Characterization of urban particulate matter by Raman spectroscopy

Raman spectroscopic measurements of particulate matter samples collected during the Pacific 2001 field campaign in the vicinity of Vancouver, BC are presented in this report. Samples were collected using custom built samplers by drawing ∼2.5 standard litres per minute (SLPM) of ambient air through 3.0 and 0.2 mum pore size silver membrane filters mounted in tandem. Four samples were collected per day at both the Slocan Park and Langley sites on the schedule: 8--10 AM, 12--2 PM, 4--6 PM and 7 PM--7 AM. The filters were analyzed in the laboratory using a Raman spectrometer which measured Raman shifts from 500--3500 cm-1. Raman spectra obtained from these samples were characteristic of soot particles. Apart from the three graphite/carbon bands (D, G and D') many samples also had bands characteristic of sulphate, nitrate and other inorganic components. In the present work, we report the quantification of the relative intensities of these bands and the implications of these results on the composition of the particles.; As the laser interrogates a very small surface volume of the sample the amount of sample analyzed by Raman Spectroscopy is unknown. Furthermore differences in sampling times (three 2 hour samples were collected during the day whereas one 12 hour sample was collected at night) also had to be taken into account. Therefore the day time results obtained by Raman spectroscopy have been compared with results obtained by other atmospheric scientists using conventional monitoring techniques. These initial results show good agreement between our results and the amount of sulphate measured by an Aerodyne Aerosol Mass Spectrometer (AMS) and denuder/filterpack assemblies. Organic carbon (OC) and elemental carbon (EC) measured by thermal/optical methods were also compared with the Raman results. Poor correlation was obtained between both data sets. However ratios of the amounts or intensities of similar species, i.e. the ratio of area under the graphite bands to the area under the sulphate band and the ratio of the amount of total carbon (TC = OC + EC) with the amount of sulphate measured by denuder/filterpack assemblies, were found to correlate well. Air mass analyses were performed using the Hybrid Single Particle Langranian Integrated Trajectories (HYSPLIT4) code to determine the origin of the air masses arriving at the study sites during the study period. These highlight the importance of meteorology and knowledge of the location and nature of possible pollution sources in understanding the data obtained during air quality studies.