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Several regions of the United States, particularly in the west, experience poor air quality during winter meteorological conditions that confine local emissions in shallow layers near the surface. However, different regions have widely different responses to local emissions in the winter. Sparsely populated oil and gas producing regions are subject to ozone pollution, while urban regions experience very low ozone but high levels of particulate matter comprised primarily of ammonium nitrate. These disparate issues are related both chemically and meteorologically. Recent field studies investigating processes relevant to winter air quality include , the , and the .