Abstract. We investigated the ozone pollution trend and its sensitivity to key
precursors from 1990 to 2015 in the United States using long-term EPA Air Quality System (AQS)
observations and mesoscale simulations. The modeling system, a coupled
regional climate–air quality model (CWRF-CMAQ; Climate-Weather Research Forecast and
the Community Multiscale Air Quality), captured well the summer
surface ozone pollution during the past decades, having a mean slope of
linear regression with AQS observations of ∼0.75. While the
AQS network has limited spatial coverage and measures only a few key
chemical species, CWRF-CMAQ provides comprehensive simulations to enable
a more rigorous study of the change in ozone pollution and chemical
sensitivity. Analysis of seasonal variations and diurnal cycle of ozone
observations showed that peak ozone concentrations in the summer afternoon
decreased ubiquitously across the United States, up to 0.5 ppbv yr−1 in major
non-attainment areas such as Los Angeles, while concentrations at certain
hours such as the early morning and late afternoon increased slightly.
Consistent with the AQS observations, CMAQ simulated a similar decreasing
trend of peak ozone concentrations in the afternoon, up to 0.4 ppbv yr−1, and
increasing ozone trends in the early morning and late afternoon. A monotonically
decreasing trend (up to 0.5 ppbv yr−1) in the odd oxygen (Ox=O3+NO2) concentrations are simulated by CMAQ at all daytime hours.
This result suggests that the increased ozone in the early morning and late
afternoon was likely caused by reduced NO–O3 titration, driven by
continuous anthropogenic NOx emission reductions in the past decades.
Furthermore, the CMAQ simulations revealed a shift in chemical regimes of
ozone photochemical production. From 1990 to 2015, surface ozone production
in some metropolitan areas, such as Baltimore, has transited from a
VOC-sensitive environment (>50 % probability) to a
NOx-sensitive regime. Our results demonstrated that the long-term
CWRF-CMAQ simulations can provide detailed information of the ozone
chemistry evolution under a changing climate and may partially explain the
US ozone pollution responses to regional and national regulations.
Two centuries of settlement and urban development in the United States
