For a few days late last month, "a cocktail of high atmospheric pressure, little wind and peak farming season emissions” left London with more-polluted air than Beijing, Bloomberg News reported.

These worse-than-Beijing episodes are likely to occur more and more frequently — not because London’s air is getting worse, but because Beijing’s has gotten so much better.

The concentration of particles less than 2.5 micrometers in size, known as PM2.5, in the air over Beijing remains well higher on average than in London. The city’s air quality is also quite variable, with unfavorable weather conditions in winter and early spring still bringing episodes of terrible pollution. But huge ongoing differences in air quality between Beijing and the major cities of the developed world seem to have become a thing of the past, while other big cities in the developing world now dominate the pollution charts.