Although Beijing has started to release its fine-particulate (PM2.5) data, there is an additional point of concern: Beijing's current classification system currently classifies "light pollution" at the same level as the US classification for "hazardous."
This raises a point of concern about grading scales. If two groups are using two different scales to measure the same thing, then it is not likely that the values will overlap. This is easily seen with many things that we might deal with in life: metric vs. US/British measures; shoe sizes; clothing sizes; etc. While I wear a size 11 (or 11.5) in the US, I wear a size 46 (or 48) in Europe. While I wear a shirt size of "Large" in the US, it changes to XXL or XXXL in Japan (I even have a XXXXXL vest that I bought in Japan that is only slightly too big for me). If we live between the two systems of measurement, then we understand how to convert (or estimate) the rough value of the "other" scale (at least in the range that we most often encounter it). Therefore, I know how to "feel out" the rough temperature equivalents between F and C in the range that I experience them (0F to 90F or -18C to 32C); I know how to feel out the rough conversion from kph to mph (especially good when I'm driving in much of the world, since driving is the only time when I encounter speeds above 50kph). So that's pretty clear; if you rarely encounter a different scale, then your proficiency of mental conversion is going to be slower and less accurate. Finally, I have often found that if I use a particular metric for a long time, then I am likely to start thinking in terms of that metric than in terms of converting from my "original" one. Therefore, after having lived outside the US for several years, I was mentally converting temperature readings from F to C so that the number would make sense to me. Now, however, I don't really need to do the mental conversion; the value of the F reading makes sense in its self.
Well, that's all well-and-good, but what does this have to do with Chinese air pollution? Well... the values for temperature and shoe size don't come as subjectively named categories. We could, of course, convert air temperatures into a scale going from "blistering cold," to "searing hot". However, you and I would start to disagree on where the cut-offs should be. This is why I generally respond to question of, "is it cold outside," with, "Well, I don't think it's cold" (or, "it might be a little cold for you," if I happen to know or can guess at the other person's preference).
China's air pollution standard (at least how it currently stands) uses a different classification system than the US. It's analogous to your mother's definition of "cold" from your own.... except that it's a national standard. Of course, it's a national standard based on a measured number, but often - when it's something that a person is not really comfortable using the raw number - people will gravitate to the category bin label. This is one source of the problem, because the two scales are completely different. (I couldn't find a comparative chart between the US and Chinese PM2.5 scales, sorry.) The stated difference between the two scales is more like my stated range of "tepid" being analogous with another person's definition of "blistering cold." (I am reminded of a Costa Rican student who was experiencing her first semester of Michigan fall/winter weather: she came to class one early October day dressed in a full parka while most were wearing light jackets, and I showed up in shorts. If you had asked her if it was cold and asked me if it was cold, our answers would have been vastly different.) This wouldn't be such a problem if the measurements were merely stated perceptions of temperature, but they are actually more important than that: they are public health advisories.
Presumably, Beijing's classification system does go to "hazardous" and presumably the US scale does go to "light pollution" (or even "no measurable pollution"), but - even if they are using the same technique to measure PM2.5 in the air - it is clear that the cut-offs for the categories are very different. However, the credibility problem comes when people look at the measurement of "light pollution" vs. "hazardous" and recognize that what they are experiencing is much closer to what the other government is telling you. ...and China knows this. China also knows that the people - no matter how used to smog they may have become - know that the air is polluted and the water is polluted, regardless of what category name the use. Okay, so Beijing calls it "light pollution"; the meaning of "light pollution" will therefore mean something equivalent to, "shitty air." In isolation, having such a category name wouldn't mean very much to credibility, since people will merely do the conversion in their head (much like converting shirt sizes, mentioned above). However, when that categorization scheme is placed next to another one that makes more sense, then people are likely to start distrusting the one that makes less sense.
Therefore, yes, Beijing can say that it's air pollution levels are "light" or even "moderate". It can meet whatever environmental or public health policy goals by meeting the requirements of having a certain number of days that are classified as having "light" air pollution, but the reality is that if the definition of "light" pollution is, in actuality orders of magnitude above unpolluted conditions, then the only thing that you're fooling is the documentation; the only thing that you're fooling is the propaganda; the only thing that you're fooling is the official rhetoric... and those who believe. Do that in isolation and you might well get away with it (witness the mental dissonance that went on during the Cultural Revolution), but do that while a rival government gives away information that better matches what people experience, and you've got a serious credibility problem on your hands.
If you remember, this is what was popularly stated that George W. Bush's "Clear Skies Initiative" would have done: moved the goal posts on metrics of acceptability such that we could have had "acceptable" conditions under "Clear Skies" that would have been described as some level of "unacceptable" under the existing framework.
In short: You can improve air quality from "bad" to "good" by using a scale that shows current quality as "good". Don't be surprised, though, when people don't believe you. Don't be surprised, either, when they believe another government's scale that matches up more closely with their perceptions.
Story from PhysOrg.