90 chamber investigations of secondary organic aerosol as cloud condensation nuclei

Monday, 7 July 2014
W.T. Hesson, University of Manchester, Manchester, United Kingdom; and A. Buchholz, P. Connolly, R. Alfarra, and G. McFiggans

Clouds have a significant impact upon weather and climate and through this affect human health and food supply(IPCC, 2007). The number of cloud condensation nuclei available is a major determining factor in the number concentration of droplets in a cloud which controls cloud brightness and lifetime (Albrecht, 1989; Topping, Connolly, & McFiggans, 2013; Twomey, 1977). This in turn affects the Earth's albedo and thus temperature. Therefore it is clear that changes in cloud caused by anthropogenic activity will affect radiative forcing. Non-volatile particles acting as cloud condensation nuclei have been studied extensively and are well-understood(Kohler, 1936; McFiggans et al., 2006). Questions still remain, however, about particles comprising of material which partitions between the gas and aerosol phase i.e. semi-volatile material. It is thought that cloud droplet formation can be significantly enhanced by the condensation of semi-volatile compounds alongside water. (Topping et al., 2013); however, observational evidence of this potentially important process is not available at the present time. The aim of this work is to determine whether the effect of semi-volatile co-condensation on cloud droplet formation can be observed in an atmospherically relevant chamber setting. We investigate droplet formation on semi-volatile secondary organic aerosol (SOA) particles generated in the Manchester Aerosol Chamber (MAC) from a variety of anthropogenic (1,3,5-trimethylbenzene) and biogenic (α-pinene, limonene and β-caryophyllene) precursors. SOA was produced under in the presence of tropospherically relevant light intensity and with ~100ppb of NOx and ~40ppb of ozone as initial conditions. Details of MAC operation and chamber description are available are available in the literature (Alfarra et al., 2012). During the formation and growth of the SOA particles, measurements were taken with a differential mobility particle sizer (DMPS) and condensation particle counter (CPC) to monitor the number and size distribution of the SOA particles A cloud condensation nuclei counter (CCNc) was used to measure the CCN activity of the SOA particles. In further experiments, using similar conditions, a HTDMA (Humidified Tandem Differential Mobility Analyser) was used to measure the hygroscopic growth factors of size selected aerosol at 90% relative humidity. This data enables us to apply the kappa-Koehler approach of Petters and Kreidenweis (Petters & Kreidenweis, 2007) to determine the two values of kappa that characterize the hygroscopicity of the aerosol: one for sub-saturated conditions and one relevant to supersaturated conditions. Differences in these two parameters may be indicative of the influence of semi-volatile vapours playing a role. After photochemical aging of between 1.5 - 2.5 hours, the aerosol sample from the experiments using a CCNc experiments were transferred to the Manchester Ice Cloud Chamber (MICC). The size distribution and number in MICC were measured with an scanning mobility particle sizer (SMPS) and CPC between depressurisations. After the transfers the modal particle sizes were 80-150 nm and total concentrations were 2500-7500/cc. Consecutive depressurisation experiments were performed between ~1000mbar and ~700mbar causing cloud formation which was measured with a WELAS optical particle counter. Data from an example depressurisation is shown in Figure 1. These data are compared to cloud parcel model calculations using the aerosol properties determined above to answer the question of whether the effect of co-condensing vapours on CCN is observable in a realistic chamber setting. Acknowledgements This work was supported by funding from the Natural Environment Research Council and is part of the ACID-PRUF project. References Albrecht, B. A. (1989). Aerosols, Cloud Microphysics and Fractional Cloudiness. Science, 245(4923), 1227–1230. Alfarra, M. R., Hamilton, J. F., Wyche, K. P., Good, N., Ward, M. W., Carr, T., … McFiggans, G. B. (2012). The effect of photochemical ageing and initial precursor concentration on the composition and hygroscopic properties of β-caryophyllene secondary organic aerosol. Atmospheric Chemistry and Physics, 12(14), 6417–6436. doi:10.5194/acp-12-6417-2012 IPCC. (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. (M. T. and H. L. M. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, Ed.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Kohler, H. (1936). 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