Low temperatures enhance organic nitrate formation: evidence from observations in the 2012 Uintah Basin Winter Ozone Study

Nitrogen dioxide (NO2) and total alkyl nitrates (Sigma ANs) were measured using thermal dissociation laser-induced fluorescence during the 2012 Uintah Basin Winter Ozone Study (UBWOS) in Utah, USA. The observed NO2 concentration was highest before sunrise and lowest in the late afternoon, suggestive of a persistent local source of NO2 coupled with turbulent mixing out of the boundary layer. In contrast, Sigma ANs co-varied with solar radiation with a noontime maximum, indicating that local photochemical production combined with rapid mixing and/or deposition was the dominant factor in determining the 6 AN concentrations. We calculate that Sigma ANs were a large fraction (similar to 60%) of the HOx free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of Sigma ANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.