The amounts of pollution produced by tar sands excavation sites are comparable to those of a medium-sized city or a large power plant, according to a new study by Environment Canada, the nation’s environmental agency. Using satellite remote sensing observations, scientists found elevated levels of nitrogen dioxide and sulfur dioxide across a 19-by-31-mile region of surface mining. In addition to corresponding to “locations of significant emissions from large mining operations,” the researchers found that pollution levels are increasing. In the case of nitrogen dioxide, levels increased by about 10.4 percent annually between 2005 and 2010. “It stands out above what’s around it, out in the wilderness,” said Chris McLinden, a research scientist with Environment Canada and lead author of the study published in Geophysical Research Letters.