The regional- to synoptic-scale transport of trace gases from North America to the western North Atlantic Ocean (WNAO) is largely controlled by midlatitude cyclones. The four primary airstreams that compose these cyclones, the warm conveyor belt (WCB), cold conveyor belt (CCB), dry airstream (DA), and post cold front (PCF) airstream, exhibit characteristic trace gas mixing ratios that vary seasonally. The present study compares ozone and CO mixing ratios measured in these four airstreams during spring 1996 and late summer/early autumn 1997. The three main influences on this seasonal variation of ozone and CO are surface emissions heterogeneity, photochemistry, and stratosphere/troposphere exchange efficiency. The more southerly springtime cyclone tracks account for nearly 50% of the increase of lower troposphere CO from late summer/early autumn to spring. The remainder of the variation is due to the seasonal cycle of background CO. Stratosphere/troposphere exchange occurs in every DA; however, the seasonal cycle of ozone in the lowermost stratosphere allows greater quantities of ozone to enter the troposphere during spring. Net photochemical ozone production occurs in the late summer/early autumn WCB at all levels and in the lower troposphere PCF. In contrast, springtime net ozone production appears absent from all airstreams, with the CCB influenced by ozone destruction. Ozone and CO are greater in spring, but the relative mixing ratios between airstreams are roughly the same in both seasons. NO_x/CO emissions ratios vary across the midlatitudes according to socioeconomic factors. It is expected that the emissions variation influences the ozone production efficiency of the cyclone airstreams that draw from these regions.