it was really great about a week ago. what happened? mainland manufactoring started again after holiday or local source?
242 in Tung Chung!!
It's basically down to wind direction. Check the windy.com site during particularly high or low pollution days- the pattern is easy to spot. Also just take a look at the size of the China coastline and compare it to the length of the NT border with the mainland. You're looking at something like 800-1000km against 50km.
"Local sources" don't suddenly blanket the entire territory in a uniform smog.
If you look on windy.com and aqicn.org/city/hongkong/ right now, you can see the correlation.
The wind from Guangzhou/Dongaun is blowing right down the middle of the Pearl River Delta. The AQICN site shows that the worst/purple air is dotted along both sides of the delta.