Abstract. Concurrent measurements of the altitude profiles of the concentration of cloud
condensation 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 an
instrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to the
onset of the Indian summer monsoon (ISM) of 2016. The experiment was
conducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and Regional
Aerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurements
covered coastal, urban and arid environments. In general, the CCN
concentration was highest in the central IGP, decreasing spatially from
east to west above the planetary boundary layer (PBL), which is
∼1.5 km for the IGP during pre-monsoon period. Despite this, the CCN
activation efficiency at 0.4 % supersaturation was, interestingly,
the highest over the eastern IGP (∼72 %), followed by that
in the west (∼61 %), and it was the least over the
central IGP (∼24 %) within the PBL. In general, higher
activation efficiency is noticed above the PBL than below it. The central
IGP showed remarkably low CCN activation efficiency at all altitudes, which
appears to be associated with high black carbon (BC) mass concentration
there, indicating the role of anthropogenic sources in suppressing the CCN
efficiency. These first-ever CCN measurements over the western IGP, encompassing
“the Great Indian Desert” also known as “the Thar Desert”, showed high CCN
efficiency, ∼61 % at 0.4 % supersaturation, indicating the
hygroscopic nature of the dust. The vertical structure of CCN properties is
found to be air mass dependent, with higher activation efficiency even over
the central IGP during the prevalence of marine air mass. Wet scavenging
associated with precipitation episodes seems to have reduced the CCN
activation efficiency below cloud level. An empirical relation has emerged
between the CCN concentration and the scattering aerosol index (AI), which
would facilitate the prediction of CCN from aerosol optical properties.