Quantitative Analysis of the Parameters Affecting In-Cabin to On-Roadway (I/O) Ultrafine Particle Concentration Ratios

A mass-balance indoor particle dynamic model was adopted and modified to investigate how on-roadway ultrafine particle (UFP, diameter < 100 nm) concentrations and vehicle ventilation settings affect UFP levels inside vehicles. The model was first parameterized focusing on a mechanistic, simulation-based interpretation of in-cabin data reported in Zhu et al. (2007). Under three different ventilation conditions, (1) Fan off-recirculation (RC) off, (2) Fan on-RC off, and (3) Fan on-RC on, the modeled UFP in-cabin to on-roadway (I/O) ratios were found to be 0.40, 0.25, and 0.10, respectively, and agree with the experimental data very well. Then, analysis focused on how the model input parameters, such as ventilation settings, vehicle speed, filtration, penetration, deposition, and human respiration, affect I/O ratios in broader categories of vehicle cabin microenvironments. Under condition (1), the modeled I/O ratios increased linearly, up to ∼25%, within the literature reported penetration factor range. The I/O ratios were directly proportional to vehicle speeds and inversely affected by deposition coefficients. Under condition (2), the airflow rate of mechanical ventilation was the predominant factor and I/O ratio was inversely proportional to the airflow rate in the range of 0–108 m 3 /h -1 , and directly proportional to the airflow rate in the range of 108–360 m 3 /h -1 Under condition (3), the effects of penetration factor, deposition coefficient, and vehicle speed were comparable. The modeled I/O ratio was inversely proportional to the airflow rate from mechanical ventilation in the whole range (0–360 m 3 h-1) with 10–20% influence depending on the particle size.