Abstract
Abstract. The cloud condensation nuclei (CCN) properties of ammonium sulfate particles mixed with organic material condensed during the hydroxyl-radical-initiated photooxidation of isoprene (C5H8) were investigated in the continuous-flow Harvard Environmental Chamber. CCN activation curves were measured for organic particle mass concentrations of 0.5 to 10.0 μg m−3, NOx concentrations from under 0.4 ppbv up to 38 ppbv, particle mobility diameters from 70 to 150 nm, and thermodenuder temperatures from 25 to 100 °C. At 25 °C, the observed CCN activation curves were accurately described by a Köhler model having two internally mixed components, namely ammonium sulfate and secondary organic material. The modeled physicochemical parameters of the organic material were equivalent to an effective hygroscopicity parameter κORG of 0.10±0.03, regardless of the C5H8:NOx concentration ratio for the span of >200:0.4 to 50:38 (ppbv:ppbv). The volatilization curves (i.e., plots of the residual organic volume fraction against temperature) were also similar for the span of investigated C5H8:NOx ratios, suggesting a broad similarity of particle chemical composition. This suggestion was supported by limited variance at 25 °C among the particle mass spectra. For example, the signal intensity at m/z 44 (which can result from the fragmentation of oxidized molecules believed to affect hygroscopicity and CCN properties) varied weakly from 6 to 9% across the range of investigated conditions. In contradistinction to the results for 25 °C, conditioning up to 100 °C in the thermodenuder significantly reduced CCN activity. The altered CCN activity might be explained by chemical reactions (e.g., decomposition or oligomerization) of the secondary organic material at elevated temperatures. The study's results at 25 °C, in conjunction with the results of other chamber and field studies for a diverse range of conditions, suggest that a value of 0.10±0.05 for κORG is representative of both anthropogenic and biogenic secondary organic material. This finding supports the use of κORG as a simplified yet accurate general parameter to represent the CCN activation of secondary organic material in large-scale atmospheric and climate models.
Highlights
The growth of atmospheric particles to form cloud droplets influences climate through indirect effects on cloud brightness and lifetime and changes in patterns of precipitation (Lohmann and Feichter, 2005; IPCC, 2007; Andreae and Rosenfeld, 2008)
Predictions of which atmospheric particles act as cloud condensation nuclei (CCN) are complicated by their diverse composition
∗ 0.4 ppbv is the minimum detection limit of the instrumentation. ∗∗ No CCN data are available for experiment 6. ∗∗∗ Concentrations of O3 are an upper limit because interference from absorption at 254 nm by hydrogen peroxide is not taken into account. ∗∗∗∗ Lower limit of yield because no corrections were made for wall loss
Summary
The growth of atmospheric particles to form cloud droplets influences climate through indirect effects on cloud brightness and lifetime and changes in patterns of precipitation (Lohmann and Feichter, 2005; IPCC, 2007; Andreae and Rosenfeld, 2008). The influence of NOx concentration on the yield of particle mass for isoprene photooxidation was reported previously (Presto et al, 2005; Dommen et al, 2006; Kroll et al, 2006; Carlton et al, 2009), and the observed changes in yield reported in those studies for varying VOC-to-NOx ratios suggested significant variability in the chemical composition of secondary organic material. This variability, which we hypothesized could imply changes in CCN activity, provided our motivation for the study described . Particle conditioning by use of thermodenuders has been discussed in several recent laboratory studies (Paulsen et al, 2006; An et al, 2007; Jonsson et al, 2007; Stanier et al, 2007), and the reported effects on hygroscopic properties motivated us in this study to test for significant changes in CCN activity (Meyer et al, 2009; Poulain et al, 2009)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
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 call
