Wintertime halogen chemistry in polluted, inland, urban environments

Atmospheric reactive halogens, including chlorine atoms (Cl?), alter atmospheric composition and air quality, by oxidizing volatile organic compounds and facilitating the production ozone and particulate matter. An important Cl? source is nitryl chloride (ClNO2), which is formed at night through the reaction of dinitrogen pentoxide (N2O5) with chloride-containing aerosols. Unexpected observations of inland ClNO2, coincident with poor air quality, highlight a major gap in our understanding of coupled chlorine and nitrogen chemistry in wintertime polluted, inland, urban environments. The reactive uptake of N2O5 and yields of ClNO2 and particulate nitrate produced are dependent on particle surface composition, which has not been constrained in previous field studies. Recent aircraft-based field and modeling results from the western and northeast U.S. have highlighted discrepancies between measured chemistry and that predicted based on bulk particulate matter composition and application of the current mass-based laboratory parameterization. Therefore, we hypothesize that individual particle mixing state and chloride content are critical for N2O5 fate and ClNO2 production. We propose to test this hypothesis through laboratory-based studies of authentic standards and samples, with comparison to ambient flow tube results from the reaction of synthesized N2O5 with ambient particles, with coincident individual particle source and chemical composition measurements. Salt Lake City, UT is an ideal field site to explore this chemistry due to its severe wintertime air quality and complex suite of sources of particulate chloride, including residential wood burning emissions, deicing road salt aerosol, salt-rich playa (dry lakebed) dusts, and industrial sources.
This collaborative project will combine laboratory and field measurements of N2O5 uptake coefficients, ClNO2 yields, and individual particle chemical composition, using chemical ionization mass spectrometry and single-particle mass spectrometry. Laboratory aerosol flow tube experiments will provide fundamental knowledge of N2O5 uptake and ClNO2 yields for both standard and authentic particulate chloride sources, including road salts and playa dust from Utah. These lab experiments will both inform and be informed by the proposed fieldwork in Salt Lake City, UT from Jan. ? Feb. 2022. Field measurements will focus on perturbation experiments, in which ambient particles will be exposed to N2O5 in a field-deployable aerosol flow tube, to be constructed as part of the project. The reaction of N2O5 with ambient particles will be characterized as a function of individual particle chemical composition and chloride source for comparison to the laboratory results from the same experiment methodology. We will compare our laboratory and field measurement results with those calculated using the existing parameterization, as well as use our results to improve this, or develop a new parameterization based on our project results. Broader Impacts &
The proposed project will result in an improved, process-level understanding of inland, urban atmospheric nitrogen-chlorine chemistry for improved simulations of atmospheric composition. This work contributes to the NOAA CPO goal to ?advance our understanding of Earth?s climate system? and the AC4 program?s aim for observational studies ?to support the development and improvement of models, and to inform?air pollution management efforts.? Our proposal is directly responsive to two AC4 competition priority areas: 1) ?multispecies approach to understanding the urban environment? and 2) ?participation in upcoming field efforts in U.S. cities or exploration of novel chemistry and tracers of urban air?. Fieldwork in Salt Lake City will investigate the ?harmful levels of air pollution?in the intermountain valleys in the western US?. This project will be led by two female assistant professors, mentoring two PhD students and one female postdoc, and will include public outreach.