Assessment of cabin filtration efficiency: contribution to an on-road methodology

Abstract. The use of cabin filters provides protection for passengers against outdoor pollution caused by particles (aerosols) when travelling by car. Their filtration efficiency is assessed through rigorous laboratory studies in accordance with standards. However, there are few or no comparable studies conducted under real road conditions using reference particle measurements to complement the laboratory results. In this context, our study aims to contribute to the improvement of on-road aerosol measurement by evaluating cabin filters with gravimetric measurements. To this end, a sampling line upstream of the cabin filter in an area located under the windscreen, and a sampling point downstream of the cabin filter on the front seat inside the cabin were designed. The sampling efficiency of the sampling line was calculated using a transdisciplinary work involving Computational Fluid Dynamics, wind tunnel measurements and numerical methods. The results showed that its sampling efficiency for PM₁ and PM_2.5 (i.e., particles with an aerodynamic diameter less than 1 and 2.5 µm, respectively) is greater than 94 and 77 %, respectively. Next, on road tests were carried out in a moving car in the Paris region, for two different cabin filters and during tests without a cabin filter. The average filtration efficiency was 51.4 % (n=15), 52.7 % (n=14) and 9.5 % (n=4) for PM₁ and 57.3 % (n=9), 56 % (n=9) and 32 % (n=4) for PM_2.5, respectively, for Filter A, Filter B and without a cabin filter. Lastly, optical counters were simultaneously operated with gravimetric measurements. The results showed that optical counters underestimate the concentration of PM_2.5, highlighting the importance of using a reference method to measure the particles under real road conditions.