I was in BJ for CNY last year. On NYE I was at a little party not far from the US Embassy (where the air sensor is located), and after about 8-9 pm it was insane--the air was so thick with smoke and fireworks/firecracker debris that you cut it with a knife--I kid you not. And the Embassy sensor went over 1000...which as an eyewitness in that area, I can fully believe. That night was also very still.

Most of the rest of CNY air was pretty decent, as the breezes started again and blew the stuff away. Between 2002 and 2012, I managed to be present in Beijing for only 3.5 Chinese New Years: 2005, blissful since fireworks/crackers weren't allowed in the central city; 2009, 2011 (but not the first few days), and 2012. The other years out of China entirely and on purpose.

The heating will go on regardless of population movement, as most of it originates in central plants that have to keep shoveling the coal regardless, in order to maintain constant steam pressure in the system. Factories do cut back somewhat, so the Hebei- and Shandong-produced pollution that floats over to Beijing and gets trapped is, IME, lessened.