Near-Roadway Emission of Reactive Nitrogen Compounds and Other Non-Criteria Pollutants at a Southern California Freeway Site

Abstract:

Emission control systems in light-duty motor vehicles (LDMVs) have played an important role in improving regional air quality by dramatically reducing the concentration of criteria pollutants (carbon monoxide, hydrocarbons, and nitrogen oxides) in exhaust emissions. Unintended side-reactions occurring on the surface of three-way catalysts may lead to emission of a number of non-criteria pollutants whose identity and emission rates are poorly understood. A series of near-roadway field studies conducted between 2009-2015 has investigated LDMV emissions of these pollutants with unprecedented depth of coverage, including reactive nitrogen compounds (NH₃, amines, HCN, HONO, and HNO₃), organic peroxides, and carbonyl compounds (aldehydes, ketones, and carboxylic acids). Methods to collect these pollutants using mist chambers, annular denuders, impingers, and solid-phase cartridges and quantify their concentration using GC-MS, LC-MS/MS, IC, and colorimetry were developed and validated in the laboratory and field. These methods were subsequently used in near-roadway field studies where the concentrations of the target compounds integrated over 1-4 hour blocks were measured at the edge of a freeway and at a background site 140 m from the roadway. Concentrations followed a steep decreasing gradient from the freeway to the background site. Emission factors (pollutant mass emitted per mass fuel consumed) were calculated by carbon mass balance using the difference in concentration measured between the freeway and background sites for the emitted pollutant and CO₂ as a measure of carbon mass in the vehicle exhaust. The significance of these results will be discussed in terms of emissions inventories in the South Coast Air Basin of California, emission trends at this site over the period of 2009-2015, and for NH₃, emission measurements conducted by our group and others over the period 2000-2015.