Application of CMAQ at a hemispheric scale for atmospheric mercury simulations

Abstract. Application of a regional model to study of fate and transport of a global pollutant such as mercury in the atmosphere can be challenging and improper usage of models may lead to questionable results. The difficulties in such application stem from the fact that regional models are usually used in relatively small domains and rely heavily on initial and boundary conditions (IC/BC) provided by global models where atmospheric physics and chemical mechanisms are generally diverse. This problem is particularly apparent for a persistent air pollutant such as mercury. In this study, a conventional application of the CMAQ (Community Multi-scale Air Quality) modeling system on regional scale was extended towards a hemispheric scale. Two simulations were performed using different IC/BC obtained from two global models. In terms of model evaluation, aircraft measurements of total gaseous mercury (TGM) concentration as well as mercury concentration and deposition data from ground-based measurements were used altogether in comparisons with the model simulations. The model results suggested an improvement in performance, as evidenced by a better circulation of the pollutant in Northern hemisphere relative to regional-scale simulations performed in our previous work. In this study, the simulation results using the two different inputs were found to be convergent as the simulation time progressed. The model results also suggested that BC has much weaker influence on the simulation results in a hemispheric domain than that on our previous regional assessment where BC was found to be one of the most important factors. In addition, mitigations of influences from IC/BC on model results in a hemispheric domain and implication of peaks of TGM concentration evident in aircraft measurement are also discussed.