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Modeling and Meteorology
Modeling and Meteorology
The primary purpose of the Modeling and Meteorology program is to conduct air quality modeling to identify impacts from the construction or modification of facilities that are seeking a permit to install / operate air emission sources. This includes conducting modeling or auditing modeling results from environmental consultants as required by State of Michigan Air Pollution Control Rule (R 336.1227) and/or federal rules (40 CFR §52.21).
Additional program responsibilities include:
- Simulating air quality improvement from the implementation of selected control measure strategies in those areas that are not attaining or maintaining a NAAQS.
- Assistance in the development of State Implementation Plans (SIPs)
- Assessing meteorological conditions that contribute to the occurrence of elevated pollutant levels, providing daily Air Quality Index (AQI) forecasts, and issuing air quality advisories when conditions warrant.
- Regional / Multi-State photochemical modeling
The meteorological data support document includes a station map showing the locations of our 72 available meteorological (met) stations. Hourly met data is collected at all stations and 29 of the stations also collect one-minute met data. This document includes a table with links to ZIP files which contain canned one-minute and hourly met data that can be used when performing dispersion modeling with AERMOD. Each ZIP file includes a five-year dataset (2016 to 2020) with both Surface Met Data and Profile Met Data. USEPA recommends the use of one-minute met data for input in dispersion modeling. Their reasoning is that hourly average winds calculated with the one-minute meteorological data are more reflective of actual conditions as opposed to the single two-minute observation used with the hourly met data.
AERMOD is the preferred USEPA regulatory dispersion model; therefore, only meteorological data to be used with AERMOD is available for download from this site.
When performing regulatory modeling for a stationary source, it is important to use data from the station most representative of conditions at the source location. The representativeness of a station is often measured by proximity. However, proximity alone does not guarantee that a particular station is the most representative station available. The AQD can be consulted to help identify which station is the most representative for a specific modeling demonstration.
Modeling for sources emitting criteria pollutants may be required to demonstrate compliance with National Ambient Air Quality Standards (NAAQS). If a demonstration is needed, guidelines specify that ambient backgrounds be added to predicted concentrations. Ambient backgrounds are calculated via data made available from various air monitoring stations located across the region.
The appropriate selection of a particular air monitoring station depends on several factors including, but not limited to, the following:
- proximity of the monitor to the modeled source,
- location of the monitor with respect to the modeled source (e.g. upwind or downwind based on prevailing winds),
- available years of data,
- sighting objective of the monitor,
- completeness of data, and
- availability of data from a potentially more representative monitor.
When calculating a background it is desirable that data from the last three years be used. Data from the last three years will not always be available. In these cases, the AQD can be consulted as to which alternatives can be considered.
Prevention of Significant Deterioration Baseline Dates
When modeling to demonstrate compliance with the Prevention of Significant Deterioration (PSD) increments, it is necessary to include all PSD increment-consuming sources in addition to the proposed source. All post-baseline sources emitting SO2, NO2, PM10, or PM2.5 are considered to consume increment whether or not their emission rates exceed EPA's de minimus ton per year levels. To determine whether or not an existing source consumes increment it is necessary to know the applicable baseline dates. There are two types of baseline dates: major source and minor source.
The major source baseline dates are fixed dates identified in the 1977 Clean Air Act for each of the three pollutants. Emissions associated with a modification at a major stationary source consume increment after this date.
The minor source baseline date was set by the first completed PSD application received by the Air Quality Division. Emission changes at all sources after this date affect the increment. For this reason, it is commonly referred to as the "baseline date." It is possible to expand the available increment if a post-baseline source is to reduce or eliminate the emissions of the pollutant. Minor source baseline dates are established for different areas of the state called Air Quality Control Regions (AQCRs). Note that AQCR 123 has a separate area for which PM10 has an alternate baseline date.