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Cooperative SO2 and NOx aerosol formation in haze pollution

A study of major haze pollution episodes in China in 2013 by researchers from the State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Chinese Academy of Sciences, shows that the formation of secondary sulfate aerosols from SO2 increases in the presence of NOx. Their paper is published in the journal Science China Earth Sciences.

Air pollution in China has exhibited noticeable changes over the past 30 years, shifting from point-source pollution (around factories and industrial plants) in the 1980s to urban pollution in the 1990s. Since the start of this century, air pollution has become increasingly regional and more complex. Recent research has indicated that the cooperative transition of SO2 and NOx into secondary aerosols (sulfate and nitrate) played a critical role in the haze pollution episode in China in January 2013.

The coexistence of high concentrations of primary and secondary gaseous and particulate pollutants results in numerous heterogeneous reactions occurring on the surfaces of fine particles. These reactions change the oxidizing capacity of the atmosphere, chemical compositions, and the physicochemical and optical properties of the particulate matter. The overall effect is that air pollution and haze formation is accelerated.

The study explores the cooperative transition of SO2 and NOx into secondary aerosols on the surfaces of carbon-containing particles through heterogeneous reactions. The formation of sulfates from SO2 is promoted by the existence of NOx.

Results show that as the particle size increases, the fractional contributions of secondary inorganic ions, such as sulfate and nitrate, also increase. The hygroscopicity of the particles increase and the increased water content can accelerate the gas–liquid–solid reactions of SO2 and NOx, which further increases the hygroscopicity of the particles.

These processes form a positive feedback mechanism that enhances the conversion of primary gas pollutants into secondary aerosols. Consequently, the researchers found, it is important to reduce emissions of the precursor gases of PM2.5 to reduce the overall PM2.5 concentrations in the atmosphere.

They summarized the three major findings of the study as:

  1. Atmospheric circulation, local meteorological factors, and the structure of the boundary layer, together with specific geographical conditions, were unfavorable for pollutant diffusion during the haze events.

    Strong cold air events were sparse, and the weather systems were weak in central and eastern China. Zonal upper air circulation was dominant over North China, and warm air advection existed in the lower atmosphere; in addition, a low or weak pressure field was dominant over North China, which did not favor dispersion. Increasing humidity and sustained high humidity conditions, the wind direction turning from north to south and sustained weak wind were the main meteorological causes for the severe haze pollution over the Jing-Jin-Ji area. Meanwhile, the pollutants were concentrated at the surface layer due to the intense, thick inversion layer and low mixed layer height.

  2. The abnormally high amount of NOx during the haze episodes, produced by fossil fuel combustion and vehicle emissions, played either a direct or indirect role in the rapid secondary transformation of SO2 into sulphate aerosols. SO2, NO2, and the other primary gas pollutants were transformed into secondary aerosols (e.g., sulphate and nitrate) through heterogeneous reactions on the surface of carbon-containing ultrafine particles; the change in the composition of the particles enhances their hygroscopicity and increases the size and mass concentrations of the ultrafine particles.

  3. As haze pollution becomes a regional atmospheric pollution problem, synchronous emission abatement policies that are based on integrated planning should be implemented in stages to improve the regional air quality throughout central and eastern China, especially for the Jing-Jin-Ji area. Among these abatement policies, stricter single pollutant emission standards and further emission reductions are of particular importance to reduce the synergistic effects between various pollutants.


  • Wang, Y. S., Yao, L., Wang, L. L., Liu, Z. R., Ji, D. S., Tang, G. Q., Zhang, J. K., Hu, B., Xin, J. Y. (2014) “Mechanism for the formation of the January 2013 heavy haze pollution episode over central and eastern China,” Science China Earth Sciences doi: 10.1007/s11430-013-4773-4


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