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Direct measurements of ozone response to emissions perturbations in California

Citation

Wu, Shenglun et al. (2022), Direct measurements of ozone response to emissions perturbations in California, Dryad, Dataset, https://doi.org/10.25338/B85P9B

Abstract

A new technique was used to directly measure O3 response to changes in precursor NOx 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 NOx, 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 (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 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 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.

Methods

O3 sensitivities to precursor NOx/VOC concentrations were measured in central Sacramento, CA (N 38.57, W 121.49) from April – December 2020 (222 experiement days out of a total of 251 days). Grab samples of ambient air were collected between 10:00 AM to 12:00 PM to characterize the daytime O3 formation rates in the presence of variable atmospheric mixing and regional emissions. Sensitivities were based on perturbation concentrations of approximately 8 ppb of NOx injected into chamber #1 and 8 ppb of VOC surrogates injected into chamber #3. Initial gas concentrations were measured from the full chambers in the dark over a 30 min period (10 min for each chamber). The UV lamp panels were then illuminated for 180 min and the chamber concentrations were measured in a continuous cycle of 10 min intervals over a total of seven cycles.  Each active monitoring period lasted 210 min (=30 min of dark measurements + 180 min of light measurements). Measurements in different chambers are made at different times, making it difficult to compare chamber results at the conclusion of the experiment.  It was noted that O3 concentrations within each chamber averaged in each 10 min sampling interval increased linearly over the 180 min period when the UV lights were on. A linear regression model was therefore applied to extrapolate O3 concentrations in each chamber to the end of the measurement period to facilitate direct comparisons between the basecase chamber #2 and perturbed chambers #1 and #3. O3 concentration after 3-hour UV exposure. The difference of O3 concentration after 3-hour UV exposure was calculated between chamber #1 to chamber #2, and chamber #3 to chamber #2 to quantify the O3 sensitivity. 

Usage Notes

Published dataset includes the chamber O3 and NOx concentration from daily chamber measurement. The O3 concentration was published as the linear regression coeffcient (slope and intercept) and 3hr projected concentration (calculate though the linear regression coefficient) for each bag. The NOx concentration published represent the initial NOx concentration in each bag. The detailed description about dataset usage please see README.txt. 

Funding

California Air Resources Board, Award: 19RD012

Coordinated Research Council, Award: A-121