Chemistry Reference
In-Depth Information
residual ash, plant tissues released due to incomplete combustion, or soil particles
suspended by physical mechanisms. Coarse particles are usually redeposited to
the ground close to the emission source but they can also be transported to long
distances under favorable conditions. Besides biomass burning, Ca-rich particles
are emitted from many other sources, including fossil fuel burning and the cement
and metal industries. In August to September 2002, there were also a lot of crustal
elements (Fe, Al, Cd, and Si) in both fine and coarse particles collected in
Helsinki [
14
].
Usually urban aerosol in fine fraction is neutral. However, in the smoke plumes
observed in August 2006 in Helsinki, there was an excess of cations, mainly
ammonium [
18
]. These particles can be good sites for various heterogeneous
reactions involving acidic trace gases. In August 2006 smoke particles had
transported only a short distance, and therefore there were probably still plenty of
gaseous smoke compounds that had not yet reacted with the particles.
4.3 Transformation During Transport
After release from fires, organic and some inorganic components undergo rapid or
more delayed chemical transformation in the atmosphere. The physical properties
as well as chemical composition of smoke particles may alter on the way from the
source areas (biomass burning areas) to the measurement sites in Northern Europe.
There are several reasons why particle properties change. Chemical components
can, e.g., become oxidized or substituted in particles, but also the condensation of
secondary material onto the LRT particles during the transport changes the particle
properties.
It has been suggested that levoglucosan degrades during the transport [
10
]. In
spring 2006 at Spitsbergen, the ratio of levoglucosan to carbon monoxide was more
than an order of magnitude lower than the emission factor for levoglucosan from
agricultural biomass burning determined earlier. Since the LRT smoke aerosol was
assumed not to be removed during the transport, the low emission factor for
levoglucosan indicated the degradation during the transport. It has been suggested
that levoglucosan may decay in the atmosphere, e.g., by the influence of hydroxyl
radicals [
43
].
Oxidation of organic compounds during the transport changes their water solu-
bility. Two smoke episodes detected in 2006 in Helsinki had significantly different
transport times, and therefore also the water solubility of OM. In spring the biomass
smokes were carried in the atmosphere several hundreds of kilometers in a time
period from 1 day to 4 days whereas in August 2006 the fires were only 200-500 km
from Helsinki with a minimum transport time of 5 h. In spring the contribution of
water-soluble organic compounds was higher (70%) than in August (56%)
indicating that the organic compounds were much less oxidized after the transport
of only the few hours in August, and therefore they were still largely water
insoluble [
18
].
