The natural gas supply chain contributes to air pollution in several ways, including (1) the leaking, venting, and combustion of natural gas in the course of production operations; and (2) the combustion of other fossil fuel resources or other emissions during associated operations. Emission sources include pad, road, and pipeline construction; well drilling, completion, and flowback activities; and gas processing and transmission equipment such as controllers, compressors, dehydrators, pipes, and storage vessels. Pollutants include, most prominently, methane (i.e., the principal component of natural gas) and volatile organic compounds (VOCs)— of which the natural gas industry is one of the highest-emitting industrial sectors in the United States—as well as nitrogen oxides, sulfur dioxide (SO2), and various forms of hazardous air pollutants (HAPs).