A study on aerosol-cloud condensation nuclei (CCN) activation over eastern Himalaya in India

Simultaneous measurements of condensation nuclei (CN) and cloud condensation nuclei (CCN) has been performed over a high altitude site Darjeeling (27°01′N, 88°15′E, 2200 m asl) at eastern part of Himalaya in India. The study was carried out during dry seasons (October 2015-May 2016) to investigate the temporal variability of CN and CCN concentrations and the major factors controlling CN-CCN activation. CCN concentrations measured at 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9% super saturations have been reported in this study. The number concentrations of CN ranged between 694 and 23,643 cm− 3 with an average of 6563 ± 2160 cm− 3 whereas that of CCN (at 0.5% super saturation) ranged between 262 and 13,382 cm− 3 with an average of 1761 ± 856 cm− 3 during the entire study period. CN and CCN showed prominent monthly and seasonal variations controlled by local emissions, meteorological factors and long-range transport. Indo-Gangetic Plain (IGP) was found to be the most contributing region for CN and CCN over Darjeeling mainly during winter and premonsoon. A clear indication of the contribution of aerosols from plain land regions driven by up-slope valley wind was observed during premonsoon from diurnal variability of CN and CCN. Overall, 30-32% aerosols were observed to activate to CCN during winter and premonsoon whereas 24% activation was observed during postmonsoon. Chemical nature and hence solubility of aerosols controlled CCN activation more in night-time than day-time as observed from the correlations between activation ratios and hygroscopicity parameter, k. Strong seasonal dependence of CCN and activation ratio on super saturation and aerosol loading was observed. We observed higher CN and CCN loading and lower activation ratio over Darjeeling compared to western Himalayan high altitude station suggesting higher influence of local and long-range aerosols over eastern Himalaya and significant difference in chemical nature of aerosols between these two parts of Himalaya.