Monday, February 4, 2013

Fast and Furious

One of the more interesting aspects of our current inversion is how rapidly the air quality deteriorated.  To illustrate this, check out the two photos below, one taken late Wednesday afternoon, the other Sunday afternoon.

Wednesday Afternoon
Sunday Afternoon
The ramp up of PM2.5 during this period was fairly gradual until Saturday afternoon (Groundhog Day, 2 Feb), when there was a large spike in PM2.5 as the lake breeze pushed in.

Source: Utah Division of Air Quality
Morning soundings show that temperatures at the Salt Lake City airport increased through February first, but afterward the low levels cooled quite dramatically.

Morning (0500 MST) soundings from 29 Jan – 4 Feb 2013
Another curious oddity is that last night there was very little drop in temperature and PM2.5 at the Salt Lake City sensor above.  We have seen very large nighttime drops in PM2.5 at that station during inversion events earlier this season (or even the previous day in the trace above), but last night temperatures remained fairly constant and the PM2.5 was elevated all night long.  

So many questions, so little time.  The role of topography, radiative processes, snow cover, and the Great Salt Lake in this event deserves further investigation.  Feel free to dig in and comment.  


  1. As I said in the email, the huge ramp in PM on Saturday seemingly came out of nowhere. If it blew in on NW winds then presumably it would have formed over the Lake because prior to that PM was not all that elevated. So, was there a pool of PM forming over the Lake for several days and we only saw it on Saturday due to the winds? Or, did it form rapidly on Saturday over the Lake or over the Valley?

    DAQ might be able to comment on what the afternoon peak was composed of? I suppose it is possible that the pool over the Lake allowed HNO3 to form as a terminal product of NO/NO2 chemistry. And I suppose it is also possible that NH3 from livestock areas surrounding the Lake migrated into the pool and rapidly formed NH4NO3 particulate. I'm thinking aloud here because on Friday AM I was in Tremonton and the stench of NH3 was tremendous even though it is fully snow covered and it was well below freezing. So, could those ag areas be throwing off NH3 and moving it south over the Lake? You must have NH3 to form NH4NO3 which is 50% to 75% of our typical particulate composition so looking for the NH3 source is critical. We know where the NO/NO2 & -> HNO3 comes from (combustion/cars).

    My last comment is that it might be hard to use wind directions from Hawthorne or any other single site to try and develop a good picture of possible "smog pool" sloshing back and forth according to the diurnal winds.

  2. An interesting thing comparing last evening's sounding with this morning's (00Z vs 12Z 4 Feb), is that a mixed boundary layer appears to have formed overnight below the stratus deck. It looks like the stratus mostly eliminated the diurnal cycle near the surface, but radiative cooling of the stratus deck itself resulted in a drop of 5C or more overnight in and just above this cloud layer, between about 850 and 800 mb. So mixing may have actually improved overnight in the lower valley due to this elevated radiational cooling. This may be the first night that we have really had a thick, elevated stratus deck like this in recent inversions, as most of the previous ones were relatively thin or scattered.

  3. Looking at mesowest and current temperatures, I am wondering if we are entering the point of the season where incoming radiation is burning off the inversion. The stratus deck has melted out quickly, and something interesting is the T/RH numbers on the benches vs valley. Temp and RH at SL Int'l is ~30/75, while Natural History is 39/60.

    While I know this means early degradation of powder, the incoming solar radiation would be a welcome sight in my lungs.

    1. The sun certainly does a much better job of warming the surface now than a month ago... about a 40% increase in total solar radiation now compared to the winter solstice. Also, notice how the dew point numbers have dropped at least a few degrees since yesterday. Maybe the cooling of the cloud deck overnight caused some moisture to precipitate, or perhaps some of the dry air just above this cooled enough that it was added to the mix.

  4. A bit of a struggle looking at the overnight cold front passing through. Will this be strong enough to mix the shallow inversion or will we again see a partial 'cleaning'. -GWeyman

    1. I'm leaning toward a partial mixout, but hope I'm wrong and it cracks.

  5. A weak trough may just deepen the inversion, allowing it to be less concentrated at the surface till Friday's storm.