The cloud of pollution over Beijing grew so dense this week that much of the city was rendered invisible. Hundreds of flights were grounded, auto traffic was stalled for scores of miles, and residents rushed to buy masks and air filters. Chinese officials downplayed the air-quality crisis, as usual, saying their readings showed that air pollution was “moderate.” But ordinary Chinese were complaining. A link to the air monitor on the roof of the U.S. Embassy, which showed levels “beyond index” in recent days, went viral on Chinese social media. Even Chinese newspapers, which usually refer to pollution as “fog” caused by weather, began calling it “smog” in headlines this week.