Sorry to complicate the story but dustnmud is not quite correct. It's not the pyruvic acid (pyruvate) that causes onion acridity. Pyruvate has no sulphur in it at all so it couldn't contribute any sulphydryl groups to any reactions.
No, the clip below:
Pungency in onions is derived from a number of volatile sulphur compounds. These compounds are produced when the onion cell is mechanically disrupted, bringing the enzyme alliinase into contact with the flavour precursors,S-alk(en)yl-L-cysteine sulphoxides.1,2In addition to producing volatile sulphur compounds, the enzymatic breakdown of the S-alk(en)yl-L -cysteine sulphoxides also produces stoichiometric amounts of ammonia and pyruvic acid
(http://ucanr.edu/datastoreFiles/234-247.pdf)
So it's the volatile suphur compounds that make you cry. The pyruvate is easier to measure as it also goes up under the influence of the alliinase enzyme.
