Citation:

Bullock, O R. AQUEOUS REDUCTION OF HG2+ TO HG0 BY HO2 IN THE CMAQ-MODEL. Presented at International Conference on Mercury as a Global Pollutant, Ljubljana, Slovinia, June 27-July 2, 2004.

Impact/Purpose:

The objective of this task is to improve EPA's ability to accurately predict the concentrations and deposition of air pollutants in the atmosphere that are known or suspected to cause cancer or other serious health effects to humans, or adverse environmental effects. It is an essential component of EPA's National Air Toxics Assessment (NATA), which seeks to identify and quantify the concentrations and sources of those hazardous air pollutants which are of greatest potential concern, in terms of contribution to population risk. It is a major contributor to NERL's Air Toxics Research Program.

"Air toxics" or "hazardous air pollutants" (HAPs) is a category that covers a large variety of chemicals, which range from relatively non reactive to extremely reactive; can exist in the gas, aqueous, and/or particle phases; display a large range of volatilities; experience varying deposition velocities, including in some cases revolatilization; and are emitted from a wide variety of sources at a large variety of different scales. In addition, concentrations of air toxics are needed by regulators for both short (days) as well as long (up to a year) time scales. These requirements challenge our current capabilities in air quality models far beyond the needs for other pollutants, such as ozone. The specific work being done under this task involves 1.) developing and testing chemical mechanisms which are appropriate for describing the chemistry of air toxics; 2.) incorporating these chemical and physical mechanisms into EPA's CMAQ modeling system and applying the model at a variety of scales; and 3.) developing the methods for using models to predict HAPs concentrations at subgrid or neighborhood scales; and 4.) using these tools to assess the magnitude and variability of concentrations to which urban populations are exposed.

Description:

Numerical models of atmospheric mercury are formulated based on the current understanding of mercury chemistry in air and in atmospheric water. Recent evidence that significant reduction of Hg2+ by reaction with HO2 may not actually occur in natural atmospheric water has obvious implications for the modeling of atmospheric mercury, the seriousness of which has been investigated in the context of the Community Multi-scale Air Quality (CMAQ) model. A published rate constant of 1.1x10 4 M-1s-1 for the aqueous reduction of Hg2+ by HO2 was used in the mercury version of the CMAQ model (CMAQ-Hg) that was previously tested for its agreement to observed measurements of mercury wet deposition in the United States. A modeling analysis has now been conducted to investigate the effect of removing this controversial reaction from the CMAQ-Hg model. The results show more mercury wet deposition, thus reducing the low bias previously found from the base model configuration. However, other sources for this low bias have also been identified. An examination of mercury concentrations in precipitation brings into question the apparent model improvement and highlights the need for confident determination of chemical kinetic modeling parameters.

The information in this document has been funded wholly or in part by the U.S. Environmental Protection Agency (EPA and has been developed as part of an interagency agreement with the National Oceanic and Atmospheric Administration (NOAA). It has been subjected to review by both EPA and NOAA, and has been approved for publication, although it may not necessarily reflect official Agency policy.

Record Details: