Characteristics of Aerosol Pollution Under Different Visibility Conditions in Winter in a Coastal Mega-City in China

Abstract

To investigate chemical profiles and formation mechanisms of aerosol particles in winter haze events, comprehensive measurements including hourly concentrations of PM2.5 and water-soluble inorganic ions and related gas-phase precursors were conducted via an online monitoring system from January to March of 2016 in Shenzhen, a coastal mega-city in south China. In this study, high concentrations of PM2.5, NO2 and lower levels of O3 were observed during haze periods in comparison with clear days (Visibility > 15km). The major secondary ionic species were \begin{document}${\rm NH_4^ +} $\end{document} 、 \begin{document}${\rm NO_3^ -} $\end{document} and \begin{document}${\rm SO_4^{2 - }}$\end{document} -, which varied significantly on haze and clear days. The ratio of \begin{document}${\rm NO_3^ -}$\end{document} / \begin{document}${\rm SO_4^{2 - }}$\end{document} in haze days was greater than that on clear days and tended to be larger when air pollution became more serious. At the same time, compared with previous studies, it has been found that the ratio has been increasing gradually in Shenzhen, indicating that the motor vehicle exhaust emissions have a more and more important impact on air quality in Shenzhen. Sulfur oxidation rate(SOR) and nitrogen oxidation rate(NOR) was higher during the haze period than that in clean days, indicating efficient gas to particle conversion. Further analysis shows that high concentrations of sulfate might be explained by aqueous oxidation, but gas-phase reactions might dominate nitrate formation. This study also highlights that wintertime nitrate formation can be an important contributor to aerosol particles, especially during haze periods.