Lu, Gongda; Marais, Eloise A; Vohra, Karn; Horner, Rebekah P; Zhang, Dandan; Martin, Randall V; Guttikunda, Sarath; (2025) Near‐Automated Estimate of City Nitrogen Oxides Emissions Applied to South and Southeast Asia. Journal of Geophysical Research: Atmospheres , 130 (2) 10.1029/2024jd041000.

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Abstract

Cities in South and Southeast Asia are developing rapidly without routine, up‐to‐date knowledge of air pollutant precursor emissions. This data deficit can potentially be addressed for nitrogen oxides (NOx) by deriving city NOx emissions from satellite observations of nitrogen dioxide (NO2) sampled under windy conditions. NO2 plumes of isolated cities are aligned along a consistent wind‐rotated direction and a best‐fit Gaussian is applied to estimate emissions. This approach currently relies on non‐standardized choice of upwind, downwind, and across‐wind distances from the city center, resulting in fits that often fail or yield non‐physical parameters. Here, we propose an automated approach that defines many combinations of distances yielding 54 distinct sampling boxes that we test with TROPOspheric Monitoring Instrument (TROPOMI) NO2 observations over 19 isolated cities in South and Southeast Asia. Our approach is efficient, uses open‐source software, is adaptable to many cities, standardizes and eliminates sensitivity to sampling box choice, increases success of deriving emissions from 40% to 60% with one sampling box to 100% (all 19 cities) with 54, and yields emissions consistent with the current manual approach. We estimate that the annual emissions range from 15 ± 5 mol s−1 for Bangalore (India) to 125 ± 41 mol s−1 for Dhaka (Bangladesh). With enhanced success of deriving top‐down emissions, we find support from comparison to past studies and inventory estimates that top‐down emissions may be biased, as the method does not adequately account for spatial and seasonal variability in NOx photochemistry. Further methodological development is needed for enhanced accuracy and use to derive sub‐annual emissions.

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