Observing Nitrogen Dioxide Air Pollution Inequality Using High-Spatial-Resolution Remote Sensing Measurements in Houston, Texas

Abstract

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.

COVID-19 pandemic reveals persistent disparities in nitrogen dioxide pollution

“…For many cities, there were no significant changes in NO2 disparities during the lockdowns, and marginalized communities faced higher NO2 levels during the lockdowns than nonmarginalized communities experienced prior to the lockdowns. Our findings build on Demetillo et al (4), who similarly used TROPOMI to understand environmental justice in Houston and inform drivers of inequality, and are consistent with contemporaneous studies that have analyzed long-term trends in NO2 and other air pollutants and found that, despite widespread decreases in pollution, the most exposed demographic subgroups in the 1980s and 1990s remain the most exposed in the present day (8,9).…”

“…The use of only spring 2019 for our baseline period stems from the short data record offered by the Tropospheric Monitoring Instrument (TROPOMI), and the slight increases in NO2 in parts of the Great Plains and Midwest during lockdowns (< 0.5 × 10 15 molecules per square centimeter) could reflect differences in natural (e.g., soil, lightning, stratospheric NOx ) or anthropogenic sources of NO2 between the baseline and lockdown periods. Demetillo et al (4) found that TROPOMI is capable of resolving NO2 differences between census tracts in the Houston area, and our nationwide comparison of TROPOMI NO2 with surface-level observations reveals TROPOMI's utility as a tool to understand NO2 variability (SI Appendix, Text S1 and Fig. S1).…”

“…Specifically, we determine density as the number of primary road segments within 1 km of a tract's centroid. We choose 1 km as our threshold for "nearby," as NO 2 concentrations decrease up to ∼ 50% within 0.5 km to 2 km from major roadways (4,53). Other means of quantifying traffic exist (e.g., length of roadway within a specified distance, traffic within buffer zones, sum of distance traveled) ( 59), but our approach allows for consistent use of geospatial data from the US Census Bureau.…”

“…A dverse air quality is an environmental justice issue, as it disproportionately affects marginalized and disenfranchised populations around the world (1)(2)(3)(4). Growing evidence suggests that these populations experience more air pollution than is caused by their consumption (5)(6)(7).…”

“…NO2 is also a precursor to other pollutants such as ozone and particulate matter (13). Major sources of anthropogenic NO2, such as roadways and industrial facilities, are often located within or nearby marginalized and disenfranchised communities (14,15), and disparities in NO2 exposure across demographic subgroups have been the focus of several recent studies (4,8,(16)(17)(18).…”