Citation:

PONGPRUEKSA, P., C. LIN, S. LINDBERG, C. JANG, T. BRAVERMAN, R. BULLOCK, T. C. HO, AND H. CHU. SCIENTIFIC UNCERTAINTIES IN ATMOSPHERIC MERCURY MODELS III: BOUNDARY AND INITIAL CONDITIONS, MODEL GRID RESOLUTION, AND HG(II) REDUCTION MECHANISMS. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 42(8):1828-1845, (2008).

Impact/Purpose:

The objectives of this task are to continue development and improvement of EPA's mesoscale (regional through urban scale) air quality modeling systems, such as the Community Multiscale Air Quality (CMAQ) model, as air quality management and NAAQS implementation tools. This task focuses on needed research and development of air quality models targeted for a major CMAQ model release in FY08. Model development for a broad scope of application is envisioned. For example, CMAQ will need to be able to simulate air quality feedbacks to meteorology and climate as well as intercontinental transport. The 2008 release of CMAQ is timed to coincide with EPA/OAR's and the states' needs for an improved model for assessments of progress (mid-course corrections) in the post-SIP submittal timeframe.

Description:

In this study we investigate the CMAQ model response in terms of simulated mercury concentration and deposition to boundary/initial conditions (BC/IC), model grid resolution (12- versus 36-km), and two alternative Hg(II) reduction mechanisms. The model response to the change of gaseous elemental mercury (GEM) concentration from 0 to 2 ng m^-3 in IC/BC is found to be very linear (r2 > 0.99) based on the results of sensitivity simulations in July 2001. An increase of 1 ng m^-3 of GEM in BC resulted in an increase of 0.81 ng m^-3 in the monthly average of total mercury concentration, and 1270 ng m^-2 in the monthly total deposition. IC has similar but much weaker effects compared to those of BC. Varying reactive gaseous mercury (RGM) or particulate mercury (PHg) in BC/IC has a less significant impact than for GEM. Simulation results at different grid resolutions show good agreement (slope = 0.950-1.026, r = 0.816-0.973) in mercury concentration, dry deposition, and total deposition. The agreement in wet deposition is somewhat weaker (slope = 0.770-0.794, r = 0.685-0.892). Replacing the aqueous Hg(II)-HO₂ reduction by either RGM reduction by CO (5 x 10^-18 cm³ molecule s^-1) or photoreduction of RGM (1 x 10^-5 s^-1) gives significantly better model agreement with MDN-measured deposition. Possible ranges of the reduction rates are estimated based on model sensitivity results. The kinetic estimate requires further verification by laboratory studies.

Record Details:

Planning Links: