Inferring Ozone Production Regimes over the Continental United States

Policy created to regulate surface ozone relies heavily on Air Quality Models (AQMs) as important guiding tools, and comparison with observations is crucial to validating the model's ability to represent ozone chemistry. 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. We compare Ozone Production Regimes (OPRs) identified from satellite observations and model simulations, as defined by 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 spans diurnal, rural-urban and altitudinal variations of OPRs, offering important insights for effective policy formulation.

At the time of the OMI overpass (~1:30 PM LT), we find that ozone chemistry is NOx-limited in most of the regions of CONUS, as determined from OMI column ratios. Analysis of CAMx column ratios shows similar results. In contrast, more regions, particularly urban areas, are VOC-limited when we constrain our ratio calculation to CAMx results within the planetary boundary layer (PBL). In the morning (~10 AM LT), the CAMx PBL column ratios shift even more towards the VOC-limited regime. We highlight areas of CONUS for which afternoon satellite measurements of HCHO/NO₂ are not a particularly accurate indicator of near-surface OPRs, and we highlight how ozone chemistry may change throughout the day. 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 air quality data.