Most of the US is covered by cloud and snow today (lower left), however, there is elevated PM2.5 in Pennsylvania and in California (AQI loop lower right). Mark Ruminski noted that while there was no smoke in the California central valley over previous days which had elevated PM, the clearer air today is now showing fires and smoke. He conjectured that the poor circulation in the valley may have led to burn bans earlier this week and now that the weather is better, burning starts again. That fits with pattern of emissions we saw in California last January.
In Los Angeles, there was a curious stagnant circulation of low clouds this afternoon. It is not uncommon to see low clouds off the coast, but it is not usually to see the clouds just trapped in the basin with no clouds elsewhere.
And for those that think it is winter, today's 70F temperatures in Baltimore set an all time record for December 21. The shortest day of the year, the beginning of winter and a big holiday for all you Druids. The days are getting longer now. And, yes, I know I misspelled "Smog" but firebreathing dragons are the thing right now for all you Hobbit fans.
Update: Dec. 22, 2013 19:00EST
Two areas on Sunday have elevated PM today. The San Joaquin Valley is in winter mode and is showing elevated PM in Visalia and Bakersfield (below left). You can see the haze in the lower San Joaquin Valley on AQUA's MODIS imager (below right).
Oregon is under cloud cover today, but the winner (or loser depending on your viewpoint) is Klamath Falls Oregon that reached 70 µg m-3 overnight at 1:00 AM. It is likely to be a low boundary layer and woodsmoke.