Observational Constraints on the Oxidation of NO_x in the Upper Troposphere

NO_x (NO_x ≡ NO + NO₂) regulates O₃ and HO_x (HO_x ≡ OH + HO₂) concentrations in the upper troposphere. In the laboratory, it is difficult to measure rates and branching ratios of the chemical reactions affecting NO_x 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 NO_x concentrations. These quasi-Lagrangian measurements obtained during the Deep Convective Clouds and Chemistry experiment are used to characterize the daytime rates for conversion of NO_x 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.