Suspended Particulate

Prior to 1970, industrial sources were largely responsible for the high levels of particulate measured in the Missoula valley.  In particular, unregulated industrial wood waste burning at local lumber mills produced large quantities of smoke that became trapped in the Missoula valley.

By 1974, however, strict enforcement of emission standards reduced industrial emissions in the valley by more than 90 percent. As a result, Missoula met the Federal annual average ambient air quality standard for total suspended particulate (TSP) in 1973 and again in 1975. However, violations of the daily TSP standard continued during the winter months and the respite from generally poor air quality was short.

After the oil embargo of the 1970s, wood became a popular home heating alternative in Missoula. In 1974 and 1975, local air quality officials found that wintertime particulate levels were increasing and that TSP collection filters were darker in color than those collected earlier in the decade. They suspected residential wood burning as the source of the increased particulate levels and the cause of the darker filters.

PM10

In July of 1987, the U.S. Environmental Protection Agency (EPA) passed a PM₁₀ (particulate matter with an aerodynamic diameter less than or equal to 10 microns) standard that replaced the TSP standard. The PM₁₀ standard is more protective of human health, because the 10 micron and smaller particles can reach into the small airways of the lungs, while the larger particles are trapped and expelled by the body’s defense mechanisms. The 24-hour PM₁₀ NAAQS is 150 µg/m³, not to be exceeded more than once per year on average over a three-year period. The annual PM₁₀ NAAQS was 50 µg/m³, averaged over three years.  In 2006, the EPA revoked the annual PM₁₀ NAAQS.

During the winter of 1986/1987, MCCHD conducted a chemical mass balance study (CMB) at Rose Park to apportion the sources of PM₁₀ in the valley.  Residential wood smoke contributed 47% of the PM₁₀ during the study, followed by road dust at 22.6%, motor vehicle exhaust at 10.2% and industry at 7.6%. This study confirmed that residential wood burning had replaced industry as the primary source of particulate pollution in the valley.

Missoula exceeded the annual average PM₁₀ standard in 1986 and exceeded the 24-hour PM₁₀ standard several times between 1987 and 1989.  Because of these exceedances, Missoula was designated a non-attainment area for PM₁₀ and Montana was required to submit a State Implementation Plan (SIP) to the U.S. Environmental Protection Agency by 1990 that included monitoring, emission inventories, chemical analysis of particulate to identify sources, and regulations adequate to meet the PM₁₀ standard in Missoula within three years. This plan was written by the Missoula City-County Health Department.

EPA fully approved the Missoula nonattainment plan in 1995. In the 1980s and early 1990s, Missoula PM emissions were primarily attributed to smoke from residential wood burning and fugitive dust from travel on roads.  These sources composed roughly 92 percent of the PM₁₀ emissions in the 1986/87 winter emission inventory that was submitted as part of the 1992 SIP submittal.  Point sources were responsible for a small percentage of PM₁₀ emissions.  Because road dust and residential wood smoke were the significant PM₁₀ sources, Missoula’s PM₁₀ nonattainment plan included a wood stove removal program, restrictions on wood burning devices that can be installed in the Missoula Air Stagnation Zone, emission restrictions for wood burning devices during Air Pollution Alerts and Warnings, paving requirements for new roads, driveways and parking lots, and requirements to reduce silt loading on paved roads by addressing sanding and chip sealing standards as well as street sweeping and deicer requirements.

As a result of all the community work and effort that has gone into reducing particulate emissions, Missoula has not violated a federal particulate standard since 1989.

There was a dramatic decrease in Missoula’s human-caused PM₁₀ pollution from the 1980s through 2014. These decreases have come about because of a community-wide effort to control and reduce emissions from road dust, residential wood burning and industry.  Because winter inversions that trap pollutants near the ground are typical of mountain valley topography, most control measures have been aimed at reducing winter PM₁₀ emissions.

In the summer, human-caused PM₁₀ pollution typically includes road dust, construction fugitive emissions and smoke from outdoor burning.  However, because dispersion conditions in the spring and summer months are typically good to excellent, these sources generally have minimal impact on Missoula’s air quality.

It is important to note, however, that summer wildfires regularly contribute to the worst air pollution days in Missoula.  Since the 2000, wildfires have grown more severe, and wildfire season can last into late September and early October.  Figure 2.2-2 omits wildfire data in order to accurately display the impacts Missoula’s Air Pollution Control Program has had on reducing human-caused particulate pollution.

In the winter of 1995/1996, Missoula performed another CMB study to see if the apportionment of particulate had changed since 1987 (Figure 2.2-3).  The most striking differences between the two studies were that the average PM₁₀ levels decreased from 107 µg/m³to 58.8 µg/m³, and the contribution from residential wood combustion decreased (from 47% to 10.5%), while the contribution from road dust and other vehicle-related emissions increased from 32.8% to 64.8%.

Vehicles continue to be the primary source of PM₁₀ in Missoula. A winter 2010 PM₁₀ emission inventory estimated that vehicles contribute 68% of Missoula’s winter PM₁₀ via road dust and exhaust, tire wear and brake wear.

Based on decades of proven compliance with the PM₁₀ NAAQS, in 2016, the Missoula City-County Health Department submitted a redesignation requested and limited maintenance plan to the EPA, requesting to be redesignated as in attainment for PM₁₀.  The EPA has indicated they will consider the request in fall 2017.  Once redesignated, Missoula will enter a 20-year maintenance period for PM₁₀