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 (£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, £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 £AN concentrations. We calculate that £ANs were a large fraction (∼60%) of the HOx free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of £ANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.