Sunday, December 11, 2011

Dirty Air Getting Dirtier

Looking northwest from Olympus Cove
When a persistent cold pool (a.k.a., inversion) is in place, as has been the case over the past several days, one can count on pollution levels gradually increasing over time as urban emissions and secondary pollution build up in the Salt Lake Valley.  The online PM-2.5 (total mass per cubic meter of particulate matter 2.5 microns in diameter or smaller) trend charts from the Division of Air Quality show this well.  The hourly readings (brown circles) show some diurnal variability, but the 24-h running average (black circles) shows a gradual climb over the past five days, albeit with some flattening out over the past day.

Source: Utah Division of Air Quality
A curious aspect about the evolution of PM2.5 during this event is how the amplitude of the diurnal cycle increased on the 9th.  Not being an expert on air pollution, the causes of this are unclear to me.  Perhaps one of the other weenies out there can comment.

Upper-air soundings from Salt Lake City show that this is a fairly strong cold pool.  The sounding from yesterday afternoon near the time of maximum surface temperature shows that the atmosphere was isothermal (i.e., having a constant temperature with respect to height from the surface to nearly 700 mb (near crest level).

With nocturnal cooling, we had a strong inversion in place this morning with a surface temperature of -5.5C (22F), but a temperature near 775 mb (about 1 km above the valley floor) of 1C (34F).

For as nasty as things look out there, it would probably be worse if there was snow on the ground.  Cold pools are typically stronger when there is snow on the ground, which reflects a larger fraction of solar energy back to space than bare land cover.

1 comment:

  1. I wonder where the DAQ measurements are taken from. Looking out the window and at the last observation suggests something a moderate distance south of the airport..