Peroxynitric acid (HO₂NO₂) measurements during the UBWOS 2013 and 2014 studies using iodide ion chemical ionization mass spectrometry

Abstract. In this paper laboratory work is documented establishing iodide ion chemical ionization mass spectrometry (I^- CIMS) as a sensitive method for the unambiguous detection of peroxynitric acid (HO₂NO₂; PNA). A dynamic calibration source for HO₂NO₂, HO₂, and HONO was developed and calibrated using a novel total NO_y cavity ring-down spectroscopy (CaRDS) detector. Photochemical sources of these species were used for the calibration and validation of the I^- CIMS instrument for detection of HO₂NO₂. Ambient observations of HO₂NO₂ using I^- CIMS during the 2013 and 2014 Uintah Basin Wintertime Ozone Study (UBWOS) are presented. Strong inversions leading to a build-up of many primary and secondary pollutants as well as low temperatures drove daytime HO₂NO₂ as high as 1.5 ppbv during the 2013 study. A comparison of HO₂NO₂ observations to mixing ratios predicted using a chemical box model describing an ozone formation event observed during the 2013 wintertime shows agreement in the daily maxima HO₂NO₂ mixing ratio, but a differences of several hours in the timing of the observed maxima. Observations of vertical gradients suggest that the ground snow surface potentially serves as both a net sink and source of HO₂NO₂ depending on the time of day. Sensitivity tests using a chemical box model indicate that the lifetime of HO₂NO₂ with respect to deposition has a non-negligible impact on ozone production rates on the order of 10 %.