Traffic emission factors of ultrafine particles: effects from ambient air

Ultrafine particles have a significant detrimental effect on both human health and climate. In order to abate this problem, it is necessary to identify the sources of ultrafine particles. A parameterisation method is presented for estimating the levels of traffic-emitted ultrafine particles in terms of variables describing the ambient conditions. The method is versatile and could easily be applied to similar datasets in other environments. The data used were collected during a four-week period in February 2005, in Gothenburg, as part of the Gote-2005 campaign. The specific variables tested were temperature (T), relative humidity (RH), carbon monoxide concentration (CO), and the concentration of particles up to 10 mm diameter (PM10); all indicators are of importance for aerosol processes such as coagulation and gas-particle partitioning. These variables were selected because of their direct effect on aerosol processes (T and RH) or as proxies for aerosol surface area (CO and PM10) and because of their availability in local monitoring programmes, increasing the usability of the parameterization. Emission factors are presented for 10-100 nm particles (ultrafine particles; EFufp), for 10-40 nm particles (EF10-40), and for 40-100 nm particles (EF40-100). For EF40-100 no effect of ambient conditions was found. The emission factor equations are calculated based on an emission factor for NOx of 1 g km(-1), thus the particle emission factors are easily expressed in units of particles per gram of NOx emitted. For 10-100 nm particles the emission factor is EFufp 1.8 x 10(15) x (1 - 0.095 x CO - 3.2 x 10(-3) x T) particles km(-1). Alternative equations for the EFs in terms of T and PM10 concentration are also presented.