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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.

Background: Air pollution health studies have been increasingly using prediction models for exposure assessment even in areas without monitoring stations. To date, most studies have assumed that a single exposure model is correct, but estimated effects may be sensitive to the choice of exposure model., Methods: We obtained county-level daily cardiovascular (CVD) admissions from the New York (NY) Statewide Planning and Resources Cooperative System (SPARCS) and four sets of fine particulate matter (PM 2.5 ) spatio-temporal predictions (2002-2012). We employed overdispersed Poisson models to investigate the relationship between daily PM 2.5 and CVD, adjusting for potential confounders, separately for each state-wide PM 2.5 dataset., Results: For all PM 2.5 datasets, we observed positive associations between PM 2.5 and CVD. Across the modeled exposure estimates, effect estimates ranged from 0.23% (95%CI: -0.06, 0.53%) to 0.88% (95%CI: 0.68, 1.08%) per 10 µg/m 3 increase in daily PM 2.5 . We observed the highest estimates using monitored concentrations 0.96% (95%CI: 0.62, 1.30%) for the subset of counties where these data were available., Conclusions: Effect estimates varied by a factor of almost four across methods to model exposures, likely due to varying degrees of exposure measurement error. Nonetheless, we observed a consistently harmful association between PM 2.5 and CVD admissions, regardless of model choice., (© 2021. The Author(s).)

To what extent do the short-term negative externalities of fossil fuel use traverse national borders? Transnational negative externalities are thought to motivate international environmental cooperation, but we often lack detailed data on their occurrence. Using a Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), we offer global estimates of the extent of transboundary air pollution from coal-fired power generation. In an advance of the existing literature, we attribute the air pollution experienced in different locales to specific coal-fired power plants, allowing us to evaluate the extent to which pollution from the coal industry is experienced across different jurisdictions. Our results indicate that the issue is most severe in South Asia and East Asia. When weighting by the population of "receiving" locations, India is found to be the largest emitter of transboundary air pollution, followed by China. Residents of Bangladesh are found to experience the most transboundary air pollution by a wide margin., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

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.

Prior work suggests drought exacerbates US air quality by increasing surface ozone concentrations. We analyze 2005–2015 tropospheric column concentrations of two trace gases that serve as proxies for surface ozone precursors retrieved from the OMI/Aura satellite: Nitrogen dioxide (ΩNO2; NOx proxy) and formaldehyde (ΩHCHO; VOC proxy). We find 3.5% and 7.7% summer drought enhancements (classified by SPEI) for ΩNO2 and ΩHCHO, respectively, corroborating signals previously extracted from ground‐level observations. When we subset by land cover type, the strongest ΩHCHO drought enhancement (10%) occurs in the woody savannas of the Southeast US. By isolating the influences of precipitation and temperature, we infer that enhanced biogenic VOC emissions in this region increase ΩHCHO independently with both high temperature and low precipitation during drought. The strongest ΩNO2 drought enhancement (6.0%) occurs over Midwest US croplands and grasslands, which we infer to reflect the sensitivity of soil NOx emissions to temperature. Plain Language Summary: Projected increases in drought severity and frequency for this century raise questions regarding possible impacts on air quality. Surface ozone, an air pollutant estimated to cause over 1 million annual premature deaths globally, forms when its precursor gases react in the sunlit atmosphere. These precursor gases depend on temperature and precipitation and thus can respond to drought. We analyze over a decade of satellite observations of two trace gases relevant to ozone formation and find that, on average, their concentrations increase during summer droughts in the Eastern US. While higher temperatures during droughts are usually associated with observed increases in trace gas concentrations, in the Southeast US we find increases associated with low precipitation independent of temperature. Satellite detection of these changes implies promise for application to other regions and more generally for improving mechanistic understanding of air quality responses to drought and other climate extremes. Key Points: Satellite retrievals of tropospheric NO2 and HCHO show drought enhancements of 3.5% and 7.7%, respectively, during Eastern US summersLow precipitation and high temperatures both independently drive HCHO drought enhancement (10%) in Southeast US woody savannasHigh temperatures drive NO2 drought enhancement (6.0%) in Midwest US croplands and grasslands [ABSTRACT FROM AUTHOR]