In-vehicle airborne fine and ultra-fine particulate matter exposure: The impact of leading vehicle emissions.

Airborne particulate matter (PM) concentrations inside vehicle cabins are often extremely high compared to background levels. The present study was motivated by the fact that in the last few decades, the implementation of new emission standards has led to the reduction of vehicle particle emissions. This study addresses for the first time the relationship between leading vehicle (LV) emissions and in-cabin PM exposure levels in the immediately following vehicle (henceforth called the study vehicle - SV), with particular emphasis on the role of the LV's emission reduction technologies (e.g., diesel particulate filter-DPF) as an effective risk management measure. The study was performed using an instrumented study vehicle (always to be considered as the following vehicle) on a 26-km fixed route where 10 repeated tests were conducted during 60-minute trips. On-line monitoring of the fine 0.3-1 μm and 1-2.5 μm (PM and PM) and ultra-fine particle (UFP) concentrations was performed inside the SV's car cabin with fixed ventilation settings (i.e., windows closed, air conditioning off, and recirculation fan off). Simultaneously, the license plate numbers of the LVs along the route were recorded to retrieve information pertaining to their fuel type and Euro emission standard category. The results clearly showed that the in-cabin PM levels were significantly affected by the LV's Euro emission standard. Regarding petrol-fuelled LVs, the median in-cabin particle exposure levels were statistically lower (e.g., -34% for PM) when following vehicles with stricter emission standards (in particular, Euro 6) than when following a low-emission standard vehicle (i.e., Euro 0-2). Concerning diesel-fuelled LVs, a strong and significant decrease in the in-cabin median exposure levels (up to -62%, -44%, and -48% for UFPs, PM, and PM, respectively) was observed for recent-emission standards LVs (i.e., Euro 5-6) with respect to older-emission standard LVs (i.e., Euro 0-4). A specific analysis revealed that the in-cabin median exposure concentrations of PM were highly and significantly reduced by DPF-equipped LVs. For UFPs, this resulted in a 47% reduction compared to diesel-fuelled (non-DPF) LVs. For PM, an approximate 80% reduction was observed compared to both petrol-fuelled and diesel-fuelled (non-DPF) LVs. For PM, an approximate 38% reduction was observed compared to petrol-fuelled LVs and a 46% reduction compared to non-DPF LVs.