Inter-seasonal variability in size-resolved CCN properties at Kanpur, India

We present properties of size-resolved cloud condensation nuclei (CCN) over the central Indo-Gangetic Basin (IGB) station, Kanpur, India. The measurements were done during three different seasons: spring, summer and monsoon in 2012. Size-resolved CCN efficiency spectra (20–280 nm), measured at 0.2–1% supersaturation (SS), resulted in an average activation diameter (Da) of 148–45 nm. The activation diameter was found to be maximum in spring followed by monsoon and summer at all SS except at SS = 1%. The size-averaged hygroscopicity (κ) of CCN-active particles for all SS was found to be 0.11 ± 0.03, 0.24 ± 0.13 and 0.14 ± 0.06 in spring, summer, and monsoon, respectively. κ increased with increasing particle diameter, suggesting change in chemical composition with size which further leads to change in CCN activity. In comparison to summer and monsoon, despite the presence of lower κ in spring, the higher activation fraction in Aitken mode is observed due to the shift of CN size distribution towards larger diameter. κ is observed to be lower in first half of the day (0900–1400 h) for SS = 0.2–0.5% due to relatively less photochemical activity compared to second half of the day. Size-and-time averaged κ value over all three seasons (0.16 ± 0.08) suggests dominance of organic species, as reported in previous studies.