As we have noted all week, this is the transition from ozone and particles air quality season to cleaner air. It is a great time to spend hours raking leaves instead of pouring over satellite images of a pretty clear United States. But this is also a transition to a time of low planetary boundary layer heights which make local pollution much more important. A low top of the boundary layer doesn't leave much room in the box over your head to put pollutants without having the concentrations rise markedly. In AIRNOW's AQI Sunday, we see the San Joaquin Valley forecasted to stand out with moderate particulate AQI readings. We can now expect LA, the San Joaquin Valley and Utah to figure more prominently in our posts over the new few months. All are ringed by high mountains and with no way for air in the boundary layer to escape, pollutants start to build up.
GASP's loops have to be carefully examined to signatures of pollutants because the senescence of leaves (turning yellow, then red, then brown) moves the reflected radiance from the low wavelength side to the high wavelength side of the broad GOES visible band. This causes a reflectance artifact where fall leaves look like pollution. But on Saturday, you can see an interesting region in Missouri where reflectance is not the cause. It is actually a clear air region within a regional haze level of AOD. You can see the clear air region move southeast across the state. There is a hazy line in Florida which moves NW up the state which looks to be unrelated to surface reflectivity.
And as Graham noted, there are fire signatures on the HMS site, many of which are in the Northwest. Remember that these fire detects can be fires as small as an acre in size, so slash burning and clearing of fields is a possible source for the detects. There is no smoke noted by the HMS team.
In India (below), the haze also gets trapped by mountains. Haze is backed up against the Himalayas.
Posted by Ray Hoff at November 3, 2013 12:18 PM