you can get it here in 7-Eleven or FamilyMart, you dont have to bring it from home.
if you're climbing the greatwall or going hiking, you dont need it, its pretty clean. If u're visiting the gardens you dont need it too. But if ure going to the Olympic Park, maybe, there are less trees around there.
and when u get here, just check the AQI. These days I don't use a mask, but sometimes u need it. (just in case)
Below 200 is still okay, above 200 u definitely need a mask.
southern china has better air than beijing.
you need the mask for 2 things, not just the smog, but also the pollen during spring.