Traffic related particles

• Petri Tiitta, M.Sc., Researcher

Background

Urban air pollution has a wide range of impacts including reduced visibility, acid deposition, and severe human health effects. In the urban areas, one of the most important sources for fine particles is traffic. Emission height of the vehicles is low, often quite near the breathing zone of people which may highlight unwanted health effects. Carbon dioxide (CO2), water vapour (H2O) and the oxides of the nitrogen (NOX) are the main components of combustion process of gasoline and diesel fuel. Exhaust gases also contain carbon monoxide (CO), hydrocarbons (HC), methane (CH4), nitrous oxide (N2O), sulphur dioxide (SO2) and particles. In addition to the combustion process, particulate emissions are created from the brakes, tyres, from pins, wearing out of the motor, and from the surface of the road (resuspension). Elemental carbon formed inside of the motor state at high temperature, is oxidised and agglomerates during the combustion process or a few milliseconds after that. Metal components which are added to lubricating oils condense mainly to accumulation mode particles but minority also to smaller particles. The formation of the nucleation mode immediately after the tailpipe is driven by the nucleating species (mainly hydrocarbons and sulphur) and it's degree of supersaturation (Kittelson et al., 2006). Existing accumulation mode particles (condensation sink) suppress or forbid nucleation and growth of new particles. Rapid dilution and subsequent cooling of exhaust makes nucleation more likely. After nucleation, secondary particles rapidly grow by coagulation and condensation of semi-volatile organic and sulphur compounds, the mode peaking around 15 - 20 nm. In the measurement it has been shown that a majority fraction of the volatile emissions are incombustible lubricating oil (e.g. Canagaratna et al., 2004). Remarkable fraction of both diesel and petrol-operated vehicle emissions are semivolatile compounds (Kuhn et al., 2005).

The aims and methods of study

The possibilities to study of UFP (ultrafine particles) by using traditional techniques e.g. filter sampling are limited because ultrafine particles have low mass concentration, complex composition and relatively short lifetime. To characterize composition and properties of UFP, different type of DMA techniques have been adapted e.g. hygroscopicity, volatility and organic TDMA methods (Joutsensaari et al., 2001). Tandem DMA techniques are able to give indirect information about the chemical composition of particles, even when concentration or size of the particles is too small for actual chemical analysis. Tables 1 and 2 shows a summary of instrumentation during research work.

Table 1: Summary of the aerosol and ion number and size distribution analyzers

Instrument	Operation	Size range (nm)
TWIN-DMPS Twin Differential Mobility Particle Sizer	Particle size distribution	3 - 945
CPC (TSI 3025) Particle counter	Particle number concentration	>3
EAS Electrical Aerosol Spectrometer	Particle size distribution	10 - 10000
AIS The Air Ion Spectrometer	Mobility distribution of air ions	0.46 - 40

The aim of the study is to produce good quality data and widen the understanding about traffic related aerosols. The classification of vehicle emission aerosols defined now first time also by using organic tandem technique (UFO-TDMA). Using both Differential Mobility Particle Sizer (DMPS) and Air Ion Spectorimeter (AIS) enabled also below 10 nanometer aerosol size distribution monitoring.

Table 2: Summary of the aerosol composition analyzers

Instrument	Operation	Size range (nm)
UFH-TDMA Hygroscopic Tandem Differential Mobility Analyzer	Hygroscopicity of aerosols at RH 90%	10, 20 and 50
UFV-TDMA Volatility Tandem Differential Mobility Analyzer	Volatility of aerosols at 50, 150 and 280 °C	10, 20 and 50
UFO-TDMA Organic Tandem Differential Mobility Analyzer	Organic matter identification at S = 0.8	10, 20 and 50
Aethalometer	Black carbon concentration	All particles

References:
Joutsensaari J., Vaattovaara, P., Vesterinen, M., Hämeri, K. and Laaksonen, A. (2001). A Novel tandem differential mobility analyzer with organic vapor treatment of aerosol particles. Atmospheric Chemistry and Physics, 1, 51-60.

Kittelson D.B., Watts W.F., Johnson J.P. 2006. On-road and laboratory evaluation of combustion aerosols - Part1: Summary of diesel engine results. J. Aerosol Sci. 37, 913--930.

Kuhn, T., Biswas S., and Sioutas C. (2005). Diurnal and seasonal characteristics of particle volatility and chemical composition in the vicinity of a light-duty vehicle freeway. Atmospheric Environment 39, 7154-7166.

Canagaratna M.R., Jayne J.T., Ghertner D.A., Herndon S., Shi Q., Jimenez J.L. Silva P.J., Williams P., Lanni T., Drewnick F., Demerjian K.L., Kolb C.E and Worsnop D.R. (2004) Aerosol Science and Technology, 38, 555-573.