We use an Eulerian, three-dimensional tracer model driven by European Center for Medium Range Weather Forecasting winds to study the effect of the global background on synoptic-scale tracer concentration. Inside the global domain we put a synoptic-scale domain that includes North America, the North Atlantic Ocean, as well as parts of the eastern North Pacific Ocean, Africa, and Europe; and inside the synoptic-scale domain we have a regional subdomain over North America. We determine the percentage PI of concentration inside the regional subdomain that came from outside the synoptic-scale domain for species whose only source is at continental surfaces and whose only loss is a spatially and temporally independent first-order decay with an exponential decay time &tgr;. In the annual average, PI≈0% at the surface and increases with height: at z=11.8 km, PI is 3% for &tgr;=1 day, 50% for &tgr;=5.5 days, and 60% for &tgr;=30 days. Month-to-month changes in convection over land and in horizontal transport produce a seasonal cycle in PI with a summer minimum and winter maximum, and the amplitude of this seasonal cycle in PI decreases as &tgr; increases.

The most interesting species have &tgr;s near 5.5 days, because species with &tgr;≫5.5 days have less of a seasonal cycle in PI and have less variation with height of PI in the free troposphere than for a species with &tgr;≈5.5 days, while species with &tgr;≪5.5 days have such small PIs as to be uninteresting. To help us interpret these Eulerian model results, we use simple Lagrangian models to derive three main conclusions. First, the apparent travel time from other source regions, particularly Asia, to North America decreases a factor of 2 as &tgr; decreases from 30 days to 1 day, which means that more of the 1-day lifetime species arrives than one would expect based on the amount of the longer-lived species that arrives. Second, in the regional subdomain, PI usually increases as &tgr; increases, but sufficiently remote from the sources in the subdomain, PI may decrease as &tgr; increases. Third, although the concentration imported to the middle and upper troposphere generally increases as the vertical mass flux by clouds increases, PI can be insensitive to changes in the cloud mass flux, as long as the cloud mass flux changes similarly both inside and outside the synoptic-scale domain. ¿ American Geophysical Union 1991