Characteristics of peroxyacetyl nitrate pollution during a 2015 winter haze episode in Beijing.

Peroxyacetyl nitrate (PAN) are effective indicators of photochemical pollution, and also play an important role in regional oxidant balance. Surprisingly, in recent years, PAN have also been detected under conditions that do not favor the photochemical processes. To obtain a better understanding of the mechanisms of formation of atmospheric compound pollution, this study examined the relationships between concentrations of PAN and other pollutants (e.g., ozone [O3] and PM2.5) during a winter haze episode. The observation periods were from December 31, 2015, to February 2, 2016, and from February 19, 2016, to March 4, 2016. The maximum daily concentration of PAN during haze episodes was 4-10 times higher than that during non-haze episodes. The continuous cumulative increase in PAN concentrations was the result of a combination of photochemical production during the daytime and production based on free radical chemical reactions during the nighttime. During the haze episode, the correlation between concentrations of PAN and O3 was weak, while a significant correlation was observed between PAN and PM2.5 concentrations (R2 = 0.82). This may have been due to higher concentrations of particulate matter impairing illumination, which can then inhibit the photochemical reactions that produce PAN and O3. OH radicals can replace the role of light in PAN formation, which can cause concentrations of PAN and O3 to vary independently. During the haze episode, the ratio of PAN/O3 was around 0.3, which was much higher than that during the clean period.