Particulate Matter Concentration: Causes, Standards, and Air Quality

by Scott Richter and Dr. Patrick Jones

People who have lived in the Spokane area over the past few years likely noticed the air pollution caused by smoke from wildfires during summer 2021 was not as bad as most – specifically 2017 and 2020. This year, at least locally, we were spared multiple weeks of thick, yellow-brown smoke coming from fires seemingly from everywhere but the middle of the Pacific Ocean.

During 2020, the combination of unprecedented fire seasons in both Oregon and California and the relatively common late summer easterly wind patterns actually sent wildfire smoke hundreds of miles east over the Pacific Ocean before turning west again and while greatly dissipated, eventually finding its way to Europe.

An important part of overall air quality is Particulate Matter (PM) per cubic meter. PM is defined as the solid and liquid particles found in ambient air. Spokane Trends Indicator 4.1.7 PM2.5 Concentration measures the annual average of the 24-Hour PM2.5 concentration levels based on continuous monitoring. PM2.5 measures the volume of particles less than 2.5 micrometers in diameter.

Exposure to elevated levels of PM2.5 can lead to increased cases of respiratory illnesses and illness-related mortality. When inhaled, PM2.5 is small enough to lodge deep into the lungs, damaging delicate lung tissue. While PM2.5 can affect anybody, people especially sensitive include those with existing heart and lung diseases, as well as pregnant women, young children, and the elderly.

Sources of PM are either directly emitted or the result of chemical reactions. Directly emitted particles can come from wood fires, power plants, and industrial facilities. Additionally, farming, agricultural burns, dust from roads, construction activities, all agitated by wind, create PM. Indirectly emitted particles, or secondary particles, are typically from chemical reactions traced to naturally occurring gasses or burning fossil fuels, whether from vehicle engines or power plants.

So smoke is not the only source – basically all combustion creates PM, not just wildfires.

The Clean Air Act of 1970 (42 U.S.C. §7401 et seq) requires the U.S. Environmental Protection Agency (EPA) to “periodically review the standards to ensure that they provide adequate health and environmental protection, and to update those standards as necessary.”

During the 1997 review, EPA reduced the fine particle standard from 15.0 micrograms per cubic meter (µg/m3) to 12.0 where it has remained through the latest completed review in 2012. According to EPA, “An area will meet the standard if the three-year average of its annual average PM2.5 concentration (at each monitoring site in the area) is less than or equal to 12.0 µg/m3.”

Currently, EPA is in the process of reviewing the National Ambient Air Quality Standards (NAAQS). The review has yet to be completed so the fine particle standard of 12.0 µg/m3 for PM2.5 remains in place.

NAAQS create thresholds for criteria air pollutants, which in addition to PM include carbon monoxide, lead, ozone, nitrogen dioxide, and sulfur dioxide.

Looking at the graph for Indicator 4.1.7, we can observe that at no point in the series does Spokane County have the lowest average. Spokane County is similar to Benton County, WA (Tri-Cities) and Ada County, ID (Boise). The comparison of Benton and Spokane Counties reveals more similarities than each have with Ada County. Ada County shows the two highest annual measurements in the period observed --11.95 during 2013 and 11.06 during 2007.

The highest annual average in Spokane County was 10.69 occurring during 2020, the year with the most recent annual data available. The lowest annual average in Spokane County was 5.08 during 2018.

Wildfire events causing widespread poor air quality are certainly part of this indicator, but keep in mind this is an annual average calculated by the daily averages. While these events can greatly impact us during the days or weeks they are occurring, major events decreasing air quality are the exception, not the rule. Additionally, the benchmark counties are close enough in proximity to Spokane County to have air quality diminished by the same wildfires.  

Over the 16-year span of this series, there has not been much of an increase. Looking at another indicator on Spokane Trends, 4.1.1 Air Quality Days, maybe a little contrary to thinking, but 2020 had the highest number of Good Air Days, 323, than any other year for Spokane County and overall for King and Pierce Counties.

During the summer of 2021, Spokane set new records for total days over 100 degrees in a calendar year, total days over 90 degrees in a calendar year, all-time record high temperature, the hottest month on record based on the average of the daily high temperature, all while the nighttime low temperatures remained higher than normal. Unlike the global mean surface temperature, these data do not provide any real evidence of global warming. Yet indirectly, global warming increases the chance of elevated PM2.5 by way of an increasing risk of wildfires – both in rate and intensity.