Direct measurements of ozone response to emissions perturbations in California

Abstract

A new technique was used to directly measure O₃ response to changes in precursor NOₓ and volatile organic compound (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 NOₓ, and one chamber added surrogate VOCs (ethylene, m-xylene, n-hexane). Comparing the O₃ formation between chambers over a 3-hour UV cycle provides a direct measurement of O₃ sensitivity to precursor concentrations. Measurements made with this system at Sacramento, California, between April–December 2020 revealed that the atmospheric chemical regime followed a seasonal cycle. O₃ formation was VOC-limited (NOₓ-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 (NOₓ-rich) during the fall season as the concentrations of ambient VOCs decreased and NOₓ 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 ratios from the TROPOspheric Monitoring Instrument (TROPOMI) aboard 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 NOₓ-limited regime, suggesting that a NOₓ 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 NOₓ reduction would increase O₃ concentrations. This challenging situation suggests that emissions control programs that focus on NOₓ reductions will immediately lower peak O₃ concentrations but slightly increase intermediate O₃ concentrations until NOₓ levels fall far enough to re-enter the NOₓ-limited regime. The spatial pattern of increasing and decreasing O₃ concentrations in response to a NOₓ emissions control strategy should be carefully mapped in order to fully understand the public health implications.

Files

Additional details