Source-receptor modeling using high resolution WRF meteorological fields and the HYSPLIT model to assess mercury pollution over the Mississippi Gulf Coast region

The Mississippi coastal region is environmentally sensitive due to multiple air pollution problems originating as a consequence of several developmental activities such as oil and gas refineries, operation of thermal power plants, and mobile-source pollution. Mercury is known to be a potential air pollutant in the region apart from SOX, NOX,CO and Ozone. Mercury contamination in water bodies and other ecosystems due to deposition of atmospheric mercury is considered a serious environmental concern. Identification of sources contributing for the high atmospheric mercury levels will be useful for formulating pollution control and mitigation strategies in the region.

The present study demonstrates the use of high-resolution output from the WRF (Weather Research Forecast) model as input to the HYSPLIT atmospheric dispersion model to analyze a high mercury concentration episode measured at the Grand Bay National Estuarine Research Reserve (NERR).

A high mercury concentration episode observed at the Grand Bay NERR during May 5-6, 2008 was selected as a case study. The peak concentration of reactive gaseous mercury (RGM) measured was 170 pg/m3 during this episode, an order of magnitude above the background concentrations observed at the site. The study comprises of two components, one to produce high resolution atmospheric fields (4 km) using WRF-ARW model and the other to drive the HYSPLIT dispersion model using this WRF-ARW output to generate backward trajectories from the NERR station and forward trajectories from the known elevated point sources in the region.

The ARW model was designed to have three two-way interactive nested three domains with 36-12-4 km resolutions with the inner most domain covering the region from 90.28W-84.77W and 29.38N-32.54N. The model was designed to have 43 vertical levels, including 33 levels below 500 hPa to better simulate the boundary layer characteristics. Initial and time varying boundary conditions were taken from NCEP FNL data available at 1 degree horizontal resolution. The land use and terrain fields were generated from USGS data at the nearest corresponding resolution. The model simulated meteorological fields were used to study the diurnal variations of the atmospheric fields and the characteristics of the boundary layer over the study region and are evaluated by comparison against available meteorological observations.

The HYSPLIT atmospheric dispersion model, driven by the output from WRF model, was used to obtain the Lagrangian path of trajectories from the NERR observation station. Backward trajectories were generated for every hour during May 4-7, corresponding to the episode and for one day before and after the episode. These back trajectories are used in conjunction with a regional mercury emissions inventory to identify the potential sources of mercury contributing to the high concentrations observed. Forward trajectories are also studied from each of the identified sources. Throughout the study, trajectory results using high-resolution WRF meteorological data fields are compared with trajectories estimated using coarser meteorological data, e.g., the NOAA EDAS 40km dataset.

This study is part of a larger collaborative effort between Jackson State University, NOAA, and the Grand Bay NERR to study the dispersion of atmospheric pollutants in the Gulf Coast region.