Air Quality Impact Evaluation

An Air Quality Impact Evaluation (AQIE) is an evaluation, using approved computer models, of the ambient air quality impacts that the public may be expected to be exposed to due to air pollution emissions from one or more facilities.  The pollutants emitted by the facility, the emission rates, stack parameters (e.g. heights, flow rates, temperature), building parameters, terrain, and meteorology are all taken into account to predict potential ambient impacts caused by the facility.  An AQIE may be conducted for criteria pollutants (e.g. CO, NOx, PM, SO2) or hazardous air contaminants (e.g. benzene, formaldehyde, arsenic, chromium, …).   For emissions of hazardous air contaminants, the provisions of 5-261 of the Vermont Air Pollution Control Regulations (VAPCRs) apply and those requirements can be found on the “Hazardous Air Contaminant Review” page.

An applicant for a Permit to Construct may be required to conduct an AQIE as part of the permit application in order to demonstrate that the proposed emissions from the facility will not cause or contribute to any violations of state or national ambient air quality standards.  For comparison to criteria pollutant ambient air quality standards the projected impact from the facilities are added to the existing ambient background concentrations representative of the facility location.  The ambient background data to use in this analysis is summarized and updated annually.  

Larger sources of criteria pollutants may also be required to demonstrate that the proposed emissions from the facility will not  exceed prevention of significant deterioration (PSD) increments.  Under the PSD regulations, no proposed new or modified source may have an impact greater than the respective increment, even if compliance with the ambient standard may still be demonstrated.  These provisions are intended to prevent any one source from “using up” all the available clean air such that no further projects can be permitted.  For comparison to PSD increments, only the impact from the facility’s emissions is used.  Ambient background is not included for comparison to allowed PSD increments.

While an AQIE may be required as part of the application for any Permit to Construct, AQCD policy is to use discretion when the proposed increase in emissions from the new source or modification is less than the significant emission rate provided in 5-101 of the VAPCRs and provided reasonable stack parameters are proposed.  

Pollutant Significant Emission Rate (tpy)
per 5-101 of VAPCR
Carbon Monoxide 50
Nitrogen Oxides 40
Sulfur Dioxide 40
Particulate Matter (PMtotal) 25
Particulate Matter (PM10) 15
Particulate Matter (PM2.5) 10

An AQIE shall follow the AQCD Air Quality Impact Evaluation Guidelines and 40 CFR Part 51 Appendix W - Guideline on Air Quality Models.

The AQCD is in the process of updating this page and the modeling guidance specific to Vermont requirements.  In the interim, the State of Maine has recently updated their Permit modeling website which provides a lot of information you may find useful.