Yesterday, Xinhua reported that Beijing achieved above an 80% “blue sky” rate in the first half of this year:
BEIJING, March 31 (Xinhua) — Beijing saw 73 “blue sky days”, 81.1 percent of the total, in the first three months of 2009, Beijing authorities said here on Tuesday.
The city experienced six more blue sky days than in the first quarter of last year, and 24.3 days more than the average of the last decade, said an official of the Beijing Municipal Environmental Protection Bureau.
Experts at the bureau said active cold airflows had helped particulate matter to disperse.
Efforts to reduce pollution from heating systems had paid off with January having the most blue sky days since 2000, said the official.
For those who prefer original sources, here is the notice from the Beijing EPB (in Chinese).
Let’s take a closer look at the data and examine what this means in terms of air quality.
First of all, as a quick introduction for new readers, China defines “Blue Sky Days” as days for which the Air Pollution Index (API) is 100 or below. For a detailed description of exactly what the API is, see this post. Past Beijing API data may be queried from MEP’s datacenter, but unfortunately only in Chinese.
For this post, I queried the data for 1/1/09 to 3/31/09, and ran some quick analyses to see what interesting things I could find.
Part 1: Checking Beijing’s EPB’s Numbers
The Beijing EPB claims “截至3月31日，今年累计73个达标天，占监测天数的81.1%。其中一级7天，二级66天，三级15天，四级1天，五级1天。” The second sentence says that in this time period there were 7 days of Grade 1 (API 0-50) air, 66 days of Grade 2 (API 51-100) air, 15 days of Grade 3 (API 101-200) air, and 1 day each at Grade 4 (API 201-300) and Grade 5 (API 301+).
However, by my count, there were 8 days of Grade 1 (one more than reported) 64 days of Grade 2 (two less than reported), then 15, 1, and 1 days of Grades 3, 4, and 5, respectively (identical to that reported). It seems pretty basic to me that you would want your publicly reported data summary to match your public database, so I can’t imagine what’s going on here. This is especially true because, in this case, the data indicates Beijing did better than they claimed.
The data for one day, 2/19/09, is missing from the database. If we assume that 2/19 was a Blue Sky Day, though, then at least we do indeed have 73 Blue Sky Days for the quarter.
Part 2: Converting to Pollutant Concentrations
Because API is a unitless index, in order to evaluate air quality we have to convert back to pollutant concentrations. This is a bit tricky, because the API is only reported according to whichever pollutant had the highest daily concentration, meaning that we do not have daily raw data for every pollutant. Still, we can make a rough approximation by assuming that PM10 is the limiting pollutant on all days. (Of the 89 data points, 72 (81%) were reported with PM10 highest, 9 were reported with SO2 highest, while 8 had no pollutant data because no pollutants are listed for Grade 1 air quality days.)
In any case, given the above assumption, this graph shows calculated daily PM10 concentrations for this quarter:
These data yield a quarterly average PM10 concentration of 124 ug/m^3. This is very similar to what I calculated as last year’s annual average.
124 ug/m^3 is still well above China’s annual target (100 ug/m^3) and well well above the WHO’s ideal target for developed nations (20 ug/m^3).
For those of you following along at home, here is an Excel formula for converting API to PM10 concentration (in this example, the API would be in cell E9):
This is based on equations presented at the bottom of this post.