Category Archives: Air quality

Blowing dust in Texas and Oklahoma


GOES-16 “Moisture” Infrared brightness temperature difference (10.3-12.3 µm) images, with hourly surface reports plotted in cyan [click to play animation]

Strong winds in the wake of a cold frontal passage created large areas of blowing dust across the Panhandle Plains of northwestern Texas after 16 UTC on 21 January 2018. GOES-16 “Moisture” or “split-window difference” (10.3 µm12.3 µm) images (above) showed that the leading edge of this airborne dust moved over far southwestern Oklahoma after 20 UTC. (Note to AWIPS users: the default enhancement for this GOES-16 “Moisture” Channel Difference product was changed to “Grid/lowrange enhanced” to better highlight the dust with shades of yellow)

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Cirrus” (1.37 µm) images (below) also displayed blowing dust signatures; the surface visibility was restricted to 2-3 miles at some locations, with Big Spring briefly reporting only 1/4 mile from 20-21 UTC. The dust signature was apparent on the Cirrus imagery because this spectral band can be used to detect any airborne particles that are effective scatterers of light (such as cirrus ice crystals, volcanic ash, dust/sand or haze).


GOES-16 “Red” Visible (0.64 µm) images, with hourly reports of surface weather plotted in red and surface visibility (miles) plotted in red [click to play animation]

GOES-16 Near-Infrared

GOES-16 Near-Infrared “Cirrus” (1.37 µm) images, with hourly reports of surface weather plotted in red and surface visibility (miles) plotted in red [click to play animation]

A Cirrus band is also available with the MODIS instrument on the Terra and Aqua satellites (as well as the VIIRS instrument on Suomi NPP and NOAA-20) — a comparison of Visible (0.65 µm), Cirrus (1.37 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images from Terra and Aqua (below) highlighted the differing appearance of the blowing dust features as sensed by each of those spectral bands. The airborne dust exhibited a darker signature in the Shortwave Infrared images since the small dust particles were efficient reflectors of incoming solar radiation, thus appearing warmer at 3.7 µm.

Terra MODIS Visible (0.65 µm), Cirrus (1.37 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images, with surface reports plotted in cyan [click to enlarge]

Terra MODIS Visible (0.65 µm), Cirrus (1.37 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images, with surface reports plotted in cyan [click to enlarge]

Aqua MODIS Visible (0.65 µm), Cirrus (1.37 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images, with surface reports plotted in cyan [click to enlarge]

Aqua MODIS Visible (0.65 µm), Cirrus (1.37 µm), Shortwave Infrared (3.7 µm) and Infrared Window (11.0 µm) images, with surface reports plotted in cyan [click to enlarge]

Pilot reports within 20-45 minutes after the Terra overpass time (below) revealed Moderate to Severe turbulence at an elevation of 8000 feet, just southeast of the most dense dust plume feature (highlighted by the cooler, lighter gray infrared brightness temperatures) — this was likely due to strong wind shear in the vicinity of the rapidly-advancing cold front. Farther to the southwest, another pilot report indicated that the top of the blowing dust was at 7000 feet, with a flight-level visibility of 3 miles at 10,000 feet.

Terra MODIS Infrared Window (11.0 µm) image, with a pilot report of turbulence highlighted in red [click to enlarge]

Terra MODIS Infrared Window (11.0 µm) image, with a pilot report of turbulence highlighted in red [click to enlarge]

Terra MODIS Infrared Window (11.0 µm) image, with a pilot report of dust layer top and flight level visibility highlighted in red [click to enlarge]

Terra MODIS Infrared Window (11.0 µm) image, with a pilot report of dust layer top and flight level visibility highlighted in red [click to enlarge]

Wacky Waves, Winter Warmth, and Pollution Perspectives

Kory Davis sent me the great cloud photo below taken looking east yesterday at about 8:30 AM from Snowbasin.

Photo courtesy Kory Davis
This is a wonderful example of breaking waves produced by Kelvin-Helmholtz instability, named after physicists Lord Kelvin and Hermann von Helmholtz.  Such instabilities are produced by vertical wind shear, leading to breaking waves that are similar in appearance to waves breaking on a beach.  Such instabilities occur frequently in the atmosphere, and produce turbulence, but aren't always easily seen. 

Moving on to today's weather, there's much to discuss.  First, how about our afternoon temperatures.  At 2242 UTC (3:42 PM MST), many sites along the east bench are running in the 50s, including a 56 at the University of Utah.  Alta is 43, Mountain Dell 51, and Kimball Junction 50.  THIS IS NO WAY TO RUN A WINTER.  

If you look carefully, evidence of the valley inversion is apparent, with temepratures in the 40s in Kearns, Taylorsville, and along the Jordan River north of South Jordan.  Also apparent is the strong influence of the lake breeze, with the Salt Lake Airport sitting at only 44.

How about we give the PurpleAir network a little love today and use it to take a look at the distribution of pollution around the region.  Sometimes caution is needed in interpreting the data collected by these low-cost sensors, but they look to be quite useful today.  Note that the highest PM2.5 concentrations are in the downtown area.  Lower values are found on the east bench, the west bench, and in the southern Salt Lake Valley.  One can find predominantly clean air up I-80 to the east.


The ups-and-downs in air quality over the past few days have been astounding.  The DAQ sensor at Hawthorne Elementary has had periods each day with PM2.5 at levels ranging from unhealthy for sensitive groups to good.

Source: DAQ
Yesterday in the Salt Lake Tribune, head of EPA region 8 noted that he believes that Utah's air quality can be improved, in part because of Colorado's success in Denver.  He notes, correctly, that part of the trick is figuring out the chemistry.  That's all fine and dandy, but here's my three New Year wishes that I would like to see happen now:

1. Upgrade and improve real-time air quality monitoring.  It is an embarrassment that there is only one "official" real-time monitor in Salt Lake County and that the data from this monitor is often more than an hour old.   There are large spatial and temporal variations in air quality that exist in our county (and other non-attainment counties in northern Utah) and citizens deserve better information about what they are breathing right now.   Alternatively, take greater advantage of lower-cost sensors, accounting for observational issues that arise from their design limitations.

2. More proactively work to improve air quality.  As a scientist, I always cringe at rallying cries for more research.  Yes, we do need more research, but the assumption that because we don't know everything, we know nothing is a bad one.  We know enough to move forward on new initiatives today. 

3. Show greater commitment and be results oriented.  I'm sorry, but I've lived here for over 20 years and I have heard the same song and dance for a long time.  If this state truly cared about the air pollution, we would have bent the curve years ago.  Near as I can tell, the primary difference between states that have made substantial air quality improvements (e.g., California) and those that haven't is political will and the desire to do what is difficult. 


After publishing this post, I realized I may have been overly harsh on points 2 and 3.  There have been long-term improvements in many air quality indicators, however, the worst PM2.5 events (98th percentile) have shown little trend over the past 10 years (see  Improvements basically flatlined after 2003.  The shift to tier 3 gas will hopefully help, but more can be done.  It's really a matter of whether or not you are satisfied with the status quo.  I am not.  

Skiing, Inversions, Pollution, and All That

It's been an interesting break.  After hemming and hawing about whether or not to get out of Dodge, we decided that there was enough snow and decent enough air quality that we'd gut out Christmas week at home.  By and large, I think it was a wise decision.

The cross country skiing has been pretty decent and has provided a good cleansing of my aorta.  Back country skiing?  We got in a tour the day after Christmas.  I can't say the skiing was good, but it was an education for my son who had never been out on such a hair-trigger, high-hazard day before.  I made him suffer too, with a 2000 vertical foot climb through heavily brushed slopes to start the day.  It's important that youngsters recognize it's not all fun and games out there. 

We went up for a couple of high-speed laps at Alta this morning.  It was only my 2nd day of lift served this year.  Conditions were surprisingly good for carving.  I'm always amazed that when conditions here are about as bad as they can get, it's still far better than most of the good days I had when I lived in upstate NY.  That being said, it's incredible to think that Mineral Basin is still closed and probably will be through the 1st of the year, unless Snowbird has some real tricks up its sleeve.

Despite the poor off piste coverage, the coverage on the groomers was adequate and by 11, the corrals were full and there was stiff competition for Jerry of the Day honors.  After a few laps of human-breakaway-slalom, we decided to return home and get something done.

Which brings us to the weather.

Here's your pearl of wisdom for the day.  Don't count on a pattern change until you see the whites of its eyes.  Stop looking at the 10-day forecast and buy yourself a fat bike.  Alternatively, go to Jackson.

Now maybe you can provide me with some pearls of wisdom.  Is it just me or is this the weirdest inversion ever?  Remember all that fog, stratus, and pollution on Christmas Day and Boxing Day?  Well, the fog has dissipated, we just hit 44˚F at the airport, and the PM2.5 concentrations have been up and down like a yo-yo the past few days.

Source: DAQ
I'm sure there's still plenty of gunk behind the lake breeze where the cool air over the lake won't release the goods to the free atmosphere, but this situation is far better than I could have hoped a couple of days ago.  More time at the office is needed to decipher this riddle.  Comments on what's going on appreciated.

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.  

About Yesterday’s Terrible Air Quality

It's a 2-for-1 day at the Wasatch Weather Weenies.  If you want to read about the overnight snowfall, proceed to the previous post.  If you want to read about dust, air quality, pollution, and other apocalyptic aspects of the storm, stay here.

Yesterday morning, an extremely nasty plume of dust penetrated into the Salt Lake Valley, pushing air quality to unhealthy levels.

Although Salt Lakers are aware of poor air quality during inversion events, yesterday we dealt with an entirely different beast.  During inversion events, emissions from the combustion of fossil fuels and wood, as well as a few other sources, build up within the Salt Lake Valley.  Winds are light and the strong atmospheric stability prevents the mixing of pollution vertically.  In contrast, yesterday the poor air quality occurred during strong, southerly, prefrontal winds.

The graphs below show what happened at Neil Armstrong Academy in the Northwest Salt Lake Valley.  From Tuesday morning (19 December) through just after midnight Wednesday (20 December), PM2.5 concentrations fluctuated from about 0 to 19, with the peak around midnight.  These are values that indicated good to moderate air quality with just a little bit of pollution. 

Source: MesoWest
Then, after midnight, strong southwest winds developed, scouring the valley clean.  Those winds persisted overnight and into the morning.  However, beginning after about 6 AM, PM2.5 concentrations began to climb, eventually spiking to over 200 ug/m3, which is well into unhealthy territory.  That spike occurred just ahead of the surface cold front.  PM2.5 concentrations then dropped from 203 to 10 ug/m3 in 10 minutes as the front went through and brought in cleaner air. 

Another perspective is provided by a laser ceilometer at the University of Utah.  This is a device consists of a laser that points vertically through the atmosphere.  The signal returned back to the device can be used to infer pollution concentrations and the base of clouds. 

Below is a time-height section from about 1500 MST on Tuesday through 1500 MST Wednesday.  There is some moderate pollution evident on Tuesday afternoon and evening, but the airmass becomes relatively clean overnight.  The dust plume appears shortly after 6 AM and through about 10 AM extends to about 750 meters (2500 feet) above ground level. 

Image Source: MesoWest
As the front approaches, the depth of the dust increases.  I suspect this is due to the convergence of surface winds and the lifting of air near the front lofting the dust plume to deeper heights.  If one looks carefully, you can see evidence of the dust aloft event after the clean air behind the front has moved in at low levels.

So, what the hell is going on and where is all of this dust coming from.  We had a similar event on December 3rd, but the dust wasn't as think.  One of our graduate students, Derek Malia, pointed out to me that the dust plume was very evident in satellite imagery that day.  Sure enough, you can see it in the image below, originating in the south Cedar Valley west of Utah Lake.

Source: NASA
Yesterday, similar story.  I didn't have access to as sharp of an image as on 3 December, but one could still see the plume originating over the south Cedar Valley. 

Image Source: NASA
And, one can see it as a pink streak in the dust product from GOES-16.  Note that the plume continues downstream over northeast Utah, in that area likely above the shallow post-frontal airmass. 

So, we have a good idea that this dust is coming from the south Cedar Valley.  Google Earth shows that the dust emissions could be coming from agricultural fields in that area, which are perhaps exceptionally dry for this time of year due to the drought conditions.  Another possibility is emissions from a fire-affected area.  I haven't had a chance to dig into the past fire data to examine if this is a viable hypothesis. 

Hopefully, this is an issue that will remedy itself with precipitation.  If not, perhaps it wouldn't take much to reduce emissions from that area. 

Three Surprises about Yesterday’s Weather

Richard Feynman.  Photo: Tamiko Thiel
"The only way to have real success in science is to describe the evidence very carefully without regard to the way you feel it should be. You must try to explain what's good about it and what's bad about it equally."
- Richard Feynman
The above quote is one that every forecaster should keep in mind.  A forecast is a scientific hypothesis, and improving as a forecaster requires careful evaluation of your hypotheses without cherry picking evidence or being overly kind (or harsh) on yourself.

As a scientist and as a forecaster, I get especially excited about surprises.  Things that happen that I can't explain or didn't anticipate.  In other words, they lie outside the bounds of my hypotheses.    These provide great learning opportunities and in some cases fodder for future scientific research.

Yesterday's weather provided three surprises.  The first was the haze evident in the Wasatch Mountains above the valley pollution, as discussed in depth in the previous two posts.

The second was the snow shower activity in the late afternoon and overnight.  Yes, it wasn't much, adding up to 0.13" of water equivalent and an inch of snow at Alta-Collins, but it was more than I expected.

The third was the stirring and partial mixout of pollution in the Salt Lake Valley.  Although it is faint, I can actually see Lone Peak from my office right now and observations from Hawthorne Elementary show a significant decline in PM2.5 concentrations overnight.  Great news, although air quality remains moderate.
Source: DAQ
The question then becomes why were these surprises.  In the case of the smoke from California, it was a case of inadequate situational awareness.  The smoke had clearly been blowing offshore and northward along the Pacific Coast and the thought of it actually moving inland never really crossed my mind until I was skinning yesterday morning.

The other two surprises, the snow and the pollution mixout, are related to the intensity of the short-wave trough that was dropping down the back (eastern) side of the ridge.  These are both related to the intensity of the trough, which in forecasts brushed by Utah, kept us in northerly flow at 700 mb, and dropped crest-level temperatures to only -3ºC.

Instead, the trough was further west and stronger.  It brought in more humidity, stronger flow, and dropped 700-mb temperatures to about -6ºC overnight.

This led to the somewhat more productive snowshowers, but also the cooler temperatures aloft and stronger flow helped provide some ventilation for the valley.

Could this have been anticipated?  Perhaps as a low probability possibility.  For example, I went through several SREF forecasts and could find a member or two that put out more than .10" of precipitation at Alta.  An example is below.

We don't currently mine the ensembles for guidance related to pollution mixout (my bias is snow), but perhaps if we consulted the full range of model solutions, we could find a few with a sharper trough.  The European center model, for example, had a bit of a sharper trough than the GFS and might have provided a bit better guidance.

Source: Weatherbell
Looking over all the data, chances are I wouldn't have expected the mountain precipitation and pollution mixout as a high probability outcome.  They would have been low probability outcomes, but in the range of possibilities nonetheless.  All of this illustrates the importance of assessing the full range of possibilities and not zeroing in on one forecast possibility.  After all, we live in a chaotic world.

All of that being said, I'm glad we got an angry inch of snow and a pollution stirring.

Incredible PM2.5 Observations from KSL Chopper

Following up on the previous post, our intrepid MesoWest Team has partnered with KSL-TV to collect PM2.5 observations from their helicopter, Chopper 5 (much thanks to KSL for enabling this incredible resource).  The data today is simply incredible.

Below shows one period during which the chopper was flying across the Salt Lake Valley and over portions of the Wasatch Front.  The chopper position varies, as well as its altitude, and it is clear that sometimes it it flying in clean air (blues) other times "dirty" air with much higher PM2.5 values (red).

The scattergram below illustrates all the PM2.5 measurements during that period based on altitude. Below about 2100 meters, the helicopter frequently flew in the valley pollution.  PM2.5 values were generally above 35 ug/m3.  There are, however, a few places at those elevations where the air was fairly clean.  This is not surprising given the tendency for there to be waves and other features on the inversion. 

Between about 2100 and 2600 meters, the air is mostly clean.  All observations were below about 30 ug/m3 and the vast majority were < 5 ug/m3.  

However, go higher up and there are some horrible PM2.5 values again, some as high (or higher) as in the polluted air in the valley.  This is what I suspect is the smoke from the California fires.  This smoke isn't uniform.  There are some pockets of clean air, but the PM2.5 levels are quite high in some locations.  

This is simply incredible data.  I've never seen anything like it.  Wintertime pollution trapped in the valley.  Smoke from extreme wildfires at upper levels.  Remarkable.  Thanks to the MesoWest team and KSL for making this happen.  Note that this data is freely available in real time at

Smoke from California May Have Arrived in the Mountains

Many people over the past few days have wondered if some of our pollution is from the California fires and up until today, the answer was no.

If you live in the lowlands along the Wasatch Front, the answer to that question is probably still no.  We are in a soup of locally produced pollution that is quite isolated from changes aloft.

However, in the mountains, the answer might be yes.

I went up to Alta this morning for a quick tour and workout, and was quite surprised to see some dense haze in the canyon.  This haze was especially apparent looking down canyon from the upper Albion lot.

After our climb, the view toward Heber showed some dense haze that appeared to be thicker aloft than right near the surface in the valley.

What was quite strange was how spotty and variable the haze was.  At times, the air looked quite clear in one area, but hazy in another.  One thing that was clear is that there didn't seem to be any spread of pollution from the valley to high elevations, as suggested by the photo below.

Thus, I don't think the mountain haze is from the valley pollution.  Providing further evidence of this, the morning sounding at KSLC showed an incredibly strong inversion, with the temperature within the inversion increasing more than 10ºC.  My experience with such strong inversions is that the air is typically quite clean once you are above them.

I wonder if instead some smoke from the California fires has finally crested the ridge and dropped down into northern Utah. Over the past several days, that smoke has been pushed offshore and at times northward along the Pacific Coast.  Monday's Modis image, for example, shows the smoke offshore, along the coast, and wrapping northward off the Northwest Coast.

Modis Image 11 December 2017.  Source: NASA
Yesterday, however, that smoke had spread inland across much of Washington and Oregon, and possibly across northern Idaho.

Modis Image 11 December 2017.  Source: NASA
 Thus, it is possible that the northwest flow has finally transported that smoke into our area.  As I write this, we haven't gotten a Modis overpass yet, but one will be coming soon and it should be interesting to see what it shows.

Note that this does not mean that the smoke is contributing to our problems in the valley.  The smoke is likely elevated and the inversion that prevents the pollution from entering the valley, also limits the spread of smoke aloft into the cold pool.  The smoke instead simply adds to the misery by reducing the clarity in the mountains.

Inversion Tidbits and Long-Range Prospects

Yesterday's satellite imagery summarized the ridge-dominated weather of western North America quite well with extensive fog found in the major basins, many of the valleys of British Columbia and the Northwest United States, and the Great Salt Lake Basin.  At the same time, smoke from the California Fires covered much of the offshore eastern Pacific Ocean.  If you look carefully, it appears that some of this smoke has been carried northward to the Queen Charlotte Islands.

Composite MODIS image from NASA.
Within the Salt Lake Valley, the pollution went into overdrive yesterday, with PM2.5 levels skyrocketing in the morning to unhealthy levels.  Unlike previous nights, when PM2.5 dropped considerably, levels declined only modestly overnight and remain unhealthy for sensitive groups. 

PM2.5 concentrations at Hawthorne Elementary.  Source: DAQ
Looking for a brightside?  The frosty trees make for a beautiful Christmassy scene.  

We are so desperate for weather that I feel the need to mention that there is actually a weak short-wave trough dropping down the back (eastern) side of the ridge and passing through our area Wednesday night.  

Yup, that's your weather for the week.  It will bring somewhat cooler temperatures to the mountains, perhaps helping with the snowmaking efforts and might stir the upper part of the inversion a bit.  Emphasis on might.  Low elevations will likely remained mired in pollution. 

I am a bit more optimistic that the trough on Saturday is strong enough to give us at least a partial mix out.  It's still soon to say if it will scour it all out.  Sometimes, the coldest, most polluted air at the lowest elevations can be quite stingy. 

Snowfall totals for the mountains presently look paltry.  About half the members in our downscaled NAEFS ensemble generate 2 inches or less.  A few members go for more.  A game changer is unlikely. 

The word "pattern change" is being thrown around a lot, but I bet you'll have a hard time finding anyone who can tell you what that means.  I have yet to see any indication from any ensembles that we are going to shift from the high-amplitude pattern that has dominated for weeks and in which there are very deep ridges and troughs at upper levels, to a more progressive pattern with stronger westerly flow.  Instead, there may be some shifts in the position of the ridges and troughs.  For example, some of the GEFS 10-day forecast members below have a ridge upstream of the west coast of North America, rather than near its present location along the west coast or just inland. 

Source: Penn State E-wall
Those shifts could be important if they lead to a slowly evolving but wet pattern for Utah.  However, looking at the GEFS solutions above, some might bring us some snow, others keep us dry.  Why waste time talking about this range of possibilities?  Like thermonuclear war, the best option is not to play.  

Thus, hope we get something from the trough on Saturday and at minimum hope it cracks the inversion.  It's the only slim hope we have for mountain snow over the next week.  After that, your guess is as good as mine. 

Diurnal Intricacies of the Inversion

PM2.5 concentrations during our current inversion event have shown remarkable variations from day to night.

Below is a time series of PM2.5 measured at our mountain meteorology lab at the University of Utah showing a clear long-term upward trend, but also a tendency for PM2.5 concentrations to spike just before noon, remain elevated until mid to late afternoon, and then decline.

Source: MesoWest
What are the causes of this diurnal behavior.  There are several possible contributors.

First, there is the possibility that photochemistry - chemical reactions occurring in the presence of sunlight, are contributing.  Comparison of the above plot with the incoming solar radiation below shows some relationship, with the PM2.5 exhibiting a bit of a lag relative to the solar radiation.

Source: MesoWest

Another possibility is that temperature is playing a role since it also affects the PM2.5 chemistry.  Again, there is some correlation.  

Source: MesoWest
Finally, there is the transport possibility as the winds are also changing diurnally, with a good correlation between wind direction and PM2.5 concentrations.  

Source: MesoWest
Another perspective is provided by the someone hacked-up graphs below, based on data collected at the University of Utah by our MesoWest team over the 24-hour period ending this morning at 10 AM (the hacking reflects my splicing of their multiple graphs together).  The top figure is derived using a laser that is shot vertically through the pollution.  The color fill is backscatter, a measure of how much of the laser light is reflected back to the ground, with higher values roughly correlated with greater PM2.5 concentrations (brown-white being the dirtiest air).

The plot begins on the left at 10 AM on Sunday when the local flow just shifted to predominantly westerly (some variability from SW-NW).  Surface PM2.5 concentrations during this period are quite high and, in addition, the pollution is quite deep.  At just after 1700 MST (5 PM), the flow shifts abruptly to ENE, which reflects the onset of down valley flow from Red Butte Canyon.  This marks the beginning of a gradual decline of surface PM2.5 concentrations, as well as a decrease in PM2.5 concentrations aloft.

Source: MesoWest
There is a brief lull in the wind that occurs just before 2300 MST (11 PM MDT), with the flow becoming somewhat erratic.  Without the inflow of cleaner air from Red Butte Canyon during this period, the surface PM2.5 values climb, although things don't change too much aloft.  Finally, after midnight, the ENE flow returns and PM2.5 values drop again, although there are a few spikes during the night that may correlate with declines in wind speed (I haven't bothered to check take this comment for what it is worth).

At the end of the time period, the PM2.5 values climb again, abruptly, when the flow shifts to westerly.

All of this illustrates some of the intricacies of these inversion events.  Pollution concentrations vary in the vertical (yes, there is clean air up there), although if you look carefully at the plot above, you can see that it's not as simple as polluted air near the ground and non-polluted air aloft.  There are layers.  In addition, pollutant concentrations vary horizontally and at the University of Utah one can clearly see the migration of pollutant-laden air onto campus when the wind shifts to westerly in the morning.

What role photochemistry and temperature play in all of this is unclear to me.  I suspect it plays a secondary role compared to meteorological factors, but I am not an atmospheric chemist and over the years I've learned that when all you have is a hammer, everything looks like a nail.  In other words, as a meteorologist, I might be guilty of placing too much weight on meteorological factors.

One thing to keep in mind is that not all inversions look or behave like this and even this one might behave differently in the days to come.  As a scientist, I think what we see over the next few days will be "interesting."  As a citizen, I wish the damn thing would just blow away.