Dual roles of inorganic aqueous phase on SOA growth from benzene and phenol

Benzene, emitted from automobile exhaust and biomass burning, is ubiquitous in ambient air. Benzene is a precursor hydrocarbon (HC) that forms secondary organic aerosols (SOA), but its SOA formation mechanism is not well studied. To accurately predict the formation of benzene SOA, it is important to understand the gas mechanisms of phenol, which is one of the major products formed from the atmospheric oxidation of benzene. Our chamber study found that wet-inorganic aerosol retarded the gas oxidation or phenol or benzene, and thus their SOA formation. To explain this unusual effect, it is hypothesized that a persistent phenoxy radical (PPR) effectively forms via a heterogeneous reaction of phenol and phenol-related products in the presence of wet-inorganic aerosol. These PPR species are capable of catalytically consuming ozone during a NO _x cycle and negatively influencing SOA growth. In this study, explicit gas mechanisms were derived to produce the oxygenated products from the atmospheric oxidation of phenol and benzene. Gas mechanisms include the existing Master Chemical Mechanism (MCM v3.3.1); the reaction path for peroxy radical adducts originating from the addition of an OH radical to phenols forming low-volatility products (e.g., multi-hydroxy aromatics); and the mechanisms to form heterogeneous production of PPR. The simulated gas products were classified into volatility-reactivity based lumping species and incorporated into the UNIfied Partitioning Aerosol Reaction (UNIPAR) model that predicts SOA formation via multiphase reactions of phenol or benzene. The predictability of the UNIPAR model was examined using chamber data, which were generated for the photooxidation of phenol or benzene under various experimental conditions (NO _x levels, humidity, and inorganic seed types). The SOA formation from both phenol and benzene still increased in the presence of wet inorganic seed because of the oligomerization of reactive organic species in aqueous phase. However, model simulations show a significant suppression in ozone, the oxidation of phenol or benzene, and SOA growth, compared to those without PPR mechanisms. In addition, the production of PPR is accelerated in the presence of acidic aerosol and this weakens SOA growth. In benzene oxidation, about 53 % of the oxidation pathway is connected to phenol formation in the reported gas mechanism. Thus, the contribution of PPR to gas mechanisms is less than phenol. Overall, SOA growth in phenol or benzene is negatively related to NO _x levels in the high NO _x region (HC ppbC/NO _x ppb <5). However, the simulation indicates that the significance of PPR rises with decreasing NO _x levels. Hence, the influence of NO _x levels on the SOA formation from phenol or benzene is complex under varying temperature and seed types.