A Recap of Wildfire Smoke Levels in August and September 2017

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The 2017 wildfire season is officially the worst Spokane Regional Clean Air Agency (Spokane Clean Air) has on record. During the wildfire events, the Spokane-area saw its highest concentrations of PM2.5 (fine smoke particles) and over the longest duration.

We first saw a glimpse of what was to come on Tuesday, August 1, 2017. An Air Quality Alert was issued by the National Weather Service and Washington State Department of Ecology due to increased wildfire smoke in the region.

On Friday, August 4, a shift in winds brought a thick layer of smoke from fires in British Columbia into the area. The air quality quickly deteriorated and we saw our first “unhealthy/red” Air Quality Index (AQI) reading. From this date through mid-September, the Spokane-area was periodically impacted by severe wildfire smoke.

Understanding the Pollution Levels

The federal, health-based standard for PM2.5 (combustion particles 2.5 microns in diameter and smaller) is 35 micrograms per cubic meter of air (µg/m3) averaged over a 24-hour period, midnight-to-midnight. 

The highest pollution reading due to wildfire smoke in 2017 occurred on Thursday, September 7. The 24-hour average measured was nearly five times the standard at 205.5 µg/m3. When the concentration is converted to the AQI, this was a “very unhealthy/purple” day.

In total, Spokane recorded three “very unhealthy/purple” days during the wildfire season. The last time Spokane had a “very unhealthy/purple” day on record was in 1999 for a dust storm. It’s important to note that the pollutant for the dust storm was PM10. Since monitoring for PM2.5 began, Spokane has not had an AQI reading in “very unhealthy/purple” until this year.

Listed below are the maximum 24-hour (midnight to midnight) average concentrations for PM2.5 when levels reached “unhealthy for some/orange” or above during the smoke events.

PM2.5

Date

24-hour average micrograms/cubic meter of air

Converted to 
Daily AQI Value AQI 
Category

AQI
Category

8/4/17

62.0

154

Unhealthy/Red

8/5/17

49.8

136

Unhealthy for Some/Orange

8/6/17

42.9

119

Unhealthy for Some/Orange

8/7/17

43.8

121

Unhealthy for Some/Orange

8/8/17

41.4

113

Unhealthy for Some/Orange

8/9/17

54.9

149

Unhealthy for Some/Orange

8/10/17

54.4

148

Unhealthy for Some/Orange

8/11/17

55.5

151

Unhealthy/Red

8/12/17

38.8

109

Unhealthy for Some/Orange

8/30/17

43.1

120

Unhealthy for Some/Orange

9/4/17

148.6

199

Unhealthy/Red

9/5/17

203.5

254

Very Unhealthy/Purple

9/6/17

195.4

245

Very Unhealthy/Purple

9/7/17

205.5

256

Very Unhealthy/Purple

9/8/17

110.6

179

Unhealthy/Red

9/14/17

47.5

131

Unhealthy for Some/Orange

In addition to high levels of PM2.5, Spokane also experienced elevated levels of PM10. The PM10 was largely due to the heavy smoke and some ash brought into the area from the wildfires.

The federal, health-based standard for PM10 (particles 10 microns in diameter and smaller) is 150 micrograms per cubic meter of air (µg/m3) averaged over a 24-hour period, midnight-to-midnight. According to monitors, Spokane breached this threshold on four days in September.  

Listed below are the maximum 24-hour (midnight to midnight) average concentrations for PM10 when levels reached “unhealthy for some/orange” during the smoke events.

PM10

Date

24-hour average micrograms/cubic meter of air

Converted to 
Daily AQI Value AQI 
Category

AQI
Category

9/4/17

183

115

Unhealthy for Some/Orange

9/5/17

227

137

Unhealthy for Some/Orange

9/6/17

232

139

Unhealthy for Some/Orange

9/7/17

227

137

Unhealthy for Some/Orange

While Spokane County can see elevated levels of ground-level ozone concentrations in the summer, this year’s wildfire smoke also contributed to exceedances of the pollutant.

Fires can worsen ground-level ozone levels because they release nitrogen oxides and hydrocarbons. These gases then form a chemical reaction in the presence of sunlight, causing increased levels of ground-level ozone. This reaction often occurs downwind of the fire itself.

The federal, health-based standard for ground-level ozone is 70 parts per billion (ppb) averaged over an 8-hour period, midnight-to-midnight. Spokane experienced two days that breached this threshold.

Ground-Level Ozone

Date

8-hour average micrograms/cubic meter of air

Converted to 
Daily AQI Value AQI 
Category

AQI
Category

8/4/17

0.072 ppm

105

Unhealthy for Some/Orange

9/7/17

0.071 ppm

101

Unhealthy for Some/Orange