Summertime Urban Ammonia Emissions May Be Substantially Underestimated in Beijing, China

Estimation of Ammonia Emissions over China Using IASI Satellite-Derived Surface Observations

An accurate ammonia (NH3) emission inventory is crucial for policymakers developing air pollution mitigation strategies. Both satellite observations and bottom-up estimates identify significant NH3 emission hotspots in China. However, bottom-up NH3 emission inventories are highly uncertain due to the lack of localized emission factors, while large and uncertain errors in IASI satellite NH3 columns have hindered their direct application in top-down emission inversion methods. In this study, we perform a top-down optimization of monthly NH3 emissions over China using IASI-derived surface NH3 concentrations with well-evaluated error estimates, combined with the CAMx model at a 36 km resolution. Our posterior NH3 emissions for 2020 (12.3 [10.9-13.6] Tg N yr-1) are significantly higher than prior estimates from the MEIC inventory (7.6 Tg N yr-1), which primarily underestimates emissions during the warm months in hotspot areas (e.g., NCP and MLYR). We employ multiple approaches to comprehensively evaluate our inversion results. Our study highlights that error estimates for low-value observations are a particularly critical factor in the inversion setup, significantly influencing the reliability of emission optimization.

Significant Increase in Ammonia Emissions in China: Considering Nonagricultural Sectors Based on Isotopic Source Apportionment

Isotopic source apportionment results revealed that nonagricultural sectors are significant sources of ammonia (NH3) emissions, particularly in urban areas. Unfortunately, nonagricultural sources have been substantially underrepresented in the current anthropogenic NH3 emission inventories (EIs). Here, we propose a novel approach to develop a gridded EI of nonagricultural NH3 in China for 2016 using a combination of isotopic source apportionment results and the emission ratios of carbon monoxide (CO) and NH3. We estimated that isotope-corrected nonagricultural NH3 emissions were 4370 Gg in China in 2016, accounting for an increase in the total NH3 emissions from 7 to 31%. As a result, compared to the original NH3 EI, the annual emissions of total NH3 increased by 35%. Thus, in comparison to the simulation driven by the original NH3 EI, the WRF-Chem model driven by the isotope-corrected NH3 EI has reduced the model biases in the surface concentrations and dry deposition flux of reduced nitrogen (NHx = gaseous NH3 + particulate NH4+) by 23 and 31%, respectively. This study may have wide-ranging implications for formulating targeted strategies for nonagricultural NH3 emissions controls, making it facilitate the achievement of simultaneously alleviating nitrogen deposition and atmospheric pollution in the future.