Long-term observations of cloud condensation nuclei (CCN) characteristics of atmospheric aerosols: Results from a rural location in India

Abstract

Long-term measurement of cloud condensation nuclei (CCN) is essential to assess the role of seasonally varying aerosol sources and underlying synoptical meteorological conditions in aerosol-cloud interactions. However, long-term measurement of CCN is limited over the Indian region and globally. This study provides first-ever long-term observations of CCN concentrations at different supersaturation (SS) values ranging from 0.2 to 1.0% using a CCN counter from a rural location of Gadanki, India (13.5°N, 79.2°E, and 375 m AMSL) during October 2019–August 2023. The total CCN concentration (NCCN) was the highest during pre-monsoon season (2847 ± 1148 #/cc), followed by winter (2463 ± 1066 #/cc), post-monsoon (2124 ± 1145 #/cc), and monsoon (1099 ± 740 #/cc) at 0.4% SS. Diurnal variations in NCCN showed bi-modal distribution with a relatively weak peak at around 8:00 h (Local time, LT) and a substantial rise at about 20 h LT. However, a systematic increase in NCCN was observed during the daytime in all the seasons except for the monsoon. The two peaks in diurnal variation of NCCN were associated with CCN of sizes less than 2 μm. The peak diameter of the mono-modal size distribution of CCN shifted towards the higher SS, implying the growth of small aerosols and available CCN. The broad peak indicates that most aerosol particles over Gadanki grow at the SS of 0.4%. Estimates of the k-parameter (Twomey's empirical fit) show the aerosols over Gadanki are relatively more hygroscopic during the monsoon than in winter, with their respective k-values of 0.7 and 0.4. Further, the diurnal variation in the k-parameter suggests significant variability in hygroscopicity within a day during monsoon and winter. Comparison with other locations suggests the CCN concentrations over Gadanki lies between those of polluted urban, coastal, and high-altitude stations.