Abstract
The aim of this study is to assess particle number concentrations (PNCs) and distributions (PNDs) in a car cabin while driving. Further objectives include the determination of the influence of particle transformation processes on PNCs, PNDs and estimation of PNC related exposure. On-board measurements of PNCs and PNDs were made in the 5–560 nm size range using a fast response differential mobility spectrometer (DMS50), which has a response time of 500 ms. Video records of the traffic ahead of the experimental car were also used to correlate emission events with measured PNCs and PNDs. A total of 30 return trips was made on a 2.7 km route during morning and evening rush hours, with journey times of 7 ± 2 and 10 ± 3 min, respectively. The average PNC for the set of morning journeys, 5.79 ± 3.52 × 104 cm−3, was found to be nearly identical to the average recorded during the afternoon, 5.95 ± 4.67 × 104 cm−3. Average PNCs for individual trips varied from 2.42 × 104 cm−3 to 2.18 × 105 cm−3, mainly due to changes in the emissions affecting the experimental car (e.g. when the experimental car was following another vehicle). The largest one second averaged PNC during a specific event, 1.85 × 106 cm−3, was found to be over 30-times greater than the overall average of 5.87 ± 4.06 × 104 cm−3. Correlation of video records and concentration data indicated that close proximity to a preceding vehicle led to a clear increase in PNCs of freshly emitted nucleation mode particles. The evolution of normalised PNDs demonstrated that dilution was the dominant transformation process in the car cabin. The deposition of inhaled particles in the lung was estimated on the basis of either the size-resolved distribution or the total PNC. In general, the two methods yielded similar results but differences up to 30% were noted in some cases, with the latter method giving the lower values. Overall, the results reflect the importance of size-resolved measurements for deriving accurate evaluations of exposure rates, as well as identifying emissions from nearby traffic as the cause of short-term elevations of PNCs and hence dose rates.