Chemistry Reference
In-Depth Information
downtown Athens (3 km from the city centre) and is influenced by traffic emissions.
High spatial homogeneity has been observed for all three ions at these two locations
as indicated by the significant correlations with slopes close to 1, denoting rela-
tively small contribution from local sources. Indeed for the PM
1
fraction, the
correlations (r) between LYK and GOU were 0.93 for SO
4
2
and NH
4
+
with
slope of 1.1, while 0.81 for NO
3
with slope of 1.0. For PM
10
the correlations (r)
were 0.94 for SO
4
2
with slope of 0.9 and 0.81 with slope of 0.9 for NO
3
.
The temporal variability for each of the main ions is also studied and compared
to a regional background site (Finokalia; FKL) to assess the importance of regional
and local sources.
Non-sea-salt sulphate (nss-SO
4
2
): nss-SO
4
2
presents a prominent peak in
winter (only in GAA) and summer (in all sites; Fig.
6a, b
). The summer peak
could be related to enhanced photochemistry, lack of precipitation, low air mass
renovation at regional scale or the increment of the summer mixing layer depth
favouring the regional mixing of polluted air masses [
58
]. Note also that during
summer air masses are almost exclusively originating from Central/Eastern Europe
which bring higher levels of SO
2
than the rest of the wind sectors [
7
,
59
]. The
secondary maxima of SO
4
2
concentration commonly recorded during the winter
could concur with the anticyclonic pollution episodes as also indicated by the high
nitrate levels [
5
,
6
].
In the case of nss-SO
4
2
there is a clear decreasing gradient from urban to
suburban and natural sites (5.3, 5.0 and 4.2
gm
3
in PM
1
for GOU, LYK and
FKL, respectively). In addition a clear seasonal variation is observed in the GAA/
FKL ratio regarding nss-SO
4
2
. Indeed during the warm season the GAA/FKL
nss-SO
4
2
ratio ranges between 1.1 and 1.25, indicating that sulphur levels above
Greece are largely controlled by long-range transport and processes evolving at a
large spatial scale. On the other hand, during winter the GAA/FKL nss-SO
4
2
ratio
significantly increases ranging from 1.4 to 1.5. This behaviour indicates significant
contribution from local anthropogenic sources (combustion of sulphur-rich diesel
for domestic heating) within the GAA during the cold-season.
Nitrate (NO
3
): As expected higher contributions of nitrate are found at the
urban and suburban sites compared to the background site due to the presence of
local sources of NO
x
in conjunction with thermodynamic conditions producing
stable ammonium nitrate (PM
10
: 2.7, 2.5 and 1.7
m
gm
3
;PM
1
: 0.8, 0.8 and
m
gm
3
, for LYK, GOU and FKL, respectively; Fig.
6c, d
). In the GAA,
NO
3
presents strong seasonal variability in both PM
1
and PM
10
, with higher
values during colder months, which, as in the case of SO
4
2
,
are likely to originate
from local pollution sources and especially vehicular traffic. On the contrary, no
clear seasonal trend is observed at FKL. The summer minimum of NO
3
in the
GAA, which is more prominent in the fine mode, is due to instable ammonium
nitrate formation during that period [
68
,
69
]. No ammonium nitrate formation
occurs during the warm season due to high temperature in agreement with
Eleftheriadis et al. [
15
].
Nitrate (NO
3
) behaves differently at FKL and the GAA. At FKL on average,
about 94% of particulate nitrate (NO
3
) is associated with coarse particles, strongly
0.1
m
