Atmospheric aerosols are tiny particles suspended in the air. They range in size from nanometers to tens of micrometers in diameter. The size of the particles affects both their residence times in the atmosphere and their physical and chemical properties. Primary particles are emitted to the atmosphere as particles, eg. from combustion processes, sea spray, wind-blown dust, pollens, volcanic dust. Secondary particles are formed in the atmosphere by gas-to-particle conversion. Particles from combustion sources contain elemental carbon, organic carbon and trace metals. Freshly formed particles in the atmosphere contain sulphate, nitrate, ammonium and secondary organics. The sizes of the so called fine particles are less than 2.5 µm in diameter.

Aerosol particles influence climate directly by absorbing incoming solar radiation or scattering it back to space and indirectly by acting as cloud condensation nuclei (CCN) for cloud and fog droplets and thus affecting droplet concentrations, the optical properties and the lifetimes of clouds (Charlson et al., 1992). There are still large uncertanties in estimating their effects on the radiation balance of the Earth (IPCC, 2001). Additionally, aerosols cause visibility problems and acidify rainwater thus damaging lakes and plant life. Particularly fine and ultrafine particles have recently become of great interest due to their adverse health effects (Dockery and Pope, 1994). Biological mechanisms are not yet well known, neither which physical and chemical property of particles (number concentration, size, surface area, mass, chemical composition) has a key role in forming harmful effects for health.

Urban air quality is mostly dominated by traffic emissions that are characterised by large number of sources with low source heights. Automotive engines are major sources of fine particles. Due to the emission regulations and advanced technology, the specific mass emissions (in g/km) have been decreased markedly, about one order of magnitude since the 1970's, both for passenger cars and heavy-duty engines). Although particulate mass emissions from spark ignition engines with new technology are typically 10-100 times lower than those of diesel engines, considerable increase in the number concentrations of nanoparticles (d<30nm) from spark ignition engine has been recently reported (Kittelson, 1998).

Traffic particles consist mainly of elemental carbon, organic carbon and sulphate compounds (Jokiniemi et al., 2000). They also included some metals.


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