Zonal variations in the vertical distribution of atmospheric aerosols over the Indian region and the consequent radiative effects

Abstract

Abstract. The vertical structure of atmospheric aerosols over the Indian mainland and the surrounding oceans and its spatial distinctiveness and resultant atmospheric heating are characterised using long-term (2007–2020) satellite observations, assimilated aerosol single scattering albedo, and radiative transfer calculations. The results show strong, seasonally varying zonal gradients in the concentration and vertical extent of aerosols over the study region. Compared to the surrounding oceans, where the vertical extent of aerosols is confined within 3 km, the aerosol extinction coefficients extend to considerably higher altitudes over the mainland, reaching as high as 6 km during pre-monsoon and monsoon seasons. Longitudinally, the vertical extent is highest around 75∘ E and decreasing gradually towards either side of the study region, particularly over peninsular India. Particulate depolarisation ratio profiles affirm the ubiquity of dust aerosols in western India from the surface to nearly 6 km. While the presence of low-altitude dust aerosols decreases further east, the high-altitude (above 4 km) dust layers remain aloft throughout the year with seasonal variations in the zonal distribution over north-western India. High-altitude (around 4 km) dust aerosols are observed over southern peninsular India and the surrounding oceans during the monsoon season. Radiative transfer calculations show that these changes in the vertical distribution of aerosols result in enhanced atmospheric heating at the lower altitudes during the pre-monsoon, especially in the 2–3 km altitude range throughout the Indian region. These results have strong implications for aerosol–radiation interactions in regional climate simulations.

3D assimilation and radiative impact assessment of aerosol black carbon over the Indian region using aircraft, balloon, ground-based, and multi-satellite observations

“…A rigorous description about CALIOP data screening can be found in Kala et al (2022) and is not repeated here.…”

“…The assimilated AAOD thus obtained (Pathak et al, 2019) provides only columnar values, and no information on its vertical distribution, which is known to be heterogeneous with structures and elevated layers (e.g. Satheesh et al, 2008;Babu et al, 2011b;Kala et al, 2022). Hence, a mere extension of surface BC measurements to higher altitudes, assuming a constant mixing ratio within the ABL and exponentially decreasing thereafter, will not be realistic over the Indian region.…”

“…Several assimilation methods are in use for combining scattered in situ observations with the gridded background data, such as the successive correction methods (SCMs; Kalnay, 2003;Lewis et al, 2006) and the weighted interpolation method (WIM), which follows the Cressman method (Cressman, 1959). These assimilation methods are based on a specified radius of interest around the in situ observation, and the variances are minimized within this radius of interest.…”

“…The assimilated product evaluated using 3D-Var is known to have lesser estimates of variance compared to the background or observational data (Kalnay, 2003;Lewis et al, 2006), also leading to smaller uncertainties in the assimilated product. Since the 3D-Var is not a constrained model, the final assimilated product may contain negative values of assimilated absorption coefficient (kasm); in which case, those specific grids alone would be replaced by the corresponding kbg values.…”

“…These anthropogenic BC emissions arise mainly from coal-based thermal power plants (Prasad et al, 2006), vehicular emissions, small-scale industrial emissions, burning of litter, and biofuel burning for domestic cooking (Reddy and Venkataraman, 2002;Di Girolamo et al, 2004). During MAM, the IGP region is influenced by BC produced from agricultural residue burning and forest fires and the continental inflow of polluted aerosols to this region , whereas the crop residue burning has also been cited as a contributing cause during ON (Badarinath et al, 2004;Jain et al, 2014;Kaskaoutis et al, 2014).…”