Screening Approach to Account for Secondary PM_(2.5) in Stationary Source Modeling

摘要

Recent guidance from United Stated Environmental Protection Agency (EPA) suggests that PM_(2.5) modeling should account for secondary impacts as it relates to formation of [NH4]_2SO_4 (ammonium sulfate) and NH_4NO_3 (ammonium nitrate) particulates that result from precursor SO_2 and NOx gaseous emissions. The guidance from EPA suggests a tiered approach for determining which sources would be important to consider when assessing secondary PM_(2.5) concentrations, but the guidance lacks specifics as to how the evaluations should be conducted if they are needed. This paper presents a screening methodology that uses a first-order conversion rate method to estimate the formation of secondary particulate sulfates and nitrates from emissions of primary sulfur dioxide (SO_2) and oxides of nitrogen (NOx), respectively. Estimates of the effective hourly transformation rate are calculated by using either modeling results or evidence from historical field studies. The modeling approach could use the ratios of the CALPUFF modeled concentrations of secondary species to primary species and the corresponding transport time. We also review numerous historical field studies and provide recommendations for look-up table screening-level %/hr conversion rates of gas to particle formation from the field study findings. Hour-of-day and seasonally varying frequency distributions of the modeled hourly transformation rates are then used to compute a conservatively high transformation rate that can be applied in a simple first-order conversion equation to estimate the corresponding secondary particulate emission rate from a source given a specified level of primary SO_2 and NOx emissions. The calculated secondary PM_(2.5) emissions (which vary with transport time) are then added to the primary PM_(2.5) emissions and modeled within AERMOD. Although CALPUFF has the ability to model the near-source impacts of secondary particulate, CALPUFF is not the EPA-approved model for near-field applications within 50 kilometers for New Source Review (NSR) applications. Recent attempts to use CALPUFF to regulatory model applications in the near-field have been met with resistance by EPA. As such, the screening-level methodology presented could use CALPUFF as a "tool" to develop chemical transformation rates, but relies upon the dispersion of AERMOD to model the additional secondary particulate. This methodology is intended for use in the New Source Review (NSR) permit application process and does not involve use of state-of-the-science whole-atmosphere models such as CMAQ or CAMx. These whole-atmosphere models are not yet readily applied or approved for NSR permitting. CALPUFF may be more appropriate in this context because it is an EPA guideline model long-range transport, acid deposition and regional haze modeling. The study outlines the methodology and presents a case study of the approach for a natural gas power plant, which under current EPA draft PM_(2.5) modeling guidance would be required to assess PM_(2.5) impacts and "account" for secondary formation. The concept of a parameterized post-processor for AERMOD to account for secondary particulate formation is also discussed as possible future work.