In 1994 and 1995 concentrations of PCBs, transnonachlor, atrazine and mercury, were measured in both the air and the water. Atmospheric monitoring stations were located at nine locations around Lake Michigan. Fewer stations were located in the northern half of the lake because it was expected that less of these chemicals would be seen there. Also, samples were taken several times a year on the bow of U.S.EPA's research vessel Lake Guardian at various locations around the Lake. We did indeed see higher concentrations of the chemicals in the air in the southern half of Lake Michigan.
Pollutants can enter and leave Lake Michigan in many ways.
However, pollutants can also leave Lake Michigan by volatilization - gasses evaporating from the water. These processes are often in balance, so that at any given time pollutants are depositing as much as they are evaporating. They are essentially cycling between the air and water. It's crucial to know the amount of a chemical coming down in atmospheric deposition compared to the amount leaving the lake through volatilization. In the summer, PCB air levels are higher than water levels.