Relative role of black carbon and sea-salt aerosols as cloud condensation nuclei over a high altitude urban atmosphere in eastern Himalaya

Abstract The present study is an attempt to investigate the relative role of black carbon (BC) and sea-salt aerosols on the CCN activation over a high altitude station, Darjeeling (27.1° N and 88.15° E, 2200 m asl) at eastern Himalaya. Aerosols (CN, CCN, BC and PM2.5) were measured during premonsoon and monsoon in 2017 and 2018. A unique sampling strategy and a novel methodology were adopted that enabled us to quantitatively and separately determine the contributions of local emissions (LE), valley wind transport (VWT) and long-range transport (LRT) to BC aerosols and their role in CCN activation. On the other hand, the contributions of transported sea-salt (NaCl) aerosols to CCN activation were also determined when they interact with the local anthropogenic soluble species and when they do not. CCN (0.5% super-saturation) concentrations were found to be increased when BC aerosols were more aged (~ 80 cm−3 and 218 cm−3 increase in CCN for 1 μg m−3 increase in BCLE and BCLRT with activation ratios of 0.17 and 0.55 respectively). Local anthropogenic acidic species (SO42−/H2SO4 (g) and NO3−/HNO3 (g)) interact with NaCl resulting to Cl− depletion. Cl− depletion was increased with the increase in NO3− + SO42−(45% for1 μg m−3increase in NO3− + SO42−) that in turn sharply decreased the AR of NaCl (0.04 for 1% increase in Cl- depletion). On the other hand, higher the NO3− + SO42−, higher were the CCN activation of transported BC which could be due to the soluble coating on BC. The important and interesting fact is that when transported and interacted with anthropogenic soluble species, BC aerosols (though hydrophobic) act as much better CCN than NaCl (though hydrophilic).