According to Bloomberg News, London had more toxic air than Beijing for a few days late last month, due to "a cocktail of high atmospheric pressure, little wind, and peak farming season emissions." These worse-than-Beijing incidents are likely to become more common — not because London's air is growing worse, but because Beijing's has improved so much.


The average concentration of PM2.5 particles in the air above Beijing remains much higher than in London. The city's air quality is also highly variable, with particularly bad pollution episodes occurring during poor weather conditions in the winter and early spring. However, significant persistent variations in air quality between Beijing and major developed-world cities appear to have faded, while other major developing-world cities increasingly dominate the pollution charts.


China became known as the "air pollution capital of the world" during its rapid economic growth in the 1990s and 2000s, according to the scientific journal The Lancet. Beijing became a global byword for unclean skies, despite the fact that pollution was considerably worse in some of the country's inland industrial areas.


This was thanks in part to the United States State Department, which erected an air monitor on the roof of its Beijing embassy in 2008 and began tweeting hourly PM2.5 readings. The automatic tweets included ratings of whether the air was "good," "moderate," "unhealthy for sensitive groups," "unhealthy," "extremely unhealthy," or "dangerous" based on data from the US Environmental Protection Agency. The concentration topped the EPA's 500-point scale on one day in 2010, prompting a code in the embassy's computer to designate the air quality as "crazy terrible."


The readings, as well as the international attention they created, were embarrassing to the Chinese government, which asked in 2012 that the US and other foreign embassies stop publicizing such material, claiming that doing so constituted interference in the country's internal affairs. Instead of following through, Beijing significantly increased its own air-quality testing and reporting systems and stepped up its anti-pollution efforts. The @beijingair account of the US Embassy is still active, but it now offers the same story as official Chinese statistics - that of significant improvement over the past decade.


Beijing's air has been cleaned mostly by substituting natural gas for coal in residential heating, industrial use, and electricity generation. Restrictions on car and truck use and emissions, as well as the city's prominence as an early user of electric vehicles, have all played a part. According to a recently completed analysis by scientists from the United Kingdom and China, Beijing produces significantly less pollution than expected, with the majority of its bad air days now being caused by contaminants drifting in from rural areas and other cities to its south. Beijing is also subjected to sandstorms that blow in from the Gobi Desert, though they have become less common, thanks in part to the huge Great Green Wall reforestation work.


Beijing is the country's capital and one of its wealthiest cities. Not everyone in China has the resources to take such measures – for one thing, the country lacks sufficient natural gas. However, China is moving toward cleaner energy sources in general. Although coal continues to dominate energy generation, the trend is apparent.


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