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The frequency of wildfires in the western United States has escalated in recent decades. Here we examine the impacts of wildfires on ground-level ozone (O 3 ) precursors and the O 3 -NO x -VOC chemistry from the source to downwind urban areas. We use satellite retrievals of nitrogen dioxide (NO 2 ) and formaldehyde (HCHO, an indicator of VOC) from the Tropospheric Monitoring Instrument (TROPOMI) to track the evolution of O 3 precursors from wildfires over California from 2018 to 2020. We improved these satellite retrievals by updating the a priori profiles and explicitly accounting for the effects of smoke aerosols. TROPOMI observations reveal that the extensive and intense fire smoke in 2020 led to an overall increase in statewide annual average HCHO and NO 2 columns by 16% and 9%. The increase in the level of NO 2 offsets the anthropogenic NO x emission reduction from the COVID-19 lockdown. The enhancement of NO 2 within fire plumes is concentrated near the regions actively burning, whereas the enhancement of HCHO is far-reaching, extending from the source regions to urban areas downwind due to the secondary production of HCHO from longer-lived VOCs such as ethene. Consequently, a larger increase in NO x occurs in NO x -limited source regions, while a greater increase in HCHO occurs in VOC-limited urban areas, both contributing to more efficient O 3 production.

Understanding the local-scale spatial and temporal variability of ozone formation is crucial for effective mitigation. We combine tropospheric vertical column densities (VCD Trop ) of formaldehyde (HCHO) and nitrogen dioxide (NO 2 ), referred to as HCHO-VCD Trop and NO 2 -VCD Trop , retrieved from airborne remote sensing and the TROPOspheric Monitoring Instrument (TROPOMI) with ground-based measurements to investigate changes in ozone precursors and the inferred chemical production regime on high-ozone days in May-August 2018 over two Northeast urban domains. Over New York City (NYC) and Baltimore/Washington D.C. (BAL/DC), HCHO-VCD Trop increases across the domain, but higher NO 2 -VCD Trop occurs mainly in urban centers on ozone exceedance days (when maximum daily 8 h average (MDA8) ozone exceeds 70 ppb at any monitor in the region). The ratio of HCHO-VCD Trop to NO 2 -VCD Trop , proposed as an indicator of the sensitivity of local surface ozone production rates to its precursors, generally increases on ozone exceedance days, implying a transition toward a more NO x -sensitive ozone production regime that should lead to higher efficacy of NO x controls on the highest ozone days in NYC and BAL/DC. Warmer temperatures and enhanced influence from emissions in the local boundary layer on the high-ozone days are accompanied by slower wind speeds in BAL/DC but stronger, southwesterly winds in NYC.

In China, emissions of ozone (O 3 )-producing pollutants have been targeted for mitigation to reduce O 3 pollution. However, the observed O 3 decrease is slower than/opposite to expectations affecting the health of millions of people. For a better understanding of this failure and its connection with anthropogenic emissions, we quantify the summer O 3 trends that would have occurred had the weather stayed constant by applying a numerical tool that "de-weathers" observations across 31 urban regions (123 cities and 392 sites) over 8 years. O 3 trends are significant ( p < 0.05) over 234 sites after de-weathering, contrary to the directly observed trends (only 39 significant due to high meteorology-induced variability). The de-weathered data allow categorizing cities in China into four different groups regarding O 3 mitigation, with group 1 exhibiting steady O 3 reductions, while group 4 showing significant ( p < 0.05) O 3 increases. Analysis of the relationships between de-weathered odd oxygen and nitrogen oxides illustrates how the changes in NO x , in anthropogenic volatile organic compounds (VOCs), and reductions in fine particulate matter (PM 2.5 ) affect the O 3 trends differently in these groups. While this analysis suggests that VOC reductions are the main driver of O 3 decreases in group 1, groups 3 and 4 are primarily affected by decreasing PM 2.5 , which results in enhanced O 3 formation. Our analysis demonstrates both the importance of and possibility for isolating emission-driven changes from climate and weather for interpreting short-term air quality observations.

Urban ozone (O 3 ) formation can be limited by NO x , VOCs, or both, complicating the design of effective O 3 abatement plans. A satellite-retrieved ratio of formaldehyde to NO 2 (HCHO/NO 2 ), developed from theory and modeling, has previously been used to indicate O 3 formation chemistry. Here, we connect this space-based indicator to spatiotemporal variations in O 3 recorded by on-the-ground monitors over major U.S. cities. High-O 3 events vary nonlinearly with OMI HCHO and NO 2 , and the transition from VOC-limited to NO x -limited O 3 formation regimes occurs at higher HCHO/NO 2 value (3 to 4) than previously determined from models, with slight intercity variations. To extend satellite records back to 1996, we develop an approach to harmonize observations from GOME and SCIAMACHY that accounts for differences in spatial resolution and overpass time. Two-decade (1996-2016) multisatellite HCHO/NO 2 captures the timing and location of the transition from VOC-limited to NO x -limited O 3 production regimes in major U.S. cities, which aligns with the observed long-term changes in urban-rural gradient of O 3 and the reversal of O 3 weekend effect. Our findings suggest promise for applying space-based HCHO/NO 2 to interpret local O 3 chemistry, particularly with the new-generation satellite instruments that offer finer spatial and temporal resolution.