Friday, November 16, 2018

Local Pollution or Smoke?

Looking toward the central Wasatch Mountains from the Avenues Foothills 3:30 PM Friday 16 November 2018
A lot of people have asked me if the haze in the valley is smoke from the fires in California.  There is probably a small amount of smoke that is in our area, but for the most part, what you are seeing in the valley is pollution of our own doing.

The vast majority of the smoke from the California fires remains over the central Valley, Bay Area, and offshore Pacific ocean, although there is some that extends eastward across the Great Basin.  This is illustrated by the HRRR-SMOKE analysis below for near-surface smoke concentrations.  Note that the HRRR estimates near-surface smoke concentrations to be 4 ug/m3 or less over northern Utah.   

Source: ESRL
I'm not sure how much to trust that estimate, but let's go with it.  PM2.5 concentrations on the University of Utah campus today have been running around 14 to 20 ug/m3, much higher than the HRRR smoke estimates.  Note that over the past two days the values have fluctuated dramatically, dropping to as low as zero overnight and then rising during the day.  This reflects the development of a down-canyon flow in Red Butte Canyon (the observing site sits at the mouth of the canyon), which brings clean air from aloft (above the smog layer evident in the picture above) at night.  This is another reason why I'm skeptical about smoke being a strong contributor to our current pollution.  We simply don't have much evidence of high smoke concentrations aloft.

Source: MesoWest
Finally, November IS part of our wintertime smog season.  The sun angle right now is plenty low and comparable to that in late January.  Further, we do have a persistent cold pool in place over the Salt Lake Valley.  Yesterday afternoon, for example, a series of stable layers just above 5000 feet prevented pollution from the valley from mixing with the atmosphere aloft.  If there were snow on the ground, we'd have a whopper of an "inversion" event. 

Source: SPC
So, as I like to say, we have met the enemy and it is us.  The good news is that the weak cold front coming through late tomorrow should stir things up.  However, we go back to ridging early next week, so consider taking the bus or alternative modes of transportation if you can do it.  


  1. Hello, I was wondering if you knew the units of measure for the 'short term aqi' scale used by KSL on their tv news/website versus the ug/m^3 scale in your graph above. For example, KSL is stating aqi values around 100 for much of the SL valley this evening, whereas the U. of U. Mountain Metlab is stating about 20 ug/m^3. Thank you.

    1. The image on the left of will show you the PM2.5 and the AQI to show you the conversion.

      I get a bit of a laugh about the AQI. Apparently using actual values with units wouldn't work, so they replace those with a unitless scale that makes no sense and is completely meaningless. I bet maybe 1% of the population knows what the AQI scale means.


  2. Thanks. I misinterpreted AQI values as PM 2.5 measurements while on a recent trip to New Delhi, which misled me to believe the air quality was worse than it was (although still really bad). Next time I'll be better informed!