Chemistry Reference
In-Depth Information
The histograms show a clear influence of multiple aerosol sources (clean and
polluted) and with a high seasonal cycle for smaller particle sizes. Both stations have
a very widely spread histogram in all seasons suggesting high variability in
concentrations and multiple types of air masses. Most of the Mace Head N
100
histogram is located at low concentrations with approximately 200 particles cm
3
,
but with a second mode at about order on magnitude greater concentrations. This is
well in line with previous studies from the both stations showing the importance of
the difference between air masses arriving over the relatively clean Atlantic Ocean
contrasted by polluted air masses arriving fromBritain and mainland Europe [
22
,
23
].
Neither of the stations have strong seasonal signals in CCN-sized concentrations.
3.2.5 Mountain Stations (See Fig.
6
)
At high-altitude sites (defined as measurement height approximately 1,000 m above
mean sea level, note that the categorization is slightly changed from [
18
] to include
some of the intermediate-height stations), particle number size distributions are
similar, even though the stations are located in different parts of the continent. The
particle number concentrations were low compared to nearby lowland sites, as most
of the aerosol sources are on surface. The particle number size distributions generally
show bimodal behavior, although the modes are overlapping at some of the stations.
The seasonal cycle is similar at all sites, with greater concentrations during summer,
especially for particles over 70 nm in diameter. The variability is considerable
especially in summertime, suggesting a range of different types of air masses -
most likely boundary layer air during daytime and clear tropospheric air during
nighttime. Overall, the concentrations are smaller the higher the station is located.
All of the N
30-50
histograms at mountain sites have similarities, with almost log-
normal shapes, with clear concentration tails towards greater concentrations and
similar seasonal cycles. The winter conditions, probably more representative of the
free troposphere, were characterized by lower concentrations. The summertime
histograms show greatest concentrations, probably due to the planetary boundary
layer and/or valley winds influence. The greatest concentrations were generally
observed during daytime. The median concentrations of N
30-50
varied between
79 cm
3
for Jungfraujoch station in Switzerland (JFJ, altitude 3,580 m) and
418 cm
3
for Schauisland (SSL, altitude 1,210 m).
For the N
50
and N
100
particles, in most mountain sites the greatest concentrations
were observed during daytime summer and autumn. This daytime effect could be
connected with air masses arriving from lower altitudes bringing more polluted air
from below [
24
]. The concentrations are all strongly skewed towards lower
concentrations. The lowest concentration tails of the distributions are probably
indicative of concentrations of the free tropospheric air. The concentrations of the
stations with highest altitude had a more pronounced clean mode with N100
concentrations below 100 cm
3
.
