Reactive Oxidized Nitrogen at a High Mountain Site in the North China Plain: Chemical Speciation and Temporal Variations During 2007–2023

Total reactive oxidized nitrogen (NO_y) and its speciated compounds, such as nitric oxide (NO), nitrogen dioxide (NO₂), peroxyacetyl nitrate (PAN), and particulate nitrate (pNO₃⁻), play a major role in tropospheric chemistry, regional air quality, and climate change. However, little is known regarding the chemical speciation and long-term trends of NO_y in China. In this study, we analyzed the temporal variations in NO_y and speciated compounds during eight intensive field campaigns and 6 years of continuous measurements during 2007–2023 at Mount Tai in the North China Plain (NCP). The measured concentration levels of NO_y were significantly higher than those at other remote high-altitude sites globally. NO₂ was the most abundant species (24%–42%), followed by pNO₃⁻ (18%–34%) and PAN (7%–12%). Over the study period, the concentrations of NO_x, NO_y, and NO_z exhibited a significant downward trend, while pNO₃⁻ exhibited a significant upward trend. Vertical transport from the planetary boundary layer (PBL) substantially influences the atmosphere at Mount Tai, with air masses from both PBL and free troposphere containing different chemical compositions of NO_y. The lower free troposphere above the NCP contained relatively high abundances of PAN and pNO₃⁻. This study represents the first comprehensive analysis of decadal trends in NO_y and its chemical speciation in China's high-altitude atmosphere. Our findings underscore the necessity for additional efforts to mitigate photochemical and secondary aerosol pollution caused by nitrogen species at high altitudes in China.