Thursday, January 6, 2011

PM2.5 Evolution - Need Help from an Atmospheric Chemist

In Salt Lake County, we are now a few days into red burn conditions, or the more specific scientific classification of "gross."

The Utah Division of Air Quality provides quasi-real-time PM2.5 data from Salt Lake City on their web site.  It is very interesting to examine the evolution over the past 5-6 days.

In particular, there is a nearly monotonic increase in PM-2.5 concentrations from New Years Day through January 4th.  Beginning on the 5th, however, a large diurnal cycle appears, with lower concentrations at night, and higher concentrations during the day.  The NO concentrations go up dramatically on these two days, with a diurnal cycle that is out of phase with the PM2.5.  Note also that the diurnal cycle increased on the 5th as the mid level clouds became less wide spread.  So we have a situation where at night and in the early morning we have a stronger surface based inversion, higher NO, but lower PM2.5.

I need help from someone with a background in atmospheric chemistry and/or PM observations to explain this.  Any volunteers?

One thing to keep in mind with a trace like this is the dangers of extrapolating short term trends.  The low PM2.5 concentrations overnight and in the morning, for example, are not an indication of improving air quality.  This afternoon we have the highest PM2.5 concentrations of the event (at least at the observing site).


  1. There are problems with the instrumentation. WIth higher RH the units can freeze up at night and not perform properly. There are numerous problems with real-time PM instruments esp in cold climates. I wouldnt put too much money on the validity of the apparent diurnal PM behavior.

  2. Judging by what I have found on my car the last couple of mornings, I have an idea where some of the PM2.5 is going. Seriously, though, I wonder if the formation of actual fog such as we had the last two night helps to deposit or precipitate out a lot of the particles. This probably would not affect gases such as NO fumes? Would be interesting to compare nights with and without fog development.