I'm glad I took the time to tunnel into twitter because the data seemingly reaffirms my subjective observations from my rooftop. Unfortunately, my VPN is crapping out right now so I'm unable to check out Saturday's data, but I'm in complete agreement with the hourly data from today. Late this morning, when I observed the all of that "fog", beijingair was reporting "very unhealthy" PM 2.5 levels. But, over the course of the day, the situation steadily improved to the point that they're claiming that it's only unhealthy for people with health problems. Indeed, I have a faint view of the CCTV Tower (west 3rd ring) again; this morning, I could barely see the CBD (less than a mile away).

ETA: I managed to get into twitter again. Comparing 11a today with 2p Saturday, it appears that PM 2.5 levels were 13 times greater.