Inferring near-surface ozone production regimes: Insights from using satellite retrievals over the contiguous US

Ozone production regime Smog Formaldehyde to NO2 ratio CAMx CONUS QA4ECV OMNO2 OMHCHO OMI PBL Policy Air quality model Surface ozone regulation TEMPO Surface ozone regulation policies rely heavily on air quality models, such as CAMx, for guidance. Comparison with observations is crucial to evaluating a model’s ability to represent ozone production chemistry. Identifying factors influencing surface ozone formation is complicated because ozone photochemical production rates are non-linearly dependent on concentrations of precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). We compare ozone production regimes (OPRs) identified from satellite observations and model simulations, as defined by the ratio of column formaldehyde to nitrogen dioxide (FNR, HCHO/NO2). We performed CAMx simulations for summer 2016 over the Contiguous United States (CONUS) and compared output against two OMI NO2 and HCHO retrievals. Our analysis spans diurnal and altitudinal variations of OPRs, offering important insights for effective policy formulation. At the time of the OMI overpass oz(~1:30 p.m. LT), OPR is NOx-limited over most of the CONUS, as determined from OMI column ratios. Analysis of CAMx column ratios shows similar results. In contrast, more regions are VOC-limited when we constrain our ratio to within the Planetary Boundary Layer (PBL). In the morning (~9 a.m. LT), the CAMx PBL column ratios shift towards VOClimited regime compared to the afternoon. We highlight areas of the CONUS where satellite measurements of FNR may not be an accurate indicator of near-surface OPRs. Air quality regulations based on satellite obser­ vations should consider the diurnal variations of surface OPRs and assess how well their ratios represent nearsurface OPR. Our results have implications for interpretation of TEMPO data for policy relevant applications.