I guess China is improving. They're better off than India.

The yellow color in California is likely due to pollution blowing in from China. California has the strictest clean air standards in the nation, but according to a study described in this article, "pollutants from China have caused a 65% increase in the Western Ozone – or, as it’s also referred to, smog. One study shows that 29% of the particulates in smog in San Francisco come directly from China’s coal plants." (The Wall Street Journal article is a better source but that article is behind a paywall.)