Abstract
Abstract. Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground observational facility situated in the rain-shadow region of the Indian subcontinent are illustrated. These observations were part of the Cloud Aerosol Interaction Precipitation Enhancement Experiment (CAIPEEX) during the Indian summer monsoon season (June to September) of 2018. Observations are classified as dry–continental (monsoon break) and wet–marine (monsoon active) according to the air mass history. CCN concentrations measured for a range of supersaturations (0.2 %–1.2 %) are parameterized using Twomey's empirical relationship. CCN concentrations at low (0.2 %) supersaturation (SS) were high (>1000cm-3) during continental conditions and observed together with high black carbon (BC∼2000ngm-3) and columnar aerosol loading. During the marine air mass conditions, CCN concentrations diminished to ∼350cm-3 at 0.3 % SS and low aerosol loading persisted (BC∼800ngm-3). High CCN activation fraction (AF) of ≅0.55 (at 0.3 % SS) was observed before the monsoon rainfall, which reduced to ≅0.15 during the marine air mass and enhanced to ≅0.32 after that. There was mostly monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of ≅85 nm, with least (≅9 %) contribution from nucleation mode (<30 nm) particles persisted before the monsoon, while multimode NSD with ≅19 % of nucleation mode particles was found during the marine air mass. Critical activation diameters (dcri) for 0.3 % SS were found to be about 72, 169, and 121 nm prior to, during, and after the marine conditions, respectively. The better association of CCN with aerosol absorption, and the concurrent accumulation mode particles during continental conditions, points to the possibility of aged (oxygenated) carbonaceous aerosols enhancing the CCN activity prior to the marine conditions. An enhancement in CCN concentrations and k values during the daytime along with absorption Ångström exponent was observed during the marine conditions. Best closure obtained using measured critical diameter and ammonium sulfate composition during continental conditions emphasizes the role of aged aerosols contributing to the accumulation mode, enhancing the CCN efficiency. The overestimation of CCN and less hygroscopicity of accumulation mode aerosols during the marine air mass indicate the role of size-dependent aerosol composition in CCN activity during the period.
Highlights
Atmospheric aerosol particles (APs), emitted from both natural and anthropogenic sources, affect the radiation budget as well as the hydrological cycle of Earth, mainly through its direct and indirect effects
Before the onset of monsoon, under the influence of continental air mass, the mean black carbon (BC) values were above 2000 ng m−3, which decreased to very low values (∼ 746 ng m−3) during the marine conditions
It is found that the pollutedcontinental conditions transform into a polluted marine condition by the onset of the Indian summer monsoon (ISM) with a significant change in aerosol size distribution and composition affecting the cloud nucleating properties
Summary
Atmospheric aerosol particles (APs), emitted from both natural and anthropogenic sources, affect the radiation budget as well as the hydrological cycle of Earth, mainly through its direct and indirect effects. Those APs or condensation nuclei (CN) which act as the cloud condensation nuclei (CCN) at a specific supersaturation (SS) can indirectly affect the climate by altering the cloud microphysical properties. The SS measurements are seldom possible, and the large disagreements between the CCN and cloud droplet number concentration remain elusive (Moore et al, 2013). Jayachandran et al.: Cloud condensation nuclei characteristics during the Indian summer monsoon
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