Air quality control agencies measure pollutants in the ambient air to determine whether an area is meeting EPA’s air quality standards. Ambient air monitoring is conducted via networks of monitors located throughout the country that measure actual concentrations of various air pollutants, including sulfur dioxide, nitrogen dioxide, ozone, lead, particulate matter, carbon monoxide, and several toxic air pollutants. In addition, new standards from EPA require the establishment of a monitoring network near major roadways in metropolitan areas across the country that will measure near roadway emissions of nitrogen dioxide, carbon monoxide, and particulate matter. Agencies also use air quality modeling to project emissions of pollutants from stationary and mobile sources. Modeling is used to help identify appropriate air quality controls during the permitting process, develop state implementation plans for meeting air quality goals, and, in some cases, determine air quality in areas where a monitor is not available.