The N oxidation ratio, [HNO3>/[NOx>, in the free troposphere is measured at ~1 to 9 (averaging ~5), consistently lower than the values of 15--100 which known theory suggests to be reached rapidly. This discrepancy casts doubt on current models' abilities to predict effects of NO sources on remote [NO>, and so also [O3> and [OH>. HNO3-to-NOx recycling processes involving HCHO in solution are appealing explanations for the ratio. Known chemical theory are illustrated using a Lagrangian box model of the mid-troposphere which simulates characteristic episodes from NOx input to HNO3 washout. Ratio and budget constraint equations for NOy can usefully restrict conceivable explanations of missing chemistry of fast nitrogen cycling; most explanations could be called ''fast-cycles'' or ''re-NOx-ification.'' These equations also show how current global models may suggest spuriously good N oxidation ratios and errors in OH. Aerosol or cloud droplets reactions with HCHO may produce (a) formic acid (which needs an appropriate source) and NOx, or alternatively, (b) hydroxymethyl nitrate (or methyl dinitrate), which might supply a large, variable amount of ''missing NOy'' not currently measurable except by NOy instruments. ¿ American Geophysical Union 1994