Not to take credit for these answers, but I saw a couple great responses (one from @gretchengehrke, ) from this thread on which you originally asked, and wanted to just be sure they're linked-to from here: https://groups.google.com/forum/#!topic/plots-airquality/0gn3lm9wUFc
Just a small point Glen, which you may already know since you have thought to include meteorology on board, but the biggest driver of many regional air pollution levels tends to be the atmospheric boundary layer, with traps also an issue. The boundary layer is generally low in morning and during temperature inversions. With the turbulence created by solar warming, post sunrise, the boundary layer tends to rise rapidly during the day, expending the total area in which pollutants may be dispersed and thereby decreasing effective density. Of course, photochemical action also helps create new secondary PM2.5 as organics get oxidized and denser, increasing partitioning to PM, midday or a little later. Since this takes time it can be considerably downwind of the sources. This plus geographic traps, like mountains, is basically an important reason why San Bernardino Riverside is so polluted to the east of LA. Again, you may already know all of this but perhaps some on the list do not.
What Wig says is absolutely right, and some of the most tragic and deadly air quality events have been during temperature inversions when pollution has been socked in cities (like the London fog episode in the early 1950s). It might actually be a very meaningful and memorable experience for your students to observe how particulate matter decreases above the boundary layer (which, depending on where are, is often ~ 1000 m during the day, so might be doable with a balloon kit).
Another factor to consider is how humidity changes with altitude. Most (all?) real-time or near-real-time particulate matter monitors use an optical system to count particles. Basically, they shine a light into a chamber and measure the light refracted, assuming it is due to particles. This makes these systems very sensitive to humidity though, as water drops will also refract the light. I'm really not sure how much this would impact measurements, but since absolute humidity decreases with altitude, you might get a more accurate particle count at higher altitudes. Anyway, it's just something to think about.