(203d) Relating Cloud Condensation Nuclei Activity, Volatility, and Droplet Growth Kinetics of Terpene Secondary Organic Aerosol

Cloud droplets form on a subset of atmospheric particles (aerosol) called cloud condensation nuclei (CCN). The concentration of these CCN in the atmosphere influences the radiative properties of clouds, thus having an effect on the Earth's climate. Cloud-particle interactions are one of the major challenges in quantifying the indirect climate forcing (Houghton et al., 2001; IPCC, 2007). Understanding CCN activation is an important step towards understanding cloud formation and properties. While inorganic particles are well understood (Köhler, 1936; Pruppacher and Klett, 1997; Svenningsson et al., 2006), many questions still remain about the influence of organic particulate material on CCN activity. A class of compounds called monoterpenes is estimated to make up 11% of the global natural VOC flux of 1150 Tg carbon per year (Guenther, 1995). Sesquiterpenes, while less abundant as a precursor, are important because of potentially large aerosol yields (Griffin, 1999) due to high molecular weight oxidation products.

The ability of secondary organic aerosol (SOA) particles formed during the ozonolysis of beta-caryophyllene, alpha-pinene and other monoterpenes to act as cloud condensation nuclei (CCN) was investigated using a static CCN counter and a cylindrical continuous-flow streamwise thermal gradient CCN counter developed by Droplet Measurement Technologies (DMT). Secondary organic aerosol (SOA) was produced from the dark reaction of either beta-caryophyllene, alpha-pinene or monoterpene mixtures (alpha-pinene, beta-pinene, limonene and 3-carene) with ozone in a 12 m³, temperature-controlled smog chamber. The initial terpene concentrations were in the 10-30 ppb range and an excess of ozone was used. The CCN concentrations, activation diameter and droplet growth kinetic information were monitored online as a function of supersaturation for several hours and their changes with age were quantified. Additionally, filter samples were collected for analysis of water soluble organic carbon (WSOC). Filter samples allow for the investigation of surfactant characteristics and estimation of thermodynamic properties, such as molecular weight. Volatility of sesquiterpene (beta-caryophyllene) SOA was probed using a thermodenuder.

Both fresh and aged monoterpene SOA are quite active as CCN. The initial concentrations of ozone and monoterpene precursor do not appear to affect the activity of the resulting SOA. However, reactions of the hydroxyl radicals produced during the monoterpene ozonolysis lead to further oxidation of the SOA material and an improvement of their CCN properties with time. A decrease in CCN activation diameter for alpha-pinene SOA of approximately 3 nm hr^-1 was observed as the aerosol continued to react with oxidants. Hydroxyl radicals further oxidize the SOA particles thereby enhancing the particle CCN activity with time. The initial concentrations of ozone and monoterpene precursor (for concentrations lower than 40 ppb) do not appear to affect the activity of the resulting SOA.

Köhler Theory Analysis (KTA) is used to infer the molar mass of the monoterpene SOA sampled online and offline from atomized filter samples. The estimated average molar mass of online monoterpene SOA was determined to be 180 ± 55 g mol^-1 (consistent with existing monoterpene SOA speciation studies) assuming complete solubility. KTA suggests that the aged aerosol (both from alpha-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 65%). CCN activity measurements of the monoterpene SOA mixed with (NH₄)₂SO₄ suggest that the organic can depress surface tension up to 10 nM m^-1 (with respect to pure water). The droplet growth kinetics of monoterpene SOA samples are similar to (NH₄)₂SO₄. The CCN activation diameter of alpha-pinene and mixed monoterpene SOA can be modelled to within 10-15% of experiments by a simple implementation of Köhler theory, assuming complete dissolution of the particles, no dissociation into ions, a molecular weight of 180 g mol^-1, a density of 1.5 g cm^-3, and the surface tension of water.

The model sesquiterpene system, beta-caryophyllene, shows that the CCN contain small amount of hygroscopic material. Although beta-caryophyllene produces higher yields than monoterpene systems the aerosols are less effective as CCN in the atmosphere. A major finding in this work is that the hygroscopic component of beta-caryophyllene SOA is volatile. This leads to discrepancies between CCN instrumentation not observed in monoterpene systems. The finding is potentially quite important for atmospheric measurements impacted by diurnal temperature cycles and thus, potentially, CCN activity diurnal cycles. Evidence of a kinetic barrier to droplet activation was revealed from a comparison of droplet growth kinetics of the CCN and the fraction of WSOC. Filter samples show that the WSOC fraction of the SOA is composed of low molecular weight (<200 g mol^-1) compounds that are slightly surface active. This is similar to the findings for monoterpene SOA systems indicating that exact speciation knowledge might not be required for predictive understanding of CCN.

Griffin, R. J., Cocker, D. R., Flagan, R. C., and Seinfeld, J. H.: Organic aerosol formation from the oxidation of biogenic hydrocarbons, J. Geophys. Res., 104, 3555?3567, 1999.

Guenther, A., Hewitt, C. N., Erickson, D., Fall, R., Geron, C., Graedel, T., Harley, P., Klinger, L., Lerdau, M., McKay, W. A., Pierce, T., Scholes, B., Steinbrecher, R., Tallamraju, R., Taylor, J., and Zimmerman, P.: A global model of natural volatile organic compound emissions, J. Geophys. Res., 100, 8873--8892, 1995.

Houghton, J., Ding, Y., Griggs, D., Noguer, M., van der Linden, P., Dai, X., Maskell, K. and Johnson, C.: Climate Change, 2001: The Science 1035 of Climate Change, Cambridge University Press, New York, Cambridge, UK, 2001.

IPCC (Intergovernmental Panel on Climate Change): Changes atmospheric constituents and in radiative forcing in: Climate change 2007: The physical science basis, Cambridge University Press, 2007.

Köhler, H.: The nucleus in and the growth of hygroscopic droplets, Trans. Faraday Soc. 32, 1151--1161, 1936.

Pruppacher, H. R. and Klett, J. D.: Microphysics of Clouds and Precipitation, Kluwer, Dordrecht, 1997.

Svenningsson, B., Rissler, J., Swietlicki, E., Mircea, M., Bilde, M., Facchini, M.C., Decesari, S., Fuzzi, S., Zhou, J., Monster, J., Rosenorn, T.: Hygroscopic growth and critical supersaturations for mixed aerosol particles of inorganic and organic compounds of atmospheric relevance, Atmos. Chem. Phys., 6,1937?1952, 2006.