Abstract:

Ammonia (NH₃), the only major alkaline gas in the atmosphere, plays a crucial role in the formation of secondary inorganic aerosols, such as ammonium nitrate and ammonium sulfate, through neutralization with acidic precursors (e.g. SO₂ and NO_x). These aerosols can contribute up to 50% of the PM_2.5 mass concentration in urban China, thus making NH₃ a key component in the formation of air pollution. Unlike SO₂ and NO_x, which have been substantially reduced in China since 2013 owing to stringent emission control policies, NH₃ concentrations have remained stable or have even increased in some urban areas. This trend challenges the effectiveness of current pollution mitigation efforts and highlights the importance of identifying and managing NH₃ sources, particularly in cities where fine particulate matter pollution remains a persistent health and environmental concern. Although agricultural activities, particularly fertilizer application and livestock, are recognized as the dominant sources of NH₃ emissions at the national scale, recent studies suggest that fossil fuel combustion, especially vehicular traffic, is increasingly important in densely populated urban areas. Recently, the relative importance of agricultural and non-agricultural sources of NH₃ emissions has remained a subject of ongoing debate, particularly in urban areas. While non-agricultural sources may account for a relatively minor share of total NH₃ emissions at regional or broader scales, they can exert a disproportionately significant influence on PM_2.5 formation, particularly in densely populated metropolitan areas with high traffic intensity. However, these non-agricultural sources are often underrepresented or entirely omitted in regional emission inventories due to a lack of measurement data, making it difficult to support effective policy interventions for their reduction. This study addresses this knowledge gap by integrating ambient NH₃ concentration measurements with stable nitrogen isotope (δ_15N-NH3) analysis to apportion emission sources across multiple urban microenvironments in Beijing during winter. Ambient NH₃ was measured using ALPHA passive samplers deployed at 17 strategically selected sites across Beijing, comprising 12 roadside locations along major transportation corridors and five non-roadside sites representing diverse urban land-use types, including university campuses, forest parks and peri-urban agricultural areas. Weekly measurements were conducted from December 2016 to January 2017, and five samples were collected for each measurement. For three representative weeks, the collected NH₃ samples were further analyzed for δ_15N values using a BrO⁻ oxidation-NH₂OH reduction protocol coupled with PT-IRMS detection. Ambient NH₃ concentrations during the study period ranged from 2.1 μg·m⁻³ to 32.9 μg·m⁻³, with an overall mean of (14.3±0.8) μg·m⁻³. The δ_15N-NH3 values ranged between −40‰ and −25‰, with a mean of −32.8‰±1.1‰. Roadside sites consistently exhibited higher NH₃ concentrations and less negative δ_15N values than non-roadside sites, indicating a stronger influence from isotopically enriched sources, such as traffic exhaust. Isotope mixing model simulations, informed by the measured δ¹⁵N values of NH₃ and isotopic signature values of major emission sources, suggested that non-agricultural sources contributed approximately 54%±14% of total NH₃ in the urban atmosphere during the monitoring period, with a range of 32%‒74%. At roadside locations, the non-agricultural contribution was even higher, reaching 74% at certain sites. These results demonstrate a clear spatial pattern of NH₃ pollution, where proximity to major roadways significantly increases both the concentration and isotopic enrichment of atmospheric NH₃. In contrast, two campus sites (i.e. China Agricultural University and Beijing Normal University), dominated by residential, administrative, and educational activities, are subject to limited and low-intensity anthropogenic NH₃ emissions. Hence, relatively low NH₃ concentrations and δ_15N values were observed in this study. Forest park sites (i.e., Olympic Forest Park and Baiwangshan Forest Park) showed the lowest NH₃ levels, likely due to NH₃ uptake by vegetation and minimal direct emissions from these areas. The agricultural site located on Beijing’s urban fringe had low NH₃ levels, which was consistent with the suppressed agricultural activity during the cold season. The δ_15N approach provides several methodological advantages for the source apportionment of atmospheric NH₃. Unlike chemical speciation alone, isotopic measurements offer source-specific fingerprints that can be used to resolve overlapping emission contributions, particularly in complex urban environments such as Beijing. Although our observations provided dense spatial coverage, it is important to consider the potential isotopic fractionation associated with diffusion sampling. In this study, we recognized that the parameter f plays a critical role in equilibrium isotopic fractionation during the gas-to-particle conversion of nitrogen. Therefore, we adopted the expected relationship between the initial δ_15N-NH3 and varying f values to mitigate the influence of isotopic fractionation. For source apportionment purposes, NH₃ and NH₄⁺ should be treated as a unified species (i.e., NH_x), with full consideration given to isotopic fractionation occurring during the NH₃-NH₄⁺ conversion. However, the low temporal resolution of our weekly measurements may obscure short-term variations in δ¹⁵N associated with NH_x partitioning and the influence of atmospheric conditions on their kinetics of uptake. This highlights the need for future studies to implement synchronized δ¹⁵N measurements of gaseous NH₃ and particulate NH₄⁺, along with higher-resolution monitoring, to enable more reliable source apportionment using refined stable isotope techniques. Our findings underscore the increasingly prominent role of non-agricultural NH₃ emissions, particularly traffic-derived emissions, in the atmospheric nitrogen budget and PM_2.5 formation in Chinese megacities. This aligns with national trends in vehicle ownership, which have risen dramatically over the past decade, leading to increased exhaust emissions. Given the health hazards posed by sustained dependence on fossil fuels and the shift toward sustainable development, a transition in the energy sector is both urgent and inevitable. In conclusion, this study provides isotopic evidence that non-agricultural sources, particularly vehicular emissions, substantially contribute to urban atmospheric NH₃ concentrations in Beijing during winter. The elevated NH₃ concentrations and δ_15N-NH3 values at roadside sites highlight the growing influence of traffic emissions and suggest that urban NH₃ pollution can no longer be attributed solely to agricultural activities. To effectively address urban NH₃ pollution, future policies should broaden the current agricultural-focused control strategies by incorporating non-agricultural NH₃ emissions, e.g. transportation and waste management sectors. Moreover, integrating isotope-based field monitoring with dynamic atmospheric transport models may provide better constraints on urban NH₃ emission inventories and support the co-design of pollution control and public health protection strategies. Such efforts are essential for meeting China’s dual objectives of air quality improvement and sustainable urban development in the context of its national carbon neutrality targets.

Key words: ammonia, urban area, nitrogen isotopes, source apportionment, air quality

摘要：

氨气（NH₃）作为大气中唯一已知的高浓度碱性活性氮气体，其与酸性前体物（如SO₂、NO_x）反应形成的二次无机气溶胶是大气细颗粒物（PM_2.5）的主要组成部分，对城市空气质量与人类健康具有重要影响。准确认知大气NH₃污染特征与来源，是科学落实NH₃减排、推进大气污染防治的重要基础研究。然而，城市地区大气NH₃主要来源仍存在争议，特别是对非农业源排放贡献的认知不足。在北京城区设置17个采样点，利用ALPHA被动采样器开展冬季连续5周的大气NH₃浓度及其δ_15N的多点监测，并结合氮稳定同位素溯源方法开展来源解析，揭示城区大气NH₃污染特征与来源。 结果显示，NH₃周质量浓度范围为2.1-2.9 μg·m⁻³，平均质量浓度为（14.3±0.8）μg·m⁻³，δ_15N-NH3监测值为−32.8‰±1.1‰。马路点的NH₃浓度与δ_15N-NH3普遍高于非马路点。源解析结果表明，非农业源（交通源与废弃物源）对NH₃的贡献约为54%±14%，可能范围为32%-74%。在相同的气粒转化系数下，马路点的非农业源贡献比非马路点高了6%-37%，以交通源为主。研究指出人为源NH₃排放，尤其是交通排放，对北京城区大气NH₃浓度有显著影响，这表明了加强非农业源NH₃排放控制，对缓解城市地区大气污染和改善空气质量具有重要现实意义。

关键词: 氨气, 城区, 氮同位素, 来源解析, 空气质量