Seasonal changes in the transport of pollutants into the Arctic troposphere‐model study

In association with Tropospheric Ozone Production about the Spring Equinox (TOPSE) measurement campaign a regional episodic chemical transport model is used to study the seasonal mechanisms of transport of pollutants from their Northern Hemisphere emission regions into the remainder of the troposphere. The model simulates the strong seasonal cycle for CO and hydrocarbons that agrees well with TOPSE measurements. In this study we use the isentropic perspective to analyze transport during different seasons and from different emission regions. Simulations with diagnostic tracers are conducted to quantify (1) the contribution of different emission regions to tracer distribution and (2) the importance of cross‐isentropic transport. In the high latitudes during winter, in agreement with previous studies, the European and Siberian emission sources are the largest contributors to the diagnostic tracer distributions in the lower troposphere due to large‐scale circulation patterns, low temperatures at the source and presence of a cold stable boundary layer that facilitates diabatic cooling. North American emissions are located further south, often south of the polar front, and are on average emitted at higher potential temperatures. Owing to their predominant transport over the relatively warm Atlantic, they experience strong diabatic heating due to the general instability in the air column and heavy precipitation in the storm track. During the summer months the pollution from all emission regions is more likely to be diabatically transported to higher potential temperatures and diluted. In addition, direct transport of pollutants into the Arctic is less frequent during summer due to the differences in the large‐scale circulation patterns.