During the May 2014 meeting of the Board, two awards were granted for financial assistance to attend & present at the 2014 A&WMA Annual Conference on the Environment.  The following two individuals are presenting their respective topics, this year, at ACE.  Congratulations!

Abstract

As the federal air quality standards get stricter, and oil and gas production in upwind states and Canada rapidly expand, downwind states like Minnesota will face more challenges in future ozone air quality management. It is becoming very important to consider the transport of ozone and its precursors from neighboring states when designing effective air pollution control strategies. The purpose of this study is to evaluate the contribution of sources outsides the state versus contribution of local sources to the ozone levels in Minnesota during summer time. More specifically, this work will quantify the magnitude of U.S. ‘background’ ozone and regionally transported ozone, and their influence on local air quality.

We use a state of the art 3D global chemical transport model (GEOS-Chem) to perform nested grid (0.5º×0.667º) simulations over the United States. We conduct a standard simulation (base run) and a perturbed run with Minnesota local sources turned off, including anthropogenic sources of CO and NOx, and both anthropogenic and biogenic sources of VOCs. The simulated ozone from the perturbed run can be considered to represent the contribution of sources outside Minnesota, and the differences between these two runs can be used to approximately represent the contribution of Minnesota sources alone. Note that due to the non-linear nature of the ozone response to the changes of its precursor emissions, this approach cannot perfectly reflect the contribution of each source, but can still provide a good approximation.

The methodology developed in this research could be applied to other states for cross-border air pollution transport problems. It can also be applied to study other pollutants such as particulate matter. The results will provide scientific basis for regional ozone air quality management and emission control strategies.

Abstract

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 NO2 dispersion modeling relates to the conversion of NOx to NO2 concentrations. Appendix W specifies a three tiered approach to account for this conversion. Though, most facilities with significant NOx emissions use the Tier 3 approach to characterize NOx to NO2 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 NO2 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 NO2 NAAQS, the chances of the 98th percentile monitored concentration occurring at the same time as the meteorology to generate the 98th 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 NOx to NO2 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.