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

The chemistry of peroxynitric acid (HO2 NO2 ) and methyl peroxynitrate (CH3 O2 NO2 ) 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 240 K, we calculate that about 20% of NOy in the mid- and high-latitude upper troposphere is HO2 NO2 . Under these conditions, the reaction of OH with HO2 NO2 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≡HO2 NO2 +CH3 O2 NO2 +PAN+PPN+. . . ) aboard the NCAR C-130 research aircraft. We infer the sum of HO2 NO2 and CH3 O2 NO2 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 NOy and other nitrogen oxide measurements confirms the importance of HO2 NO2 and CH3 O2 NO2 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 (HO2 NO2 +CH3 O2 NO2 ) to NOy are highly temperature dependent: on average 30% of NOy at 230 K, 15% of NOy at 240 K, and <5% of NOy above 250 K. 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=1×10−5 s−1 ) agreed with the observed sum of HO2 NO2 +CH3 O2 NO2 to better than 35% below 240 K where the concentration of these species is largest.