We present optimal estimates of the emission patterns for trichloroethene (TCE,CHCl&dblbnd;CCl2), tetrachloroethene (perchloroethylene, CCl2&dblbnd;CCl2), and trichloromethane (chloroform, CHCl3) utilizing hourly gas chromatographic measurements at Nahant, Massachusetts (approximately 10 km northeast of Boston). Our analysis combines the measurements with back trajectory information obtained from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT-4) model (National Oceanic and Atmospheric Administration Air Resources Laboratory, Silver Spring, Maryland). Using a Kalman filter inverse method and an analytical solution of the continuity equation to estimate the effects of eddy diffusion, we calculate the surface emissions on a 1¿¿1¿ grid for the selected species necessary to optimally match the observations. These emissions are compared with the estimates determined by the Reactive Chlorine Emissions Inventory (RCEI) working group of the International Global Atmospheric Chemistry Program Global Emissions Inventory Activity (GEIA). The new emissions scenarios computed here provide an observation-based assessment for comparison with the RCEI emissions inventories for the northeastern United States and southeastern Canada. Results indicate that the RCEI estimates of the anthropogenic emissions of these chemicals in the geographical domain studied differ from our optimal estimates but generally lie within the estimated error of these optimal estimates. Results are sensitive to the assumed vertical distributions and hence to the assumed vertical mixing rates. The current accuracy achievable through this observation-based technique (¿45%) is limited in large part by the uncertainty in the vertical distribution of these compounds in the troposphere over highly emitting regions. Within this accuracy the optimal estimates of emissions presented here indicate that the emissions for many grid cells do not need to be corrected significantly from the 1990 estimates presented in the RCEI. However, for trichloroethene and tetrachloroethene we calculate large statistically significant decreases in emissions for some highly populated urban locations on the East Coast and correspondingly large increases for some less populous grid cells in Pennsylvania and New York State. Only anthropogenic sources of trichloromethane (representing roughly 11% of estimated global emissions) were gridded in the RCEI inventory and included in the initial inventory used here. We find that these anthropogenic emissions are, as expected, too low toexplain the observations and that most grid cells require small increases and several cells require substantial increases (~5--10 nmoles m-2 h-1) to produce emissions estimates that are consistent with observations. This is reasonable given current knowledge of natural land sources (e.g., soil emissions) and a large oceanic source for this compound inferred from previous oceanic observations. ¿ 2000 American Geophysical Union