It's In The Fog



I've had a lot of fun the last two weeks looking at plots of temperature and pollution profiles gathered by automobile in Fairbanks, AK during an ice-fog episode on February 6th of this year. The big question, of course, is where does so much sulfate come from? (I'm predisposed to blame aqueous sulfur chemistry in the sub-freezing, yet still liquid, fog and aerosol droplets, but I don't have the data to back up that assertion). But the profiles are interesting as well for what they reveal about pollution transport within fogs.

The most interesting part is the role of turbulence at the very bottom of the ice fog, and the way it creates an accumulation layer of pollutants.

As the ice fog thickens, the effective surface for IR radiation “lifts off” the valley bottom, allowing a steep vertical temperature gradient to occur on the valley floor and permitting convection to occur there. This kind of convection is common in fogs in other places (e.g., the Central Valley, the Great Basin, etc.) but it wasn’t clear to me until now whether the ice fog was thick enough to allow it to occur in Fairbanks too. These temperature profiles show that convection can occur, at various times and places, and stop at other times and places, making for a complicated geography of pollution.

What the turbulence does is drive pollution from high emission, higher turbulence zones (on the valley floor) upwards, into more stable air (on the hillsides). So, an accumulation layer can form, almost like a bathtub ring around the valley, and pollution can move from one neighborhood into the next, and appear where one wouldn’t expect it to be found, based on expectations of emission density. Similarly, the ice fog is thin enough that the pollution pumping can stop as well, and then maybe start up again later.

I discussed all this this morning, at cartoonish length, in the morning meeting, provoking Craig and Gabe to recommend John Carpenter's 1980 horror movie "The Fog" (starring John Carpenter, Jamie Lee Curtis, Adrienne Barbeau & Janet Leigh).