Significant impact of heterogeneous reactions of reactive chlorine species on summertime atmospheric ozone and free-radical formation in north China.

Heterogeneous reactions of NO, O, OH, ClONO, HOCl, ClNO, and NO, with chlorine-containing particles are incorporated in the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of heterogeneous reactions of reactive chlorine species on ozone and free radicals. Changes of summertime ozone and free radical concentrations due to the additional heterogeneous reactions in north China were quantified. These heterogeneous reactions increased the O, OH, HO and RO concentrations by up to 20%, 28%, 36% and 48% for some regions in the Beijing-Tianjin-Hebei (BTH) area. These areas typically have a larger amount of NOx emissions and a lower VOC/NOx ratio. The zero-out method evaluates that the photolysis of ClNO and Cl are the major contributors (42.4% and 57.6%, respectively) to atmospheric Cl in the early morning hours but the photolysis of Cl is the only significant contributor after 10:00 am. The results highlight that heterogeneous reactions of reactive chlorine species are important to atmospheric ozone and free-radical formation. Our study also suggests that the on-going NOx emission controls in the NCP region with a goal to reduce both O and secondary nitrate can also have the co-benefit of reducing the formation Cl from ClNO and Cl, which may also lead to lower secondary organic aerosol formation and thus the control of summertime PM in the region.