Smog has choked parts of China's northern city of Harbin, Heilongjiang province, forcing schools and highways to shut and disrupting flights. An index measuring PM2.5, or particulate matter with a diameter of 2.5 micrometers (PM2.5), reached a reading of 1,000 in some parts of Harbin, home to some 11 million people, state-run news agency Xinhua reported.