Sally E. Pusede

The impact of emissions on atmospheric chemistry requires a detailed chemical understanding of their transformation and fate. However, there are many sources of potential uncertainty in modeling the life cycle and transformation of biogenic volatile organic compounds (BVOC). This study investigates the sensitivity of a 0-D box model to several sources of potential uncertainty at a southeastern U.S. forest site over the summer season, including mixing ratios of critical trace oxidants (O3 and NO2), planetary boundary layer height (PBLH), and the factors that control emission rates. Within this framework, our results indicate that the model is relatively insensitive to the tested O3 input. Meanwhile NO2 input, evaluated here as a bounding comparison between rural and urban conditions, produces a substantial effect on modeled BVOC mixing ratios. Differences among currently available PBLH data sets emerged as a major source of overall uncertainty of comparable or greater magnitude than the NO2 bounding test. Daytime and nighttime model-observation correlation and bias varied considerably depending on PBLH data set and compound, with the largest discrepancies and lowest correlations occurring at night, highlighting particular challenges in modeling nocturnal BVOC mixing ratios. Compound-specific biases also varied substantially: isoprene and α-pinene, the two best-studied compounds, showed moderate bias across most scenarios, while sabinene showed substantially larger, consistent underprediction, suggesting poorly constrained emission factors for this compound specifically. These results highlight the sensitivities of this modeling framework to PBLH, oxidant mixing ratios, and emission factor uncertainty, and underscore the value of observationally constraining PBLH and isomer-resolved BVOC mixing ratios to improve current biogenic emissions modeling.

Carbon monoxide (CO) and nitrogen dioxide (NO₂) are combustion pollutants that disproportionately harm communities of color in US cities. While distributive inequalities are established, we have lacked observational constraints on variability in air pollution mixtures and the sources that drive exposure disparities. We present the first application of the TROPOspheric Monitoring Instrument (TROPOMI) to describe census tract-scale CO inequalities and population-weighted NO₂/CO enhancement ratios (ΔXNO₂/ΔXCO) in six major US urbanized areas (UAs). Because CO has a long atmospheric lifetime, we develop an approach to describing CO inequalities using column-averaged dry-air mole fractions normalized by the daily urban variation (XCO_norm). Annual daily mean inequalities in XCO_norm are significant in some UAs, e.g., Los Angeles, Phoenix, New York City, and Chicago, where they are 7–11% for Black, 7–15% for Latino, and 1–11% for Asian residents compared to non-Hispanic/Latino white residents; however, in Houston and Atlanta, inequalities in XCO_norm are near or indistinguishable from zero. We report direct satellite evidence of cumulative exposure burdens as CO and NO₂ disparities are correlated across cities (r = 0.89). We compare inequalities on weekdays and weekends and with conditions that differently affect short- and long-lived, primary and secondary species, with evidence suggesting that CO inequalities are influenced not only by gasoline vehicles but also chemical production. TROPOMI ΔXNO₂/ΔXCO indicate that combustion mixtures in primarily Black, Latino, and Asian neighborhoods are disproportionately influenced by heavy-duty diesel vehicles (HDDVs), even on weekends. To address exposure disparities, efforts should focus on differences in source composition not just magnitude. Spatial discrepancies between ΔXNO₂/ΔXCO and the 2020 National Emissions Inventory (NEI20) suggest the NEI20, at least without downscaling, underpredicts the extent to which Black, Latino, and Asian neighborhoods are affected by high NO₂/CO sources, e.g., HDDVs. Such errors in the NEI20 potentially direct attention and resources away from HDDV controls with environmental justice benefits.

In Los Angeles, air pollution disproportionately impacts communities of color and low-income residents. Routine city-wide measurements of hazardous air pollutants (HAPs), of concern for health and contributing to urban air pollution, are notably lacking. In this study, we use the highest spatially resolved (∼2 km) measurements of emissions and concentrations ever reported of HAPs while covering a whole megacity and combine observations with US Census information. We observe higher concentrations and emissions of 17 measured HAPs, such as benzene, naphthalene, and p-chlorobenzotrifluoride (PCBTF), in California-designated Disadvantaged Communities (DACs) and census tracts with low-income Hispanics and Asians. These groups share an unequal burden from traffic-related emissions, with benzene, nitrogen oxides (NO_x), and carbon monoxide (CO) concentrations up to 60% higher. However, in DACs and census tracts with large Hispanic populations (>50%), we observe toluene-to-benzene emission ratios above 3, pointing to inequalities in other HAPs primarily caused by non-traffic emission sources such as industry and solvents. In these communities, regulatory inventories also significantly underestimate emissions. We find that efforts to address HAP inequalities and environmental justice concerns in Los Angeles will need to consider contributions from volatile chemical products, which represent a growing source of emissions driving inequalities in impacted communities.

Industrialized swine facilities adversely affect the health and well-being of Eastern North Carolina residents in the U.S. and are an issue of environmental racism. Concentrated animal feeding operations (CAFOs) emit various harmful and noxious air pollutants, including ammonia (NH₃). There are limited measurements of CAFO-related air quality, contributing to disputes around its severity. We use NH₃ vertical column densities from the space-based Infrared Atmospheric Sounding Interferometer (IASI) to report systematic, distributive inequalities in NH₃ column enhancements (ΔNH₃ columns), equal to NH₃ columns less an observationally determined tropospheric background. Population-weighted block group-scale ΔNH₃ columns are higher by 27 ± 3% for Black and African Americans, 35 ± 3% for Hispanics and Latinos, and 49 ± 3% for American Indians compared to non-Hispanic/Latino whites in Eastern North Carolina (April-August 2016-2021). Surface winds and air temperature influence block group-scale NH₃ distributions, with higher absolute NH₃ inequalities for all groups on calm days and for Black and African Americans and Hispanics and Latinos on hot days, consistent with effects from NH₃ volatization downfield of facilities from, e.g., manure-covered fields, particles, and other surfaces. ΔNH₃ columns correspond spatially with permitted swine facilities, with residents living multiple kilometers from swine CAFOs chronically exposed to elevated NH₃. Trends in NH₃ columns over 2008-2023 are driven by regional-scale atmospheric processes rather than localized NH₃ changes in CAFO emissions. Results are discussed in local decision-making contexts that have broad relevance for air quality issues without protective federal regulatory standards.

In Houston, Texas, nitrogen dioxide (NO₂) air pollution disproportionately affects Black, Latinx, and Asian communities, and high ozone (O₃) days are frequent. There is limited knowledge of how NO₂ inequalities vary in urban air quality contexts, in part from the lack of time-varying neighborhood-level NO₂ measurements. First, we demonstrate that daily TROPOspheric Monitoring Instrument (TROPOMI) NO₂ tropospheric vertical column densities (TVCDs) resolve a major portion of census tract-scale NO₂ inequalities in Houston, comparing NO₂ inequalities based on TROPOMI TVCDs and spatiotemporally coincident airborne remote sensing (250 m × 560 m) from the NASA TRacking Aerosol Convection ExpeRiment-Air Quality (TRACER-AQ). We further evaluate the application of daily TROPOMI TVCDs to census tract-scale NO₂ inequalities (May 2018-November 2022). This includes explaining differences between mean daily NO₂ inequalities and those based on TVCDs oversampled to 0.01° × 0.01° and showing daily NO₂ column-surface relationships weaken as a function of observation separation distance. Second, census tract-scale NO₂ inequalities, city-wide high O₃, and mesoscale airflows are found to covary using principal component and cluster analysis. A generalized additive model of O₃ mixing ratios versus NO₂ inequalities reproduces established nonlinear relationships between O₃ production and NO₂ concentrations, providing observational evidence that neighborhood-level NO₂ inequalities and O₃ are coupled. Consequently, emissions controls specifically in Black, Latinx, and Asian communities will have co-benefits, reducing both NO₂ disparities and high O₃ days city wide.

Urban air pollution disproportionately harms communities of color and low-income communities in the U.S. Intraurban nitrogen dioxide (NO₂) inequalities can be observed from space using the TROPOspheric Monitoring Instrument (TROPOMI). Past research has relied on time-averaged measurements, limiting our understanding of how neighborhood-level NO₂ inequalities co-vary with urban air quality and climate. Here, we use fine-scale (250 m × 250 m) airborne NO₂ remote sensing to demonstrate that daily TROPOMI observations resolve a major portion of census tract-scale NO₂ inequalities in the New York City-Newark urbanized area. Spatiotemporally coincident TROPOMI and airborne inequalities are well correlated (r = 0.82-0.97), with slopes of 0.82-1.05 for relative and 0.76-0.96 for absolute inequalities for different groups. We calculate daily TROPOMI NO₂ inequalities over May 2018-September 2021, reporting disparities of 25-38% with race, ethnicity, and/or household income. Mean daily inequalities agree with results based on TROPOMI measurements oversampled to 0.01° × 0.01° to within associated uncertainties. Individual and mean daily TROPOMI NO₂ inequalities are largely insensitive to pixel size, at least when pixels are smaller than ∼60 km², but are sensitive to low observational coverage. We statistically analyze daily NO₂ inequalities, presenting empirical evidence of the systematic overburdening of communities of color and low-income neighborhoods with polluting sources, regulatory ozone co-benefits, and worsened NO₂ inequalities and cumulative NO₂ and urban heat burdens with climate change.

Reductions in anthropogenic emissions have drawn increasing attention to the role of the biosphere in O₃ production chemistry in U.S. cities. We report the results of chemical transport model sensitivity simulations exploring the relative impacts of biogenic isoprene and soil nitrogen oxides (NOx) emissions on O₃ and its temporal variability. We compare scenarios with high and low anthropogenic NOx emissions representing the reductions that have occurred in recent decades. As expected, summertime O₃ concentrations become less sensitive to perturbations in biogenic isoprene emissions as anthropogenic NOx emissions decline. However, we demonstrate for the first time that across policy relevant O₃ nonattainment areas of the United States, O₃ becomes more sensitive to perturbations in soil NOx emissions than to identical perturbations in isoprene emissions. We show that interannual variability in soil NOx emissions may now have larger impacts on interannual O₃ variability than isoprene emissions in many areas where the latter would have dominated in the recent past.

Air pollution disproportionately burdens communities of color and lower‐income communities in US cities. We have generally lacked city‐wide concentration measurements that resolve the steep spatiotemporal gradients of primary pollutants required to describe intra‐urban air pollution inequality. Here, we use observations from the recently launched TROPospheric Ozone Monitoring Instrument (TROPOMI) satellite sensor and physics‐based oversampling to describe nitrogen dioxide (NO₂) disparities with race, ethnicity, and income in 52 US cities (June 2018–February 2020). We report average US‐urban census tract‐level NO₂ inequalities of 28 ± 2% (race‐ethnicity and income combined), with many populous cities experiencing even greater inequalities. Using observations and inventories, we find diesel traffic is the dominant source of NO₂ disparities, and that a 62% reduction in diesel emissions would decrease race‐ethnicity and income inequalities by 37%. We add evidence that TROPOMI resolves tract‐scale NO₂ differences using relationships with urban segregation patterns and spatial variability in column‐to‐surface correlations.