Secondary organic aerosol (SOA) formation in the atmosphere: Selected issues related to nucleation, partitioning, and pre-existing aerosols

SOA formation from monoterpene photochemical reactions in the presence of ozone (O3) or nitrogen oxides (NOx) is of great interest due to its potential contribution to the atmospheric fine particulate matter (<2.5 mum). Atmospheric fine particulate matter can affect mangy aspects of the environment from climate change to adverse human health effects. Many studies have demonstrated that SOA can he formed via gas/particle partitioning of semivolatile organic carhops (SOC) from monoterpene photochemical reaction on background aerosols in the atmosphere. G/P partitioning of SOC strongly depends on the chemical and physical properties of substrate aerosols. Because of its complexity, it is uncertain how existing atmospheric aerosols will impact SOA formation.; In this study, some of the influences of background aerosols on SOA formation from monoterpene photochemical reactions are studied using outdoor smog chamber experiments under the natural sunlight. Diesel soot particles were used as background aerosols and alpha-pinene as the SOA source. An increase in the polarity of diesel soot particles was observed during photochemical reactions by monitoring G/P partitioning behavior of deuterated alkanes. During SOA formation in the presence of diesel exhaust, abnormally high particle phase concentrations of aldehyde were observed and compared to theoretical predictions.; This was due to heterogeneous acid-catalyzed reactions in the diesel soot particle phase. The G/P partitioning behavior of aldehyde compounds were further studied with atmospheric acid aerosols (diesel soot particles and wood smoke) by comparing experimentally obtained G/P partitioning coefficients of various aldehyde compounds to the theoretical estimations.; Nucleation precursors from alpha-pinene + O3 reactions were also studied with flow reactor linked to a Teflon bag chamber. A correlation of particle formation with reacted alpha-pinene was observed over a time scale of 6 seconds. Nucleation precursors are discussed with respect to product formed in the initial phase of the reaction. Nucleation could be suppressed by scavenging thermally stabilized Criegee intermediates (SCI) with different carboxylic acids as well as water. Experimental results showed the nucleation rate is different, depending on the type of SCI scavenger. These results strongly imply that SCIs are key compounds involved in the initial steps of nucleation from alpha-pinene ozonolysis.