You may have noticed, if you’re from around here, that we did not exactly have an “Arizona Highways sky” yesterday. The sky was an awful whitish color tinged with blue (I’m fussy here). Below are examples of smog from the ground and from satellite. from yesterday afternoon. (Where there are clouds, BTW, the satellite can’t tell if there is smog under them.)
As you can see, the sky is not good over Catalinaland in this mid-afternoon shot from yesterday. And, it was not aloft in a layer as we have sometimes seen, but is partially obscuring the hills and mountains in the distance. It is also what we would call a well-mixed smog, which indicates its not generated locally. In that case of a very local source, there would be obvious thick and thin regions. Yesterday, it was homogenous, pretty much the same looking smog in every direction. (Tends to look worse toward the sun.)
In the aerosol satellite image, you can see we are in a blob of smog as indicated by the light turquoise area over Arizona and extending into northern Mexico. BTW2, dark blue in this graphic is a good sky, while a red one is awful. Our smog level here, as indicated by a parameter we call, “aerosol optical depth” (AOD), was around o.3 or so according to the satellite image below (probably on the high side). AOD measurement doesn’t know the height of the smog; its just how much is in the air between the top of the atmosphere and the ground.
For comparison, if the sun is NOT visible due to smog, the AOD would be about 4.0. Yikes! A really clean sky has an AOD of less than o.o5; its a really blue one and on such a day, from here you would be able to see Mt. Humphreys near Flagstaff if it went up to 40,000 feet or so, it is THAT clean. (You can actually see the tops of Cumulonimbus clouds near Flagstaff in the summertime from here in Catalina on these kinds of days with no intervening clouds.)
So where is it all coming from, I’d like to know?
To get an idea, we go to our trusty NOAA Air Resources Lab and their “HYSPLIT” model and run back trajectories for the air at the levels we estimate the smog is at. Of course, you can do this yourself; in fact, I have to say that it seems like I have to do everything for you.
Anyway, below are a couple of plots for various heights above the ground from ARL, 500 m (1500 feet), 2000 m (6600 feet), and 3000 m (10,ooo feet) for the past 4 days before the air arrived here.
You can forget about the one that starts at 3000 m (green line) above ground level in the Pacific. These plots also estimate the amount of lifting and descent the air went through, and you can see that the green parcel was caught up into a low center, got spun around in a circle, thrown upward to above 20,000 feet (7 km) and then descended on the back (west side) of the low. In going upward so much, clouds and precip forming in the upward moving air would have removed all untoward aerosols, so we can pretty much rule out something from Asia having crossed the Pacific, as happens from time to time.
Looking at those lower level trajectories (500 and 2000 m above ground level, blue and red lines in the plot) the likely culprit is found. Those ones represent back trajectories in much slower moving air that arrived over us, air that apparently spent a day or two in northern Mexico before getting here. That the of smog over us extended into northern Mexico and you start to think that the smoke probably was captured there and then eased northward across the border. This seems to be hinted at in the cloud wind field available through the University of Wisconsin and the US Navy’s Research Lab here. There are a couple of “wind barbs” (yellow colored ones) in northern Mexico that suggest southerly and southeasterly winds at levels where the smog was. Not an airtight case, but it probably drifted up from Mexico.
Enough about smog!
