Role of Aerosol Physicochemical Properties on Aerosol Hygroscopicity and Cloud Condensation Nuclei Activity in a Tropical Coastal Atmosphere

Simultaneous measurements of cloud condensation nuclei (CCN) number concentration (between 0.20 and 1% supersaturation), particle number size distributions (PNSDs, 9.8 to 414 nm), and nonrefractory submicron aerosol (NR-PM1.0) chemical composition were carried out from a tropical coastal location Thumba, in the southwest tip of peninsular India. The observations were carried out during the winter (28 January to 22 February 2019) to examine the role of changes in aerosol physicochemical properties on aerosol hygroscopicity and CCN activity. The contrasting diurnal variation between the number concentrations of total aerosol (N-CN), CCN (N-CCN), the mass fraction (MF) of organics, the geometric mean diameter (GMD) of the PNSDs, CCN activation ratio (AR = N-CCN/N-CN), and overall hygroscopicity (kappa(total)) of aerosols highlighted the variations in aerosol and CCN characteristics within a day, due to the combined effect of local atmospheric boundary layer dynamics, mesoscale sea-land-breeze circulation, and varying source strength. Between the sea- and land-breeze regimes, a significant difference is observed in N-CN (similar to 5217 +/- 3423 and 8872 +/- 3837 cm(-3) during the sea and land breeze, respectively), CCN ARs (0.64 +/- 0.21 and 0.48 +/- 0.13 at 0.40% supersaturation), GMD (110 +/- 16 and 95 +/- 12 nm) submicron aerosol chemical composition (MF of organics similar to 0.60 +/- 0.10 and 0.78 +/- 0.09), and kappa(total) (0.37 +/- 0.06 and 0.23 +/- 0.07). An estimate of kappa(organics )based on f(44) [which is the ratio between the mass-to-charge ratios (m/z) at 44 and the total organics signal in the component mass spectrum] revealed highly hygroscopic organics (kappa(organics) similar to 0.24 +/- 0.03) during the sea breeze/daytime and relatively less hygroscopic organics during land breeze/nighttime (kappa(organics) similar to 0.15 +/- 0.04) due to varying formation pathways and atmospheric processing. Along with the varying values of the inferred critical diameter (78 +/- 24 and 101 +/- 19 nm) from PNSDs, these observations highlighted the distinct CCN-activation ability of the aerosol systems during the sea-land breeze regimes. The higher CCN activation during the sea breeze/daytime was aided by the larger and more hygroscopic particles, whereas opposite conditions existed during the land breeze/nighttime. Further, in the CCN dosure study using the kappa-Kohler theory, accurate CCN estimations (<10% bias) were obtained when the assumed kappa(organics) (>0.2 and >0.1 at 0.40% supersaturation for the sea- and land-breeze regimes, respectively) were close to the f(44)-derived kappa(organics), irrespective of the mixing state (internal/external mixing) assumptions. In the case of particles with similar size distributions, the CCN activation depicted a dependence on the oxidation levels of the organics for particle populations with sizes >= 100 nm. Our results highlighted the importance of realistic kappa(organics) (i.e., representation of the extent of oxidation/aging of organics) in the improved CCN estimation over the organics-dominated coastal environment.