Observational Constraints on the Oxidation of NO_x in the Upper Troposphere

Abstract

NO_x (NO_x ≡ NO + NO_2) regulates O_3 and HO_x (HO_x ≡ OH + HO_2) 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 NOx to different peroxy nitrates, the sum of alkyl and multifunctional nitrates, and HNO_3. We infer the following production rate constants [in (cm^3/molecule)/s] at 225 K and 230 hPa: 7.2(±5.7) × 10^(–12) (CH_3O_2NO_2), 5.1(±3.1) × 10^(–13) (HO_2NO_2), 1.3(±0.8) × 10^(–11) (PAN), 7.3(±3.4) × 10^(–12) (PPN), and 6.2(±2.9) × 10^(–12) (HNO_3). The HNO_3 and HO_2NO_2 rates are ∼30–50% lower than currently recommended whereas the other rates are consistent with current recommendations to within ±30%. The analysis indicates that HNO_3 production from the HO_2 and NO reaction (if any) must be accompanied by a slower rate for the reaction of OH with NO_2, keeping the total combined rate for the two processes at the rate reported for HNO_3 production above.

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