Atmospheric aerosol properties at a semi-rural location in southern India: particle size distributions and implications for cloud droplet formation

Particle number size distributions in the diameter range of 5 nm-34 mu m were measured in tropical southern India during the transition season from May (pre-monsoon) to June-July (monsoon). Averaged over the entire measurement period, the particle number size distributions were bimodal with a total particle number concentration of N-Tot = (4.5 +/- 2.7) x 10(3) cm(-3) (arithmetic mean +/- standard deviation). The ratio of Aitken to accumulation mode particle number concentrations (N-Ait/N-Acc) ranged from 1.06 to 2.07, increasing from May to June-July due to a pronounced decrease in the accumulation mode particles with the onset of the monsoon. Cloud condensation nuclei concentrations at 0.4% supersaturation -(CCN0.4) were calculated under the assumption of an average continental hygroscopicity parameter of kappa = 0.3. The calculated -CCN0.4 concentrations are mostly sensitive to variations in N-Acc and vary accordingly between the May and June-July. Using a cloud parcel model, we investigated the formation of cloud droplets using the measured average particle number size distributions, updraft velocities up to 20 m s(-1), and initial aerosol particle number concentrations up to 25,000 cm(-3). We found different regimes of CCN activation and cloud formation for the month of May (shifting from aerosol-limited regime to transitional regime) and for June-July (transitional regime), which was primarily due to the variability of particle number concentrations in the accumulation mode.