NPF-induced CCN enhancement over an urban location in Eastern Himalayan Foothills: A campaign-based study

The New Particle Formation (NPF) mechanism plays a significant role in regulating the cloud condensation nuclei (CCN). Here, we present results from a campaign mode study during 3 January - 4 February 2023 obtained from simultaneous measurements of particle number concentration within a size range 10–300 nm (N_10-300) using the scanning mobility particle sizer (SMPS) and CCN concentrations (N_CCN) using a CCN Counter (CCNc) over an urban location, Dibrugarh, in the Eastern Himalayan Foothills. The mean diurnal variation of N_CCN at supersaturations (SS) 0.2 %–1.0 % exhibits similar variation to the total number concentration of aerosols (N_10-300) measured by the SMPS. Both exhibit higher concentrations during nighttime and lower during daytime, primarily influenced by atmospheric boundary layer (ABL) dynamics. We have identified four NPF events during the study period. The new particle burst appeared below 25 nm, while the growth process sustained for 12 to 17 h at the growth rate 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. Further, the enhancement factor of CCN due to NPF (E_N_CCN) was estimated to examine the aerosol-CCN interaction. E_N_CCN was found to vary between 2 and 8 for the four NPF events at the SS range 0.2–1 %, which are comparable to those over many urban places across the globe.