AbstractMost regional three‐dimensional chemical transport models neglect gaseous elemental mercury (GEM) oxidation by bromine (Br) radicals and Br chemistry. In this study, the Community Multiscale Air Quality model with its default mercury module (CMAQ‐Hg) was modified by implementing a state‐of‐the‐art algorithm depicting Hg reactions coupled with Br chemistry (CMAQ‐newHg‐Br). Using CMAQ‐newHg‐Br with initial and boundary concentrations (ICs and BCs) from global model output, we conducted simulations for the northeastern United States over March–November 2010. Simulated GEM mixing ratios were predominantly influenced by BCs and hence reflected significant seasonal variation that was captured in the global model output as opposed to a lack of seasonal variation using CMAQ‐Hg's default constant BCs. Observed seasonal percentage changes (i.e., seasonal amplitude [=maximum – minimum] in percentage of the seasonal average) of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) were 76% and 39%, respectively. CMAQ‐newHg‐Br significantly improved the simulated seasonal changes in GOM and PBM to 43% and 23%, respectively, from 18% and 16% using CMAQ‐Hg. CMAQ‐newHg‐Br reproduced observed Hg wet deposition with a remarkably low fractional bias (FB; 0.4%) as opposed to a −56% to 19% FB for CMAQ‐Hg simulations. Simulated Hg dry deposition using CMAQ‐newHg‐Br excluding the GEM + OH reaction agreed well with studies using inferential methods and litterfall/throughfall measurements, and the discrepancy varied over 13%–42%. This study demonstrated the promising capability of CMAQ‐newHg‐Br to reproduce observed concentrations and seasonal variations of GEM, GOM and PBM, and Hg wet and dry deposition fluxes.