The Influence of NOx and HCHO in Hot Extremes Observed from Space on Urban Populations and Ozone: A Case Study in Houston, TX (USA)

Prolonged high temperature events (hot extremes) have increased globally and on a regional scale. Traditional data processing methods to understand air quality changes in hot extremes may not adequately describe variations in directly emitted air pollutants like nitrogen oxides (NO_x) or volatile organic compounds (VOCs) and then their influence on secondary species like ozone (O₃). In Houston, TX (USA), the TROPOMI NO₂ (4.92%) and HCHO (21.25%) column observe an increase in hot extremes, but spatially and temporally the increase and resulting air quality vulnerability on populations is not uniform and can go unrecognised. Lee and Wang (2023) developed a clustering algorithm CLASP (CLustering of Atmospheric Satellite Products) rooted in density-based clustering which can capture variation and describe high magnitude features captured in TROPOMI observations on their spatial, magnitude, and temporal axes. CLASP qualified variation in hot extremes driven by biogenic and anthropogenic variables, by identifying increases in the magnitude and frequency of high magnitude hotspots in Houston. CLASP readily identified unique column increases in hot extremes which could be attributed to a regional airport and a peaking power plant. Population-weighted TROPOMI columns together with CLASP inform how Hispanics and Blacks or African Americans observe higher weighted population columns, and that these groups experience a higher frequency and magnitude of NO₂ and HCHO compared to other population groups, which intensifies in hot extremes. Surface ozone sees an overall increase of 14.80% in hot extremes, but similarly, increases are not homogenous as different extreme periods see varying degrees of change. In taking an observational approach to describe ozone air quality, we show how CLASP can help inform of variation captured in the TROPOMI column as well as identified limitations which prompts a need to further untangle how hot extremes effect ozone air quality in a region that may experience further exacerbated extremes.