Constraints on NO x emission in Thailand using GEMS satellite data

Nitrogen oxides (NOx=NO+NO2) are key pollutants that contribute to ozone and secondary aerosol formation, posing environmental and health risks. Accurate simulation and forecasting of NO x pollution is essential for developing mitigation strategies. Local inventories in Thailand are infrequently updated, leading researchers to use global inventories such as CAMS-GLOB-ANT for simulation. Global inventories carry uncertainties due to assumptions in emission factors, outdated activity data, and coarse temporal resolution. To address these limitations, this study applies a top-down approach to update NO x emissions in Thailand using the iterative finite difference mass balance (IFDMB) method. Tropospheric NO 2 vertical column densities (VCDs) from GEMS version 3 are integrated with the WRF-Chem to refine CAMS-GLOB-ANT emissions for September 2023. The simulations with posterior emissions are evaluated against TROPOMI NO 2 VCDs and surface NO x concentration. Results show that the baseline simulation overestimates NO 2 VCDs across Thailand compared with GEMS, except in North Thailand. GEMS reports substantially higher NO 2 than TROPOMI – particularly over Lampang – reflecting retrieval uncertainty associated with the a priori profiles used in version 3. Consequently, IFDMB reduces NO x emissions across most regions but increases in the North. The increase is attributed primarily to a retrieval artifact rather than to a genuine emission underestimate. Baseline NO x emissions in Thailand are 35 852 Mg per month with North Thailand and 30 699 Mg per month without North. Our posterior emissions over three IFDMB schemes are 25 994, 34 808, and 36 870 Mg per month with North, and 15 376, 26 523, and 27 087 Mg per month without North. These adjustments improve model bias and error relative to GEMS. However, when evaluated against TROPOMI, we find an increase in the bias for North Thailand, likely due to discrepancies between GEMS and TROPOMI retrievals. These discrepancies highlight the importance of future retrieval algorithm improvement and calibration across satellite products. Comparisons to surface observations indicate that IFDMB shifts the NO x peak to later than observations. This is because observations are strongly influenced by local transportation sources, which are hard to observe and retrieved by GEMS and the model, respectively.