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Mat-Su Butte Site

Last Updated 04/05/2019

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Butte - View From North
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Butte - View From West

Views from Site in Four Cardinal Directions

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Butte - Looking West

Monitoring Siting Criteria for the Harrison Ct. (Butte) Air Quaity Monitoring Site


The Code of Federal Regulation Title 40 Part 58 covers Ambient Air Quality Surveillance. Appendix E to Part 58—Probe and Monitoring Path Siting Criteria for Ambient Air Quality Monitoring contains specific location criteria applicable to State and Local air Monitoring Stations (SLAMS) ambient air quality monitoring probes and inlets. These siting criteria are necessary to ensure the uniform collection of compatible and comparable air quality data. The probe siting criteria discussed in Appendix E must be followed to the greatest extent possible, although there may be situations where some deviation from the siting criteria may be necessary. Specific siting criteria that are phrased with a “must” are defined as requirements and exceptions must be approved through the waiver provisions. However, siting criteria that are phrased with a “should” are defined as goals to meet for consistency but are not requirements.

The Harrison Ct (Butte) air monitoring site houses continuous PM10 and PM2.5 Met One Beta Attenuation Monitors (BAM) with size selective inlets. Below is a list of criteria from Appendix E to 40 CFR 58 applicable to these types of particulate matter monitors. Note, only references to probes or inlets will be addressed as any reference to monitoring paths are not applicable because the instruments in use are not open path monitors.

Horizontal and Vertical Placement

“The probe or at least 80 percent of the monitoring path must be located between 2 and 15 meters above ground level for all O3 and SO2 monitoring sites, and for neighborhood or larger spatial scale Pb, PM10, PM10-2.5, PM2.5, NO2, and CO sites”

The Butte site is classified as a neighborhood scale. As such, the inlet is required to between 2-15m above ground. The instrument inlets are approximately 1.5 m above the surface of the trailer and approximately 4 meters above the ground.

“The probe or at least 90 percent of the monitoring path must be at least 1 meter vertically or horizontally away from any supporting structure, walls, parapets, penthouses, etc., and away from dusty or dirty areas.”

The inlets for both monitors are more than 1m from any other structure.

Spacing from Minor Sources

“If a monitoring site is to be used to determine air quality over a much larger area, such as a neighborhood or city, a monitoring agency should avoid placing a monitor probe, path, or inlet near local, minor sources. The plume from the local minor sources should not be allowed to inappropriately impact the air quality data collected at a site. Particulate matter sites should not be located in an unpaved area unless there is vegetative ground cover year round, so that the impact of windblown dusts will be kept to a minimum.”

The Butte monitoring site is located at the end of a cul-de-sac. The road surface is gravel. While gravel roads are a source of PM10, the monitoring site experiences very little traffic as it only serves three homes. The amount of PM10 generated by vehicle traffic on Harrison Ct can be considered insignificant, as is windblown gravel from the road surface. The gravel road surface does not experience sufficient mechanical disturbance to loosen material for low wind speeds to pick up sufficient dust to impact the measurements.

The primary occurrences of elevated PM10 concentrations are due to windblown dust events causing glacial silt and particles from the Knik riverbed to be transported to the monitoring site. These events typically occur in the spring and fall. The wind speeds associated with these events are high enough to transport the particulates long distances and clearly cover a larger geographic area than just the cul-de-sac. The monitoring equipment at the site collects hourly data for particulate matter, wind speed, and wind direction among others and the onset of these windblown events is clearly visible in the data. The monitoring site was initially chosen specifically to capture windblown dust impacts to the larger community from the Knik River.

During summer months when the road is dry enough to produce PM10 (and not covered by snow), the monitoring site usually does not measure sufficiently high levels of PM10 to adversely affect the design value of the site. (The design value is the measured value which is compared to the EPA PM10 standard.)

While vehicle traffic past the monitor produces low levels of PM10, the emissions are part of the background levels for the site, but are not the main contributor to elevated PM10 levels, which can be close to the PM10 standard. The amount of PM2.5 generated from gravel roads is only a fraction of the PM10 and therefore even less of an issue when considering the siting requirements of a PM2.5 monitoring site.

Spacing from Obstructions

“A probe, inlet, or monitoring path must have unrestricted airflow in an arc of at least 180 degrees. This arc must include the predominant wind direction for the season of greatest pollutant concentration potential.”

There are no structures or buildings nearby the Butte monitoring site, except for trees to the west of the site. For requirements about spacing from trees see the section below.

Spacing from Trees

“Trees can also act as obstructions in cases where they are located between the air pollutant sources or source areas and the monitoring site, and where the trees are of a sufficient height and leaf canopy density to interfere with the normal airflow around the probe, inlet, or monitoring path. To reduce this possible interference/obstruction, the probe, inlet, or at least 90 percent of the monitoring path must be at least 10 meters or further from the drip line of trees.”

At the Butte site, the trees to the west of the site are white spruce that have grown up since the initial siting of the monitoring site. The east and north of the trailer are free of any obstruction, and the south is mostly free of obstruction during the spring, fall (PM10), and winter (PM2.5) when high PM levels occur. Additionally, the trees do not act as a solid wall to create obstruction of air moving to the site. The wind sensor at the site measures winds from all directions, as demonstrated by this example of a windrose for 2018.

Spacing from Roadways

“Spacing for Particulate Matter (PM2.5, PM10, Pb) Inlets. (a) Since emissions associated with the operation of motor vehicles contribute to urban area particulate matter ambient levels, spacing from roadway criteria are necessary for ensuring national consistency in PM sampler siting.”

“For the middle scale site, a range of acceptable distances from the roadway is shown in figure E-1 of this appendix. This figure also includes separation distances between a roadway and neighborhood or larger scale sites by default. Any site, 2 to 15 meters high, and further back than the middle scale requirements will generally be neighborhood, urban or regional scale."

The Butte site is located on the end of a cul-de-sac with very limited traffic impacts. The nearest roadways with documented traffic counts are South McKechnie Loop and the Old Glenn Highway, which are each greater than 20 meters from the monitoring site. The Alaska Department of Transportation publishes annual average daily traffic counts on the Old Glenn Highway close to the Butte monitoring site as 3,277 vehicles per day and 154 vehicles per day on South McKechnie Loop.