Diurnal coupling of total column nitrogen dioxide and aerosol optical depth: Insights from the Pandora Asia network

To improve understanding of air quality in Asia and support satellite validation, the Korean government launched the Pandora Asia Network (PAN) in 2021. The high temporal resolution and simultaneous retrievals of nitrogen dioxide (NO 2 ) and aerosol optical depth (AOD) from the PAN allow for detailed analysis of aerosol and precursor variability. This study evaluated NO 2 and AOD levels at various sites across major Asian cities with different air quality regulations, using consistent quality measurements from Pandora and satellite data. Generally, AOD from the Visible Infrared Imaging Radiometer Suite (VIIRS) Deep Blue product showed good agreement with the PAN retrievals, whereas TROPOspheric Monitoring Instrument (TROPOMI) generally underestimated NO 2 levels relative to Pandora, especially in highly polluted cities such as Dhaka and Singapore. The NO 2 -to-AOD ratio (NAR), which assesses aerosol characteristics associated with NO 2 sources, was higher in major cities such as Seoul and Singapore, whereas lower values were observed in areas with uncontrolled or limited combustion activity. Differences in NO 2 measurements between TROPOMI and Pandora led to discrepancies in NAR between satellite and ground-based data, particularly in Singapore, Islamabad, and Dhaka. Using high-frequency PAN sampling, we calculated the daily correlation coefficient between AOD and NO 2 (r d ) at each site. High values of r d were observed in Islamabad and Dhaka, where ammonia (NH 3 ) and NO 2 levels were elevated; both are important tracers of nitrate aerosol formation. This study estimated the NO x emission contribution factor to AOD (NCF) based on a statistical relationship among r d , NO 2 , NH 3 , and water vapor (H 2 O), which can serve as a diagnostic tool for evaluating aerosol emission sources across Asian countries.