at least one recent scientific study has shown that the city can, under the right conditions, “form its own smog”.
The study by Hong Kong and Macau air scientists argued that a rapid build-up of particulate matter in the air – a key component of smog – was possible even in the absence of northerly winds that can transport pollutants from afar.
http://www.scmp.com/news/hong-kong/health-environment/article/2076236/hong-kong-can-create-its-own-smog-researchers-say