Thursday, December 21, 2017

About Yesterday's Terrible Air Quality

It's a 2-for-1 day at the Wasatch Weather Weenies.  If you want to read about the overnight snowfall, proceed to the previous post.  If you want to read about dust, air quality, pollution, and other apocalyptic aspects of the storm, stay here.

Yesterday morning, an extremely nasty plume of dust penetrated into the Salt Lake Valley, pushing air quality to unhealthy levels.

Although Salt Lakers are aware of poor air quality during inversion events, yesterday we dealt with an entirely different beast.  During inversion events, emissions from the combustion of fossil fuels and wood, as well as a few other sources, build up within the Salt Lake Valley.  Winds are light and the strong atmospheric stability prevents the mixing of pollution vertically.  In contrast, yesterday the poor air quality occurred during strong, southerly, prefrontal winds.

The graphs below show what happened at Neil Armstrong Academy in the Northwest Salt Lake Valley.  From Tuesday morning (19 December) through just after midnight Wednesday (20 December), PM2.5 concentrations fluctuated from about 0 to 19, with the peak around midnight.  These are values that indicated good to moderate air quality with just a little bit of pollution. 

Source: MesoWest
Then, after midnight, strong southwest winds developed, scouring the valley clean.  Those winds persisted overnight and into the morning.  However, beginning after about 6 AM, PM2.5 concentrations began to climb, eventually spiking to over 200 ug/m3, which is well into unhealthy territory.  That spike occurred just ahead of the surface cold front.  PM2.5 concentrations then dropped from 203 to 10 ug/m3 in 10 minutes as the front went through and brought in cleaner air. 

Another perspective is provided by a laser ceilometer at the University of Utah.  This is a device consists of a laser that points vertically through the atmosphere.  The signal returned back to the device can be used to infer pollution concentrations and the base of clouds. 

Below is a time-height section from about 1500 MST on Tuesday through 1500 MST Wednesday.  There is some moderate pollution evident on Tuesday afternoon and evening, but the airmass becomes relatively clean overnight.  The dust plume appears shortly after 6 AM and through about 10 AM extends to about 750 meters (2500 feet) above ground level. 

Image Source: MesoWest
As the front approaches, the depth of the dust increases.  I suspect this is due to the convergence of surface winds and the lifting of air near the front lofting the dust plume to deeper heights.  If one looks carefully, you can see evidence of the dust aloft event after the clean air behind the front has moved in at low levels.

So, what the hell is going on and where is all of this dust coming from.  We had a similar event on December 3rd, but the dust wasn't as think.  One of our graduate students, Derek Malia, pointed out to me that the dust plume was very evident in satellite imagery that day.  Sure enough, you can see it in the image below, originating in the south Cedar Valley west of Utah Lake.

Source: NASA
Yesterday, similar story.  I didn't have access to as sharp of an image as on 3 December, but one could still see the plume originating over the south Cedar Valley. 

Image Source: NASA
And, one can see it as a pink streak in the dust product from GOES-16.  Note that the plume continues downstream over northeast Utah, in that area likely above the shallow post-frontal airmass. 

So, we have a good idea that this dust is coming from the south Cedar Valley.  Google Earth shows that the dust emissions could be coming from agricultural fields in that area, which are perhaps exceptionally dry for this time of year due to the drought conditions.  Another possibility is emissions from a fire-affected area.  I haven't had a chance to dig into the past fire data to examine if this is a viable hypothesis. 

Hopefully, this is an issue that will remedy itself with precipitation.  If not, perhaps it wouldn't take much to reduce emissions from that area. 

1 comment:

  1. I took a picture of the dust from the 11/16 event while flying into SLC. Right from the spot you mention.