Friday, December 30, 2016

A Look at Inversion and Air Pollution Intricacies

As Yogi Berra once said, "you can observe a lot just by watchin'" and that's especially true during inversions.

Let's begin with a few key points of nomenclature and interpretation.  First, an inversion is a layer in the atmosphere in which temperature increases with height.  It is important to recognize that some poor air quality events in the Salt Lake Valley are not associated with true inversions.  Because density varies with both temperature and pressure, a dry atmosphere is stable until the temperature decreases at a rate of about 1ºC/100 meters altitude.  Periods in which the temperature decreases slowly with height can be strongly stable and associated with elevated air pollution levels.  The term "inversion" is often applied colloquially to such periods, even though that is meteorologically incorrect.

Second, the inversion is not the cause of poor air quality.  An inversion simply limits vertical transport and mixing.  It is a benign atmospheric feature.  It is the emission of primary and secondary pollutants produced by the burning of wood and fossil fuels (there are other pollutant sources, but those are the big ones) that is the cause of the poor air quality.

Finally, the layer of pollution is not necessarily an indicator of the inversion layer.  Often, the pollution is trapped beneath the inversion or within the inversion.  The inversion itself can extend well above the pollution layer.

With these points in mind, let's have a look at today's inversion and air pollution.  The figure below shows the profile of temperature (red line) above Salt Lake City this morning.  Note that in this figure, a line of constant temperature (dotted lines) is "skewed" rather than oriented up and down.  This morning, a bonafide inversion was present at low levels and extended from very near the surface where it was -7ºC to 2192 meters (7200 feet) where it was 3ºC.  There are, however, two additional stable layers above this inversion, which extend to 2781 meters (9125 feet).

The photos below were taken in the Avenues Foothills at about 2:15 PM at an elevation of 1780 meters (5840 feet).  The bulk of the smog is confined below this level.  Thus, very clearly the smog is confined within the inversion layer illustrated above.  The atmosphere remains highly stable above the smog layer.  If one looks carefully at the photo below (click to enlarge) one can see some additional layers of pollution that are trapped with in the higher portion of the inversion or the surmounting stable layers.  Finally, while the smog layer is fairly well defined, it clearly has a diffuse top as some smog is exchanged vertically.

Another complication in this event is the formation of fog (clouds at the ground) or stratus (layered clouds above the surface).  In the panoramic photo below, one can see that most of the eastern Salt Lake Valley is covered by smog, but clouds are present over downtown.  

Here's a closeup looking into the mouth of City Creek Canyon.  You can very clearly see the stratus, but also how there is smog over the stratus and smog within the mouth of City Creek Canyon that is cloud free.

All of this makes for a very complicated stew of pollution and clouds, especially since clouds can greatly complicate air chemistry processes.  I won't touch that as my knowledge of air chemistry is just enough for me to get into trouble.

I will, however, show that in some parts of the valley, the air this afternoon is quite heinous.  At Hawthorne Elementary on 700 East, for example, we spiked to unhealthy levels this afternoon (red region), although they have since dropped to unhealthy for sensitive groups (orange region).

Very high values are also being observed at the University of Utah, which was previously at or above the pollution, but is now enveloped in it.  Values this afternoon spiked to over 80 ug/m3, similar to the peak at Hawthorne.



  1. Is the development of stratus clouds due to the pooling of cooler air and condensation? I always wonder what exactly I'm looking at during inversion events - is it purely smog or a mixture of smog and clouds as your pictures clearly illustrate? Seems like areas outside of Utah (Jackson, Wyoming comes to mind) experience cloudy inversions, but without substantial particulate matter, as in the Salt Lake Valley.

    1. The moisture sources for the clouds include the Great Salt Lake, sublimating snow, and even vehicles (water vapor is one by products of fossil fuel combustion). The pollution does change the characteristics of the clouds some (probably causing them to reflect more sunlight), but we would see fog and stratus in this valley even if there was no pollution.

  2. I looked up the altitude of our address to see if we live above the pollution "inversion" (at least the worst of it). If we live in Cottonwood at 5,190 feet, are we out of the smoggiest part? Is there a map or website that shows the elevation of the pollution/inversion on a daily basis?

    1. It's very difficult to generalize these events for any location on the benches. Much depends on the depth of the pollution and whether or not it is a cloudy or simply smoggy event. Over the past couple of days, for example, the benches (at least at my 5000 ft elevation) transitioned from clear to polluted. Unfortunately, the measurements from Hawthorne Elementary are often not representative of the benches. There is a group deploying lower cost sensors throughout the valley (, but the sensors measure the number of particles rather than their size and composition and my impression is that they overestimate by a fairly high ratio the actual PM2.5 concentrations. Ideally, high quality sensors should be distributed throughout the valley and available in real time, but unfortunately that is not the case.