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

Abstract

Ammonia (NH 3 ) is critical to the nitrogen cycle and PM 2.5 formation, yet a great deal of uncertainty exists in its urban emission quantifications. Modelunderestimated NH 3 concentrations have been reported for cities, yet few studies have provided an explanation. Here, we explore reasons for severe WRF-Chem model underestimations of NH 3 concentrations in Beijing in August 2018, including simulated gas-particle partitioning, meteorology, regional transport, and emissions, using spatially refined (3 km resolution) NH 3 emission estimates in the agricultural sector for Beijing-Tianjin-Hebei and in the traffic sector for Beijing. We find that simulated NH 3 concentrations are significantly lower than ground-based and satellite observations during August in Beijing, while wintertime underestimations are much more moderate. Further analyses and sensitivity experiments show that such discrepancies cannot be attributed to factors other than biases in NH 3 emissions. Using site measurements as constraints, we estimate that both agricultural and non-agricultural NH 3 emission totals in Beijing shall increase by ∼5 times to match the observations. Future research should be performed to allocate underestimations to urban fertilizer, power, traffic, or residential sources. Dense and regular urban NH 3 observations are necessary to constrain and validate bottom-up inventories and NH x simulation.

Atmospheric NH3 in urban Beijing: long-term variations and implications for secondary inorganic aerosol control

“…This will require further observational research on atmospheric NH 3 in urban Beijing in the future. In addition, existing studies have demonstrated that emission inventories have underestimated atmospheric NH 3 emissions in the Beijing urban area (Xu et al, 2023), and the assessment results have varied across emission inventories . Additionally, given the limited research years of current emission inventories, the observed differences between long-term trends in monitored NH 3 concentrations and NH 3 emissions require continued attention in the future.…”

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

“…Despite the dominant role of agricultural sources of NH 3 at global or regional scales, recent observations have revealed that NH 3 concentrations are higher in densely populated urban areas that have low agricultural activity. − This finding posed a challenge to simulations driven by the current NH 3 emission inventories (EIs), as they could not accurately reproduce the observed high NH 3 concentrations in urban areas. − Meanwhile, more and more isotopic source apportionment results pointed out that nonagricultural activities, such as fossil fuel combustion, industrial processes, on-road traffic, , slip from power plants, and waste, presented a larger contribution to atmospheric NH 3 concentration in urban areas as compared to the agricultural sectors. − As shown in Table S1, during nonpollution periods, the contribution of nonagricultural sources to atmospheric NH 3 concentrations in the regions across China varied from 14 to 76%, and these values even reached 49–97% during haze periods. − However, the relative contributions of nonagricultural and agricultural sources to the total NH 3 emissions from the isotopic source apportionment results significantly differed from those derived from the bottom-up anthropogenic NH 3 EIs (e.g., the Multi-resolution Emission Inventory for China, MEIC) at the city level (Figure S1). − For instance, isotopic source apportionment revealed that fossil fuel was a significant source of NH 3 at an urban site in Beijing, contributing as much as 76%, whereas the corresponding value was only 35% based on the NH 3 EI .…”