Atmospheric mixing ratios of carbon monoxide (CO) and perchloroethylene (PCE, C₂Cl₄) were measured above the canopy at Harvard Forest, MA every half-hour for three years starting in January 1996. Pollution enhancements are strongly correlated with winds from the southwest, the direction of the New York City – Washington, D. C. corridor, as compared to background levels observed during northwest winds travelling from Canada. We establish the ratio of CO to PCE pollution enhancements by wind direction, by season, and by year and use these results to test the quality of county-level and national source emission inventories for these two gases. The EPA carbon monoxide emission county-level inventories and the McCulloch and Midgley sales-based national-level PCE release estimates are found to be in accord with our independent observations of urban/industrial releases. For the New York City – Washington, D. C. corridor the inventory-based CO/PCE emissions ratio of 638 (kg/kg) for 1996 falls well within the range of observationally-based CO/PCE pollution plume ratios of 388 to 706 (kg/kg) and is only 18% higher than the observed mean of 521 ± 90 (kg/kg). On the basis of this agreement, PCE emission estimates for 1997 and 1998 are derived from the CO inventory emissions values and the observed CO/PCE ratios in pollution plumes for those years; despite the call for voluntary cutbacks, urban/industrial emissions of PCE appear to be on the rise.