CNY has around 3 heavy fireworks days, and a quick google search should reveal the exact dates. If you happen to be in Beijing during these days, I encourage you to stay indoors at a hotel with an awesome HVAC system (Marriott NE is the best I know of). For the rest of CNY, the air really isn't all that bad (because there are less people/cars in town). The mask thing really won't help your wife much (apart from a placebo effect). You might also want to consider a side trip to one of China's cleaner cities (e.g. Dalian, Qingdao, Xiamen).