Impact of environmental variables on the reduction of nitric acid by proxies for volatile organic compounds emitted by motor vehicles

Özet

Recent work has identified nitric acid (HNO3) as a potential precursor of nitrous acid (HONO), which is animportant source of oxidants that regulate ozone and particulate pollution. Recent work in our laboratoryhas indicated that the reduction of HNO3 to HONO can occur homogeneously in the presence of surrogatesfor volatile organic compounds (VOCs) emitted by motor vehicles. This study focuses on theimpact of environmental variables on the rate of formation of HONO in this process. The observed basecase (25.0 C and ~20.0% relative humidity (RH)) HONO formation rate was 0.54 ± 0.09 ppb h1, valuescomparable to enhancements observed in HONO during morning rush hour in Houston, TX. The rate wasenhanced at lower temperatures of ~20.0 C, but the rate remained statistically similar (1s) for experimentsconducted at temperatures of 25 C, 30 C, and 35 C. The assumption that multiple reactivecomponents of the VOC mixture react with HNO3 is supported by this observation, and the relativeimportance of each reactive species in the reaction may vary with temperature. The enhanced rate atlower temperatures could make the proposed reaction mechanism more important at night. The formationrate of HONO does not change substantially when initial HNO3 concentration is varied between400 and 4600 ppt, suggesting that the concentration of reactive VOCs was the limiting factor. Thereduction of HNO3 to HONO appears not to occur heterogeneously on the aerosol surfaces tested. Thepresence of ~120 ppb of ammonia has no observable impact on the reaction. However, it is likely that UVirradiation (l ¼ 350 nm) decreases the formation rate of HONO either by consuming the reactive VOCsinvolved or by directly interfering with the reaction. The “renoxification” of less reactive HNO3 to morereactive HONO has significant implications for daytime ozone and particulate pollution.