I saw this article at BoingBoing, and had to look it up.
From Wikipedia:
The Aroma of Tacoma, also known as the Tacoma Aroma, is a putrid and unpleasant odor associated with Tacoma, Washington, United States.[1] The smell has been described as similar to the odor of rotten eggs.[2] The odor is not noticeable throughout the city, but is rather concentrated in the Tacoma tideflats and is frequently smelled by motorists traveling that section of Interstate 5.[2]
Saint John, New Brunswick also used to have a terrible smell from the pulp mill. Last time I was there I didn’t notice it, so maybe they have cleaned it up.
It’s specific to chemicals used in the Kraft process for making wood pulp. Other Mills might use at different process, with their own issues (like SO2 gas).