Wednesday, April 6, 2016

How Does PM2.5 Pollution Vary with Elevation in the Salt Lake Valley?

Although the wintertime PM2.5 period is now past us, a number of you have asked how much the air quality varies with elevation in the Salt Lake Valley.  I'll answer this question like a scientist.  It depends.

First, let me note that the best situation for knowing the air quality before you recreate or go outside is to have high-quality real-time monitoring at many locations across the Salt Lake Valley.  The Division of Air Quality does not provide this.  We get data in real time (usually an hourly average with a bit of a delay) from Hawthorne Elementary and that is it.  The air quality can, of course, be considerably different in other areas of the valley.

The University of Utah has recently installed sensors at a few locations as well as on TRAX trains, which helped during this past winter.  There is also the grassroots network.  Overall, these sensors helped last winter, but the coverage remains incomplete and we still still need to vet the purple air sensors, which seem to overestimate PM2.5 concentrations.  There's still more work to be done until we have a good idea about the PM2.5 concentrations in your neighborhood.

With that in mind, let's talk about what we do know about the air quality dependence on elevation?  In January and February 2011, several University of Utah and DAQ scientist collected PM2.5 measurements at several locations in the Greater Avenues area during several poor air-quality episodes.  Twenty-four average PM2.5 concentrations from these locations are plotted below and show that in most episodes there was a trend toward lower concentrations.

Note, however, that there are variations in the size (and even sign) of the decrease.  In other words, it depends.  In the 4 Feb event, for example, PM2.5 was higher at 1500 m than it was at 1300 m.  In addition, these are 24-hour averages and so they are more representative of the long-term PM2.5 exposure than what you might get when you go outside to recreate at a particular time of day.  In the morning, one sometimes gets clear air near the mouths of the canyons and on the upper benches due to local slope and valley flows transporting air from higher elevations, whereas in the afternoon, those locations are in the gunk.  This leads to a lower 24-hour average, but if you are recreating in the afternoon, you are breathing lousy air.

In addition, there is a tremendous amount of variability in the characteristics of air pollution from event to event.  Much depends on the strength and height of the capping stable layer or inversion and the local meteorology.  For example, there are situations when the pollution is trapped in a shallow layer and the air is much cleaner on the benches than along the valley floor.

On the other hand, there are times when even the upper-elevation benches are enveloped in smog.

Source: KSL
My personal rules of thumb are if I'm above the smog, I recreate without concern.  If I'm in the smog, then I generally assume that the PM2.5 levels on the bench are probably near what they are to those measured on the valley floor and make decisions accordingly.  Every now and then I will identify a situation where the bench PM2.5 levels are not as bad as on the valley floor and I will opt to recreate. Nearly always in these situations I've seen a very recent PM2.5 observation from the U or TRAX line to support this conclusion.


  1. The Guys from the LAIR group at the U (Dept of Atmospheric Sciences) are doing quite a bit of work at different elevations via KSL TV's helicopter
    See link for more

    Great blog and look forward to reading it each day.

    1. Yes, great work. Thanks for sharing. That data will prove quite useful.

    2. This is nice but what I want to know is the level of pollution in the high Wasatch (8000 to 11000 feet) during Salt Lake valley inversions. I know the air is terrible down here when visibility is affected even during a quarter mile, but how much better is it up high? I'm sure the particulate levels are less; does the much better visibility at high altitude correlate with lower ozone levels as well, and by how much? -- tom d.

    3. To my knowledge, there are no regular air quality measurements made in the Wasatch at that elevation.

      Most of the time during valley wintertime PM2.5 episodes, the air quality above 8000 feet (sometimes lower) is good. There are, however, sometimes events where pollution can creep to the base of the resorts. Usually these are cases where the inversion is elevated, and the PM2.5 levels are moderate, or they are short lived.

      Ozone is a different beast as in the Salt Lake Valley high episodes occur in the summer. It is very possible that ozone levels in the mountains during those events are as high as (or possibly higher) than those on the valley floor. The chemistry of ozone is such that it can reach higher values at times in rural or wilderness areas adjacent to the sources of ozone precursors (e.g., cities).


    4. Thank you for this information. Seems like it would be useful to have some figures for air quality measurements at higher elevations. The air today (12/9/17) up Big Cottonwood (above the Spruces)appeared clear; visibility was good. Coming back down into the city was...discouraging. And since I live in the city, even if I make trips up to the tops of the canyons 2-3 times a week, still that amounts to only about 20 hours a week of fresh air, which leaves the other 148 hours to be breathed down here. -- tom d.