Enhancement of Cloud Condensation Nuclei during NPF events over a location in Eastern Himalayan Foothills

The contribution of newly formed aerosol particles to cloud condensation nuclei (CCN) via gas-to-particle (GTP) conversion is highly uncertain. Here, we present results from a one-month simultaneous measurement of aerosol number concentration (N_10-299.6) and CCN concentrations (N_CCN) over a seemingly unpolluted location, Dibrugarh, in the Eastern Himalayan Foothills, during the winter of 2023. The average diurnal variation of N_CCN at different supersaturations is in line with the scanning mobility particle sizer (SMPS)-measured N_10-299.6 with a systematic diurnal variation of highest (lowest) concentrations during nighttime (daytime) under the influence of planetary boundary layer (PBL) dynamics. We have identified four new particle formation (NPF) events during the study period, with a frequency of ~13% of the study days. The distinct mode of average PNSD at the lower size regime (<25 nm) determines the NPF burst. Later, they continue to grow through coagulation and condensation processes with a growth rate ranging from 4.5 to 7.2 nm h⁻¹. The growth process begins with coagulation, followed by condensation, which becomes the dominant mechanism in the formation of CCN. Moreover, the enhancement factor of CCN due to NPF (E_N_CCN) was estimated to examine the aerosol-CCN interaction and was found to vary between 2.32 to 7.74 for all four NPF events at the supersaturation range of 0.2-1%. These values are in line with many urban places across the globe. However, state-of-the-art instruments and longer temporal analyses of CCN concentrations, as well as NPF precursor dynamics, are required to evaluate the seasonality and in-depth understanding of the processes.