Archive for August, 2010

counting grade 1 air quality days – a new metric for evaluating Beijing’s air quality?

Monday, August 30th, 2010

Earlier this month, the Economist featured a nice piece about the ineffectiveness of current efforts to improve China’s environmental quality, including some discussion of recent decreases in air quality. The authors cite the official Ministry of Environmental Protection announcement for overall air quality averages in cities across the country, but run their own numbers for Beijing specifically, arriving at this conclusion:

Residents of Beijing, who hoped the clear skies they enjoyed during the 2008 Olympic Games would persist, have also resumed their grumbling. Smog is back with a vengeance…

Official data show a diminishing share of grade 1 (“excellent”) air-quality days since the games.

The claim is supported with this chart:


This chart caught my attention because it’s the first time I’ve seen “percentage of Grade 1 air quality days” used as an indicator of air quality. Usually, data for Grade 1 and Grade 2 are presented together. The reason is because China’s ambient air quality standard calls for urban air quality to meet only the Grade 2 level. (“Blue Sky Days” are days which have Grade 1 (excellent) or Grade 2 (good) air quality by the Chinese standards.) Charting percentage of days meeting Grade 1 or Grade 2 yields a very different impression:

number grade 1 and 2 through july 2010

Using this metric of Grade 1 or 2, there is no apparent decrease in air quality from 2008 to 2009, the drop from 2009 to 2010 seems minimal, and the overall situation appears much less dire (the percentage of passing days floats above 70% as opposed to 10%).

Two questions come to mind here. First, is it misleading for the Economist to use this unconventional metric? Maybe. The article neither describes what Grade 1 means nor justifies why “percentage of Grade 1 days” is an appropriate / better indicator of air quality, although Grade 2 is discredited using an comparison to the BeijingAir Twitter feed:

The American embassy in Beijing, to the annoyance of local officials, issues frequent air-quality readings for its part of the city. These, based on the presence of fine particulates, mostly ranged from “moderate” to “unhealthy” in the 24 hours after midday on July 31st. But the government called that period grade 2, or “good”. Incredibly, to anyone familiar with China’s perennially grey urban landscapes, fully 91% of days in 113 big cities in the first half of this year were described as “blue sky”.

Second question: is it correct to talk about percentage of Grade 1 days? That’s tough to answer. The technical answer is no, it’s not, because any use of “number of days” or “percentage of days” meeting a single standard is statistically meaningless. (For a short and nerdy explanation of this, see this post). That having been said, though, I do believe that scrutinizing Beijing’s air quality by looking at Grade 1 – instead of Grades 1 and 2 – days provides a much more reasonable snapshot of the actual air quality in the city. For one thing, China’s air quality limits for Grade 1 are much closer to internationally-recognized standards. For another, one would assume that there is lower risk for data manipulation around the Grade 1/2 border than around the Grade 2/3 border.

I raised the idea of looking closer at Grade 1 earlier this year. Maybe it’s time for a more in depth analysis, but I’ll save that for another post.

official data shows air quality worsening in china

Wednesday, August 4th, 2010

Last week, multiple media outlets (including Xinhua, the New York Times, and the Wall Street Journal) reported on MEP’s admission of worsening air quality during the first half of 2010. Here, I will take a closer look at how this was reported, what exactly this means, and what is happening in Beijing specifically.

First of all, the original announcement from MEP is here (Chinese only). Both Xinhua* and the Wall Street Journal made major mistakes in their reporting of the announcement. Both reported that the average atmospheric particulate level rose to 0.091 mg/m^3 from 0.002 mg/m^3 last year. This is incorrect. Doesn’t anyone check original sources any more? The MEP report says “可吸入颗粒物浓度同比上升0.002毫克/立方米。” This means that the PM level is 0.002 mg/m^3 higher than last year’s, not that last year’s value was 0.002 mg/m^3. Someone pointed this out in the comments section of the Wall Street Journal article (which claims the increase is “humongous”), but it still hasn’t been corrected.

The Wall Street Journal also made the mistake of claiming that “the first half of 2010 had the worst air quality since 2005.” This is also not true. MEP’s announcement, which the WSJ story links to, says, “自2005年以来,环保重点城市空气质量优良天数比例首次出现下降,可吸入颗粒物浓度首次上升。” The correct interpretation here is that the first half of 2010 is the first period since 2005 that there has been a decrease in air quality from one year to the next. In other words, the trend of improving air quality has changed. At least Xinhua got this one right:

Economic recovery has partly caused the country’s air quality to fall in the first half of the year, the first such fall since 2005, figures from environmental authorities showed on Monday…”It was the first time for these cities to record a fall in the number of days with good air quality and a rise in the concentration of inhalable particles since 2005,” ministry spokesman Tao Detian said.

Unfortunately, the WSJ’s mistakes are already bouncing around the blogosphere.

Of course, despite the mistakes I’ve pointed out, the story is still quite significant. After years of official statistics showing improving air quality, MEP’s air quality data now shows a very slight increase in ambient particulate matter concentration. The data MEP gives are average pollution levels for 113 major cities, which is a little strange, since pollution levels vary widely across China. Although the averages are an interesting snapshot (and it is significant that MEP is reporting this bad news), these averages are not very meaningful; they say nothing about regional trends / changes or population exposure. This is an important area for more detailed research.

MEP’s announcement also gives averages in percentages of days meeting Class I and II air quality standards (so-called “Blue Sky Days“), but this metric is meaningless, as described previously on this blog.

What is more interesting is looking at data on individual cities. After all, most people spend most of their time in a single city. Plus, local and regional pollution control programs may vary from place to place. For Beijing specifically, I ran my own analysis of Beijing’s API data for the first half of the year using data I downloaded from MEP’s datacenter. After converting API to PM10 concentration using the methodology described here, I calculated the average PM10 concentration for the first half of 2010 in Beijing to be 124 ug/m^3. I also produced the following figure:

Beijing 2000-2010a

The figure shows that Beijing’s air quality, using average PM10 concentration as an indicator, has not shown improvement over the period 2008-2010. This trend is a continuation of the stagnant pollution levels I described earlier this year. Although MEP’s 113-city average (91 ug/m^3) is below China’s ambient air quality standard (100 ug/m^3), Beijing’s air quality remains well above China’s own standard, which is well, well above the WHO’s.

For those of you keeping track of Blue Sky Days, the Beijing EPB announced that there were 140 Blue Sky Days in the first half of the year, which, despite the increase in air pollution, somehow means Beijing is still on track to meet its Blue Sky Day goal. But this is a subject for another post.

Anyone who read this blog over the period 2008 to 2009 will know that I often repeated a two-part mantra: Beijing’s air quality is getting better, but we have a long way to go. I always felt that acknowledging the first part was critical to making progress working with China, and I was happy to see the New York Times article last October making this exact claim (headline: “Beijing’s Air is Cleaner, but Far from Clean;” my analysis here) It is frustrating and sad that this is no longer the case, even by China’s official data.

*To be fair to Xinhua, their mistake was only one word. Their report states, “The amount of inhalable particles, a major air pollution index, was also 0.091 milligrams per cubic meter in these cities, rising from 0.002 milligrams per cubic meter over the same period last year, the ministry reported.” The problem in this sentence is the one word “from,” which significantly changes the meaning (from incorrect to correct) if removed.