<p>Severe and persistent haze events&#160;in northern China, characterized by high loading of fine aerosol especially of secondary origin, negatively impact human health and the welfare of ecosystems. However, current&#160;knowledge&#160;cannot fully explain the formation of this haze&#160;pollution. Despite field observations of elevated levels of reactive halogen species (e.g., BrCl, ClNO<sub>2</sub>, Cl<sub>2</sub>, HBr) at several sites in China,&#160;the influence of halogens&#160;(particularly bromine)&#160;on haze pollution&#160;is largely unknown.&#160;Here, for the first time, we compile an emission inventory of anthropogenic bromine and quantify the collective impact of halogens on haze pollution in northern China. We utilize a regional model (WRF-Chem),&#160;revised to incorporate&#160;updated&#160;halogen chemistry and anthropogenic chlorine and bromine emissions&#160;and validated by measurements of atmospheric pollutants and halogens,&#160;to show that halogens enhance the loading of fine aerosol in northern China (on average by 21%) and especially its secondary components (~130% for secondary organic aerosol&#160;and ~20% for sulfate, nitrate, and ammonium&#160;aerosols). Such a significant increase is attributed to the enhancement of atmospheric oxidants&#160;(OH, HO<sub>2</sub>, O<sub>3</sub>, NO<sub>3</sub>, Cl, and Br) by halogen chemistry, with a significant contribution from previously unconsidered bromine.&#160;These results show that higher recognition of the impact of anthropogenic halogens shall be given in haze pollution research and air quality regulation.</p>
