Tuesday, September 27, 2016

Degraded Air Quality in Late September

Dude, I can barely see the Oquirrhs!
I hope this isn't a harbinger of things to come, but there is actually an elevated air quality event underway over Salt Lake with some characteristics similar to what we see during the winter.

As everyone is aware, late last week and last weekend and deep upper-level trough and associated cold-air intrusion rumbled through Utah.  The maximum temperature on Friday was only 51ºF.

As the trough moved downstream, a high-amplitude upper-level ridge built rapidly over the western United States.  This led to significant warming in the mid- and upper-levels of the atmosphere at a rate faster than solar heating could warm the colder air near the surface.

As a result, yesterday afternoon, the atmosphere over the Salt Lake Valley was capped by a series of stable layers.  The lowest of these stable layers was only a few hundred meters above the valley floor.

Source: Storm Prediction Center
Weather camera images yesterday showed a clear layer of gunk over the Salt Lake Valley with a pronounced top.  Look toward Lone Peak in the image below.

Source: MesoWest
And the buildup of pollution is evident in the time series below, which shows that we are now in the moderate air quality category for PM2.5.

Such conditions are a bit unusual for September, but there are probably three issues at play.  The first is the depth of the weekend trough and strength of the cold air, followed by the building of a strong ridge and associated rapid warming aloft.  Second, we received a great deal of rain Friday and Saturday, leading to high soil moistures.  As a result, a portion of the sun's energy that often would be partitioned into heating the ground and atmosphere is being used for evaporation and transpiration.  Finally, we simply have to realize that we have more people living here and driving farther than every before.  I suppose there might also be some smoke sources out there, although I'm unaware of any major incidents in the immediate area.



  1. Dew point values are pretty high over northern Utah compared to the surrounding air mass, and have been rising each afternoon due to evaporation during the day. So I agree, this is limiting surface heating a fair amount.