Observational constraints on upper tropospheric NOx emissions, lifetime, and oxidative products
Nitrogen oxides (NOx ≡ NO + NO2) regulate tropospheric ozone (O3) production rates. In the upper troposphere (~8 – 15 km above ground level), where O3 is an important greenhouse gas, there are few detailed measurements of NOx and its oxidation products. As a result, the chemical reactions that involve NOx are poorly characterized under the low temperature conditions in this region of the atmosphere. For the reactions that have been studied under these conditions (e.g., daytime nitric acid, or HNO3, and pernitric acid, or HO2NO2, production), the results from various experiments indicate a 20 – 50% disagreement for the rate constants, and the other important NOx oxidation reactions (production of acyl peroxy nitrate, like PAN and PPN, and alkyl and multifunctional nitrates) have not been well characterized for the conditions characteristic of the upper troposphere. Besides the poorly understood NOx oxidation rates, recent calculations have indicated there is an important upper tropospheric NOx oxidation product (methyl peroxy nitrate, or CH3O2NO2) that has not been measured in the atmosphere. These uncertainties in the products and oxidation rate constants affect the characterization of the input of NOx from lightning.
In this dissertation, I report observations obtained during two airborne field campaigns, the Deep Convective Clouds and Chemistry (DC3, May – June, 2012) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS, August – September, 2013) experiments, and use these observations to investigate the reaction products and rate constants for the oxidation of NOx to less reactive reservoirs. The observations focused on fresh lightning emissions in deep convective outflow, and the subsequent chemical aging of the outflow downwind.
First, I present the first ambient observations of CH3O2NO2, and recommendations on how to measure upper tropospheric in situ NO2 with minimal interferences from the thermal decomposition of CH3O2NO2 during sampling. I show that CH3O2NO2 is ubiquitous in the upper troposphere and is as important NOx oxidative product as HNO3. Then, using observations from one quasi-Lagrangian flight during DC3, I derive constraints on the daytime NOx oxidative rate constants for the reactions that remove upper tropospheric NOx. The reactions include the production of CH3O2NO2, HO2NO2, PAN, PPN, alkyl and multifunctional nitrates, and HNO3. These constraints indicate that NOx lifetime is longer than currently believe due to the daytime HNO3 and HO2NO2 production rate constants being 30 – 50% slower than currently assumed. Finally, the implications of the longer lifetime are used to show that lightning NOx emission rates are at least 33% larger than current estimates. As a consequence, model predictions indicate O3 in the upper troposphere increase by 5 – 10% with a resulting increase in radiative forcing.