Journal article

Mixing state and compositional effects on CCN activity and droplet growth kinetics of size-resolved CCN in an urban environment

Padró L, Moore R, Zhang X, Rastogi N, Weber R, Nenes A ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 21 (2012) pp. 10239-10255

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Aerosol composition and mixing state near anthropogenic sources can be
highly variable and can challenge predictions of cloud condensation
nuclei (CCN). The impacts of chemical composition on CCN activation
kinetics is also an important, but largely unknown, aspect of cloud
droplet formation. Towards this, we present in-situ size-resolved CCN
measurements carried out during the 2008 summertime August Mini
Intensive Gas and Aerosol Study (AMIGAS) campaign in Atlanta, GA.
Aerosol chemical composition was measured by two particle-into-liquid
samplers measuring water-soluble inorganic ions and total water-soluble
organic carbon. Size-resolved CCN data were collected using the Scanning
Mobility CCN Analysis (SMCA) method and were used to obtain
characteristic aerosol hygroscopicity distributions, whose breadth
reflects the aerosol compositional variability and mixing state.
Knowledge of aerosol mixing state is important for accurate predictions
of CCN concentrations and that the influence of an externally-mixed,
CCN-active aerosol fraction varies with size from 31% for particle
diameters less than 40 nm to 93% for accumulation mode aerosol during
the day. Assuming size-dependent aerosol mixing state and size-invariant
chemical composition decreases the average CCN concentration
over-prediction (for all but one mixing state and chemical composition
scenario considered) from over 190-240% to less than 20 %. CCN
activity is parameterized using a single hygroscopicity parameter,
kappa, which averages to 0.16 +/- 0.07 for 80 nm particles and exhibits
considerable variability (from 0.03 to 0.48) throughout the study
period. Particles in the 60-100 nm range exhibited similar
hygroscopicity, with a kappa range for 60 nm between 0.06-0.076 (mean of
0.18 +/- 0.09). Smaller particles (40 nm) had on average greater kappa,
with a range of 0.20-0.92 (mean of 0.3 +/- 0.12). Analysis of the
droplet activation kinetics of the aerosol sampled suggests that most of
the CCN activate as rapidly as calibration aerosol, suggesting that
aerosol composition exhibits a minor (if any) impact on CCN activation

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  • L. T. Padró

  • R. H. Moore

  • X. Zhang

  • N. Rastogi

  • R. J. Weber

  • A. Nenes

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