Altitude profiles of cloud condensation nuclei characteristics across the Indo-Gangetic Plain prior to the onset of the Indian summer monsoon

摘要

Concurrent measurements of the altitude profiles of the concentration of cloudcondensation nuclei (CCN), as a function of supersaturation(ranging from 0.2 % to 1.0 %), and aerosol optical properties(scattering and absorption coefficients) were carried out aboard aninstrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to theonset of the Indian summer monsoon (ISM) of 2016. The experiment wasconducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and RegionalAerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurementscovered coastal, urban and arid environments. In general, the CCNconcentration was highest in the central IGP, decreasing spatially fromeast to west above the planetary boundary layer (PBL), which is∼1.5 km for the IGP during pre-monsoon period. Despite this, the CCNactivation efficiency at 0.4 % supersaturation was, interestingly,the highest over the eastern IGP (∼72 %), followed by thatin the west (∼61 %), and it was the least over thecentral IGP (∼24 %) within the PBL. In general, higheractivation efficiency is noticed above the PBL than below it. The centralIGP showed remarkably low CCN activation efficiency at all altitudes, whichappears to be associated with high black carbon (BC) mass concentrationthere, indicating the role of anthropogenic sources in suppressing the CCNefficiency. These first-ever CCN measurements over the western IGP, encompassing“the Great Indian Desert” also known as “the Thar Desert”, showed high CCNefficiency, ∼61 % at 0.4 % supersaturation, indicating thehygroscopic nature of the dust. The vertical structure of CCN properties isfound to be air mass dependent, with higher activation efficiency even overthe central IGP during the prevalence of marine air mass. Wet scavengingassociated with precipitation episodes seems to have reduced the CCNactivation efficiency below cloud level. An empirical relation has emergedbetween the CCN concentration and the scattering aerosol index (AI), whichwould facilitate the prediction of CCN from aerosol optical properties.