Observing the chemistry of ground-level O3 from space

Abstract:

Ambient exposure to ground-level ozone (O₃) is identified as a leading risk factor for the global disease burden. O₃ production over urban areas is non-linearly dependent on the availability of its precursors: nitrogen oxides (NOx) and volatile organic compounds (VOCs). A major challenge in lowering ground-level O₃ in urban areas is to determine the local chemical conditions in urban areas that control O₃ production (NOx-limited or VOC-limited). Satellite remote sensing, which fills a spatial gap in ground-based networks, is playing an increasingly important role in atmospheric chemistry. We identify chemical regimes in which ground-level O₃ forms using two-decade (1996 - 2016) observations of O₃ related species (NO₂ and HCHO) from three satellites (GOME, SCIAMACHY and OMI). Multi-satellite HCHO/NO₂ captures the timing and location of the transition from VOC-limited to NO_x-limited O₃ production regimes in major U.S. cities. Our findings suggest promise for applying space-based HCHO/NO₂ to interpret local O₃ chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution. As an example, we use TROPOMI HCHO/NO₂ to identify changes in O₃ chemistry over Los Angeles during the COVID-19 lockdown. We find that the emission reductions caused by regional lockdown lead to large changes of O₃ sensitivity to precursor emissions, which aligns with the changes of O₃ weekend effect observed from ground-based monitors.