Trends of trace gases and aerosol over 2003-2023 at Mount Tai, northern China

China has been experiencing fast-paced urbanization and industrialization as well as stringent air pollution control in the past decades, which are expected to cause drastic changes in the anthropogenic emissions of primary air pollutants (e.g., SO₂, NOx and VOCs). Long-term observations are fundamental to the assessment of response of atmospheric composition to the changing anthropogenic emissions, which are, however, very limited in China. Here we integrated the observational data of trace gases and aerosol obtained during 2003-2023 at Mount Tai – the peak of the North China Plain (1534 m above sea level), a highly polluted region of China. The data were analyzed to understand the long-term changes of a variety of trace gases and aerosol properties such as ozone (O₃), PM_2.5 composition, particle number and size distribution, new particle formation and growth parameters, and O₃ depleting substances (ODS). Surface O₃ concentrations showed a significant increasing trend in summertime with a rate of ~2 ppbv yr^-1, despite the persistent decrease in NOx emissions since 2012, and can be attributed to the increasing VOCs and O₃ production efficiency. Sharp reduction in SO₂ emissions have resulted in significant decrease of sulfate in PM_2.5, whilst nitrate showed a strong increasing trend. A multi-phase chemical box model illustrated that the reduced SO₂ and sulfate enhanced nitrate formation by lessening the aerosol acidity and facilitating the partitioning of HNO₃ to the particle phase. The apparent formation rate of new particles in spring has increased at Mt. Tai, while the particle growth rate significantly decreased. The contributions of new particles to the cloud condensation nuclei (CCN) were also decreasing. The ODS regulated by the Montreal Protocol (MP) showed a significant downward trend, but the MP-controlled and unregulated halocarbon species showed overall upward trends. We will also present the results about the impacts of COVID lockdown on the regional air quality as observed at Mt. Tai.