July 11, 2021

Heatwave Haze

 During the recent heatwave, which peaked in western Washington, western Oregon, and British Columbia on June 28th, many noticed an unusual haze, particularly at lower elevations west of the Cascades.   

And this was not wildfire smoke:  it was something else.

To illustrate, here is an image from the Space Needle Panocam on June 28th around noon.  The image directed towards the Olympics really shows the brownish haze layer.

And a picture around the same time from northern Kitsap County looking towards Whidbey Island (from Greg Johnson's Skunk Bay Weather site) showed the murk.  Yuck.

The haze was even visible in some early morning visible satellite images (see below for June 28th).

So what was this hazy miasma over western Washington and British Columbia?  It wasn't wildfire smoke, as shown by the NOAA HRRR Smoke analysis at noon Monday, June 28th.

I suspected it was photochemical SMOG and checked with air quality expert Phil Swartzendruber of the Puget Sound Clean Air Agency and he concurred.

What is photochemical smog?   

It occurs when sunlight (the ultraviolet component) interacts with human-emitted nitrogen oxides and volatile organic chemicals (emitted by plants or produced by some polluting industries) to produce a witches brew of chemicals including ozone, nitric acid, aldehydes, peroxyacetyl nitrates (PANs), and other secondary pollutants.   Some of these chemicals are associated with particles that scatter and absorb sunlight, producing haze.

The production of smog is enhanced by warm temperatures and lots of sunshine and BOTH were optimally available during the heatwave period.  Nitrogen oxides can be produced by combustion in fossil-fuel-driven engines.

Ozone in Our Air

Ozone is a wonderful thing to have in the stratosphere, where it helps protect us from ultraviolet radiation. But at the surface it is a bad thing, irritating our lungs and damaging materials such as rubber.

Ozone can be produced by photochemical reactions as noted above, but can also be generated in other ways, such as by wildifres.

Here are ozone values at Seattle and Enumclaw, in the Cascade foothills.  You will notice the ozone levels steadily increasing during the heatwave at both locations.  There is also a strong diurnal (daily) variation, with sunlight and warm temperatures during the day increasing ozone levels.    
But after the heatwave was over and cool, marine air moved in, ozone levels plummeted.

The Environmental Protection Agency's excellent AIRNOW website provided maps of ozone on Monday (see below).  I have included both their analysis and observations at individual locations.

Ozone levels were high around Puget Sound and near Vancouver, BC., but generally low in eastern Washington, where there was plenty of sun and warm temperatures.   The reason?  They lacked the nitrogen oxides and organic chemicals needed for ozone production.

How unusual were the ozone values observed in Seattle?   

Below is a plot of daily ozone during the past year at the Seattle Beacon Hill site.  June 28th  of this year was the highest.  Next, highest?   During the September 2020 wildfires.

High summer ozone is generally rare in Seattle because of the relatively cool, clean marine air that generally floods into our region on most summer days.  Interestingly, the highest summer values are generally observed in the Cascade foothills (e.g., Enumclaw or the nearby Pack Forest), where nitrogen oxides from Seattle and other urban areas combine with organic chemicals emitted by vegetation.


  1. I appreciate an update on this. Thanks, Cliff.

    This is just anecdotal, but the haze seemed to stick around after the heat dissipated. In fact, looking west toward the Olympics from my Beacon Hill location, today is the first time the mountains look fairly sharp.

    I do have SODO between the mountains and me, so our local air quality is probably worse than average. Still, it's nice to see them clearly again. I don't remember such persistent haze last summer.

  2. Our house is at 1400 feet in the foothills of the Coast Range west of Portland. The June 27th NWS forecast for our location, 108F, was quite close to what we measured, 106F, similarly for June 26th. However, the forecast for the next day was off by quite a bit. As I recall, the forecast was for 87F but we measured only 75F. This trend kept up for the next 3 or 4 days, with measured temps much lower than forecast.

    I wonder if the discrepancy was due to the haze. I was reminded of last summer's fire smoke that caused temperatures to be much lower than forecast.

  3. I vaguely remember Ronald Reagan tying smog in southern California to trees, and being crucified on one of them for the remark. Maybe this is what he had in mind?

  4. A curious omission: cars! Cars and other vehicles do not emit ozone, but they do emit chemicals that react to form ozone especially in high head and strong sunlight situations. Personal automobiles are the largest single polluter. https://www.adeq.state.ar.us/air/planning/ozone/cars.aspx

  5. And how many drivers will say, "yes, and I'll do my part by not driving so much... " Ha! They got to get their kids to the private school somehow.

  6. Dr. Mass--are you aware that the high temperatures has cooked mussells and clams alive before they arrived to dinner plates?

  7. I know it's now a few days past, but yes, I did notice the slight haze over the heatwave and recall you mentioning it at the time.

    That said, I also am beginning to notice that as we head into mid July, the overall light gets a bit hazy in general, and that I contribute to the general dust from being so dry and it gets more pronounced as we head into late summer and becomes most noticeable in the afternoons as we are doing that morning marine layer, afternoon sun thing this summer, more so, so far than I recall in recent summers.

    Anyway, something I've noticed for a long time, very cognizant of this change as we move through summer.

  8. Any thoughts on the raging wildfires going on right now? I'm curious how smoke season will look like in the Pacific Northwest this year. Seems like it won't be pretty.


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