DOI: 10.25338/b85p9b
title: Direct measurements of ozone response to emissions perturbations in California

description: 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 &gt; 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.
description: O<sub>3</sub> sensitivities to precursor
 NO<sub>x</sub>/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
 O<sub>3</sub> formation rates in the presence of variable
 atmospheric mixing and regional emissions. Sensitivities were based on
 perturbation concentrations of approximately 8 ppb of
 NO<sub>x</sub> 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 O<sub>3</sub>
 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
 O<sub>3</sub> 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. O<sub>3</sub>
 concentration after 3-hour UV exposure. The difference of
 O<sub>3</sub> concentration after 3-hour UV exposure was
 calculated between chamber #1 to chamber #2, and chamber #3 to chamber #2
 to quantify the O<sub>3</sub> sensitivity. 
description: Published dataset includes the chamber O<sub>3</sub>
 and NO<sub>x</sub> concentration from daily chamber
 measurement. The O<sub>3</sub> 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 NO<sub>x</sub> concentration published represent
 the initial NO<sub>x</sub> concentration in each bag. The
 detailed description about dataset usage please see README.txt.