Evaluation of the SO_2 and NO_x offset ratio method to account for secondary PM_(2.5) formation

摘要

The U.S. Environmental Protection Agency (EPA), state and local agencies have focused their efforts in assessing secondary fine particulate matter (aerodynamic diameter ≤ 2.5 μm; PM_(2.5)) formation inprevention of significant deterioration (PSD) air dispersion modeling. The National Association of Clean Air Agencies (NACAA) developed a method to account for secondary PM_(2.5) formation by using sulfur dioxide (SO_2) and nitrogen oxides (NO_x offset ratios. These ratios are used to estimate the secondary formation of sulfate and nitrate PM_(2.5) These ratios were first introduced by the EPA for nonattainment areas in the Implementation of the New Source Review (NSR) Program for Particulate Matter Less than 2.5 Micrometers (PM_(2.5)), 73 FR 28321, to offset emission increases of direct PM_(2.5) emissions with reductions of PM_(2.5) precursors and vice versa. Some regulatory agencies such as the Minnesota Pollution Control Agency (MPCA) have developed area-specific offset ratios for SO_2 and NO_x based on Comprehensive Air Quality Model with Extensions (CAMx) evaluations for air dispersion modeling analyses. The current study evaluates the effect on American Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) predicted concentrations from the use of EPA and MPCA developed ratios. The study assesses the effect of these ratios on an electric generating utility (EGU), taconite mine, food processing plant, and a pulp and paper mill. The inputs used for these four scenarios are based on common stack parameters and emissions based on available data. The effect of background concentrations also evaluates these scenarios by presenting results based on uniform annual PM_(2.5) background values. This evaluation study helps assess the viability of the offset ratio method developed by NACAA in estimating primary and secondary PM_(2.5) concentrations. An alternative Tier 2 approach to combine modeled and monitored concentrations is also presented.