First, go out as little as possible and don't do strenuous exercise outdoors. Don't expose your skin directly to smog.
Second, wear a mask to cover the acne area when you go out in foggy days. Cotton is the first choice, and non-woven fabrics can cause some people to be allergic.
The third is to improve skin immunity. Due to the lack of sunshine in foggy days, it is easy to cause insufficient vitamin D production in the human body. So eat more soy products, high-quality milk, fresh vegetables and fruits, drink more water and eat less irritating food.