The present study examines in detail the photochemistry of the important hydrocarbon intermediate HCHO during the 1993 Fritz Peak/Idaho Hill OH Photochemistry Experiment. Measurement-model relationships for HCHO were studied employing a box model with input measurements of hydrocarbons, photolysis frequencies, NOx, and HOx measured at the Idaho Hill sampling site during clean westerly flow. Tunable diode laser absorption and long-path UV-visible absorption spectroscopic techniques provided the high-quality time-resolved ambient HCHO database necessary for this study. For clear sky conditions during clean westerly flow, midday HCHO measurements and model calculations were in agreement to within 12% under various realistic model scenarios. This is significant since it places constraints on the OH concentrations as well as the characteristics of potential missing hydrocarbons necessary to balance measured and modeled OH. Measurement relationships between HCHO and CH3CHO were also investigated; polluted air from the Denver-Boulder front range resulted in the following linear regression: =(0.62¿0.20)ppbv+(1.83¿0.15)CH3CHO. The slope reflects the large if not dominant influence of primary and biogenic emissions, and the intercept reflects the existence of HCHO sources which are not sources of CH3CHO. Clean westerly flow produced a significantly different relationship with no obvious correlation, and this is consistent with smaller biogenic sources of CH3CHO compared with the much larger known biogenic sources of HCHO.¿ 1997 American Geophysical Union