Houston, Texas is a major U.S. urban and industrial area where poor air quality is unevenly distributed and a disproportionate share is located in low-income, non-white, and Hispanic neighborhoods. We have traditionally lacked city-wide observations to fully describe these spatial heterogeneities in Houston and in cities globally, especially for reactive gases like nitrogen dioxide (NO 2 ). Here, we analyze novel high-spatial-resolution (250 m × 500 m) NO 2 vertical columns measured by the NASA GCAS airborne spectrometer as part of the September-2013 NASA DISCOVER-AQ mission and discuss differences in population-weighted NO 2 at the census-tract level. Based on the average of 35 repeated flight circuits, we find 37 ± 6% higher NO 2 for non-whites and Hispanics living in low-income tracts (LIN) compared to whites living in highincome tracts (HIW) and report NO 2 disparities separately by race ethnicity (11−32%) and poverty status (15−28%). We observe substantial time-of-day and day-to-day variability in LIN-HIW NO 2 differences (and in other metrics) driven by the greater prevalence of NO x (NO + NO 2 ) emission sources in low-income, non-white, and Hispanic neighborhoods. We evaluate measurements from the recently launched satellite sensor TROPOMI (3.5 km × 7 km at nadir), averaged to 0.01°× 0.01°using physics-based oversampling, and demonstrate that TROPOMI resolves similar relative, but not absolute, tract-level differences compared to GCAS. We utilize the high-resolution FIVE and NEI NO x inventories, plus one year of TROPOMI weekday−weekend variability, to attribute tract-level NO 2 disparities to industrial sources and heavy-duty diesel trucking. We show that GCAS and TROPOMI spatial patterns correspond to the surface patterns measured using aircraft profiling and surface monitors. We discuss opportunities for satellite remote sensing to inform decision making in cities generally.

In US cities, the concentrations of many air pollutants have been observed, modeled, and inferred to be higher in neighborhoods where residents are primarily people of color and have lower household incomes (e.g.,

Drought conditions affect ozone air quality, potentially altering multiple terms in the O 3 mass balance equation. Here, we present a multiyear observational analysis using data collected before, during, and after the record-breaking California drought (2011−2015) at the O 3 -polluted locations of Fresno and Bakersfield near the Sierra Nevada foothills. We separately assess drought influences on O 3 chemical production (PO 3 ) from O 3 concentration. We show that isoprene concentrations, which are a source of O 3 -forming organic reactivity, were relatively insensitive to early drought conditions but decreased by more than 50% during the most severe drought years (2014−2015), with recovery a function of location. We find drought− isoprene effects are temperature-dependent, even after accounting for changes in leaf area, consistent with laboratory studies but not previously observed at landscape scales with atmospheric observations. Drought-driven decreases in organic reactivity are contemporaneous with a change in dominant oxidation mechanism, with PO 3 becoming more NO x -suppressed, leading to a decrease in PO 3 of ∼20%. We infer reductions in atmospheric O 3 loss of ∼15% during the most severe drought period, consistent with past observations of decreases in O 3 uptake by plants. We consider drought-related trends in O 3 variability on synoptic time scales by analyzing statistics of multiday high-O 3 events. We discuss implications for regulating O 3 air pollution in California and other locations under more prevalent drought conditions.

Urban air pollution disproportionately harms communities of color and low-income communities in the U.S. Intraurban nitrogen dioxide (NO 2 ) 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 NO 2 inequalities co-vary with urban air quality and climate. Here, we use fine-scale (250 m × 250 m) airborne NO 2 remote sensing to demonstrate that daily TROPOMI observations resolve a major portion of census tract-scale NO 2 inequalities in the New York City−Newark urbanized area. Spatiotemporally coincident TRO-POMI 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 NO 2 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 NO 2 inequalities are largely insensitive to pixel size, at least when pixels are smaller than ∼60 km 2 , but are sensitive to low observational coverage. We statistically analyze daily NO 2 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 NO 2 inequalities and cumulative NO 2 and urban heat burdens with climate change.