Evaluation of CMAQ Coupled With a State‐of‐the‐Art Mercury Chemical Mechanism (CMAQ‐newHg‐Br)

Abstract

Most 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.

Is oxidation of atmospheric mercury controlled by different mechanisms in the polluted continental boundary layer vs. remote marine boundary layer?

“…The rate of reaction 2 is expected to be predominantly controlled by NO 2 in more polluted regions, and possibly HO 2 in more remote regions, considering the apparently relatively small influence of the nature of Y, compared to that of its concentration, on the reaction rate (Dibble et al 2012, Jiao and Dibble 2017). Regional modeling over the northeastern United States by Ye et al (2018) indicated relatively good seasonal and diurnal agreement for GEM oxidation and produced GOM and PBM when applied with the updated multistep bromine-induced mechanism (Ye et al 2016), excluding GEM oxidation by OH and O 3 . Nevertheless, as discussed in Si and Ariya (2018), there are indications that in the lower troposphere, particularly in the boundary layer, GEM oxidation is more complex and its initiation may predominantly occur by OH, O 3 and other oxidants, as suggested by earlier studies, instead, or in addition to oxidation by halogen radicals.…”

Understanding factors influencing the detection of mercury policies in modelled Laurentian Great Lakes wet deposition

“…Similar model biases, particularly summer underestimates of wet deposition, have been reported in several modelling studies in this region. 25,33,73 Underestimates of precipitation in GEOS-5 in the Midwest, summertime prevalence of deep convective thunderstorms that more effectively scavenge upper-troposphere Hg(II) (and which are not resolved in a global-scale models like GEOS-Chem), and model underestimates of upper tropospheric Hg(II) may contribute to these wet deposition underestimates. 33,73,74 Lower snow collection efficiency, compared to rain, of MDN samplers has also been hypothesized to contribute to wintertime differences between observed and modelled values.…”

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Pre-Reactive Complexes and a Bifurcation

“…This ignorance introduces large uncertainties in global and regional models of atmospheric mercury oxidation. [5][6][7][8] Field and modeling studies implicate atomic bromine (Br•) as the species initiating oxidation of Hg(0) in bromine-rich environments, including the marine boundary layer and polar regions during the atmospheric mercury depletion events. [9][10][11] Furthermore, recent global models 9,12,13 assert that oxidation by Br•, alone, can account for the extent of global oxidation of Hg(II) in the atmosphere.…”

“…Indeed, using rate constants computed by Jiao and Dibble, 20 models which include these reactions have found improvements in model-measurement agreement of seasonal variations and deposition fluxes of mercury on both the regional 8 and global scales. 12 Notably, an extension of the CMAQ regional model 8 reproduced for the first time the diurnal pattern of a gaseous oxidized mercury (GOM, mostly Hg(II)) daytime maximum at a marine site.…”

“…Indeed, using rate constants computed by Jiao and Dibble, 20 models which include these reactions have found improvements in model-measurement agreement of seasonal variations and deposition fluxes of mercury on both the regional 8 and global scales. 12 Notably, an extension of the CMAQ regional model 8 reproduced for the first time the diurnal pattern of a gaseous oxidized mercury (GOM, mostly Hg(II)) daytime maximum at a marine site. Although other reactions of BrHg• have been considered (addition to Br• and •OH, 15 addition to halogen oxides 19,21 and ROO•, 20 hydrogen abstraction from sp 3 -hybridized carbons, 22 and addition to sp 2 -hybridized carbons 22 ), these reactions are much less important due to significantly lower reactant concentrations than NO 2 and HO 2 , 12 or due to unfavorable reaction enthalpies (for hydrogen abstraction and addition).…”