Trucks and buses have dropped NOx emissions by 98% in the past 10 years, according to the California Air Resources Board (CARB). At the same time, sulfur emissions have dropped by 97%. According to the Executive Director of the Diesel Technology Forum, more than 80% of all United States freight is transported by diesel-powered vehicles. The Forum also notes that it would take 60 diesel trucks to generate the same amount of diesel emissions as one truck did before 1988.
Business Week credits these reductions in NOx emissions to more efficient diesel engines and Selective Catalytic Reduction (SCR) technology. SCR is the more popular technology among manufacturers, as they struggle to meet the new emissions standards. SCR technology operates through the use of Diesel Exhaust Fluid (DEF).
The Environmental Protection Agency (EPA) governs NOx emissions, and limits the average NOx emission of a diesel truck to 0.07g/mile.