Observations from the SEAC^4RS aircraft campaign over the southeast United States in August–September 2013 show NO/NO_2 concentration ratios in the upper troposphere that are approximately half of photochemical equilibrium values computed from Jet Propulsion Laboratory (JPL) kinetic data. One possible explanation is the presence of labile NO_x reservoir species, presumably organic, decomposing thermally to NO_2 in the instrument. The NO_2 instrument corrects for this artifact from known labile HNO_4 and CH_3O_2NO_2 NO_x reservoirs. To bridge the gap between measured and simulated NO_2, additional unaccounted labile NO_x reservoir species would have to be present at a mean concentration of ~40 ppt for the SEAC^4RS conditions (compared with 197 ppt for NOx). An alternative explanation is error in the low‐temperature rate constant for the NO + O_3 reaction (30% 1‐σ uncertainty in JPL at 240 K) and/or in the spectroscopic data for NO_2 photolysis (20% 1‐σ uncertainty). Resolving this discrepancy is important for understanding global budgets of tropospheric oxidants and for interpreting satellite observations of tropospheric NO_2 columns
