NOx Loss and Lifetime Via Its Reaction with HOx in the Upper Troposphere

Chemical production of ozone (O3) in the upper troposphere, where it is an important greenhouse gas, is controlled by NOx concentrations. NOx in the upper troposphere has several sources (lightning, transport from stratosphere, convection, and chemical production from reservoirs). However its primary sink is the production of HNO3. Recent studies have indicated that HNO3 production through the reaction of OH with NO2 is occurring at a slower rate in the atmosphere than in models. Other studies have indicated that the production of HNO3 has an additional source through the reaction of HO2 with NO. The Deep Convective Clouds and Chemistry (DC-3) experiment provided an opportunity to study the loss of NOx and simultaneous production of HNO3 at the high altitudes and low temperatures at the core of the current debate. In-situ measurements of OH, HO2, NO, NO2, and HNO3 were made with Lagrangian sampling in the outflow of convective storms. We observe a rate of HNO3 production (and NOx removal) directly from the measurements, finding the rate of NOx loss to be slower than current recommendations. We speculate the reasons for the discrepancy are due to the temperature dependence of the HOONO channel in the reaction of OH with NO2. The observations also put an upper limit on the production of HNO3 by the reaction of HO2 with NO. Those observations show NOx has a longer lifetime in the upper troposphere than indicated by current models. Thus, the NOx concentration is higher, and we calculate that O3 is produced more rapidly in the upper troposphere than in those same models.