In urban areas of the United States, levels of NO2 fell, mainly because of environmental regulations that reduced emissions from motor vehicles and power plants. Declines are most evident in the eastern United States, though densely populated western cities such as Los Angeles (56 percent), Phoenix (54 percent), Atlanta (48 percent), New York (45 percent) also saw levels drop significantly. NO2 concentrations increased slightly in the Williston Basin (North Dakota), the Permian Basin (west Texas), and the Eagle Ford Shale (southern Texas)—areas where natural gas and oil production has increased dramatically due to horizontal drilling and high-volume hydraulic fracturing. Aura observed declining NO 2 levels over most European cities, likely due to the tightening of vehicle emission standards. The cites in Europe that experienced the largest decreases were Madrid (48 percent), Lisbon (47 percent), and Barcelona (44 percent). The story in the Middle East was mixed. In Iraq, N...