Refining satellite-based HCHO/NO₂ diagnostics by isolating secondary HCHO: Enhanced mapping of ozone formation sensitivity
Journal
Science of The Total Environment
Journal Volume
983
Start Page
179704
ISSN
0048-9697
Date Issued
2025-06-25
Author(s)
Abstract
Persistent ozone pollution continues to challenge urban air quality management, even as primary pollutant emissions decline. Conventional satellite retrievals of formaldehyde (HCHO) do not distinguish between direct emissions and secondary formation through VOC oxidation, often leading to inaccurate classifications of ozone sensitivity regimes. In this study, we refine the satellite-based HCHO/NO₂ diagnostic by isolating the secondary component of HCHO using high-resolution TROPOMI observations coupled with ground-based measurements in Taichung, Taiwan. A multiple regression model incorporating carbon monoxide (CO) and odd oxygen (Ox = O₃ + NO₂) is employed to differentiate primary and secondary HCHO quantitatively. Primary HCHO contributions in urban areas reach up to 40 % during winter months, highlighting the critical importance of source separation for accurate sensitivity analysis. The resulting modified HCHO/NO₂ ratio (formaldehyde-to‑nitrogen dioxide ratio, FNR) exhibits a significantly stronger correlation with observed surface ozone concentrations and identifies distinct seasonal and spatial patterns in ozone sensitivity. Specifically, urban and suburban areas within the Taichung Basin are found to be predominantly VOC-limited, while coastal industrial zones exhibit transitional characteristics that necessitate targeted NOₓ emission controls. The modified FNR threshold values established in this study (1.29–2.70) differ significantly from previously reported generic thresholds, emphasizing the importance of region-specific diagnostic frameworks. This refined approach not only enhances our understanding of regional ozone formation dynamics but also provides a robust tool for guiding emission control strategies in complex urban environments. The methodology holds promise for broader application in regions with comprehensive satellite and ground-based monitoring capabilities, thereby advancing real-time air quality assessment and management.
Subjects
Air quality management
NOₓ–VOC regimes
Ozone sensitivity
Satellite-based diagnostics
Secondary formaldehyde
Publisher
Elsevier BV
Type
journal article