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

Abstract. Nitrogen dioxide (NO₂) and total alkyl nitrates (ΣANs) 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, ΣANs 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 ΣAN concentrations. We calculate that ΣANs were a large fraction (~60%) of the HO_x free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of ΣANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.