Air-borne in-situ measurements of aerosol size distributions and BC across the IGP during SWAAMI

Abstract. During the South-West Asian Aerosol Monsoon Interaction (SWAAMI) experiment, collocated air-borne measurements of aerosol number-size distributions in the size (diameter) regime 0.5 to 20 µm and black carbon (BC) mass concentrations were made across the Indo-Gangetic Plains (IGP), for the first time, from three distinct locations, just prior to the advent of Indian Summer Monsoon over the IGP. These measurements provided an east-west transect of region-specific properties of aerosols as the environment transformed from mostly-arid conditions of western IGP (represented by Jodhpur, JDR) having dominance of natural aerosols to the Central IGP (represented by Varanasi, VNS) having very high anthropogenic emissions, to the eastern IGP (represented by the coastal station Bhubaneswar, BBR) characterized by a mixture of the IGP outflow and marine aerosols. Despite these, the aerosol size distribution revealed an increase in coarse mode concentration and coarse mode mass-fraction (fractional contribution to the total aerosol mass) with increase in altitude across the entire IGP, especially above the well-mixed region. Consequently, both the mode radii and geometric mean radii of the size distributions showed an increase with altitude. However, near the surface and within the atmospheric boundary layer (ABL), the features were specific to the different sub-regions; with highest coarse mode mass fraction (FMC ~ 72 %) in the western IGP and highest accumulation fraction in the Central IGP with the eastern IGP coming in-between. The elevated coarse mode fraction is attributed to mineral dust load arising from local production as well as due to advection from the west. This was further corroborated by data from Cloud Aerosol Transportation System (CATS) onboard International Space Station (ISS), which also revealed that the vertical extent of dust aerosols reached as high as 5 km during this period. Mass concentrations of Black Carbon (BC) were moderate (~ 1 µg m−3) with very little altitude variation up to 3.5 km, except in the Central IGP (VNS) where very high concentrations were seen near the surface and within the ABL.