Beijing is pretty humid in the summer, I’m not sure where you get dry from (although my first summer in Beijing wasn’t until 2002, not quite thirty years ago). Dust storm season is spring and pollution season is winter (especially when they get inversions). Fall is the best season to tour in Beijing if you want clean air and no dust, the air won’t start getting bad until November, although moving all the farmers away from coal heating has improved things a lot (they still have bad inversions, but like LA and Salt Lake City). Two strokes have been banned for awhile, electric bikes were already taking over in 2002, you already could no longer find the tuktuks that I remember from my first trip in 1999.