a similar level to those bought 15 years earlier under real-world tests; hence, diesel cars emit around 20 times more nitrogen oxides than petrol cars. Modern on-road diesel engines typically use selective catalytic reduction (SCR) systems to meet emissions laws, as other methods such as exhaust gas recirculation (EGR) cannot adequately reduce NOx to meet the newer standards applicable in many jurisdictions. Auxiliary diesel systems designed to remediate the nitrogen oxide pollutants are described in a separate section below. Moreover, the fine particles (fine particulate matter) in diesel exhaust (e.g., soot, sometimes visible as opaque dark-colored smoke) has traditionally been of greater concern, as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines. These especially harmful particulate contaminants are at their peak when such engines are run without sufficient oxygen to fully combust the fuel; when a diesel engine runs at idle, enough oxygen is usually present to burn the fuel completely. (The oxygen requirement in non-idling engines is usually satisfied using turbocharging.). From the particle emission standpoint, exhaust from diesel vehicles has been reported to be significantly more harmful than those from petrol vehicles. Diesel exhausts, long known for their characteristic smells, changed significantly with the reduction of sulfur content of diesel fuel, and again when catalytic converters were introduced in exhaust systems. Even so, diesel exhausts continue to contain an array of inorganic and organic pollutants, in various classes, and in varying concentrations (see below), depending on fuel composition and engine running conditions. Exhaust gas composition according to various sources Chemical classes The following are classes of chemical compounds that have been found in diesel exhaust. Specific chemicals The following are classes of specific chemicals that have been found in diesel exhaust. §Includes all regioisomers of this aromatic compound. See ortho-, meta-, and para-isomer descriptions at each compound's article. Regulation To rapidly reduce particulate matter from heavy-duty diesel engines in California, the California Air Resources Board created the Carl Moyer Memorial Air Quality Standards Attainment Program to provide funding for upgrading engines ahead of emissions regulations. In 2008, the California Air Resources Board also implemented the 2008 California Statewide Truck and Bus Rule which requires all heavy-duty diesel trucks and buses, with a few exceptions, that operate in California to either retrofit or replace engines in order to reduce diesel particulate matter. The US Mine Safety and Health Administration (MSHA) issued a