Monday, June 28, 2021

It isn't wildfire smoke in Western WA. Its called Biogenic Secondary Organic Aerosol

Say what?

First, there is no major infusion of wildfire smoke into Western WA. Satellite imagery shows hardly a cloud or wisp of smoke over the last 3 days. However several fine particle pollution monitors have been showing air in the Moderate to Unhealthy for Sensitive Groups range. 

So whodunit?

The ridge of high pressure over the state giving us this extreme heat is also responsible for forcing air downward and trapping pollutants within a shallow layer. This is known as a subsidence inversion. Since winds are relatively light, all the pollution generated by human activities tends to accumulate over time. But that's not all.

Trees also emit organic gases such as terpenes and isoprene, more so on hot days. These pollutants of biogenic origin undergo various complex chemical reactions in the atmosphere, giving rise to small particles known as Secondary Organic Aerosols (SOA). "Secondary" because the aerosols are not released directly from sources but formed as a result of subsequent physical and chemical processes in the atmosphere. The diurnal nature of the elevated pollution levels over the last few days are consistent with how we expect biogenic SOA to behave. A WSU air quality model is showing the same thing.

 Biogenic emissions are also one of the ingredients of ground level ozone, aka "smog" formation. They react with oxides of nitrogen (mostly from traffic and industrial sources) under the right meteorological conditions to form ozone. All those factors came together yesterday and will do so again today to cause high ozone levels in the Cascade Foothills of King and Pierce counties. Expect air to be in the Unhealthy for Sensitive Groups or even Unhealthy range on account of this. An Air Quality Alert is in effect. 

Skip the tech talk. When will it go away?

Starting tomorrow. 

As the thermal trough shifts inland and reduces western WA temperatures somewhat, biogenic emissions reduce and so does SOA formation. So we expect fine particle pollution levels to be around the Good/ Moderate level from tomorrow onward if there are no new fires. Please see the forecast map above. Ozone formation will also relax a bit. [Side note: please do your part to prevent fires during this time of extreme fire risk. There are Red Flag Warnings about this]. 

What about eastern WA?

A 20,000 acre brushfire near the town of Lind is causing quite a bit of smoke locally. Winds are initially shifting the plumes westward, before driving it toward the Spokane area starting Wednesday. If the fire is still burning, that is. Moderate air is expected, with worse conditions closer to the fire.


  1. Didn’t expect to check the blog so early this year for air quality! I know it’s early but any thoughts how this historic event will affect wildfire season? Last year we had a “perfect storm” of conditions over Labor Day. This seems to be another event in that same vein.

    1. Hello, well the fuels are certainly drying out fast and these events "prime the pump", but predicting when, where and how large fires will pop up is not easy. We are starting out with drought conditions in eastern WA (dryer than last year in the western US), and are expecting a warmer and dryer than normal summer.

    2. Thank you Ranil as always for your insight. Seems like we really are teeing things up for the season

  2. Oh wow- I was convinced this must be smoke outside! Thank you for the interesting info, I would have never known!

  3. Thanks for the update, I did notice on the hot afternoons the air had a stronger tree/piney smell than usual. Looks like fires starting in BC and Northern CA - hoping we don't get any of that smoke blowing in our direction. Wish I could be more hopeful about smoke season by I am getting prepared by ordering replacement filters for my air purifier. Hopefully others will do the same and not wait till last minute when they might be sold out at local stores.

  4. What is causing hazy skies in North central WA / Okanogan valley?

    1. A mix of smoke from a few (mostly distant) fires, plus any lingering biogenic SOA described in the post above. This smoke appears to be affecting much of central WA but isnt expected to get bad.

    2. Ok thank you. I didn't see any marked fires nearby. But it does seem like haze from smoke. Thanks!


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