Role of Relative Humidity in the Secondary Organic Aerosol Formation from High-NOx Photooxidation of Long-Chain Alkanes: n-Dodecane Case Study

The role of relative humidity (RH) in secondary organic aerosol (SOA) formation from high-NOx photooxidation of long-chain alkanes was investigated by performing simulation chamber experiments on n-dodecane (C12H26). This molecule was chosen as a model compound for the class of long-chain alkanes. The experiments span a wide range of RH conditions from <1 to 70%. The humidity was found to reduce the SOA production yield by a factor of 2, from extremely dry (<1%) to humid (RH >= 5%) conditions. Measurements of major oxidation products in the gaseous and aerosol phases revealed the effect of water on a series of multiphasic reaction mechanisms and SOA production. Under very low RH, an acid-catalyzed "dehydration" reaction of polyfunctional molecules leads to lower volatility products. This pathway was found to be inhibited from dry to ambient RH conditions, explaining the SOA yield reduction. In addition to the previously established carbonyls and hydroxycarbonyls, novel tracers, that is, lactone and carboxylic acid, were identified using authentic standards, providing evidence for water-sensitive multiphasic pathways. Among the species formed via the dehydration channel, we find not only volatile species but also very reactive intermediates (e.g., dihydrofurans) which partition back to the gas phase. Owing to its double bond, it can be further oxidized to lower volatility secondary products, which forms a complex multiphasic scheme involving many condensation reaction-evaporation steps.