Short Title: North American OCS sink Index Terms: 0322 Constituent sources and sinks, 0345 Pollution--urban and regional, 0365 Troposphere--composition and chemistry, 0368 Troposphere--constituent transport and chemistry Key Words: Carbonyl sulfide (OCS), Carbon Dioxide (CO 2), terrestrial sink, sources. 2 Abstract: An extensive set of carbonyl sulfide (OCS) observations were made as part of the NASA Intercontinental Chemical Transport Experiment -North America (INTEX-NA) study, flown from 1 July to 18 August 2004 mostly over the eastern United States and Canada. We use these data to show that summertime OCS mixing ratios at low altitude were dominated by a surface sink, and were highly correlated with CO 2 . In marked contrast to the 2001 early springtime Transport and Chemical Evolution over the Pacific (TRACE-P) experiment, which sampled Asian outflow, anthropogenic OCS emissions were dominated by this draw-down, although evidence for local emissions were observed on some low altitude flight legs. The INTEX-NA observations are combined with the STEM regional atmospheric chemistry model for a top down validation of bottom up OCS surface fluxes. In preparation for 4 dimensional variational inversion, this manuscript summarizes INTEX-NA observations. The STEM model is applied to simulate OCS using the best available surface fluxes (1 degree, monthly, 8 sectors), fixed boundary conditions, and no chemical reactions. Initial STEM results suggest a 200% underestimation of the OCS sink.