How is the “airshed” being defined?
The “airshed” or spatial area for analyzing emissions from NGS is defined differently for different pollutants and requirements:
For the National Ambient Air Quality Standards’ criteria air pollutants (NO2, SO2, PM, PM10, PM2.5, and CO) excluding ozone, the airshed extent is commonly determined by established policies and regulations. The air quality rules for permitting point sources require an assessment of the impact of emissions to a “significant impact level” or to a concentration that is established by the U.S. Environmental Protection Agency as being significant to public health or welfare. The airshed for this analysis will be determined by air modeling.
Ozone is formed in the atmosphere by combining emissions of nitrogen oxides and volatile organic compounds, in the presence of sunlight. Ozone production is affected by temperature, humidity and other factors. To calculate impacts of ozone formation, a regional air model will be developed to focus on capturing the emissions and chemical reactions in the atmosphere. It is anticipated that the airshed for this analysis would include northern Arizona, southern Utah, the southwestern corner of Colorado, and the northwestern corner of New Mexico; however, the exact extent of this airshed will be established through modeling.
How does the airshed definition impact other resources?
Experts evaluating other resources for the EIS will establish thresholds of impact for their particular resource. These would include ecological resources, human health, sensitive areas, acid deposition, and “Class I areas”. The thresholds could include deposition rates or air quality concentrations.
These thresholds of impact are provided to the air quality modeling specialists, who then use dispersion modeling (representative meteorology and emissions) to depict the impacts in comparison to those thresholds. This modeling depiction establishes an “airshed” within which the impacts are above that threshold and, therefore, may be of concern. The modeling specialists then further calculate the magnitude of impacts over that threshold in the airshed for use in the analysis effort for these other resources.