Abstract
Abstract. 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 overprediction (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, κ, 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 κ range for 60 nm between 0.06–0.076 (mean of 0.18 ± 0.09). Smaller particles (40 nm) had on average greater κ, 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 kinetics.
Highlights
The ability of aerosol particles to act as cloud condensation nuclei (CCN) depends on their size and composition (e.g., Twomey, 1977; Dusek et al, 2006; Wang, 2007)
The data were used to assess CCN closure for a number of chemical composition and mixing state scenarios often taken in atmospheric models to predict CCN concentrations
The results of the CCN closure scenarios considered here indicate that knowing the chemical composition and mixing state of aerosols allows the accurate prediction of CCN concentrations
Summary
The ability of aerosol particles to act as cloud condensation nuclei (CCN) depends on their size and composition (e.g., Twomey, 1977; Dusek et al, 2006; Wang, 2007). Kinetic activation delays could have an important impact on cloud droplet number and size distribution (e.g., Chuang et al, 1997; Nenes et al, 2001; Feingold and Chuang, 2002; Lance et al, 2004) To test whether such limitations exist, in-situ field studies compare the size of the activated droplets from the CCN being studied against those obtained from calibration salt aerosol known to exhibit rapid activation kinetics (e.g., Sorooshian et al, 2008; Bougiatioti et al, 2009, 2011; Lance et al, 2009; Murphy et al, 2009; Padroet al., 2010; Cerully et al, 2011). This study focuses on the droplet growth kinetics of urban aerosols by comparing the droplet size of the atmospheric CCN to those of (NH4)2SO4 calibration aerosol
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