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The summertime surface ozone (O3) concentrations over Southeast Michigan (SEMI) often exceed 70 ppbv. However, the associated O3 formation regime is still not well known. In this study, we examined the chemical drivers of O3 exceedances in SEMI, based on the Michigan-Ontario Ozone Source Experiment (MOOSE) field campaign during the period of May 20 – June 30, 2021. We employed a zero-dimensional (0-D) box model, which was constrained by measurements of meteorology and trace gas concentrations during MOOSE. Our model simulations demonstrated that the formaldehyde to nitrogen dioxide ratio (HCHO/NO2) for the transition between the VOC- and NOx-limited O3 production regimes was 3.0 ± 0.3 (mean ± 1σ) in SEMI. The midday (12:00-16:00) averaged HCHO/NO2 ratio during MOOSE was 1.62 ± 1.03, suggesting that O3 production in SEMI was likely limited by VOC emissions. Our study has significant implications for air quality policy and the design of effective O3 pollution control strategies through ground-based HCHO/NO2 measurements and model simulations.
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The long-term trend and production sensitivity change in the US ozone pollution from observations and model simulations
Abstract. We investigated the ozone pollution trend and its sensitivity to keyprecursors from 1990 to 2015 in the United States using long-term EPA Air Quality System (AQS)observations and mesoscale simulations. The modeling system, a coupledregional climate–air quality model (CWRF-CMAQ; Climate-Weather Research Forecast andthe Community Multiscale Air Quality), captured well the summersurface ozone pollution during the past decades, having a mean slope oflinear regression with AQS observations of ∼0.75. While theAQS network has limited spatial coverage and measures only a few keychemical species, CWRF-CMAQ provides comprehensive simulations to enablea more rigorous study of the change in ozone pollution and chemicalsensitivity. Analysis of seasonal variations and diurnal cycle of ozoneobservations showed that peak ozone concentrations in the summer afternoondecreased ubiquitously across the United States, up to 0.5 ppbv yr−1 in majornon-attainment areas such as Los Angeles, while concentrations at certainhours such as the early morning and late afternoon increased slightly.Consistent with the AQS observations, CMAQ simulated a similar decreasingtrend of peak ozone concentrations in the afternoon, up to 0.4 ppbv yr−1, andincreasing ozone trends in the early morning and late afternoon. A monotonicallydecreasing 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 lateafternoon was likely caused by reduced NO–O3 titration, driven bycontinuous anthropogenic NOx emission reductions in the past decades.Furthermore, the CMAQ simulations revealed a shift in chemical regimes ofozone photochemical production. From 1990 to 2015, surface ozone productionin some metropolitan areas, such as Baltimore, has transited from aVOC-sensitive environment (>50 % probability) to aNOx-sensitive regime. Our results demonstrated that the long-termCWRF-CMAQ simulations can provide detailed information of the ozonechemistry evolution under a changing climate and may partially explain theUS ozone pollution responses to regional and national regulations.
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- Award ID(s):
- 1639327
- PAR ID:
- 10157798
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 20
- Issue:
- 5
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 3191 to 3208
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-20-3191-2020
