Wednesday, January 18, 2017

The Inversion Tightens the Noose

Salt Lake Valley residents awoke to a shallower but dirtier layer of pollution this morning
In my previous post issued on Monday (Why Deep Cold Pools Are "Good"), I discussed how deep valley cold pools and pollution layers are actually "good" compared to shallow ones.  When the inversion (a layer in which temperature increases with height) is elevated, pollution is distributed through a deeper layer, resulting in lower pollution concentrations.

Over the past two days, however, the inversion has lowered, tightening the noose on the Salt Lake Valley.  You can clearly see this in the soundings from Monday morning (top) and this morning (bottom).  In the Monday morning sounding, the inversion was about 1 km above the valley floor, and the atmospheric temperature profile allowed for mixing through that 1-km deep layer.  In contrast, this morning the inversion is based pretty much on the valley floor.  

Source: SPC
As a result, pollution concentrations at low elevations have increased.  Note in particular how they went into overdrive yesterday.  Yes, there are a few ups and downs, but for the most part the valley floor (Hawthorne Elementary) is observed much higher PM2.5 concentrations from yesterday afternoon through this morning than observed the prior days.  
Source: DAQ
There is some good news.  An approaching storm should lead to improving air quality tomorrow.


  1. I usually expect snow to drive out the air pollution, but I notice that even as the snow has been falling today Cache Valley's PM 2.5 levels have continued a dramatic rise that started yesterday. Out of curiosity I checked Salt Lake, which looks like it remained pretty stable and now has started to drop. Any idea why?

    1. I can only provide a hypothesis, and it is not necessarily a good one.

      It is still about 10ºF cooler in Logan than Salt Lake. Salt Lake's PM2.5 only recently dropped in the early afternoon. I suspect that temperatures in Salt Lake combined with cooling aloft got to a point where vertical mixing became more vigorous. Logan, being colder, hasn't quite reached that point.

      That being said, there may be better hypotheses out there.