We studied the sources, compositions and size
distributions of aerosol particles during long-range transport
(LRT) PM2.5 episodes which occurred on 12–15 August, 26–
28 August and 5–6 September 2002 in Finland. Backward
air mass trajectories, satellite detections of fire areas and dispersion
modelling results indicate that emissions from wild-
fires in Russia and other Eastern European countries arrived
in Finland during these episodes. Elemental analyses using
scanning electron microscopy (SEM) coupled with energy
dispersive X-ray microanalyses (EDX) showed that the proportions
of S-rich particles and agglomerates (agglomeration
was caused partly by the sampling method used) increased
during the episodes, and they contained elevated fractions
of K, indicating emissions from biomass burning. These
aerosols were mixed with S-rich emissions from fossil fuel
burning during transport since air masses came through polluted
areas of Europe. Minor amounts of coarse Ca-rich particles
were also brought by LRT during the episodes, and
they probably originated from wildfires and/or from Estonian
and Russian oil-shale-burning industrial areas. Ion chromatography
analysis showed that concentrations of sulphate
(SO2− 4 ), total nitrate (NO−3 +HNO3(g)) and total ammonium
(NH+4 +NH3(g)) increased during the episodes, but the ratio
of the total amount of these ions to PM10 concentration decreased,
indicating unusually high fractions of other chemical
components. Particle number size distribution measurements
with differential mobility particle sizer (DMPS) revealed
that concentrations of particles 90–500 nm increased during the episodes, while concentrations of particles smaller
than 90 nm decreased. The reduction of the smallest particles
was caused by suppressed new particle formation due to
vapour and molecular cluster uptake of LRT particles. Our
results show that emissions from wildfires in Russian and
other Eastern European countries deteriorated air quality of
very large areas, even at distances of over 1000 km from the
fire areas