Direct measurements of ozone response to emissions perturbations in California

A new technique was used to directly measure O₃ response to changes in precursor NO_x and VOC concentrations in the atmosphere using three identical Teflon "smog chambers" equipped with UV lights. One chamber served as the baseline measurement for O₃ formation, one chamber added NO_x, and one chamber added surrogate VOCs (ethylene, m-xylene, n-hexane). Comparing the O₃ formation between chambers over a three-hour UV cycle provides a direct measurement of O₃ sensitivity to precursor concentrations. Measurements made with this system at Sacramento, California, between April 2020 – December 2020 revealed that the atmospheric chemical regime followed a seasonal cycle.  O₃ formation was VOC-limited (NO_x – rich) during the early spring, transitioned to NOx-limited during the summer due to increased concentrations of ambient VOCs with high O₃ formation potential, and then returned to VOC-limited (NO_x-rich) during the fall season as the concentrations of ambient VOCs decreased and NO_x increased. This seasonal pattern of O3 sensitivity is consistent with the cycle of biogenic emissions in California.  The direct chamber O₃ sensitivity measurements matched semi-direct measurements of HCHO/NO2 ratios from the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor (Sentinel-5P) satellite.  Furthermore, the satellite observations showed that the same seasonal cycle in O₃ sensitivity occurred over most of the entire state of California, with only the urban cores of the very large cities remaining VOC-limited across all seasons. The O₃-nonattainment days (MDA8 O₃ > 70 ppb) have O₃ sensitivity in the NO_x-limited regime, suggesting that a NO_x emissions control strategy would be most effective at reducing these peak O₃ concentrations. In contrast, a large portion of the days with MDA8 O₃ concentrations below 55 ppb were in the VOC-limited regime, suggesting that an emissions control strategy focusing on NO_x reduction would increase O₃ concentrations. This challenging situation suggests that emissions control programs that focus on NO_x reductions will immediately lower peak O₃ concentrations, but slightly increase intermediate O₃ concentrations until NO_x levels fall far enough to re-enter the NO_x-limited regime.  The spatial pattern of increasing and decreasing O₃ concentrations in response to a NO_x emissions control strategy should be carefully mapped in order to fully understand the public health implications.