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We compare results from the CMAQ photochemical model for mercury deposition in Florida with measurements from 2009 associated with the Florida Mercury TMDL project, with focus on the question: can model predictions for the impact of local versus global emissions be validated versus measurements? Model wet deposition shows reasonable agreement with measurements, and the lack of variation between sites in Florida in both model and measurements is consistent with the model prediction for limited impact from local sources. Local sources have a larger impact on dry deposition. However, the model overestimates ambient RGM by a factor of 10 versus measured values.

Atmospheric deposition of mercury (Hg) is a major process that contributes mercury loadings in ecosystems resulting in the bioaccumulation of mercury in fish and other wildlife that has prompted public health concerns. Although methods for measuring mercury that is wet deposited have been well studied, there are limited established methods for directly measuring dry deposition of atmospheric mercury. A new method using a turf surrogate surface (TSS) technique was developed to address limitations in published dry deposition collection methods. This method was deployed on a large scale for the first time during measurement intensives conducted as part of a Hg Total Maximum Daily Load (TMDL) project in Florida, USA. Wet and dry deposition of mercury were measured at 15 sites in Florida in 2009 and 11 sites in 2010. The dry deposition of mercury measured during the month-long intensives ranged from 0.6 μg/m2 to 2.1 μg/m2. Wet deposition of mercury measured during the month-long intensives ranged from 1.5 μg/m2 to 6.9 μg/m2. A North-South spatial gradient was observed for wet deposition of mercury; however, dry deposition of mercury displayed more general variability.