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Direct Measurement of Ozone Sensitivity to Oxides of Nitrogen and Volatile Organic Compounds during COVID-19 Using a Mobile Smog Chamber System
SHENGLUN WU, Toshihiro Kuwayama, Michael Kleeman, University of California, Davis
Abstract Number: 413
Working Group: Aerosol Chemistry
Abstract
Ozone (O3) is a secondary pollutant produced from a series of non-linear photochemical reactions. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are two main precursors affecting O3 formation, but the NOx to VOC ratio defines the chemical regime that determines ozone response to changing precursor emissions. Understanding the atmospheric chemical regime is a critical first step in the design of effective emissions control strategies for reducing O3 concentrations. Here we describe a field study that directly measures the sensitivity of O3 to NOx and VOC using a mobile trailer equipped with three identical Teflon smog chambers. Three parallel chambers (each 1 m3) were filled with ambient air at approximately 11am each day. One chamber was used as basecase measurement, one chamber had NOx added, and one chamber had a surrogate VOC added (ethylene, m-xylene, n-hexane). Chambers were irradiated with UV (50 W m-2) equivalent to midday summer conditions for three hours. O3, NOx, NOy, temperature, and relative humidity were measured in all three chambers each 10 min. Measurements at Sacramento from Apr - July 2020 find increased O3 formation in the VOC-perturbed chamber and lower O3 formation in the NOx-perturbed chamber on weekdays. This finding suggests that the VOC-limited ozone production dominates weekdays in Sacramento. Reduced NOx emissions associated with COVID-19 shelter-in-place orders did not significantly affect the atmospheric chemical regime. However, a significant change of O3 sensitivity from VOC-limited to NOx-limited was detected on the weekend, when NOx concentrations were reduced by a factor of approximately two. Measurements on days with different ambient temperature suggest that increasing temperatures reduce the NOx limitation and increase O3 production. The results from the current study demonstrate a system to aid in the design of emissions control programs for O3 reduction.