Impact of 3D distribution of Black Carbon aerosols in the atmosphere on aerosol radiative forcing over the Indian region

Abstract

Black Carbon (BC) aerosols are produced by the incomplete combustion of carbonaceous materials. While they constitute an insignificant mass-fraction of the composite atmospheric aerosols, their measurement is essential in estimating global and regional climate impacts due to their strong absorption of solar radiation, which causes warming of the atmosphere. Despite this, accurate knowledge of this warming (in spatial and temporal domains) is still lacking due to a lack of information on the three-dimensional distribution of BC in the atmosphere. Some modeling studies have reported that the radiative heating caused by BC aerosols can alter the pattern and intensity of the Indian summer monsoon rainfall. However, given its large geographical, temporal, and vertical variability, estimating the climate implications of BC over the Indian subcontinent is challenging. This necessitates extensive observational data sets as well as the best possible integration of current data sets. This thesis presents a comprehensive investigation of the spatial, seasonal, and vertical variations in the distribution of aerosols, including BC, over the Indian region, as well as their climate implications, using a variety of data sets, including ground-based, aircraft-based, and balloon-borne observations, along with multi-satellite and reanalysis data sets and radiative transfer modelling. Investigations on the long-term trends, in the concentration of near-surface aerosols and columnar aerosol loading from the metropolitan region of peninsular India (Bengaluru), based on measurements, have revealed contrasting yet statistically significant trends; a decrease in the mass concentrations of near-surface BC and composite aerosols and an increase in columnar loading of composite aerosols. Examination with the aid of co-located and concurrent space-borne lidar data has revealed a vertical redistribution of aerosols, resulting in an increase in the aerosol concentration at higher levels (above the atmospheric boundary layer) is partly responsible for this. The aforementioned characteristics, along with the regional and synoptic meteorology, lead to large zonal gradients in aerosol-induced atmospheric heating rates that reach up to an altitude of 6 km over the Indian region, with strong seasonality. Aerosol-induced atmospheric heating and its climate implications are mostly determined by the concentration of aerosols as well as their single scattering albedo, which in turn substantially depends on the concentration and vertical distribution of BC aerosols. To address this issue, a synergy of aircraft and balloon observations from various dedicated campaigns and continuous data from a dense network of surface observatories, as well as multi-satellite observations, were statistically assimilated to produce a spatially continuous gridded data set of aerosol absorption over the Indian mainland for the first time. The results show the dominant role of vertical lifting of aerosols, as well as spatial and seasonal changes in the vertical distribution. It also highlights the difference in estimation of atmospheric heating rates when the assimilated data sets are incorporated in radiative transfer simulations, as opposed to the conventional use of columnar data without information on the vertical variation of aerosol absorption.