Jeffrey Lewis, in the context of his analysis of recent Chinese arms control efforts, wonders if I might know something about the commercial availability in the US of aluminum powder with a diameter of less than 500 µm. My only source for such information would be that source of nearly all knowledge, Google, and a quick search does seem to indicate that even much smaller particles are commercially offered here in the US. Just pick a size: 20-80nm here, 100nm from this supplier, and these guys will sell you a kilo of the primo 80nm stuff for $1109. (That seems like a lot to me, but then again I regard the $17.99/lb. I paid for parmigiano-reggiano yesterday as highway robbery.) Heck, you can even buy it on eBay.
So in less than three minutes I was able to find three American suppliers who can provide aluminum powder well within both the dual use (200 µm) and military (60 µm) US control thresholds (see ECCN 1C111.a.1 and USML Category V(c)(5), respectively). This is not to suggest that these companies are doing anything unlawful in making domestic sales of such stuff. The export control issue only materializes when they go to ship it overseas. (Actually, on second thought, I wonder if the ATF might regulate domestic shipments of aluminum powder due to its explosive application.)
Governments around the world keep tabs on aluminum powder because it can be used as rocket or missile fuel. To pick a benign example, NASA's Space Shuttle uses aluminum powder to fuel its solid rocket boosters. The US control levels are largely derived from its obligations under the Missile Technology Control Regime, which has established a control threshold of 200 µm (pdf, p. 20). If you want to export aluminum powder smaller than that from the US to any country except Canada in some cases (for now), you are going to need a license from either the State Department or their colleagues at Commerce.
One question is how China arrived at its spherical aluminum powder particle control threshold of 500 µm, especially since in the speech Jeffrey references by Tsinghua University professor Li Bin the Chinese academic specifically notes that their regulations mirror the MTCR controls on "the 0.5 millimeters size of aluminum powder". 0.5 millimeters is identical to 500 µm, but in actuality the MTCR only kicks in below 200 µm. A clue to what happened is in the US relaxation of its controls on metal fuels, moving the controlled particle size down to 200 µm, several months following Li's 2002 comments. The US action almost certainly followed a corresponding decision at the MTCR. (I can't quite pin the date down, but the MTCR may have decided to relax its metal fuel controls as little as a month after Dr. Li's talk at their plenary session in Warsaw.) If China is still restricting metal fuel exports in the 200-500 µm particle size range then their controls now exceed those of both the MTCR and the United States.
Whether or not anyone in China actually pays the slightest attention to their government's export controls is another matter. The US officials I've heard address this subject give China some credit for establishing the legal and regulatory framework for nonproliferation, but tend to be deeply skeptical of China's capabilities and intentions in enforcing these new laws. (Former DDTC managing director Robert "Turk" Maggi commented along these lines following a talk by Chinese Ministry of Commerce official Zhou Roujun at the big London Global Trade Controls Conference in 2003.)
For further analysis of China's missile technology export controls and how they compare (or compared in 2002) to the MTCR control annex, see Phillip C. Saunders' paper (pdf) Preliminary Analysis of Chinese Missile Technology Export Control List.