Variable Emissions in Air Quality Modeling

摘要

The subject of variable emissions has received considerable attention recently due, in large part, to the introduction of new 1-hour national ambient air quality standards (NAAQS) for sulfur dioxide (SO_2) and nitrogen dioxide (NO2) and to the increased focus on attaining the particulate matter with an aerodynamic diameter less than 2.5 microns (PM2.5) 24-hour NAAQS. The small numerical levels of these standards, and correspondingly low levels of the significant impact levels (SILs), frequently lead to the need to model existing sources when seeking a permit for a new source, to make the demonstration of not "causing or contributing" to a NAAQS violation. Air quality modeling plays a key role in estimating the impacts of proposed new sources since their impact is not included in existing monitoring data. For normal operating conditions, projecting future air quality is most frequently undertaken by assuming continuous operation of the new source at a maximum emission rate. Current USEPA policies also frequently require the use of continuous operation at maximum emissions from existing, background sources as part of a NAAQS demonstration. This approach is in contrast to the approach recommended in a recently issued draft technical assistance document (TAD) for SO_2 area designations. Although the purposes of the designations TAD and an analysis conducted in support of a permit are by nature very different, the recommendations in the TAD and the focus of those recommendations are useful to consider in the development of more realistic approaches to modeling analyses for both existing and future sources.