Indoor–outdoor relationships of airborne particles and nitrogen dioxide inside Parisian buses

This study evaluated passengers' exposure to traffic air pollution inside the articulated buses of the line 91 in Paris during 10 working days in May, 2010. Twenty articulated buses were studied on 32 routes in order to determine the influence of the sampling position on the pollutant concentrations. This parameter is still poorly known for the rigid buses and is even less known for the articulated ones. However this parameter must be studied for articulated buses because the greater length may cause a pollutant concentration gradient in the cabin. Portable devices were used to measure pollutants in the presence of passengers from 8 a.m. to 9 a.m. and from 4 p.m. to 5 p.m., time periods corresponding to the peak traffic and travellers. PM2.5 mass concentration, particle number concentration between 0.3 and 20 μm and nitrogen dioxide concentration were simultaneously measured on three positions inside the buses (front, middle and rear) in order to study the spatial distribution of these compounds. These measurements inside the buses were compared to the outdoor concentrations at the same moment of the day provided by the Parisian air quality monitoring network; they were also compared to the results of a previous monitoring campaign performed in 2008. The results obtained during the 2010 campaign revealed that in-cabin NO2 mean concentrations were 1.5–3.5 times higher than the outside concentration levels; a maximum concentration of 234 ± 40 μg m−3 was found in the rear position (location of the engine and exhaust gas). Mean in-cabin PM2.5 mass concentrations varied from one week to another one, but they were globally the same at the three positions inside the instrumented buses. In order to determine the impact of outdoor levels, correlations have been calculated between the results measured inside the buses and those measured by the outdoor air monitoring stations. The highest Pearson correlation coefficient was 0.29 for NO2 data whereas the highest Pearson correlation coefficient between in-cabin PM2.5 mass concentrations and outdoor levels was 0.96. The higher indoor/outdoor correlation of PM2.5 compared to NO2 may result from the high variation of PM2.5 outdoors. Otherwise, this low indoor/outdoor correlation of NO2 can highlight a source of pollution other than the outdoor air.

► Two monitoring campaigns were conducted to analyse air quality inside Diesel buses.
► Typical buses of Paris Public Transport (RATP) were instrumented.
► Spatial distribution of traffic pollutants (PM2.5 and NO2) is studied inside the buses.
► In-cabin/outdoor concentrations were compared.