Abstract
Abstract. The ability of secondary organic aerosol (SOA) produced from the ozonolysis of α-pinene and monoterpene mixtures (α-pinene, β-pinene, limonene and 3-carene) to become cloud droplets was investigated. A static CCN counter and a Scanning Mobility CCN Analyser (a Scanning Mobility Particle Sizer coupled with a Continuous Flow counter) were used for the CCN measurements. Consistent with previous studies monoterpene SOA is quite active and would likely be a good source of cloud condensation nuclei (CCN) in the atmosphere. A decrease in CCN activation diameter for α-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 SOA sampled online and offline from atomized filter samples. The estimated average molar mass of online SOA was determined to be 180±55 g mol−1 (consistent with existing SOA speciation studies) assuming complete solubility. KTA suggests that the aged aerosol (both from α-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 65%). CCN activity measurements of the SOA mixed with (NH4)2SO4 suggest that the organic can depress surface tension by as much as 10 N m−1 (with respect to pure water). The droplet growth kinetics of SOA samples are similar to (NH4)2SO4, except at low supersaturation, where SOA tends to grow more slowly. The CCN activation diameter of α-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.
Highlights
Cloud-particle interactions are one of the major challenges in understanding indirect climate forcing (Houghton et al, 2001; IPCC, 2001)
We investigate the Cloud Condensation Nuclei (CCN) activity of fresh and aged α-pinene and mixed monoterpene secondary organic aerosol (SOA) in order to add to the growing body of evidence of understanding of monoterpene CCN activity
The CCN activity of SOA generated by the ozonolysis of αpinene and mixed monoterpenes has been measured using a static CCN counter and a Scanning Mobility CCN Analyzer (SMCA)
Summary
Cloud-particle interactions are one of the major challenges in understanding indirect climate forcing (Houghton et al, 2001; IPCC, 2001). In addition to online measurements of aerosol properties, filter SOA samples were collected and subsequently analyzed offline in the laboratory, by measuring the CCN activity of aerosol generated from the filter samples Combination of these activation experimental data with Kohler theory provides estimates of the molar volume and surfactant characteristics of the SOA water-soluble organic carbon (WSOC) fraction (Asa-Awuku et al, 2007a). These offline samples are mixed with ammonium sulfate to infer the presence and surfactant characteristics of the WSOC in the aged SOA. A simple parameterization of CCN properties of this important class of SOA is introduced
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