Untangling the Causes of Variation in ClNO2 Yield from the WINTER Campaign

Nitrogen oxides play a key role in atmospheric chemistry. In the lower troposphere they catalyze ozone (O3) production during the day, while at night they react to form nitric acid and remove O3. While these processes are well studied in summertime, winter measurements are far more limited. While summer has more active photochemistry, in winter there is greater potential for longer-range transport of pollutants as they have longer lifetimes against photochemical or heterogeneous oxidation. As part of the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign, aircraft-based measurements were made over the northeastern United States of oxidized nitrogen species, their precursors and products. At night, NOX (NO + NO2) reacts with O3 to form N2O5. The N2O5 can then be taken up onto aerosol particles where it forms either two HNO3 or HNO3 and ClNO2. The balance between these pathways is important for atmospheric chemistry the next day, as ClNO2 will photolyze to yield a NO2 molecule and a Cl radical. In contrast, HNO3 does not participate in further radical chemistry. Thus, formation of ClNO2 can lead to longer-range transport of NOX as well as radical production in less polluted areas. Laboratory studies suggest that the yield of ClNO2 is dependent on the relative concentrations of aerosol chloride and liquid water, but it is unknown whether these yields can be accurately predicted based on air mass history or measured ambient aerosol composition. The observed levels of ClNO2 varied significantly throughout the WINTER campaign from below detection limit to over 2 ppbv, while the yield of ClNO2 covered its entire possible range of 0 to 1. In this study we will use the wealth of data and wide range of observed values to constrain which factors are most important in determining ClNO2 yield and to compare these yields to recent parameterizations from laboratory studies.