A Remarkable Decline in Air Quality

With a break between low-pressure systems and snow blanketing the Salt Lake Valley, a decline in air quality was inevitable, but the speed of the decline appears to be quite remarkable.

On Saturday afternoon, we were still "in the clear" and the afternoon sounding showed only weakly stable conditions over the Salt Lake Valley up to almost 700 mb. 

Source: SPC

Thus, I'll call Sunday Day 1 of the inversion, which developed Saturday night and Sunday as temperatures warmed aloft in response to southerly and southwesterly flow.  The Sunday afternoon sounding showed temperatures at about 775 mb (about 1 km above ground level) were slightly warmer than those at valley level.  Thus, Sunday was our first inversion day. 

Source: SPC

By Monday afternoon, the inversion had strengthened further, with about a 5˚C increase in temperature from 835 to 800 mb.  Thus, emissions in the valley were effectively capped at about 5500 feet above sea level, or within 1000 feet of the valley floor. 

Source: SPC

On Sunday night, I was watching the late news and the forecast was for "good" air quality Monday and Tuesday.  I remember thinking that must be a mistake.  If that indeed was the forecast, it has busted dramatically.  On Sunday (12-1) morning and in the early afternoon, hourly PM2.5 concentrations (maroon line below) at Hawthorne Elementary were as low as 3.4 ug/m3.  However, that afternoon they bumped up and values continued to rise through yesterday afternoon when hourly concentrations peaked at 40.9 ug/m3, which rates as unhealthy for sensitive groups.  

Source: DAQ

Basically, we went from good to unhealthy for sensitive groups in about 24 hours, which is a remarkable rise.  Concentrations dropped just a bit thereafter, but sit near 30 ug/m3 this morning.  

I used to assume it would take 3.5 days to go from clean air to unhealthy for sensitive groups, but that's clearly not the case in this event.   Why is unclear, at least to me.