The skies above Beijing were uncharacteristically clear and sunny on 13 October, the day before the nation’s top wind-energy talking shop kicked off. The US embassy’s air quality index (AQI) for harmful particulate matter (PM) came out at a reasonable PM2.5 reading of just 42, posing little or no risk to human health.
But within 24 hours, the city’s notoriously noxious smog had returned and the AQI had shot up to 169 — which is considered “unhealthy” — setting the stage for a conference devoted to cleaning up China’s energy mix.
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