Stability of single particle tracers for differentiating between heavy- and light-duty vehicle emissions

To determine the size and chemical composition of particles derived from on-road vehicle emissions, individual particles were sampled continuously with an aerosol time-of-flight mass spectrometer (ATOFMS) at the Caldecott Tunnel in Northern California. In this tunnel, traffic is segregated, such that in theory only light duty vehicle emissions or a mix of heavy- (HDV) and light-duty vehicle (LDV) emissions can be sampled separately. Two studies were carried out, one in November 1997 and a second in July 2000, time periods with average ambient temperatures of 10–15 and 26–32 °C, respectively, with the instrument operating at ambient outdoor temperatures. Analysis of the chemical composition of the particles sampled in these studies shows that sampling conditions can strongly impact the determination of suitable markers for identifying particles emitted from different vehicle types during ambient studies. Differences in the results of these two studies are shown to be related to the different ambient temperatures at which the measurements were carried out. Particles sampled from HDV-influenced traffic in the 1997 study were 16 times more likely to have mass spectra containing peaks at both m/z 128 (naphthalene) and 156 (dimethylnaphthalene/naphthaldehyde) and 51 times more likely to have mass spectra containing peaks at both m/z 156 and 170 (trimethylnaphthalene/methylnaphthaldehyde) than were particles sampled from the LDV-only traffic tunnel. The peaks corresponding to m/z 156 and 170 are on average 2 and 4 times more intense in the mass spectra of the HDV-influenced data set than in the LDV-only data set. In contrast, in the 2000 experiment, the peaks at m/z 128 and 156 were present in 1% and 2% of the particles, respectively, with no preference for traffic type. These results show the usefulness of naphthalene and its derivatives for identification of the vehicle source of particulate emissions, under appropriate conditions, and highlight the fact that ambient conditions can strongly affect potential marker ions for source apportionment studies, necessitating a combination of controlled laboratory and field measurements for identification of stable marker ions.