Ultrafine Particles Inside Cars in Busy Traffic—Developing an Empirical Model

Abstract

PP-30-051 Background/Aims: NIWA has developed a mobile monitoring platform that includes the capability for indoor/outdoor measurements while driving. We have begun an iterative campaign of measurements with the aim of creating a generalizable model of in-vehicle exposure. Methods: Initial trials were used to build a simple first-order model to predict in-vehicle particle concentrations from measured external concentrations as a function of speed and ventilation settings. This model was prone to over-estimation following sudden jumps in external concentration, suggesting the lack of aerosol dynamics and variable sink terms in the model as a weakness. Extra sink and lag terms were empirically added based on drive-throughs of a busy road tunnel in Wellington. In August 2009, an improved experimental platform, incorporating CPCs with different size cuts (TSI 3007 and TSI PTrak) and measurements of carbon monoxide, made multiple peak-time journeys over 3 busy routes between suburban and central Auckland. Instrument sampling locations were varied to investigate internal aerosol mixing and sink processes within the vehicle. An additional campaign capturing measurements of CO and PNC (using a TSI 3007) in a car, on buses, and on a bicycle was conducted at peak-time over 20 days along one of the study routes. Results: These campaigns identified that external concentrations varied largely with the proximity of road junctions and congested traffic, with certain intersections providing consistent “hot-spots.” Interior PNC principally responded to exterior PNC in a manner indicative of the cabin acting like a buffer, smoothing external variability. Conclusion: The “trapping” phenomenon was very apparent, with brief encounters with a single gross-emitting vehicle leading to extended high interior exposure lasting many minutes after the encounter. Concentrations at the back seat were lower than and, lagged those in the front indicating either inhomogeneous mixing or deposition processes—or a combination of the 2.