Haze pollution caused by particulate matter suspended in the air remains a major environmental issue because of its substantial impacts on climate, air quality, and human health. It has been reported that large fractions of both organic and inorganic matter contribute to PM_2.5 (aerodynamic diameter less than 2.5 μm) and induce severe haze pollution in urban China. It is also widely recognized that the mitigation of VOC, NH₃, SO₂ and NO_x precursors aids in reducing PM_2.5, the key factor in visibility degradation. However, it is impractical to control all precursors at the same time when their geographical locations and emission strengths remain unclear. Thus, control strategies for these various precursors still remain critical challenges.  

Diagram showing "The NOx Hypothesis" (Image by Pan et al., 2016) 

Prof. WANG Yuesi and coauthors from Institute of Atmospheric Physics, Chinese Academy of Sciences propose the “NO_x Hypothesis” that NO_x mitigation will not only reduce nitrate itself but also might intervene with secondary aerosol formation in a recent publication in peer-reviewed journal of ATMOSPHERIC ENVIRONMENT.

Based on on-line and off-line aerosol measurements in urban Beijing for both clean and haze conditions, they demonstrate that the absolute mass concentration and relative percentage of nitrate increased with visibility degradation (relative humidity), whereas the variations of organics tracked the patterns of mixing-layer height and temperature (Figure 1). They propose that the increase in the relative contribution of nitrate to PM₁ observed during the early stages of haze pollution was due to new particle formation (Figure 2), whereas the nitrate formed in PM_1-2.5 during the latter stages was due to heterogeneous formation and hygroscopic growth (Figure 3).

The increasing trend of nitrate (and also sulfate and ammonium) but decreasing trends of organics during haze development, together with the increase of the NO₂/SO₂ molar ratio with increasing proximity to downtown Beijing and with visibility degradation (Figure 4), provide further evidence that controlling NO_x emissions should be a priority for improving air quality in mega cities.

Figure 1 Variations of particulate matter and meteorological factors at different visibility levels. (Image by Pan et al., 2016)

Figure 2 Chemical compositions of NR-PM₁ at different visibility levels. (Image by Pan et al., 2016)

Figure 3 Chemical compositions of filter-based PM_2.5 at different visibility levels. (Image by Pan et al., 2016)

Figure 4 Sampling site of IAP and the molar ratio of NO₂/SO₂ in Beijing (Image by Pan et al., 2016)

Additional large-scale investigation is required to adequately characterize the regional features of NO_x-induced haze pollution in China. Such studies may provide insight into the formation of critical nuclei or the subsequent growth of freshly nucleated particles and advance our understanding of the role of nitrate in new particle formation.