Abstract
Abstract. The effect of organic particle mass loading from 1 to ≥100 μg m−3 on the cloud condensation nuclei (CCN) properties of mixed organic-sulfate particles was investigated in the Harvard Environmental Chamber. Mixed particles were produced by the condensation of organic molecules onto ammonium sulfate particles during the dark ozonolysis of α-pinene. A continuous-flow mode of the chamber provided stable conditions over long time periods, allowing for signal integration and hence increased measurement precision at low organic mass loadings representative of atmospheric conditions. CCN activity was measured at eight mass loadings for 80- and 100-nm particles grown on 50-nm sulfate seeds. A two-component (organic/sulfate) Köhler model, which included the particle heterogeneity arising from DMA size selection and from organic volume fraction for the selected 80- and 100-nm particles, was used to predict CCN activity. For organic mass loadings of 2.9 μg m−3 and greater, the observed activation curves were well predicted using a single set of physicochemical parameters for the organic component. For mass loadings of 1.74 μg m−3 and less, the observed CCN activity increased beyond predicted values using the same parameters, implying changed physicochemical properties of the organic component. A sensitivity analysis suggests that a drop in surface tension must be invoked to explain quantitatively the CCN observations at low SOA particle mass loadings. Other factors, such as decreased molecular weight, increased density, or increased van't Hoff factor, can contribute to the explanation but are quantitatively insufficient as the full explanation.
Highlights
Quantifying the potential of atmospheric particles to form cloud droplets is necessary for estimating the radiative forcing due to aerosol indirect effects, which at present have large uncertainties (IPCC, 2007)
Atmospheric particles, contain a significant mass fraction of complex organic compounds, and several laboratory studies using environmental chambers have been recently conducted to describe the cloud condensation nuclei (CCN) activation of secondary-organic-aerosol (SOA) particles produced by the oxidation of monoterpenes and sesquiterpenes (Hartz et al, 2005; VanReken et al, 2005; King et al, 2007; Prenni et al, 2007; Duplissy et al, 2008; Engelhart et al, 2008)
Agreement between the observations and the predictions of the Kohler model led to the conclusion that a surface tension of water was consistent with the observed activation of the mixed organic-sulfate particles
Summary
Quantifying the potential of atmospheric particles to form cloud droplets is necessary for estimating the radiative forcing due to aerosol indirect effects, which at present have large uncertainties (IPCC, 2007). Agreement between the observations and the predictions of the Kohler model led to the conclusion that a surface tension of water was consistent with the observed activation of the mixed organic-sulfate particles This finding was in agreement with several previous studies (Hegg et al, 2001; Ervens et al, 2005; Hartz et al, 2005) but in apparent disagreement with an assumption of a reduction in surface tension by particle-phase organic molecules (Facchini et al, 1999; Svenningsson et al, 2006; Engelhart et al, 2008). Observations of CCN activity at high SOA particle mass loadings suggested an absence of any effect of organic molecules on surface tension, the hypothesis motivating our present study was that such effects may emerge with the changed particle composition at low mass loadings representative of atmospheric concentrations. The CCN activity was investigated for organic particle mass loadings from 0.95 to ≥95.2 μg m−3, a range across which considerable changes in the organic chemical composition can be expected based upon partitioning theory (Donahue et al, 2006; Shilling et al, 2009)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
