Significant Impact Area and Vulnerable Zone Screening Analysis

Abstract

Significant impact area, and vulnerable zone, both circular and delimited by prescribed ground-level concentration values, appear in several environmental regulations. The radius of the circle is often estimated by atmospheric dispersion models in an iterative approach using various grid distances which are applied until the maximum distance is found where the concentration no longer exceeds the prescribed value. In this paper, formulas are presented to estimate the radius directly. As required by Prevention of Significant Deterioration (PSD) regulations, a major new source (or modification of an existing major source) must analyze existing air quality in the vicinity of the new source, and demonstrate that emissions from the new source and other nearby sources will not violate National Ambient air Quality Standards (NAAQS) or PSD incremental limits. Detailed modeling is used to demonstrate compliance of the new source and other nearby sources with NAAQS and PSD incremental limits. Nearby sources are taken to mean any sources which would have an impact on the significant impact area (SIA) of the new source. The SIA is the circular area beyond which the impact of the new source alone is less than significance levels established by the Environmental Protection Agency (EPA). The formulas presentedmore » would be useful as conservative screening tools for short-term (e.g., 1-3 h) impact area assessments.« less