One thing I’ve noticed about the AQI for particulates is how variable it is. On any given day, my local hourly estimate from Accuweather will differ significantly from the EPA’s Airnow map. Which, in turn, differs from readings just a few miles away. For example, above, my AQI for particulates (as of 1 PM 7/6/2023 is either 63 (Airnow) or 33 (Accuweather). Or somewhere between.
And readings within a few miles go as low as 13. At the same time, the seemingly accurate meter I just bought shows “9”, sitting on my back screen porch.
At first, I chalked that up to instrumentation. Maybe particulates are hard to measure, and what I’m looking at is more-or-less instrumentation error.
Because, serious, how could the air be so different, just a few miles away? If I were to take some other measure of the atmosphere — temperature, humidity, pressure — it would vary smoothly over vast areas. E.g., if it’s 90 degrees here in Vienna, VA, there isn’t going to be a pocket of 45 degree air five miles away in the City of Fairfax. Yet you see that sort of apparent PM 2.5 disparity all the time.
So I thought, it must be poor instrumentation. Then I bought a cheap air quality meter, noted above. Not only are the readings stable from hour to hour, they are frequently in good agreement with the Accuweather numbers. They clearly respond to ambient conditions in a hurry. (The 4th of July fireworks briefly sent the meter into the “purple” AQI range, consistent with predictions from the Airnow map.) The stated accuracy of the PM 2.5 measurement is +/- 10%. All that, from a device that measures all five of the key air pollutants and costs under $75.
So, this isn’t due to instrumentation error. Or shouldn’t be. You can get reasonably reliable PM 2.5 measurements with a cheap off-the-shelf device.
Maybe my local variation is due to the presence of large local point-sources of PM 2.5. But, to a large degree, we have no large point sources of particulate emissions in this area. Largely because we are almost devoid of industry, in the DC area, and our power plants are (mostly) located outside of the metro area.
Which also matches my observation, because it’s not as if one area is consistently dirty. It’s that the readings consistently vary a lot from place-to-place in this region.
So why do the PM2.5 readings in my area appear to be so highly localized? Is there really that little mixing of the air between PM2.5 emitters, and local air?
Trying to understand how air mixes — a fool’s errand.
After about an hour of looking, I’m going to say that short of getting a graduate degree in atmospheric science, this ain’t gonna happen.
It’s surprisingly complicated, but the joker in the deck is “turbulent mixing”? Once I found out about that, I realized it was time to call it quits on trying to understand this.
First, physicists distinguish “bulk flow” (e.g., a breeze) from “diffusion processes” (molecules or particles moving through still air). In this case, the latter would be the movement of water molecules or fine particulates through still air.
So, smoke spreads out because it 1) blows smoothly downwind, and because 2) the particles diffuse outward into surrounding clean air.
That said, it also spreads due to 3) turbulent mixing. Any time the flow of air is not smooth (laminar, or layered), turbulent mixing is said to occur. This sort of mixing can apparently distribute that smoke fully and more-or-less uniformly in the adjacent clean air.
Turbulent mixing occurs a lot in the atmosphere. I’m pretty sure that it occurs at the level at which clouds form above the ground. It occurs within clouds. I occurs if sufficiently strong wind sweeps past fixed objects, e.g., tree branches. And so on. Anything sufficient energetic will kick the flow of the atmosphere from laminar flow to turbulent flow and turbulent mixing.
The bottom line is that there is no back-of-the-envelope way to determine how well PM 2.5 (including smoke) typically mixes into the surrounding atmosphere. In the end, it’s all empirical, and depends on how hard the wind is blowing horizontally, how turbulent the atmosphere is in vertical profile, and so on.
Presumably, both water vapor and PM 2.5 move at the same speed, and mix at the same rate, when it comes to bulk transport and to turbulent mixing. In both those cases, they are merely being carried along by the surrounding air.
But PM 2.5 diffuses a lot less rapidly than (say) water vapor. A theoretical rule (via Einstein and Stokes) is that rate of diffusion is inversely proportional to the radius of the particle trying to diffuse. Getting hold of some data (but not showing the calculation), that suggest that PM 2.5 diffuses about a thousand times more slowly than water vapor.
Diameter of a water molecule seems to be given as 2.75 Angstrom, where an Angstrom is 1/(10^10) meters. Ah, round down to 2.5. But PM 2.5 is in microns, or 1/(10^6) meters. This means PM2.5 particle is about 10^4 = 1000 times larger than a water molecule. Thus under this simple theory, water (humidity) diffuses through still air roughly a thousand times faster than a PM 2.5 particle would.
At the end of the day, I have no clue whether that matters or not, with regard to widely varying PM 2.5 readings across my area.
All I know is that even without big local point-sources of PM 2.5, it’s common to see big difference in (what appears to be) actual PM 2.5, across different locations in my area. Whereas for other parameters of the atmosphere — temperature, pressure, humidity — true local variation in those quantities is tiny.
Seems kind of crazy to worry about it, but there has to be some good reason why this aspect of the atmosphere is so qualitatively different from others.
Maybe Hawaii wasn’t just a nice place to hang out.
Maybe my only clue comes from the Keeling curve (above) and how that is measured. When Keeling started measuring atmospheric C02 in the late 1950s, he established his laboratory on the windward side of Mauna Loa.
And found average atmospheric C02 around 315 PPM. Currently, it’s around 422 PPM.
But the point is why he chose that locale. His goal was to get “well mixed” atmospheric gasses, and, apparently, having circa 6000 (?) miles of open ocean to windward was just the ticket for getting that.
By contrast, you can frequently find city air with C02 levels in the 1000-PPM range, near congested roads (reference). That air hadn’t had a chance to get mixed with the rest of the atmosphere.
So, maybe Keeling located there for some reason other than it’s being a nice place. Maybe you really need that much distance to ensure uniform mixing. And maybe the mere 500 miles or so between me and the nearest Canadian mega-fire isn’t enough to ensure uniform mixing of the air.
So I’m guessing that the atmosphere doesn’t mix all that uniformly. For whatever reason. And that the small-area variation in PM 2.5 is true. And that I should not expect it to get any smaller as the summer progresses.
