Particle number, particle mass and NO_x emission factors at a highway and an urban street in Copenhagen

Abstract. This paper presents measurements of traffic-generated gas and particle pollution at two sites, one near a major highway and one near a busy urban street in Copenhagen, Denmark. Both sites were equipped for a 4-week period with a set of two measurement stations, one close to the kerbside and one background station. Measurements were carried out from March to April~2008, investigating NO_x concentrations, submicrometer particle number size distribution (size range 10–700 nm), particle mass (PM_2.5, PM₁₀), and meteorological parameters. In this study we also estimate the emission factors for NO_x, particle number and particle mass using measured traffic volume and dilution rate calculated by the Operational Street Pollution Model (WinOSPM).

The mean concentrations of most of the measured pollutants are similar for the highway and the urban kerbside stations due to similar traffic density. The average concentrations of NO_x are 142 μg m⁻³ and 136 μg m⁻³ for the highway and the urban kerbside stations, respectively. These values are about 5 times higher compared to the corresponding background values. The average particle number concentration is 24 900 particles cm⁻³ and 27 100 particles cm⁻³ for the highway and the urban kerbside stations, respectively, and these values exceed those measured at the background stations by a factor of 3 to 5.

The temporal variation of the traffic contribution (difference of kerbside and background concentrations) is analysed for NO_x, particle number and mass, and it follows the traffic pattern at the urban and the highway sites. Emission factors for particle number are found to be quite similar at both sites, (215±5) 10¹² particles veh⁻¹ km⁻¹ for the highway and (187±3) 10¹² particles veh⁻¹ km⁻¹ for the urban site. Heavy duty vehicles (HDVs) are found to emit about 20 times more particles than light duty vehicles (LDVs), which is in good agreement with other published studies. Emission factors are also determined for individual particle modes identified in the size spectra. Average fleet emission factors for PM_2.5 at the highway and the urban site are 29 mg veh⁻¹ km⁻¹ and 46 mg veh⁻¹ km⁻¹, respectively. The estimated particle number and size spectra emission factors will provide valuable input for air quality and particle dispersion modelling near highways and in urban areas.