CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol

[摘要] The ability of secondary organic aerosol (SOA) produced from the ozonolysisof 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 ScanningMobility Particle Sizer coupled with a Continuous Flow counter) were usedfor the CCN measurements. Consistent with previous studies monoterpene SOAis quite active and would likely be a good source of cloud condensationnuclei (CCN) in the atmosphere. A decrease in CCN activation diameter forα-pinene SOA of approximately 3 nm hr⁻¹ was observed as theaerosol continued to react with oxidants. Hydroxyl radicals further oxidizethe SOA particles thereby enhancing the particle CCN activity with time. Theinitial concentrations of ozone and monoterpene precursor (forconcentrations lower than 40 ppb) do not appear to affect the activity ofthe resulting SOA. Köhler Theory Analysis (KTA) is used to infer themolar mass of the SOA sampled online and offline from atomized filtersamples. The estimated average molar mass of online SOA was determined to be180±55 g mol⁻¹ (consistent with existing SOA speciation studies)assuming complete solubility. KTA suggests that the aged aerosol (both fromα-pinene and the mixed monoterpene oxidation) is primarilywater-soluble (around 65%). CCN activity measurements of the SOA mixedwith (NH₄)₂SO₄ suggest that the organic can depress surfacetension by as much as 10 N m⁻¹ (with respect to pure water). Thedroplet growth kinetics of SOA samples are similar to(NH₄)₂SO₄, except at low supersaturation, where SOA tends togrow more slowly. The CCN activation diameter of α-pinene and mixedmonoterpene SOA can be modelled to within 10–15% of experiments by asimple implementation of Köhler theory, assuming complete dissolution ofthe 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.

[学科分类] 大气科学