Monday, December 25, 2017

Christmas Brings Gravity Waves and Unexpectedly Poor Air Quality

Christmas turned out to be a fascinating weather day.  The Christmas Eve warm front and precipitation left a shallow cold pool over the Salt Lake Valley with widespread fog and dense haze in the morning.  I noticed some strong gravity waves from my vantage point in the upper avenues, one of which is pictured below.  Note the bulge in the depth of the fog, similar to the crest of a wave on the water. 


Although the fog burned off in many areas, a shallow lens of haze remained over much of the valley during the afternoon. 


My thinking during the day was that this was probably predominantly natural haze as PM2.5 concentrations were quite low on Christmas Eve morning.  However, the PM2.5 observations from Hawthorne Elementary show we reached levels unhealthy for sensitive groups Christmas evening.  In fact, the increase from Christmas Eve morning is astounding.  Normally one sees an increase in PM2.5 during a developing and persistent inversion of about 10 ug/m3 per day.  What happened from Christmas Eve to Christmas was more than double that.

Source: DAQ
Clearly, the DAQ was caught with their pants down on this one as their web site, updated at 1:22 PM on Christmas Afternoon, had us in "unrestricted action." 


So, why was the PM2.5 so high?  I'm not sure why, but there are a few possibilities.  One is that in this case the inversion didn't develop aloft and descend slowly as is often the case when a ridge builds in.  Instead, it developed as the warm front warmed temperatures aloft and precipitation cooled the near-surface airmass, resulting in a shallow cold pool right from the get go.  Thus, any emissions were confined to a shallow layer.  Indeed, the sounding collected in the late afternoon on Christmas shows a very shallow inversion based right at the surface, with a well mixed layer aloft.  Little wonder the upper benches were so clear, with a different story in the valley.  


I'm not entirely satisfied with that as an explanation, however, since the buildup in pollution seemed remarkably rapid.  I wonder if the holiday "sparked" an increase in yule log burning, and that this, along with the shallow nature of the cold pool, resulted in the rapid increase in PM2.5.  Having more wood fires with such a shallow cold pool would be a recipe for a rapid pollution rise.  Being in unrestricted action, perhaps many more people than usual decided to go for the holiday fire.  

A third possible factor is that the high humidity is causing somewhat inflated values, although the sampler used for the time series above I thought was somewhat immune to this effect (some samplers are not as water droplets inflate values).

Regardless, it's unfortunate that pollution along the valley floor was so high during this event.  

16 comments:

  1. Nice to see a white Christmas in SLC! How high elevation is the photo taken? Do you have snow cover everywhere in SLC or only up on the benches?

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  2. >A third possible factor is that the high humidity is causing somewhat inflated values, although the sampler used for the time series above I thought was somewhat immune to this effect (some samplers are not as water droplets inflate values).

    For DAQ measurements, this shouldn't be the case. They weigh the pollution counts on their filters to avoid various kinds of measuring errors, including humidity (most inversion days have near 100% humidity anyway...) Unfortunately weighing the pollution requires expensive stations, which is why the DAQ has so few of them. For laser based systems (like what Purple Air uses), humidity tends to drastically overestimate totals.

    >I wonder if the holiday "sparked" an increase in yule log burning, and that this, along with the shallow nature of the cold pool, resulted in the rapid increase in PM2.5.

    This is my guess. I'm a wood burner myself and carefully watch trends. Usually wood only contributes 2 ug or so on inversion days. But on snowstorm days you have many, many more people burning, especially on Christmas day. Compare the pollution totals to 4th of July days, and also combine it with a shallow inversion, and suddenly the wood burning seems plausible. Though the only way to really know would be to measure the amount of levoglucosan (a good marker of wood burning) in the particles.

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  3. Colorado checking in. We have more snow than you. A rare event, so we must publicize it when it happens. Wolf Creek, usually a top snowfall area in CO, has terrible snow though, far below average so can't gloat too much. It's all going north this year. But Loveland Basin and many other locations in the north have more SWE than Snowbird or Ben Lomand SNOTEL sites today. All are put to shame by Wyoming so far, of course. La Nina?

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  4. The "Tower" Snotel site, located a short distance from Steamboat Springs, regularly beats all the Utah Snotel sites for snow-water-equivalent (SWE). Might take some skinning to get there though.

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  5. Utah brags a lot, mostly about how much better their geographically small, west-facing favored slopes gets "more snow than Colorado". Yeah, in most years that is true. In only small geographic areas close to SLC. Good luck building a road into the canyons that can handle big traffic. Be careful what you wish for. Denver has I-70. Total disaster.

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  6. Elevation matters.

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  7. Dear Colorado Trollers:

    I have a good friend who lived for a number of years in Boulder. When asked where he goes to ski in Colorado, he said east. As in east to the airport, so he could fly to Salt Lake.

    Our snow snobbery stems from the fact that we have 3.2 beer on tap and a pathetic nightlife. With little to look forward to in the evenings, we obsess over early morning deep power days with right-side up powder. If you had more deep powder days with true bottomless snow, you would understand. We suspect that most of the time when you do get faceshots, you are riding on the underlying surface. Sad!

    Sincerely,

    Utahns

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  8. Ha! I really do like your blog. The best imo. Keep it up. I skied once at Alta. Around December 5, 1990 or so. It was amazing. I feel your pain.

    Eldora just outside of Boulder had rare eastern Front Range pow and face-shots on Christmas. Still plenty of rocks to avoid if you could intuit their location. But pretty good for the eastern Front Range in December!

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  9. With regards to the Marine-Intermountain-Continental definition of western snowpack types, I'm wondering what you think about the sometimes very large, wet upslope events that the Front Range experiences, usually in March or April or early May. Moisture is largely from the Gulf of Mexico. Are these storms considered to be "marine"?

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    1. The snowpack climate describes the general conditions, but variability can be important.

      From a meteorological perspective, I probably wouldn't describe those Front Range events as "marine", mainly because they often feature colder, continental air at low levels. They are very difficult storms to classify in such a simple manner.

      Jim

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  10. These big sloppy wet upslopes we get in eastern Colorado almost always happen when the sun angle is high (mostly in spring). The strongest one I remember was in late March of 2003, and some moisture even spilled over into Utah. All of the moisture streamed up on the northern side (wrap-around) of a low pressure system, from the Gulf of Mexico.

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  11. Hey remember back in the day how "the Brown Cloud" was always put forth as a way to rhetorically shit on the Front Range? Worse air now in SLC and Missoula. Nobody talks about any Denver brown cloud anymore.

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    1. The situation in Salt Lake is an utter, total embarrassment. Technically, we've made progress since the 70s, when coal and wood burning was rampant, but there's been no progress in the past 20 years, in contrast to Denver. The difference is probably political will.

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  12. I agree about political will. Utah still has too many peiple who, for some reason, think wrecking their natural environment is a "conservative value". It isn't. But also, it's obviously geography. Denver is in a slight bit of a topo basin, but not like SLC or Missoula or any other town in the basin-and-range.

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  13. Denver has the vast expanse of the Great Plains to the east. All of our pollutants go that direction and are not blocked by anything like the Wasatch.

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