Based on atmospheric measurements of multiple species at Egbert, a rural site in Ontario, Canada, during summer 1988, the emission ratios of HCHO/CO and HCHO/&Sgr;NOy for area sources and secondary production of HCHO have been estimated using a modified principle component analysis technique. The technique yields three principal components that represent a photochemically aged air mass, a diurnal cycle, and fresh area emissions. The area emission component has an emission ratio CO/&Sgr;NOy=9¿3 and SO2/CO=0.005¿0.003, in agreement with NAPAP area emission data for the eastern US [Buhr et al., 1992>. The emission ratios of HCHO/CO and HCHO/&Sgr;NOy in this component are 0.0056¿0.0022 and 0.05¿0.007, respectively. If these ratios are typical of eastern North American area emissions, the total primary HCHO emission for this region will be 8¿109 moles HCHO annually based on the NAPAP CO emission inventories. Evidence of secondary HCHO production can be found in the photochemically aged component which has considerably higher HCHO/CO (0.016¿0.004) and HCHO/&Sgr;NOy (0.29¿0.03) ratios than the emission ratios. It is estimated that for every 1 ppb NOx converted to NOy, 0.4 ppb HCHO are produced for the ratio (1-NOx/NOy)<0.6; after which the relative HCHO production rate becomes smaller. Using this relative rate, the maximum total HCHO production over the eastern North America is estimated to be 1.3¿1011 moles year-1, or approximately 16 times that from primary emission.