Thursday, January 8, 2015

Swings and Variations in Inversion Pollution

There have been some fascinating swings and variations in pollution over the past few days and last night within the Salt Lake Valley.

We begin with a time series of PM2.5 concentrations at the Hawthorne Elementary site maintained by the Division of Air Quality.  Hourly PM2.5 concentrations (brown values) show large swings over the past three days with peaks in the late afternoon or evening and valleys overnight.  Yesterday evening we hit the highest values of this episode with a peak of 51.6 ug/m3.

Source: Utah Division of Air Quality
The causes of these large diurnal (i.e., daily) swings in PM2.5 concentrations have yet to be fully elucidated, but may have something to do with how sunlight, temperature, and/or relative humidity affect small particulate production.  I'll have to bow out here since I know little of chemistry and aerosol physics!

The large diurnal swings in PM2.5 are very evident in the data being collected along the Trax lines the past couple of days.  Observations collected in the 3-hour period ending at 8 PM MST last night showed very high values of 45–55 ug/m3 along the entire green line from West Valley city to the Salt Lake City Airport, consistent with that measured at Hawthorne above.

In contrast, we find lower concentrations this morning (but still in the moderate or unhealthy for sensitive groups air quality categories depending on location), as illustrated by observations collected along the red line from Daybreak to the University of Utah.  Concentrations are lowest in the Daybreak area along the West Bench, and highest along the valley floor.  Concentrations are also a bit higher at the University of Utah than at Daybreak.

It will be interesting to see how things evolve today, but I would expect to see another major rise in PM2.5 concentrations this afternoon.  This is true even on the benches.  Note how our sensor at the University of Utah climbed to a peak of 45 ug/m3 yesterday afternoon.  Although not as high as observed at Hawthorne on the valley floor, these are still high values considered unhealthy for sensitive groups.

The only way to escape the poor air quality is to head to the mountains where the air is crystal clear and the visibility essentially unlimited.



  1. Seasonal and diurnal variation of PM2.5 apparent particle density in urban air in Augsburg, Germany.
    Pitz M1, Schmid O, Heinrich J, Birmili W, Maguhn J, Zimmermann R, Wichmann HE, Peters A, Cyrys J.
    Author information

    The apparent particle density of particulate matter with aerodynamic diameter < 2.5 microm (rho2.5) was determined at an urban site in Augsburg, Germany and its correlation with chemical composition and meteorological conditions was investigated. rho2.5 showed strong day-to-day variation from 1.05 to 2.36 g cm(-3) (5 to 95% percentile), and nearly 64% of the daily variability could be explained by a multiple variable regression model. A minimum in the morning and afternoon (about 1.5 g cm(-3)), and a maximum (near 1.8 g cm(-3)) during midday was observed. The minima represent fresh primary aerosol emissions, which were related to traffic soot particles with low density due to their agglomerate structure, especially observed in the early morning hours of weekdays. The maximum is likely due to increased secondary particle production and the presence of more aged particles with the built-up of the convectively mixed boundary layer. rho2.5 has the potential to serve as a crude tracer for chemical composition and atmospheric processing and might play an important role when considering the associations between health effects and ambient particles.

    1. Bubba - Thanks for pointing out this nice study.