How Could Air Quality in Western Washington Be So Bad When the Wildfires Were Third Rate?

 The air quality was terrible in central Puget Sound and around Bellingham today. 

 Extraordinarily unhealthy with high concentrations of the kind of particles that can move deep into your lungs.

Around 6 PM, the latest purple air map showed an unpleasant story with the most serious air pollution shown in purple.  Seattle and its eastern suburbs were in terrible shape.

In fact, here in Seattle, the air quality problems today were only surpassed by the conditions during September 2020, as shown by this plot available on the Puget Sound Clean Air Agency website for its Duwamish observing sensor.

Compare the satellite picture today and that during the 2020 event (see below).   There is no contest.  WAY more smoke in 2020. 

The 2020 fires burned over a million acres in the Northwest.   Our fires this month were about 1/20 of that.

This is actually a very low fire year in Washington, measured by the area burned.

Seattle's smoke is mainly coming from two fires.  The Loch Katrine east of Duvall (about 1600 acres) and the Bolt Creek Fire (about 15,000 acres).  Neither fire has been growing much the past several days.

So why has it been so bad in Seattle?

There are four reasons: the fires' proximity, their elevation, and the inversion.

Let's start with the inversion,  in which temperature increases with height.  

On Monday, cool moved in at low levels with warm air above, creating the inversion.  Inversions act as barriers to vertical movement of air, and our inversion was roughly 3000-5000 ft above the surface.

 

As noted in my earlier blog, at first the inversion was protective, stopping smoke from distant fires from mixing down to the surface.   But inversions have their dark side:  if smoke gets underneath, it is hard to mix the smoke out.

Seattle at noon today

And that gets to the second and third points--the location and elevation of the fires.  

 The Loch Katrine fire was very, very close to the urban area and at very low levels (2500-3000 ft).   So it could inject smoke UNDER the inversion, where it would remain at high concentrations.   And close means little space to diffuse horizontally.

The Bolt Fire was also relatively close and lower elevation.   

 During the past few years, we have not seen such close-in, low-level fires near Puget Sound.

Finally, the low-intensity nature of the fires was an issue.  An intense fire can produce a powerful convective plume that injects particles high into the atmosphere--potentially breaking through the inversion.

These wimpy, smoldering fires kept the smoke low, where it was trapped near the surface by the inversion.

An intense fire can inject particles high into the atmosphere.

Fortunately, relief is close at hand.  Later tomorrow, onshore flow will increase and air quality will rapidly improve.  Real rain will come in on Friday.    A welcome change.

Finally, there are some folks and local newspapers that suggest that this smoke event is some kind of portent of the future, a symptom of global warming.   Good science suggests otherwise.  I will discuss this in a future blog.

Question:  If the Loch Katine fire started on Sept 2, and was only 2 acres a week ago, why was it not put out? Without this fire, the smoke in Puget Sound would have been MUCH less.