Review of Nitrous Oxide (N_2O) Emissions from Motor Vehicles

摘要

Nitrous oxide (N_2O) is both an ozone depleting gas and a potent greenhouse gas (GHG), having a global warming potential (GWP) value nearly 300 times that of carbon dioxide (CO_2). While long known to be a trace by-product of combustion, N_2O was not considered a pollutant of concern until the introduction of the three-way catalyst (TWC) on light-duty gasoline vehicles in the 1980s. These precious metal-containing catalysts were found to increase N_2O emissions substantially. Through extensive research efforts, the effects of catalyst type, temperature, air/fuel ratio, space velocity, and other factors upon N_2O emissions became better understood. Although not well documented, N_2O emissions from non-catalyst vehicles probably averaged 5-10 mg/mi (on the standard FTP test), while early generation TWC-equipped vehicles exceeded 100 mg/mi. As emissions control systems evolved to meet increasingly stringent criteria pollutant standards, N_2O emissions also decreased. Today's Tier 3 vehicles are required to meet a U.S. Environmental Protection Agency (EPA) N_2O tailpipe standard of 10 mg/mi. N_2O emissions from diesel engines and vehicles became of concern in the 2000s, when catalytic control devices such as diesel particulate filters (DPFs), lean oxides of nitrogen (NO_x) traps (LNTs), and selective catalytic reduction (SCR) units were introduced to control PM and NO_x. As with gasoline vehicles, N_2O emissions from diesel applications are a balance between N_2O formation and destruction within these catalytic devices. Modern U.S. light-duty diesel vehicles must comply with the same 10 mg/mi N_2O standard that applies to gasoline vehicles; modern heavy-duty diesel (HDD) engines must comply with EPA's recently established N_2O standard of 100 mg/bhp-hr. The total GWP of motor vehicle exhaust is dominated by CO_2 emissions, with N_2O contributing only 1-2%. Furthermore, the total mass of N_2O emissions from mobile sources is declining - largely due to turnover of the light-duty fleet. Consequently, N_2O emissions from motor vehicles do not represent a significant contribution to global GHGs.