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.
Charlson R.J., Schwartz S.E., Hales J.M., Cess
R.D., Coakley J.A., Hansen J.E. & Hofmann
D.J. Climate forcing by anthropogenic aerosols.
Science 255, 423-430, 1992.
Dockery D.W. and Pope C.A. Acute respiratory effects
of particulate air pollution. Annual Review of
Public Health 15, 107-132, 1994.
IPCC, Climate Change 2001: The scientific basis.
Contribution of working group I to the third assessment
report of the Intergovernmental Panel on Climate
Change, edited by J. T. Houghton, Y. Ding, D.
J. Griggs, M. Noguer, P. J. van der Linden, X.
Dai, K. Maskell and C. A. Johnson, 881 pp., Cambridge
University Press, Cambridge, United Kingdom and
New York, NY, USA, 2001.Jacobson, M.C., Hansson,
H.-C., Noone, K.J. and Charlson, R.J. Organic
atmospheric aerosols: Review and state of the
science. Reviews of Geophysics, 38, 267-294, 2000.
Jokiniemi J., Ohlström M., Kulmala, M. and
Hämeri K. Kartoitus pienhiukkastutkimuksesta
Suomessa. Tekesin Teknologiakatsaus 100/2000,
Kittelson, D.B., 1998. Engines and nanoparticles:
A review. J. Aerosol Sci. 29, 575-588.