Long-term variabilities of Aerosol Optical properties over Kanpur City

Abstract

Principal light-absorbing aerosols in the earth's atmosphere are Black Carbon (BC), Brown Carbon (BrC) and Mineral Dust. However, large uncertainties still exist in estimating their atmospheric warming effects because of their short lifetimes in the atmosphere, spatially varying emission rates, mixing processes, and removal rates from the atmosphere. The present study uses AERONET data to partition Absorption Aerosol Optical depth (AAOD) over Kanpur city from BC, BrC and Dust components and their temporal variabilities over two decades (2001 to 2019). It is found that aerosol loading (AOD550) over Kanpur city is increasing at 0.004yr-1, and the main component driving this increase is the increase in fine-mode scattering aerosol in the atmosphere indicating contributions from anthropogenic activities. The AODs are higher during the post-monsoon, winter, pre-monsoon seasons and are lower during the monsoon season due to the wet scavenging of aerosol particles by precipitation. The high AOD, AAOD and high Ångström Exponent (ÅE) during the post-monsoon and winter seasons are due to the dominance of fine-mode aerosols from biomass burning and vehicular emissions. In contrast, high AOD, AAOD with low ÅE during the pre-monsoon season are due to mineral dust. However, the AAOD reveals a decreasing trend over Kanpur city even when AOD increases. In addition, the single scattering albedo (SSA (an indicator of the relative chemical composition of absorbing and scattering aerosols) is increasing, indicating that the scattering aerosols in nature predominantly contribute to the increased AOD are increasing over Kanpur city. BC is the biggest contributor to AAOD at 440 and 550 nm, and AAODBC is also decreasing. However, it is interesting to note that the surface BC concentration over Kapur city is increasing. So, there is a discrepancy in the trends of AAODBC and surface BC concentration. A further comparison reveals that the seasonal variability is the same in both parameters, and the trend of AAODBC from 2006 to 2010 was non-negative. The trends for AAODBrC and AAODDust are very ambiguous and are negative in general. However, AAODDust showed an increase during pre-monsoon and monsoon seasons.