Twittering Bad Air Particles in Beijing

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Ng Han Guan / AP

A Chinese cyclist passes by the iconic China Central Television building in Beijing, China on June 18, 2009

Anyone stepping outside in Beijing around midday on June 18 would have noticed something slightly amiss. The sky was dark enough for cars to use their headlights, and the air was as thick as a smoky bar just before last call. After one of the cleanest springs on record, the Chinese capital's air quality took an unhealthy plunge for the worse.

You wouldn't have known it by the official numbers. The Ministry of Environmental Protection publishes air quality data online each day at noon, but only for the previous 24-hour-period. Anyone who checked on the afternoon of June 18 would have seen an air pollution figure that indicated Beijing's skies were "slightly polluted," referring to the 24 hours before.

But a glimpse from another monitoring station that gives hourly updates shows a very different picture. The U.S. Embassy operates a single station in eastern Beijing that records levels of PM2.5, fine particles considered particularly dangerous to human health. At noon that same day, the hourly measure of PM2.5 crept up into the "hazardous" range, according to U.S. Environmental Protection Agency standards, and hit a maximum value of 500 for several hours.

The embassy data is published on its own Twitter feed, and while the U.S. doesn't actively promote the information, it has slowly been getting more attention from Beijing residents concerned about the city's air quality. "The U.S. Embassy has an air quality monitor to measure PM 2.5 particulates on the Embassy compound as an indication of air quality," says Susan Stevenson, a State Department spokesperson. "This monitor is a resource for the health of the Embassy community." She cautions that citywide analyses cannot be done from a single machine, but because the embassy has the data available, it makes it available to others.

Reporting from one area, the twitter feed is hardly a complete service, but its popularity underscores some shortcomings in the official daily reports. Chinese environmental officials don't regularly release PM2.5 data, and it isn't used to calculate the daily air pollution index. Instead the government figures rely on measurements of larger PM10 particles, sulfur dioxide and nitrogen dioxide. Several cities including Beijing and Shanghai are already measuring PM2.5, the state-run China Daily reported earlier this month, and the government is now considering what standards to set for the finer particles and ozone.

Ahead of the Beijing Olympics last year environmental officials came under harsh criticism that they were tweaking pollution data to artificially raise the number of so-called "blue sky" days when emissions fall below official targets. American environmental consultant Steven Q. Andrews accused the government of switching to monitoring stations in lower pollution areas, changing the makeup of the air pollution index to focus on less prevalent pollutants, and reporting a disproportionately large number of days with pollution measurements just below the "blue sky" cutoff. Du Shaozhong, the deputy head of the Beijing Environmental Protection Bureau, denied the allegations. In a recent paper, Andrews reported on similar "blue sky" biases in several other major Chinese cities.

In the past year, the proportion of days just under the "blue sky" cutoff has decreased in Beijing. Whether that's a sign that the numbers are more accurate, or merely better gamed, is still unclear. The city's hot, humid summers and occasional sandstorms mean that air quality can turn bad with surprising speed. Without real-time reporting, the official data are more a matter of historical interest. This afternoon the Ministry of Environmental Protection reported the air pollution index for the 24 hours ending at noon on Friday was 159, or "slightly polluted." That's still pretty bad. But had you gone outside you might have thought things were much worse.