Inferring Ozone Production Regimes over the continental US

Surface ozone regulation policies depend heavily on Air Quality Models (AQMs) as vital tools, requiring validation through comparisons with observations. Identifying the factors influencing surface ozone formation is complicated because ozone photochemical production rates are non-linearly dependent on precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), with the best pollution reduction strategy usually dependent on targeting the precursor that exerts greatest control on surface ozone. Our study compares Ozone Production Regimes (OPRs) identified from both satellite observations and model simulations, using the ratio of tropospheric column formaldehyde to nitrogen dioxide (HCHO/NO2). We perform AQM simulations using CAMx for June-July-August 2016 over the Continental US (CONUS). We compare the model outputs against two versions of OMI NO2 data (KNMI's DOMINO QA4EVC and NASA-GSFC's MINDS retrievals) and two HCHO products (BIRA and NASA GSFC retrievals). Our analysis considers diurnal, rural-urban and altitudinal variations of OPRs, offering important insights for effective policy formulation. At the time of OMI overpass (~1:30 PM LT), ozone chemistry in most CONUS regions is NOx-limited, as indicated by HCHO/NO2 ratios of OMI and CAMx columns. Analysis of CAMx columns indicates that the HCHO/NO2 ratio varies with time of day and altitude with less NOx-limited conditions in the morning (10 AM LT) and in the planetary boundary layer (PBL). We highlight areas of the CONUS for which afternoon satellite measurements of HCHO/NO2 are not an accurate indicator of near-surface OPRs. Successful air quality regulations based on satellite observations targeting urban regions should consider the diurnal variations of surface OPRs and how well the satellites represent near-surface chemistry. Our results have implications for interpretation of TEMPO data, which will be available at nominally an hourly temporal resolution.