Recent progress on the role of chlorine radical in urban atmosphere

Atmospheric chlorine radical plays essential roles in tropospheric photochemical processes, such as affecting the oxidation capacity and aerosol formation. Tropospheric chlorine chemistry was initially known to be important in the marine and polar atmosphere; nevertheless, more and more recent studies have indicated that chlorine chemistry was also active in inland areas. Here, we will present the vital chlorine radical precursors, such as nitryl chloride (ClNO₂), molecular chlorine (Cl₂), hydrochloric acid (HCl) and others that were observed in the urban atmosphere of China, as well as the recent understanding on the influence from anthropogenic chlorine sources. Our recent findings have shown the increasing importance of nocturnal chlorine activation processes in producing ClNO₂ and Cl₂, which ultimately contributing to the haze formation caused by NO_x reduction. Another finding also showed that there is a potential atmospheric chlorine sink process in environment with low temperature and significant bromine chemistry. We will finally discuss the future challenges in the understanding of reactive chlorine chemistry in urban cities under the changing world.