Which air mass only occurs in winter in North America?

The Continental Arctic air mass is indeed associated with winter conditions in North America. This air mass originates over the Arctic regions, characterized by extremely cold temperatures due to the long, harsh winters experienced there. During winter, this air mass can move southward into the continental United States and Canada, bringing frigid air and significantly lowering temperatures.

In contrast, the other air masses listed tend to have different seasonal behaviors. Maritime Tropical, for example, is more prevalent in the summer and brings warm, moist air from over warm ocean waters. Maritime Arctic occurs, but it is not limited to winter and is less common. Maritime Polar typically brings cool, moist air and can occur in both summer and winter. Thus, the unique characteristic of the Continental Arctic air mass being primarily a winter phenomenon helps identify it correctly in this context.