OK, but I think major cities in the US had much larger GDP per capita than the nation as a whole too, both in 1940 and 1990. It's also not clear to me that PPP is the relevant metric here; low cost of consumer prices does not necessarily translate into cheap factory cleaning tech even if it does mean folks will be willing to pay more for clean air. Furthermore, it's not surprising that factory technology may lag economic growth more in China since China is growing faster than the US.