Wednesday, January 21, 2015

Where Is the Inversion?

Yesterday was remarkably clear and beautiful and I had the good fortune of traveling to Utah Valley University on the Frontrunner train to give a talk.  This was my first time taking the Frontrunner south from Salt Lake Station and it is a great ride, especially when you can sit there gaping at big, white mountains the entire way.  It has to be the most scenic commuter rail ride in America.

Many people asked me where the inversion was.  It's such a shame that when we get a clear day this time of year we're surprised, but that's the way it is.

So where was the inversion yesterday?  Too high to cause a major increase in pollution and smog.  Yesterday afternoon's sounding shows the inversion (red shaded area) way the heck up there above 700 mb and generally above all but the highest peaks.  Below the inversion, the atmosphere was fairly well mixed, so our pollution was dispersing nicely.  In addition, we had some light northwesterly flow to help carry it away.

Overnight, the inversion has lowered, but even today we have a weak system dropping down from the north to weaken the inversion this afternoon and keep things stirred up.  Enjoy the clear, clean air while it lasts.

Now, to answer a few of the questions posed in comments to yesterdays blog:

1. Why are there relatively few air quality sensors in Salt Lake and Utah Valleys?

The Utah Division of Air Quality maintains a number of air quality sensors in the Salt Lake and Utah Valleys, but the data is not available in real time.  It is my understanding that they will be adding real-time capabilities at a couple more sites in the Salt Lake Valley in the near future.  In my view, the historical lack of real-time sensors in the valley has reflected expense, bureaucratic barriers, and a lack of political will.

Of course, there is more to measuring and understanding air pollution than just measuring the PM2.5 concentration.  It is critical to understand the constituents of that pollution and the processes involved in generating it.  Perhaps I can find a guest blogger to address some of these issues and the efforts and costs involved for a future post as it is outside my area of expertise.

2. In Davis and Weber Counties, what holds the pollution in on the west side of the counties. Does the inversion go all the way across the lake to the mountains on the west side of the Salt Lake?

During most inversion events, the meteorological inversion, meaning the layer aloft in which temperature increases with height, extends across the Great Salt Lake and the entire Great Salt Lake Basin.  There is really nothing that holds the pollution on the west side of the Great Salt Lake.  In fact, at night, when the flow tends to be offshore (southeasterly over the Salt Lake Valley or easterly over the northern Wasatch Front), some of the pollution is transported from the urban area over the Great Salt Lake, only to return the next day as the lake breeze develops.  

I don't know, however, how far west the pollution goes or, more specifically, what the PM2.5 levels get west of the lake and in rural northwest Utah during these events.  Perhaps DAQ has looked into this.  It is an important issue for air quality control strategies given the huge size of Utah counties.  

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