Daily Cycles in Air Quality

The air quality has fluctuated dramatically at the University of Utah, on the east bench in general, and in Parley's canyon over the last two days.  

PM2.5 observations from our mountain meteorology lab site at the mouth of Red Butte Canyon show low concentrations, indicative of good air quality, through the morning of 10 January (Monday).  On Monday, the inversion and the pollution began to build.  PM2.5 concentrations peaked at about 11 ug/m3, at the upper end of good.  

Source: Mesowest

However, they then dropped in the evening, with a couple of brief spikes to higher values.  On the morning of 11 January (Tuesday) they then climbed again, this time reaching higher values and topping out at 27 ug/m3, solidly in moderate.  Then they dropped overnight again.  

These observations reflect both a trend and a daily cycle.  The trend is the buildup of pollution in the Salt Lake Valley as emissions continue to pour into the persistent valley cold pool.  The second is the daily cycle of airflow along the east bench, with cleaner easterlies developing in the evening and at night and dirty westerlies developing during the day.  

Below is the corresponding time series of wind speed and direction at the mountain meteorology lab.  Wind direction is the blue dots.  Note the tendency for west to southwest (W-SW) flow during the day and east to northeast (E-NE) flow at night.  The PM2.5 cycle in this event lags the shift between these regimes somewhat, rising slightly after the shift to W-SW and decreasing after slightly after the shift to E-NE, which reflects the time for transport to either bring in the dirty or clean air.  

Source: Mesowest

If you look carefully, you can even see that the PM2.5 spikes at night are related to brief shifts in the wind or calm periods.  

A similar trend and cycle can be seen even at Mountain Dell up in Parley's Canyon.  During the day, the polluted urban airmass is transported up the canyon, leading to an increase in PM2.5, only to be replaced by cleaner air at night.  

Source: purpleair.com

Think of this as a sloshing of the valley cold pool and concomitant pollution if you like, and a reflection of what meteorologists call thermally forced flows.  Such flows are produced by heating and cooling of sloping land surfaces and the influence of the Great Salt Lake, which warms more slowly during the day and cools more slowly at night.  So far in this event, this has led to lower PM2.5 concentrations on the east bench in the morning than over the West Valley, as illustrated by this morning's purpleair.com observations.

Source: purpleair.com.  Obtained 7:47 AM MST 12 January 2021

Although not all wintertime pollution periods feature these characteristics.  In some cases, the inversion capping the valley cold pool is more elevated, and the clean air does not flush the east bench each night.  In others, the cold pool is shallow and the highest elevations of the east bench and stay clear all day.  Nevertheless, such cycles are not unusual during air pollution events and they are worth keeping an eye out for if you are looking for clean air for exercise.