Observational Constraints on the Oxidation of NOx in the Upper Troposphere

NOx (NOx ≡ NO + NO2) regulates O3 and HOx (HOx ≡ OH + HO2) concentrations in the upper troposphere. In the laboratory, it is difficult to measure rates and branching ratios of the chemical reactions affecting NOx at the low temperatures and pressures characteristic of the upper troposphere, making direct measurements in the atmosphere especially useful. We report quasi-Lagrangian observations of the chemical evolution of an air parcel following a lightning event that results in high NOx concentrations. These quasi-Lagrangian measurements obtained during the Deep Convective Clouds and Chemistry experiment are used to characterize the daytime rates for conversion of NOx to different peroxy nitrates, the sum of alkyl and multifunctional nitrates, and HNO3. We infer the following production rate constants [in (cm3/molecule)/s] at 225 K and 230 hPa: 7.2(±5.7) × 10−12 (CH3O2NO2), 5.1(±3.1) × 10−13 (HO2NO2), 1.3(±0.8) × 10−11 (PAN), 7.3(±3.4) × 10−12 (PPN), and 6.2(±2.9) × 10−12 (HNO3). The HNO3 and HO2NO2 rates are ∼30−50% lower than currently recommended whereas the other rates are consistent with current recommendations to within ±30%. The analysis indicates that HNO3 production from the HO2 and NO reaction (if any) must be accompanied by a slower rate for the reaction of OH with NO2, keeping the total combined rate for the two processes at the rate reported for HNO3 production above.