Environmental regulations on NO_x emissions from automobiles and power plants in the United States led to a reduction in NO₂ levels between 20 and 40% over American cities from 2005 to 2016.  In fact, the only significant increases over the U.S. occurred over areas of intensive oil and natural gas extraction in western North Dakota and western Texas, indicating an increase in activities and, subsequently, NO_x emissions from flaring of natural gas and the heavy machinery used to extract the oil and natural gas.  Significant decreases in Mexico occurred over Mazatlan and Guadalajara (−15-−20%) with insignificant changes over other cities.  Mexico City had considerable spatial heterogeneity in the changes in NO₂ levels, with large increases to the west and north of the city center and large decreases in the city center and to the northeast.