Monday, March 4, 2013

One Final Insult

This weekend brought one final insult from the inversion season.  I thought we would see elevated PM2.5 levels this weekend, but I didn't expect to exceed the 24-hour National Ambient Air Quality Standard, which is exactly what happened on Sunday morning at Hawthorne Elementary.

Source: Utah Division of Air Quality
Hourly values peaked at 10 am when light precipitation was falling.  Dramatic scavenging of the PM2.5 occurred when precipitation rates increased just after noon.  At least that's what I think happened, although I haven't taken a close look at the possibility that there was an airmass change.  

As noted in an earlier post, there was a very pronounced inversion in place over the Salt Lake Valley on Sunday morning.  Here's another look at the sounding, showing a deep ~7ºC inversion.

Source: NCAR/RAL
Yup, it's March, but we are dealing with unusual snow depths in the valley and, perhaps more importantly, unusually cold temperatures over the Great Salt Lake.  The impact of the Great Salt Lake was particularly apparent at 2 PM on Saturday afternoon.  Although most stations along the east bench reported temperatures in the low 50s, it was only 47ºF at the Salt Lake Airport and 39ºF at Hat Island.

Source: Mesowest
Any pollution pushed out over the Great Salt Lake remains trapped within this cold, stable layer of air, which pushed through the Salt Lake Valley late on Saturday and appeared to linger through Sunday.  The effect of the Great Salt Lake is especially pronounced now that we have enough surface heating to warm things up and better mix the air over the Salt Lake Valley during the day.

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