some cool graphs i found at the Beijing Ministry of Environmental Protection here
1998-2015 pollution trends in Beijing for four major pollutants:
The blue line is PM10, which has been tracked regularly since 1998 and has seen significant declines over that period (46%). I ballparked the actual values and came up with a slope of that decline, which was generally a numerical decline of 3.9 um/m3 per year.
I applied that rate of decline to the PM2.5 (tracked for three years, in red above) and forecasted that should the PM 2.5 decline at a similar rate, it will be ~18 years (2033-2034) until we hit a concentration of ~10 um/m3 (the WHO standard).
2015 PM 2.5 concentrations by district
I will annotate this later, but as you can see, generally the farther north you go, the better the air gets.
Chaoyang District is sliced by two bands (88-94 basically south of the airport expressway) and (81-87 north of the airport expressway), which means the air is approximately 8% better in the northern half.
Shunyi is also bascially split in two, with residents in the southern half (aka the expat villa area within the 6th Ring Road) having the same air quality as northern Chaoyang (aka 81-87). Out beyond the 6th Ring the quality improves to the 75-80 band, which is another 8% improvement over northern Chaoyang (or a 15% improvement over south Chaoyang).
However, to really make a difference (say, 33% less or more), you'd have to go way, way out, fairly impractical for the average expat.