Friday, July 18, 2014

Ask and You Shall Receive: Insights from a New Ozone Sensor

After belly aching in yesterday's post about the need for more real-time air quality monitoring stations in the Salt Lake Valley, I learned that my colleagues in the mountain meteorology group here at the University of Utah have just installed PM2.5 and ozone sensors at our mountain meteorology lab (MTMET) on upper campus (click here to access).  At an elevation of 4996 ft, this site should make for good comparisons with the DAQ sensors at Hawthorne Elementary near the valley floor and almost 700 feet lower.  MTMET is influenced by outflow from Red Butte Canyon, and this is already proving to be quite interesting.  

Data from the past 24 hours at MTMET (courtesy shows very nicely the gradual rise in temperature during the day yesterday, with a high just above 90ºF.  Just before 2000 (8 PM) MDT, temperatures dropped abruptly to 75ºF with the onset of easterly canyon outflow, after which temperatures held steady until about midnight when the temperatures jumped abruptly when the outflow weakened. The strength of the outflow varied for the rest of the night, with temperatures bottoming out in the low 70s just before 0800 (8 AM) MDT.   

Ozone for the period spiked briefly from 1300–1400 (1–2 PM) MDT yesterday, hitting ~80 parts per billion (ppb).  Ozone overnight, even with the supposedly "clean" canyon outflow, stayed near about 50 ppm until about 0800 (8 AM) MDT when the outflow weakened and winds gradually shifted to southwest. 

We can compare this data with that from Hawthorne, which thankfully is available online again.  It shows a similar spike in ozone in the early afternoon, but lower ozone concentrations than observed at MTMET overnight.  In other words, from the standpoint (solely) of ozone concentrations, the outflow from Red Butte was "dirtier" than the air at Hawthorne.  
Source: Utah Division of Air Quality
That may sound surprising, but it is likely the result of a curious oddity of meteorology and air chemistry.  Profiles of ozone in other regions (e.g., the Los Angeles Basin illustrated below) often show the highest ozone concentrations are aloft and not at the surface (note: this doesn't necessarily mean the air at the surface is "clean"!).  
Ozone concentrations over the Los Angeles Basin from 05-07 local time, 25 June 1987.  Source: Dayan and Koch (1996).  
This may seem strange since one expects the pollution to be worst near the ground where the emissions are, but ozone is a secondary pollutant and the complexities of ozone photochemistry and meteorological turbulence lead to the strange distribution.  Further, this effect is most pronounced at night and in the early morning.   The Los Angeles data above was collected from 5-7 AM local time when ozone concentrations were less than 50 ppb near the surface, but exceeded 250 ppb aloft.

So, one hypothesis for the higher ozone at MTMET last night is that the ozone levels dropped in the valley, but the outflow from Red Butte was tapping into the higher ozone air further aloft.  

Of course, this is an are educated guess.  Comprehensive observations of meteorology and atmospheric chemistry are needed to better understand these local characteristics of our pollution.  


  1. Would it be practical to have a mobile sensor to move around the valley, and perhaps use it to test/calibrate the existing ones? I don't know much at all about the equipment but just curious about this.

    1. A colleague is presently working to have some AQ sensors on Trax trains. That should be a nice addition and is something I've hoped for (

    2. Having a completely mobile one would be so much fun though. Drive around the base of the canyons at night during an inversion and test the various outflows, or tether it to a balloon and get a vertical profile, etc. Of course these ideas are coming from someone who doesn't know how these sensors work or how much they cost.