Monday, January 30, 2017

Strong High Pressure = Bad Air

Whether they call it haze, smog, or gunk, steel yourselves as the week ahead looks like a grey and polluted one.

Salt Lake's all-time sea-level pressure record based on data I obtained from Weather Underground appears to be 31.09 inches of mercury (1052.8 mb) (surface pressure is reduced to sea level for convenience when analyzing surface maps since surface pressure analyses look like topographic maps).  Although we won't break that during the current high-pressure event, we peaked at 30.95 inches of mercury (1048.0 mb) on Saturday, which is quite high.

Source: MesoWest
Although we've retreated a bit from that maximum, strong high pressure remains firmly in control.  In addition, this morning's sounding shows a whopper of an inversion based at about 840 mb (1600 m above sea level).  Temperatures increase about 13ºC through the inversion.

Source: SPC
The one silver lining in this event is that the inversion is elevated and there is a shallow mixed layer over the valley floor, allowing for some shallow transport and mixing of pollution.  This probably helps keep valley pollution levels a bit lower than they would be if the inversion was based on the valley floor, although they are still well into the unhealthy for sensitive groups category this morning.
Source: Utah Division of Air Quality
As we suffer this pollution, I thought I would take a moment to remind everyone just how bad the pollution in this valley would be without efforts to reduce emissions over the past decades, including transitions in how we heat our homes, the use of cleaner fuels, the use of catalytic converters, etc.

 Below is a time series of mean particulate concentrations back to the winter of 1973-74, which was provided to me by Dr. David Whiteman of the University of Utah.  The magenta cyan bars are total suspended particulates, the blue PM10 (particulate matter smaller than 10 microns) and the red PM2.5 (particulate matter smaller than 2.5 microns).  Unfortunately, we did not measure PM2.5 back to 1973-74, so this is a bit of a hodge podge of available measurements, but one can see the decline in total suspended particulates and then a decrease in PM10.

Source: Dave Whiteman
Each of those declines indicates an improvement in air quality (or perhaps better put, a reduction in the severity of poor air quality).  The PM2.5 trend is a bit more unclear.  Nevertheless, it is remarkable that it is even flat given the growth in population, vehicle miles traveled, etc.

Now, just imagine what that graph would look like if we had stuck with "business as usual."  Let's not go back to the 1970s.


  1. Great blog post. It's hard to find a good graph showing particulates over time in the Salt Lake valley. That figure is well designed given the limitations. I'll be using that image more often.

    Just an idea for a future blog entry, I've always wondered how much of the inversion we see is clouds and how much is pollution. Or in other words, what would it look like to have an inversion free day but also PM2.5 pollution 40 ug/m^3? And what does it look like to have an inversion with single digit PM2.5 pollution? And is there a way to easily have a metric which says "This is fog" or "this is smog" or "this is just plain pollution"?

    1. Your question is hard to answer for our valley for a few reasons. In particular, even before cloud formation, condensation onto soluble particulate matter creates haze that serves as a visibility obstructor. This happens at relative humidities as low as 70%. At times, it can be difficult to tell what is PM, what's haze, and what is cloud. Most of our pollution events feature RH > 70%.

      There are, however, inversion free days with high PM2.5. They are caused by smoke and/or blowing dust. Without those sources, I suspect it's hard to get to 40 ug/m3 in the Salt Lake Valley without a strong stable layer or inversion.

      If you want low RH, no inversion, and high PM, some of the megacities in Europe might be the place to look.

    2. Yes, I was digging around. Seems like Germany, Poland, and much of eastern Europe would also make great test beds for this kind of data:

      Anyway, thanks for the blog posts, I find them fascinating.

  2. According to the NWS website's data for the SLC airport, the pressure only got up to 30.82 inches at the airport on Saturday morning.

    1. Warning: Gory Detail Alert.

      The 30.82 is the altimeter setting. That is also derived from reducing station pressure to sea level, but it uses the US standard atmosphere (a long-term average) to do the reduction. The numbers in my blog post above are based on sea level pressure, which is derived instead using the surface temperature (most of the time, although there can be exceptions). If you pull up the page noted above, you'll see that the sea level pressure was 1048.1 or 1048.2. For the purpose of comparing to the sea level record, that is the value to use. I had 1048.0, and suspect the minor difference is due to truncation on the conversion.

      The sea level pressure, which is typically derived using the observed surface temperature when the altimeter setting was 30.82 is listed was 1048.1 and 1048.2. That is off by 0.1 or 0.2 mb from the figure I reference in the post probably because of truncation in my conversion.

      Unfortunately, reducing to sea level basically requires inventing an atmosphere that doesn't exist, and it leads to discrepancies like this. Using station (surface) pressure is probably a better option, but I didn't have access to those records this morning.


    2. Interesting. Thanks for the info.

  3. At least we have a good baseline to compare going forward should any environmental regulations be removed by the new administration. :0 Sorry, couldn't resist....

  4. It would be nice to see you actually admit for once that there are things the state government could be doing to improve the air but chooses not to.

    This comes off as an apology for a legislature that is more than content to let its citizens choke all the while suggesting that there is nothing they could possibly do.

    1. You mean like this:

      The simple fact of the matter is that we have made progress on the air quality front. That does not mean that we should be satisfied with that progress. My comments in the above post were more to say that we don't want to get rid of what we've done to make that progress, rather than to say we're doing OK. Yes, the state can and should be doing more. On that I agree.

  5. Interesting comment on reposted 1973-2013 timeseries of PM data. >>Iain Hueton: It wouldn't hurt to add another layer of data on top of this that shows a gradual increase of US life expectancy from 70.8 to 78.8 between 1970 and today (other factors matter, but AQ is a big one).