Tuesday, November 23, 2021

The Cross-Valley Contrast in Air Quality

Yesterday morning we discussed how a meteorological inversion had developed, but that the air quality was still good to moderate.  There just wasn't time yet for emissions to accumulate within the valley cold pool.

However, by late afternoon, it was clear that the air quality was declining, with smog evident over the Salt Lake Valley as the sun set over the Oquirrhs.  

This morning, one can still see a veil of smog over the valley.  It seems to be particularly bad over the western valley.  

The way that sunlight is scattered by pollution is such that one needs to be cautious assuming that what looks more polluted is more polluted, but that's not the case this morning.  Purple air observations confirm what one infers from the photo above with a clear increase in PM2.5 concentrations across the Salt Lake Valley.  On the east bench, PM2.5 concentrations are at good to moderate levels, whereas in the West Valley, they are higher and in some cases in the unhealthy for sensitive groups category.  

The purple air sensors sometimes read high that appears to be the case today. However, if we look at observations from PM2.5 sensors operated by the Department of Atmospheric Sciences on TRAX trains we still see the increase in PM2.5 concentrations across the valley.  Values are not as high as suggested by PurpleAir, but still max out very near the threshold for unhealthy for sensitive groups.  

Source: https://utahaq.chpc.utah.edu/. 

The situation above is not uncommon.  The reality is that the air quality in the West Valley is often worse than on the east bench.  This is a consequence of several factors including spatial patterns of emissions, elevation, and circulations such as cleaner canyon outflow jets in the morning on the east side.  

The good news is that we have a front coming in tonight, so this episode should be short lived.  The bad news is that ridging will predominate through at least the middle of next week.  Lack of valley snow cover will help, but expect to see declining air quality after Thanksgiving.  

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