Evaluation and modelling of the size fractionated aerosol particle number concentration measurements nearby a major road in Helsinki – Part I: Modelling results within the LIPIKA project

[摘要] A field measurement campaign was conducted near a major road"Itäväylä" in an urban area in Helsinki in 17–20 February2003. Aerosol measurements were conducted using a mobile laboratory"Sniffer" at various distances from the road, and at an urban backgroundlocation. Measurements included particle size distribution in the size rangeof 7 nm–10 μm (aerodynamic diameter) by the Electrical Low PressureImpactor (ELPI) and in the size range of 3–50 nm (mobility diameter) byScanning Mobility Particle Sizer (SMPS), total number concentration ofparticles larger than 3 nm detected by an ultrafine condensation particlecounter (UCPC), temperature, relative humidity, wind speed and direction,driving route of the mobile laboratory, and traffic density on the studiedroad. In this study, we have compared measured concentration data with thepredictions of the road network dispersion model CAR-FMI used in combinationwith an aerosol process model MONO32. For model comparison purposes, one ofthe cases was additionally computed using the aerosol process model UHMA,combined with the CAR-FMI model. The vehicular exhaust emissions, andatmospheric dispersion and transformation of fine and ultrafine particleswas evaluated within the distance scale of 200 m (corresponding to a timescale of a couple of minutes). We computed the temporal evolution of thenumber concentrations, size distributions and chemical compositions ofvarious particle size classes. The atmospheric dilution rate of particles isobtained from the roadside dispersion model CAR-FMI. Considering theevolution of total number concentration, dilution was shown to be the mostimportant process. The influence of coagulation and condensation on thenumber concentrations of particle size modes was found to be negligible onthis distance scale. Condensation was found to affect the evolution ofparticle diameter in the two smallest particle modes. The assumed value ofthe concentration of condensable organic vapour of 10¹² molecules cm⁻³was shown to be in a disagreement with the measured particle sizeevolution, while the modelling runs with the concentration of condensableorganic vapour of 10⁹–10¹⁰ molecules cm⁻³ resulted inparticle sizes that were closest to the measured values.

[学科分类] 大气科学