The Difference between the Inversion and the Haze/Pollution Layer

Many people equate pollution in the Salt Lake Valley with the inversion, but they are different but related phenomenon.

Understanding the difference between the two explains unusual days like yesterday when haze and smog filled the Salt Lake Valley through a relatively deep layer, extending up to about 7500 feet.  This occurred not because the inversion was deep, but because the inversion was elevated.  In addition, the atmosphere below the inversion featured weak stability, enabling pollution to mix through a relatively deep layer.  

The image below includes a photo taken yesterday morning from Alta Ski Area and the sounding taken that afternoon at the Salt Lake City International Airport.  Haze and smog filled the Salt Lake Valley to an altitude of about 7500 feet where there was a sharp boundary to clearer air (indicated by the dotted line).  The afternoon sounding showed that in that haze and smog layer, the stability was relatively weak. Meteorologists refer such a region as a mixed layer because the weak stability promotes vertical mixing.  

Sounding source: SPC

However, at about 7500 feet (7400 feet in the sounding), there was a pronounced inversion in which temperature increased rapidly with height.  This put a lid on the mixed layer, preventing mixing with the free atmosphere aloft.  Thus, pollution wasn't trapped very near the surface, but instead was able to mix to the top of the mixed layer at 7400 feet, but no higher.   

This situation is similar to what commonly happens in the Los Angeles basin where the marine boundary layer typically features a mixed layer at low levels capped by an inversion.  

The reality is that the situation yesterday from an air quality standpoint was probably better than if the inversion were in its usual position very near the valley floor.  When that occurs, pollution is trapped in an even shallower layer.  

I'm interested to see how things evolve today.  Overnight, the mixed layer was destroyed for two reasons.  First, temperatures near the valley floor cooled, while those between the surface and the elevation of the top of yesterday's mixed layer (i.e., near 7400 feet) increased.  This has produced a sounding that is very stable.  

Source: SPC

If there was snow on the ground and there wasn't a trough approaching, I would say our goose is cooked.  However, the lack of snow on the ground will enable the sun to produce a shallow mixed layer today (although probably not as deep as yesterday).  Thus, I suspect the haze/pollution layer won't be as deep today, but will probably envelop the upper benches this afternoon.  Hopefully the trough passage tomorrow and tomorrow night will scour things out and hit the reset button on air quality.