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.
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|
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.
|Image Source: NASA|
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.