over IGP (NCP) sites, with the highest (lowest) percentage of departures in AOT440 found over Beijing (Karachi). We further identified major aerosol types which showed the dominance of biomass burning, urban-industrial followed by the mixed type of aerosols. In addition, single scattering albedo (SSA), asymmetry parameter (ASP), volume size distribution (VSD), and complex aerosol refractive index (RI) showed significant temporal and spectral changes, illustrating the complexity of aerosol types. At last, the annual mean direct aerosol radiative forcing at the top, bottom, and within the atmosphere for all sites were found in the range from −17.36 ± 3.75 to −45.17 ± 4.87 W m−2, -64.6 ± 4.86 to −93.7 ± 10.27 W m−2, and 40.5 ± 6.43 to 68.25 ± 7.26 W m−2, respectively, with an averaged atmospheric heating rate of 0.9–2.3 K day−1. A large amount of anthropogenic aerosols showed a significant effect of heating (cooling) on the atmosphere (surface) results obviously, due to an increased rate of atmospheric heating. Therefore, the thermodynamic effects of anthropogenic aerosols on the atmospheric circulation and its structure should be taken into consideration for future study over the experimental sites. [Display omitted] •The 18-year climatology of AOT and radiative forcing is carried out at six AERONET sites in Southeast Asia.•AOT revealed significant seasonal and annual heterogeneity, being enhanced (reduced) during JJA (DJF) seasons.•Trend in AOT showed an increasing (decreasing) pattern on annual scale over IGP (NCP) sites.•Cluster technique used to differentiate between various types of pollutants based on their optical parameters.•Increased rates of ARF and its heating effect in the atmosphere were found in Southeast Asia. Long