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Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements
Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of α-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors ≤1.05 was observed at RH<98% RH. At RH>98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4–6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activation is steeper than what would be expected for a constant value of the hygroscopicity. Combining measurements of hygroscopic growth and activation, it was found that the surface tension that has to be assumed to interpret the measurements consistently is greater than 55 mN/m, possibly close to that of pure water, depending on the different SOA-types produced, and therefore only in part accounts for the discrepancy between hygroscopic growth and CCN activity observed for SOA particles in the past.
Hydration increases the lifetime of HSO5 and enhances its ability to act as a nucleation precursor – a computational study
Recent experimental findings indicate that HSO5 radicals may play a key role in the nucleation of atmospheric SO2 oxidation products. HSO5 radicals are metastable intermediates formed in the SO2 oxidation process, and their stability and lifetime are, at present, highly uncertain. Previous high-level computational studies have predicted rather low stabilities for HSO5 with respect to dissociation into SO3+HO2, and have predicted the net reaction HSO3+OH→SO3+HO2 to be slightly exothermal. However, these studies have not accounted for hydration of HSO5 or its precursor HSO3. In this study, we have estimated the effect of hydration on the stability and lifetime of HSO5 using the advanced quantum chemical methods CCSD(T) and G3B3. We have computed formation energies and free energies for mono- and dihydrates of OH, HSO3, HSO5, SO3 and HO2, and also reanalyzed the individual steps of the HSO3+O2→HSO5→SO3+HO2 reaction at a higher level of theory than previously published. Our results indicate that hydration is likely to significantly prolong the lifetime of the HSO5 intermediate in atmospheric conditions, thus increasing the probability of reactions that form products with more than one sulfur atom. Kinetic modeling indicates that these results may help explain the experimental observations that a mixture of sulfur-containing products formed from SO2 oxidation by OH radicals nucleates much more effectively than sulfuric acid taken from a liquid reservoir.
Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol - Part 2: Theoretical approaches
We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of α-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.