Mixing state and compositional effects on CCN activity and droplet growth kinetics of size-resolved CCN in an urban environment
Aerosol composition and mixing state near anthropogenic sources can be\nhighly variable and can challenge predictions of cloud condensation\nnuclei (CCN). The impacts of chemical composition on CCN activation\nkinetics is also an important, but largely unknown, aspect of cloud\ndroplet formation. Towards this, we present in-situ size-resolved CCN\nmeasurements carried out during the 2008 summertime August Mini\nIntensive Gas and Aerosol Study (AMIGAS) campaign in Atlanta, GA.\nAerosol chemical composition was measured by two particle-into-liquid\nsamplers measuring water-soluble inorganic ions and total water-soluble\norganic carbon. Size-resolved CCN data were collected using the Scanning\nMobility CCN Analysis (SMCA) method and were used to obtain\ncharacteristic aerosol hygroscopicity distributions, whose breadth\nreflects the aerosol compositional variability and mixing state.\nKnowledge of aerosol mixing state is important for accurate predictions\nof CCN concentrations and that the influence of an externally-mixed,\nCCN-active aerosol fraction varies with size from 31% for particle\ndiameters less than 40 nm to 93% for accumulation mode aerosol during\nthe day. Assuming size-dependent aerosol mixing state and size-invariant\nchemical composition decreases the average CCN concentration\nover-prediction (for all but one mixing state and chemical composition\nscenario considered) from over 190-240% to less than 20 %. CCN\nactivity is parameterized using a single hygroscopicity parameter,\nkappa, which averages to 0.16 +/- 0.07 for 80 nm particles and exhibits\nconsiderable variability (from 0.03 to 0.48) throughout the study\nperiod. Particles in the 60-100 nm range exhibited similar\nhygroscopicity, with a kappa range for 60 nm between 0.06-0.076 (mean of\n0.18 +/- 0.09). Smaller particles (40 nm) had on average greater kappa,\nwith a range of 0.20-0.92 (mean of 0.3 +/- 0.12). Analysis of the\ndroplet activation kinetics of the aerosol sampled suggests that most of\nthe CCN activate as rapidly as calibration aerosol, suggesting that\naerosol composition exhibits a minor (if any) impact on CCN activation\nkinetics.