Praise to the Chinese authorities for their successful efforts to reach the target of 245 Blue Sky Days (aka Clear Sky Days) of 245 in 2007, and yes it were exactly 245 days out of the 364 published values (SEPA) as reported by China Daily, despite the record breaking horrible last week of the year; also see my earlier post about this amazing feat.
Further analysis of these daily API values show an average API of 100.69 which is a bit better than 2006 (average=110) and slightly worse than 2005 (average=99); i wouldn’t say there is a visible trend in this yearly averages (also see previous post). Below is a graph of the entire year (click on image for larger file).
I cut off the graph at 200, above which EPA regards as ‘very unhealthy’ (purple level) and triggers a health alert in the USA. We had 11 days like this in 2007, but i did not hear any alerts. In Hong Kong this is the ’severe’ black level.
The green line at API 50 is the guideline for WHO (see this post, this is for the 24hr average; the WHO actually recommends 20 as guideline for annual average) and the upper limit in the EU (see this page). We stayed under this limit 32 days in 2007, that is less than 1 in 11 days. The red line at API 100 is the border of the famous Blue Sky Days, and the level up to which the authorities consider the air good enough for athletic competition. This level also mysteriously matches the annual average for 2007, which could also lead us to the conclusion that logically 50% of the days should be higher than 100 and 50% of the days lower (assuming a ‘normal’ distribution - Laplace-Gauss ‘bell’ curve), but nature in Beijing does not seem to obey this law as i have reported here. We got 67% below the average. Wikipedia on this ‘bell’ type of distribution:
The importance of the normal distribution as a model of quantitative phenomena in the natural and behavioral sciences is due to the central limit theorem. Many psychological measurements and physical phenomena (like noise) can be approximated well by the normal distribution. While the mechanisms underlying these phenomena are often unknown, the use of the normal model can be theoretically justified by assuming that many small, independent effects are additively contributing to each observation.
If anybody can give me a good reason why the distribution of API values over 1 year would not look like a bell curve, please let me know. The New York Times (29 Dec 2007) has mentioned this issue as follows:
Beijing has had 65 days that rated between 95 and 100. That bulge just inside the break point has attracted attention on Web sites and even at one foreign embassy, which compiled a statistical analysis casting doubt on the Blue Sky results, though the embassy’s officials refuse to discuss the findings.
Du Shaozhong, deputy director of Beijing’s Environmental Protection Bureau, said the ratings were not manipulated. “People used to ask me if the ratings are scientific, or if we are playing any tricks,” Mr. Du said. “But this is most advanced equipment in the world.”