Vertical structures of physical and chemical properties of urban boundary layer and formation mechanisms of atmospheric pollution

Air quality in China has been continuously improved since the implementation of “Atmospheric Pollution Prevention and Control Action Plan” in 2013. However, the mass concentrations of fine particles particularly in Beijing, Tianjin and Hebei are still much higher than the National Ambient Air Quality Standard (35 μg m−3 as an annual average), and severely polluted events also occur occasionally in autumn and winter seasons. In recent years, Chinese research scientists have made significant progresses in vertical measurements of air pollution, formation mechanisms and source apportionment of atmospheric aerosol, and also the interactions between aerosol and atmospheric boundary layer, which greatly support the scientific prevention and control of atmospheric pollution. Despite this, many questions on atmospheric pollution have not been well addressed yet. This paper first summarizes the recent research progresses in vertical measurements of air pollution in China, in particular, the measurements from the research platform of the Beijing 325 m meteorological tower. In addition to the routine long-term measurements of boundary layer (turbulence, CO2 and H2O flux) and meteorological parameters (temperature, relative humidity, wind speed and wind direction) at 7 and 15 heights, respectively, this platform can have simultaneous real-time measurements of aerosol particle composition (organics, sulfate, nitrate, ammonium, chloride, and black carbon), gaseous species (CO, SO2, and O3), particle number size distributions, and optical properties (extinction and absorption coefficients) at ground level and 260 m, and also the continuously vertical measurements from ground to 260 m using the tower container equipped with specific instruments. Then, the vertical differences of aerosol composition in different seasons in Beijing, and their interactions with boundary layer are discussed. Also, the response of aerosol chemistry to regional source emission control are elucidated based on vertical measurements during the Asia-Pacific Economic Conference (APEC) summit in 2014 and the 70th anniversary parade in 2015. This paper also reviews the latest research progress on formation mechanisms of secondary inorganic aerosol (nitrate and sulfate) and secondary organic aerosol, and the life cycles of severe haze events in China. In particular, the heterogeneous formation of sulfate during severe haze episodes strongly depends on pH values of aerosol liquid water content. While techniques for direct measurements of aerosol particle pH are essential yet challenging, laboratory experiments are more needed now to prove the hypothesis of sulfate formation mechanisms. Compared with inorganic species, the formation and evolutionary mechanisms of secondary organic aerosol (SOA) are far more complex in polluted environments, and their roles in haze formation need to be better characterized. Finally, this paper lists some future recommendations for air pollution measurements (e.g., from long-term to vertical measurements in terms of significant emission changes in the near future), formation mechanism of secondary aerosol, and molecular composition and physical and chemical properties (phase, volatility, oxidation state, etc.) of organic aerosol.