Quantification of diesel exhaust gas phase organics by a thermal desorption proton transfer reaction mass spectrometer

摘要

A new approach was developed to measure the total abundance of long chainalkanes (C and above) in urban air using thermal desorption with aproton transfer reaction mass spectrometer (PTR-MS). These species areemitted in diesel exhaust and may be important precursors to secondaryorganic aerosol production in urban areas. Long chain alkanes undergodissociative proton transfer reactions forming a series of fragment ionswith formula CH. The yield of the fragment ions is a functionof drift conditions. At a drift field strength of 80 Townsends, the mostabundant ion fragments from C to C -alkanes were 57, 71and 85. The PTR-MS is insensitive to -alkanes less than C butdisplays an increasing sensitivity for larger alkanes. Higher drift fieldstrengths yield greater normalized sensitivity implying that the protonaffinity of the long chain -alkanes is less than HO. Analysis ofdiesel fuel shows the mass spectrum was dominated by alkanes(CH), monocyclic aromatics, and an ion group with formulaCH ( 97, 111, 125, 139). The PTR-MS was deployed inSacramento, CA during the Carbonaceous Aerosols and Radiative Effects Studyfield experiment in June 2010. The ratio of the 97 to 85 ion intensitiesin ambient air matched that found in diesel fuel. Total diesel exhaustalkane concentrations calculated from the measured abundance of 85ranged from the method detection limit of ~1 μg m to100 μg m in several air pollution episodes. The total diesel exhaustalkane concentration determined by this method was on average a factor of 10greater than the sum of alkylbenzenes associated with spark ignition vehicleexhaust.