Measurements of the sum of HO₂NO₂ and CH₃O₂NO₂ in the remote troposphere

Abstract. The chemistry of peroxynitric acid (HO₂NO₂) and methyl peroxynitrate (CH₃O₂NO₂)is predicted to be particularly important in the upper troposphere where temperatures are frequently low enough that these compounds do not rapidly decompose. At temperatures below 240K, we calculate that about 20% of NO_y in the mid- and high-latitude upper troposphere is HO₂NO₂. Under these conditions, the reaction of OH with HO₂NO₂ is estimated to account for as much as one third of the permanent loss of hydrogen radicals. During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign, we used thermal dissociation laser-induced fluorescence (TD-LIF) to measure the sum of peroxynitrates (PNs HO₂NO₂+CH₃O₂NO₂+PAN+PPN+...) aboard the NCAR C-130 research aircraft. We infer the sum of HO₂NO₂ and CH₃O₂NO₂ as the difference between PN measurements and gas chromatographic measurements of the two major peroxy acyl nitrates, peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN). Comparison with NO_y and other nitrogen oxide measurements confirms the importance of HO₂NO₂ and CH₃O₂NO₂ to the reactive nitrogen budget and shows that current thinking about the chemistry of these species is approximately correct. During the spring high latitude conditions sampled during the TOPSE experiment, the model predictions of the contribution of (HO₂NO₂+CH₃O₂NO₂) to NO_y are highly temperature dependent: on average 30% of NO_y at 230K, 15% of NO_y at 240K, and 5% of NO_y above 250K. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid. A model that includes IR photolysis (J=1x10^-5s^-1) agreed with the observed sum of HO₂NO₂+CH₃O₂NO₂ to better than 35% below 240K where the concentration of these species is largest.