Measurement report: Assessing the ammonia characteristics over a high-altitude mountain site in Shanxi province, China: a comparison with the observations in the North China Plain

Abstract. Ammonia (NH₃) acts as the dominant alkaline gas and plays a crucial role in atmospheric chemistry, thereby influencing air quality and ecological systems. Previous NH₃ measurement studies have primarily focused on near-ground environments or relied on passive sampling methods; however, continuous, high-resolution NH₃ observations at high-altitude sites remain scarce. This study investigated NH₃ characteristics at a high-altitude mountain site (WTM) in northern China, using high-resolution and real-time measurement data spanning a full annual cycle. It further conducted comparative analyses with a regional background site (SDZ) and an urban site (BMS) to better understand the regional features of NH₃ in northern China. A multi-method approach was employed, integrating in situ NH₃ measurements, meteorological data analyses, Convergent Cross Mapping (CCM), Potential Source Contribution Function (PSCF), and WRF-Chem modeling, to identify NH₃ source regions and clarify underlying transport mechanisms. The results indicated that NH₃ emissions from the North China Plain (NCP) can reach WTM and SDZ through distinct circulation patterns: mountain-plain circulations for WTM and mountain-valley circulations for SDZ. Notably, despite significant differences in altitude, geography, and pollutant transport mechanisms between WTM and SDZ, regional agricultural emissions in the NCP were the dominant factor driving the similarity in NH₃ levels at the two sites. This study enhances the understanding of how surface emissions influence NH₃ concentrations at high-altitude mountain site and highlights the critical role of NCP emissions in influencing regional NH₃ levels, thereby providing insights for formulating strategies to mitigate regional NH₃ pollution.