Innovative Dispersion Modeling Practices to Achieve a Reasonable Level of Conservatism in AERMOD Modeling Demonstrations

摘要

The advent of the short term NAAQS has prompted us to reassess the level of conservatism commonly used in dispersion modeling analyses. One area of conservatism in dispersion modeling relates to the assumption that a given emission unit is in operation at its maximum capacity every hour of the year. This overly conservative assumption is evident when performing dispersion modeling analyses dealing with emission units such as emergency or peaking engines that are transient in operation. Another important element in NO_2 dispersion modeling relates to the conversion of NO_x to NO_2 concentrations. Appendix W specifies a three tiered approach to account for this conversion. Though, most facilities with significant NO_x emissions use the Tier 3 approach to characterize NO_x to NO_2 conversion. However, the Tier 3 methods (i.e., OLM and PVMRM) are complex and additional information such as ambient ozone concentrations and in-stack ratios is required to calculate NO_2 concentrations. Another element of conservatism in NAAQS demonstrations relates to the combining of predicted (modeled) concentrations from AERMOD with observed (monitored) concentrations. Normally, some of the highest monitored observations are combined with AERMOD results yielding a very conservative total concentration. For example, in the case of the 1-hour NO_2 NAAQS, the chances of the 98th percentile monitored concentration occurring at the same time as the meteorology to generate the 98* percentile ambient concentration is extremely low. Therefore, assuming that both events happen at the same time is overly conservative. The current case study addresses three areas of conservatism in current dispersion modeling practices including: the use of a maximum constant emission rate for equipment that is transient in operation; the current methods to account for NO_x to NO_2 conversion; and the use of an overly conservative background concentration. The case study evaluates a facility comprised of four reciprocating internal combustion engines (RICE) that operate under a peak shaving agreement of up to 250 hours per year. The modeling evaluation includes a comparison of modeled concentrations obtained using the current modeling methods and those obtained applying the following methods: the Emission Variability Processor (EMVAP); the updated Ambient Ratio Method (ARM2), and the combinations of EMVAP and the ARM2 Method. Justification for the use of a reasonable background concentration to combine with the AERMOD predicted concentration is also presented. The use of these methods, if accepted by regulatory agencies, can help facilities demonstrate compliance in dispersion modeling analyses while still being protective of the NAAQS.