Reactive oxidized nitrogen at a high mountain site in the North China Plain : chemical speciation and temporal variations during 2007–2023

Abstract

Total reactive oxidized nitrogen (NOy) 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 NOy in China. In this study, we analyzed the temporal variations in NOy 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 NOy 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 NOx, NOy, and NOz 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 NOy. 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 NOy 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.