how clean were april and may?

This past spring, the months of April and May in Beijing were reported as the cleanest April / May in a decade. (Sources for April: China Daily and Beijing EPB, see also my previous post; sources for May: Xinhua and Beijing EPB.)

Independent of the quantitative results, these reports seem to confirm what a lot of people have been mentioning to me, that this past spring has seemed surprisingly clean.

Let's evaluate the truth in all of this. The following graph shows average API*, average PM10 concentration, and number of Blue Sky Days for the period April-May from 2005 through 2009:

From these numbers, the results are pretty clear: the period April-May 2009 in Beijing was indeed significantly better in terms of air quality than the same period in any of the previous four years. (I could have looked farther back, but I decided only to look at five years total for this analysis.)

Here are some comparisons of 2009 vs 2005-2008 averages:

I think it's probably fair to say that the air quality this April and May was 30-40% better than the average air quality during the same period over the previous four years.

As usual, we should celebrate the progress while being mindful of the significant improvements still required. My calculated average PM10 concentration for this period, 117 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). It is also well above my estimate for the average PM10 concentration during last year's two-month Olympic period (79 ug/m^3).

Related:
Summary of Beijing's 2009 first quarter air quality
Summary of Beijing's 2008 air quality
Update on fall air quality in Beijing

*I don't really like averaging API, because it can lead to some misleading results (further discussion in this post), but despite that I still think it has value as an indicator here.