Using Satellite Observations to Diagnose the Nonlinear Ozone-NOx-VOC Chemistry over Global Urban Areas

Ozone (O₃) near the ground is an air pollutant harmful to human health and ecosystems. It is well established that O₃ formation throughout much of the troposphere is largely controlled by the availability of NO_x, but in urban areas with high NO_x emissions, O₃ formation can be VOC-limited or in transition or aerosol inhibited. Satellite instruments have provided continuous global observations of O₃ precursors, namely NO₂ and formaldehyde (HCHO, a proxy for VOCs), for nearly three decades. Satellite-observed ratio of HCHO to NO₂ reflects the relative availability of reactivity-weighted VOCs and NO_x, which has been proposed as an indicator of O₃ sensitivity, but uncertainties remain as to whether satellite HCHO is indicative of VOCs, and how aerosols affect the radical loss pathway and thus classification of O₃ production regimes. Here we present our recent work on building a space-based global O₃ production regime indicator over global urban areas. We couple a global chemical transport model with a steady-state model to derive the regime threshold values and evaluate the uncertainties of the regime threshold values due to uncertain column-to-surface relationships of NO₂ and HCHO, varying relationship between HCHO and organic reactivity, varying chemical environments and potential aerosol influence. Next, we apply the derived regime threshold to multi-instrument (GOME, SCIAMACHY, OMI, TROPOMI) satellite observations of NO₂ and HCHO, from which we identify the O₃ production regimes over urban areas and their spatial and temporal variations. Our results provide insights into the O₃ production regimes over global cities, including regions with sparse monitors, which could guide emission control strategies.