A Comprehensive Coupled Model for Tropospheric Halogen Chemistry: Evaluation of Impacts on Tropospheric Ozone, Hydroxyl Radical (OH), and Mercury

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The fate of gaseous emissions is determined by the oxidative capacity of our atmosphere that controls the production of ozone (a criteria pollutant and short lived greenhouse gas), reactive mercury (a toxic pollutant) and the lifetime of methane (a greenhouse gas). The conventional view that hydroxyl radical is the dominant oxidant has been altered by observations that show that halogens (chlorine, bromine and iodine) from natural and anthropogenic sources could also play an critical role as oxidants. However, these processes are not included in current models limiting their ability to predict future impacts. The proposed research will develop a global scale atmospheric chemistry model with this new halogen chemistry that will be evaluated with observations to enable predictions with proper validation and uncertainty quantification.The proposal addresses the role of halogen radical photochemistry on the production of tropospheric ozone (a short lived greenhouse gas), the hydroxyl radical (the major oxidant) and the toxic pollutant Mercury that is uncertain but potentially very important. The research will understand the fundamental mechanisms that cause the outstanding disagreements between observations of key atmospheric oxidants and pollutants and model simulations that limit our ability to predict future changes. To achieve this it will implement a coupled halogen (Cl-Br-I) chemistry photochemical mechanism using existing laboratory measurements within the state-of-the science community GEOS-Chem model framework and evaluate it by the large data sets (from aircraft, ground and space) of atmospheric composition as a function of latitude, longitude, altitude and season that are now available (including from NSF funded airborne field campaigns). This comprehensive model-measurements integration will be used to critically diagnose specific signatures of halogen radicals on ozone, hydroxyl radical and mercury, quantify uncertainties and identify gaps that will enable a new understanding of the role of halogen radicals on tropospheric chemistry to better quantify (1) radiative forcing by ozone, (2) the factors controlling ozone affecting air quality, oxidative capacity and methane lifetime and (3) the impacts of mercury on ecosystems.