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 240 K, we calculate that about 20% of NO_y in the mid and polar 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 (<font face="Symbol">S</font>PNs equivanlent HO₂NO₂ + CH₃O₂NO₂ + PAN + PPN + ...), aboard the NCAR C-130 research aircraft. We infer the sum of HO₂NO₂ and CH₃O₂NO₂ as the difference between <font face="Symbol">S</font>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. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid.