Long-term (2008-2017) analysis of atmospheric composite aerosol and black carbon radiative forcing over a semi-arid region in southern India: Model results and ground measurement

摘要

To achieve an in-depth understanding of radiative forcing due to aerosols is a crucial challenge for climate change studies. The first-ever long-term measurement of direct shortwave composite and black carbon aerosol radiative properties over a semi-arid region, Anantapur, in southern India is presented. Long-term variations in Aerosol Optical Depth (AOD) and Black Carbon (BC) mass concentration from December 2007 to November 2017 are discussed with specific emphasis on intra-seasonal variation in aerosol optical properties, meteorology, transport pathways, and their implications for direct short wave radiative forcing over Anantapur. The intraseasonal mean AOD showed strong seasonal dependence with the highest (0.47 +/- 0.03) during summer and lowest (0.28 +/- 0.03) during the monsoon. Meanwhile, the intra-seasonal mean (+/- sigma) BC mass concentration was about 3.57 +/- 0.45, 2.60 +/- 0.58, 1.22 +/- 0.18 and 2.24 +/- 0.28 mu g m(-3) during winter, summer, monsoon and postmonsoon respectively. Furthermore, there is an obvious temporal variation in intra-seasonal BC mass concentration during the dry season (winter and summer). To be more specific, the intra-seasonal mean (+/- sigma) BC mass concentration before 2012 (after 2012) during the dry season was about 3.37 +/- 0.7 mu g m(-3 )(2.80 +/- 0.58 mu g m(-3)), respectively. Concentration weighted trajectory analyses (CWT) revealed that the air masses originated from the continental and polluted environments located in the central and northern parts of India (except monsoon), in regulating BC mass concentration over measurement location. Further, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) derived aerosol vertical extinction profiles (532 nm) showed that majority aerosols (250 Mm(-1)) are confined within 2 km from the surface during winter while in summer particles are distributed throughout the profile (similar to 6 km) with extinction coefficient varying between 200 and 250 Mm(-1). The Santa Barbara Discrete Ordinate Radiative Transfer (SBDART) model estimated intra-seasonal mean direct shortwave composite aerosol radiative forcing (DARE) in the atmosphere (ATM) was about 31.13 +/- 3.36, 34.82 +/- 3.89, 17.10 +/- 1.15, and 17.44 +/- 1.81 Wm(-2) during winter, summer, monsoon and post-monsoon, respectively. The positive signs of ATM forcing in all seasons indicate a warming of the atmosphere, and the corresponding heating rate was around a factor of two higher during the dry season (0.92 +/- 0.12 Kday(-1)) than the wet season (monsoon and post-monsoon) (0.49 +/- 0.04 Kday(-1)). The intra-seasonal mean BC forcing in ATM before 2012 (After 2012) during the dry season was about 24.14 +/- 2.85Wm(-2) (20.09 +/- 2.59Wm(-2)), respectively. The contribution of BC alone to the composite forcing during the study period over the station was similar to 68%. These findings would be helpful for regional climate studies and making air pollution control policy over the region.