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.
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.
Another possibility is that temperature is playing a role since it also affects the PM2.5 chemistry. Again, there is some correlation.
Finally, there is the transport possibility as the winds are also changing diurnally, with a good correlation between wind direction and PM2.5 concentrations.
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.
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.