Estimating the contributions of mobile sources of PAH to urban air using real-time PAH monitoring

Motor vehicles are a significant source of airborne polycyclic aromatic hyd rocarbons (PAH) in many urban areas. Traditional approaches used in determi ning the relative contributions of individual vehicle types to the total am ount of PAH in air have been based on the analysis of integrated samples of airborne particles and gases for the presence of chemical tracers indicati ve of the vehicles from which the chemicals derived. As an alternative, we have used a photoelectric aerosol sensor (PAS) capable of measuring PAH lev els in real-time in the emissions plumes from motor vehicles. We placed the PAS near a traffic-light in Kenmore Square, a busy crossroads in downtown Boston (MA, USA). A video camera co-located at the site recorded the vehicl es passing the sensor, and this record was correlated with the PAS data. Du ring a 5-day monitoring period (similar to 59 h) in the summer of 1998, ove r 34 000 motor vehicles were counted and classified and over 24000 PAS read ings were recorded (frequency 1/8.6 s). The composition of the vehicle popu lation was 94% passenger vehicles, 1.4% buses, 2.6% small trucks, 1.3% medi um trucks, 0.35% large trucks, and 0.45% garbage and construction trucks. I n analyzing the PAS data, it was assumed that the highest PAS measurements - those that exceeded the 95% critical level of the 5-min moving average of all the PAS measurements - were indicative of primary vehicular emissions. We found that similar to 46% of the mass of particle-bound PAH (i.e. simil ar to 46% of the integrated area under the PAS signal vs. time plots) was a ttributable to primary emissions from motor vehicles passing the sensor. Of this, 35-61% was attributable to passenger vehicles (cars, pickup trucks, and sports utility vehicles) and 39-65% was attributable to non-passenger v ehicles [buses (14-23%). small trucks (12-20%), medium trucks (8.4-14%), la rge trucks (2.9-4.8%) and garbage and construction trucks (1.9-3.2%)]. Our results suggest that on a per vehicle basis. buses and trucks - the majorit y of which run on diesel fuel - emitted greater amounts of particle-bound P AH than passenger vehicles. Overall, we found that real-time photoelectric aerosol sensing (in combination with video photography) is useful for estim ating the contributions of airborne PAH from different vehicle types. Due t o the physical constraints of our monitoring site and the high volumes of t raffic, however, it was not possible to uniquely attribute PAS signals to i ndividual vehicles. (C) 2001 Elsevier Science B.V. All rights reserved.