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SENSITIVITY OF THE CMAQ MERCURY MODEL TO GAS-PHASE OXIDATION CHEMISTRY
Citation:
BULLOCK, R. AND T. BRAVERMAN. SENSITIVITY OF THE CMAQ MERCURY MODEL TO GAS-PHASE OXIDATION CHEMISTRY. Presented at 8th International Conference on Mercury, Madison, WI, August 06 - 11, 2006.
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:
Simulations of the Community Multi-scale Air Quality (CMAQ) model for mercury have shown the vast majority of the mercury deposited in the United States to be in the form of oxidized mercury. However, most of this simulated oxidized mercury was the result of atmospheric oxidation rather than direct industrial emissions from domestic sources. The simulated atmospheric oxidation of mercury by reaction with ozone and hydroxyl radical was critical to this finding. The importance of both of these gas-phase oxidation reactions has recently been called into question in a peer-reviewed journal article. Given the uncertainty about the actual rates of these reactions under atmospheric conditions, test simulations of the CMAQ model were performed with both reactions deactivated to see how the simulated deposition flux and source attribution might change. This new modeling shows that simply removing these reactions from the simulation has a dramatic effect, significantly lowering the wet deposition flux to values well below observed levels. These findings illustrate the critical importance of accurate chemical kinetics information to any such modeling assessment of mercury source attribution.