Anomalous HNO3/NOx ratio of remote tropospheric air: Conversion of nitric acid to formic acid and NOx?

The N oxidation ratio, HNO 3 / NO x , 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 O 3 and OH. HNO 3 ‐to‐NO x 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 NO x input to HNO 3 washout. Ratio and budget constraint equations for NO y , can usefully restrict conceivable explanations of missing chemistry of fast nitrogen cycling; most explanations could be called “fast‐cycles” or “re‐NO x ‐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 NO x , or alternatively, (b) hydroxymethyl nitrate (or methyl dinitrate), which might supply a large, variable amount of “missing NO y ” not currently measurable except by NO y instruments.