CO = carbon monoxide; CO2 = carbon dioxide; DDD = triple difference-in-differences; DID = difference-in-differences; EMS = environmental management system; NO2 = nitrogen dioxide; NOx = nitrogen oxides; O3 = ozone; PM = particulate matter; PM10 = particulate matter smaller than 10 microns; PM2.5 = particulate matter smaller than 2.5 microns; RD = regression discontinuity; SO2 = sulfur dioxide;