Direct measurements of ozone response to emissions perturbations in California

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.