Evidence of heterogeneous HONO formation from aerosols and the regional photochemical impact of this HONO source
Largemissing daytimeHONO sources have been reported bymany previous studies around the world. PossibleHONO sources include ground heterogeneous conversion, aerosol heterogeneous formation, soil emission, and photochemical production. In this study, a consistent 1D framework based on regional and 3D chemical transportmodels (CTMs)was used to analyze the unknown daytimeHONO sources in 14 cases worldwide.We assume that the source ofHONO fromaerosols is throughNO2 hydrolysis (not including its oxidation products) and that non-localmixing effect is negligible. Assuming all themissing unknownHONOsource is from the ground, it would imply a NO2-to-HONOground heterogeneous conversion exceeding 100% in daytime, which is unphysical. In contrast, a strongR reaching up to 0.92 is found between the unknownHONO sources and the products of aerosol wet surface area and short-wave radiation. Because the largest unknowndaytime HONO sources are found inChina due to high concentrations of aerosols andNO2, we derive an optimizedNO2 uptake coefficient on the basis of thesemeasurements. The 3DCTMsimulations suggest that in some regions of central, eastern, and southwestern (e.g. SiChuan province)China, the aerosolHONO source has the greatest effects on ozone (>10 ppbv) andOH (>200%) inwinter. In January, the simulated particulate sulfate level over these three regions increases by 6–10 μgm after including the aerosol-HONO source, which helps reduce the previousmodel underestimation of sulfate production inwinter. Additionalmeasurement studies that target the daytimeHONO sources will be essential to a better understanding of themechanisms and resulting effects on atmospheric oxidants.