New Year's Eve or Halloween?

It may be New Year's Eve, but its pretty scary out there, if you are in the valley.

In the mountains, it was a spectacular day, with the polluted grey in town helping to keep the crowds down at the resorts.  From the bluebird skies along Alta's High Traverse, the valley stratus looked quite benign.

However, it looked far more sinister during the descent down Little Cottonwood Canyon, especially as we approached the nose of stratus protruding up the canyon.

However, as ominous as it looks, the development of the stratus is a good thing as it typically leads to a deepening of the pollution layer and mixing through a deeper layer.  Instead of pollution being confined to very near the valley floor, it mixes through a few hundred meters of depth and that helps keep ground-level PM2.5 concentrations lower.  It also reduces the photochemical production of PM2.5 in the afternoon.

One can really see this in the PM2.5 trace from Hawthorne elementary.  Note how concentrations today were generally lower than yesterday, without the afternoon peak. 

Source: Utah Division of Air Quality

On the other hand, if you are on the upper benches, your air quality might be worse today as you may have been above the pollution yesterday, but in it today.