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

Laboratory work is reported here 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 NOy detector (NOy CaRDS). Photochemical sources of these species were used for the calibration and validation of the I− CIMS instrument for detection of HO2NO2. A dual inlet system was developed to determine differences in the instrument response when using a heated inlet dissociator (150 °C) and a "cold" room-temperature inlet. HO2NO2 was detected as I-HO2− (m/z 160), NO3− (m/z 62) and I-HO2NO2− (m/z 206). The I− CIMS normalized sensitivity to peroxynitric acid was 2.0 Hz pptv−1 with a detection limit (3σ) of 40 pptv via detection of the I-HO2− (m/z 160) cluster ion using an inlet dissociator at a temperature of 150 °C. Alternatively, PNA was detected via I− CIMS with a cold inlet at both the NO3− (m/z 62) and I-HO2NO2− (m/z 206) ions with normalized detection sensitivities of 144 and 0.4 Hz pptv−1 respectively. The cold inlet sensitivity of iodide CIMS towards the detection of HO2 radicals, also via detection at the I-HO2− cluster ion, a potential HO2NO2 interference, was approximately 2.6 Hz pptv−1 with an instrumental detection limit (3σ) of 20 pptv. Ambient observations of HO2NO2 using I− CIMS were made 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 significant difference os 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 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%.