Sunday, January 9, 2011

Intriguing Observations of Urban Pollution Mix Out

As promised, an upper-level trough and baroclinic zone have moved through northern Utah, weakening the inversion and scouring out some, but not all of the pollution resident over the valley.  Smog and haze can still be seen from several locations around the valley.

Looking north toward downtown from Cottonwood Heights
Panorama from the Avenues
PM2.5 concentrations provided by the Division of Air Quality also show that the air is "cleaner" in a relative sense.  PM2.5 concentrations peaked at nearly 100 ug/m3 yesterday, but lowered overnight.  Nevertheless, PM2.5 concentrations lingered near the NAAQS standard through this morning before dropping below 20 ug/m3 at mid afternoon.  Given the spatial variability in haze and smog this afternoon, there may be locations around the valley with higher concentrations.  

Because of the weak flow, the removal of polluted air from the Salt Lake Valley has been slow and one can see evidence of urban pollution in the Wasatch Mountains today.  For example, there was quite a bit of haze at times in White Pine Canyon today.  

Further evidence of the transport of urban pollution into the Wasatch Mountains is provided by CO2 measurements from the top of the Snowbird tram.  CO2 concentrations climbed from near unpolluted background values of ~392 ppm late in the day yesterday (Jan 08) to ~396 ppmm today, with a spike to 404 ppm.  I suspect that the spike reflects the movement of air with considerable urban influence over Snowbird.  

Today provides a nice example of the gradual removal of urban pollution by vertical and horizontal transport and mixing.  It provides a nice opportunity to observe processes that usually operate much more quickly.  


  1. Thanks for the discussion and great pictures, it is very helpful for us PCAPS folks. It is really interesting how gradual the mix-out was, it seems like strong baroclinic zones are usually associated with stronger flow. I wonder how much pollutants will stay around and pool back in the valley before we become strongly inverted again and back in unhealty air by Tuesday.

  2. I guess the only thing to do is get out and run tomorrow and then keep our fingers crossed. The DAQ is calling for a red-air alert by Tuesday.

    The gradual mixout today was interesting. I kept thinking that there's more to this than lapse rate when one looks at pollution.

  3. This episode is very similar to the December-January period last winter. We had numerous "almost" mix-outs, but none strong enough to completely clear out the PM2.5.

    Sounding structure from yesterday evening through this AM shows a pretty decent inversion remaining at 700mb as of 12Z.. This evening's sounding is much less stable but the sfc-700mb lapse rate still sits at 7.6c/km. I think that the weak winds throughout the period combined with only modest instability just havent been enough to completely replace the Valley air.

    DAQ's WRF-CMAQ modeling consistently shows that it is horizontal transport that really cleans the air out. WRF is rarely able to produce the extreme stability that we observe yet the CMAQ chemistry model can build PM2.5 concentrations. When CMAQ falls apart and prematurely cleans the PM2.5 out it is almost always due to an increase in horizontal winds and not entirely due to vertical stability. The WRF-CMAQ model certainly has issues, but perhaps it can offer valuable clues.

  4. Very interesting. The solution to pollution is dilution. That's about all you do when the mixing depth increases without much horizontal flow. Sounds like the CMAQ simulations support this.