| Photochemical pollution has become a serious environmental issue in China. As the main components of photochemical smog, ozone(O3) and peroxyacetyl nitrate(PAN) could cause damage to human health, plants and animals. Photochemical pollution has become a regional phenomenon, impacting not only the mega cities, but also rural and background regions. To learn the chemical and dynamic mechanism of O3 and PAN in the North China region, which suffers severe air pollution, the long-term observations of O3, PAN and other reactive trace gases were made at an urban site(CMA) in the megacity Beijing, the Shangdianzi station(regional background), the Gucheng and Raoyang stations(rural sites) in Hebei Province.The observational results from Beijing indicate that the concentration of surface O3 was much lower in winter than that in spring because of the weaker radiation and the strong titration of nitric oxide(NO) in winter. However, the concentration of PAN was high in both seasons. There was nearly no correlation between O3 and PAN in winter. This can be attributed to influences of both chemical and meteorological processes. On the one hand, the thermal decomposition of PAN was weakened in cold winter, while large amount of O3 was titrated by NO; on the other hand, the stable weather condition promoted the accumulation of PAN and nitrogen oxides(NOx), while rapid downdrafts caused by cold front from northwest brought high-O3 air mass to the ground. The correlation increased in spring as the radiation was becoming stronger gradually and photochemical reaction was able to produce more of the oxidants, but the horizontal and vertical transport was still important. Interestingly, the wintertime levels of PAN and aerosol were significantly correlated, which remains to be understood.Data from Shangdianzi shows that the concentrations of surface O3 in winter and spring were both higher than those in urban Beijing, while PAN was much lower due probably to the lacking of NOx. The correlation between O3 and PAN was negative in winter and positive in spring, which seemed to be closely related to horizontal and vertical air mass transport. Higher PAN was usually accompanied by higher NOx occurring under polluted condition, while higher O3 appeared in relatively clear condition.Observations at Gucheng and Raoyang were both made in summer. The results show that the summer levels of PAN at both rural sites were lower than that in urban Beijing in winter. The diurnal variations of O3 and PAN were similar at the two sites and both oxidants were positively correlated. However, the concentration of O3 was lower and PAN was higher at Gucheng. It may be related to the difference of the VOCs levels and the dynamic mechanism influencing the two sites. Gucheng was affected greater by regional transport while Raoyang was significantly influenced by both photochemistry and transport.During the observation at Gucheng, there were several cases with rapid increase of O3 and sharp decrease of PAN and other reactive trace gases when convection processes occurred at night. We analyzed simultaneous meteorological data and found surface pseudo-equivalent potential temperature(θse) decreased dramatically coinciding with the steep increase of wind speed in all the cases. It suggests that the convective downdrafts transported air with higher O3 and lower θse from upper atmosphere to the surface layer. As the concentration of O3 mostly increased to 60-80 ppbv, and the levels of origins of downdrafts were estimated as from lower to mid free-troposphere, it implied that O3 maintains vertically around 60-80 ppbv in the mid and lower free-troposphere in the North China in summer and early autumn. This view is supported by vertical profiles of ozone observed over Gucheng using an unmanned aircraft. Furthermore, O3 increase by convective downdrafts is able to impact a larger area of the North China region. |
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