summary of beijing's 2008 air quality

Happy New Year! I thought I would start off the year with a brief look back at Beijing's air quality during 2008.

On December 31st, Xinhua reported that Beijing had achieved 274 "Blue Sky Days" in 2008. This was well in excess of the 2008 goal of 256, and even well above the 2009 goal of 259. But what does it mean in terms of air quality and human health? Let's take a closer look at the data to find out.

First of all, according to my tally, Beijing actually only achieved 272 Blue Sky Days in 2008, with one data point (9/6) missing. Even if we assume the 9/6 sky was blue though, that only amounts to 273, not the reported 274. What's going on here? FYI, I performed my tally by first downloading Beijing's 2008 API data (available by querying MEP's datacenter) then counting the number of days with API 100 or below. Am I doing something wrong here?

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Update 1/15/09: In reviewing the data, I realized that MEP's 2008 API database is missing two data points - 9/6 and 6/4. Assuming both of these days were Blue Sky Days yields 274. I missed this the first time around because I forgot that I should be looking for 366 total data points (leap year!) not 365.
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In any case, as I have written about before, the "Blue Sky Day" metric is problematic for several reasons. Perhaps what bothers me most about it is that it tells us nothing about actual air quality; increasing annual numbers of Blue Sky Days does not necessarily mean better air quality(1). To evaluate air quality, we need numbers for daily / annual concentrations of air pollutants. Although the Beijing EPB publishes annual pollutant concentrations in the Beijing Environmental Annual Reports, the 2008 report won't be available until this summer. So we need to improvise:

Starting from the database of 2008 API values, I converted back to daily PM10 concentrations using the formulas at the bottom of this post. I assume that the primary pollutant on all days is PM10(2). Averaging over the year I get:

2008 Average PM10 concentration for Beijing: 123 ug/m^3.

The good news? This is a 17% improvement over last year. The bad news? The PM10 concentration is still over six times higher than the WHO annual target of 20 ug/m^3:


During the Olympics, Beijing saw a 50% reduction in air pollution as the city enjoyed its cleanest air in ten years. Clearly, the success of the anti-pollution campaigns was a driving force behind 2008's relative improvement over years past. At the same time, though, we have a long way to go, and the considerable pollution of the city even in a "successful" year like 2008 should not be underestimated.


(1)Here, I'm not referring to data biasing. Rather, I'm simply considering the fact that Blue Sky Days are binary, as opposed to being a concentration value or gradual scale. Consider this extreme situation: if every day in one year had an API of 100, though the number of Blue Sky Days would be 365, the average annual PM10 concentration (indicating air quality) would actually be worse than it was in 2007 or 2008 in Beijing.

(2) This assumption is slightly problematic because for APIs below 50 the primary pollutant is not listed, although for APIs above 50 the primary pollutant is almost always PM10. To estimate the accuracy of this method, I used it on the 2006 and 2007 daily API databases and calculated an annual average PM10 concentration result for each year that deviated from the Beijing EPB's reported values by well under 1%. Therefore, I think the assumption is pretty reasonable.