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

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%.