Effects of NO₂ and RH on secondary organic aerosol formation and light absorption from OH oxidation of ο-xylene

Ο-xylene is an important aromatic volatile organic compound (VOC) in the atmosphere over urban areas. In this work, the effect of nitrogen dioxide (NO₂) concentration and relative humidity (RH) on the mass concentration of secondary organic aerosols (SOA) formed from ο-xylene OH oxidization was investigated in a photooxidation chamber. The ο-xylene SOA mass concentration increased from 54.2 μg m⁻³ to 127.2 μg m⁻³ during dry conditions, but decreased from 177.7 μg m⁻³ to 146.5 μg m⁻³ during high RH conditions when the initial NO₂ concentration increased form 0 ppbv to about 900 ppbv. An increase in the ratio of [NO₃⁻]/[Org] and a decrease in the oxidation state of carbon (OS_C) of SOA suggested that acid-catalyzed heterogeneous reaction was responsible for enhancing SOA formation with increasing NO₂ concentrations in dry conditions. In contrast, in humid conditions, the high molecular diffusion capacity of SOA could promote the reactivity of OH towards the interior of SOA, and the enhancement of nitrous acid (HONO) formation under high NO₂ conditions could promote the SOA aging processes and be responsible for the decreasing trend of SOA formation with NO₂. Light absorption by SOA was also measured, and both NO₂ and RH enhanced the mass absorption coefficient (MAC_{λ = 365 nm}) value for the optical properties of ο-xylene SOA. The highest MAC_{λ = 365 nm} value of ο-xylene SOA was 0.89 m² g⁻¹, observed during humid conditions with an initial NO₂ concentration of 862 ppbv, which was 3.9 times higher than in the experiment conducted in the absence of NO₂ under dry conditions. The formation of nitrogen-containing organic compounds (NOCs) and humic-like substances (HULIS) were responsible for the increased MAC_{λ = 365 nm} values of ο-xylene derived SOA. This study provides new insight into the effect of NO₂ on SOA formation through the change in ο-xylene photooxidation under different RH conditions, and the complex effect of multiple environmental factors on SOA formation was also important and should not be ignored.