Environmental Engineering Reference
In-Depth Information
2.4. Backwards trajectories
Origins of air parcels that arrived at the two receptor points (Sites 1 and 2) have
been identified using the two-dimensional backwards trajectories downloaded
from http://www.emep.int. Backwards trajectories are calculated by tracking an air
parcel every 2 h for 96 h backwards in time, four times per day (at 00, 06, 12 and
18 h UTC). The trajectory calculation was based on the wind fields at 925 hPa of
PARLAM-PS model at 50 × 50 km
2
.
2.5. Mud rain event detection
Mud rains associated with Saharan dust intrusions over Croatia were identified on
the basis of daily data of AI. When the points closest to the both sampling sites
simultaneously had AI > 0.7 then the daily maps of AI and backwards trajectories
were investigated for the possible Saharan source. In the end precipitation
chemistry analysis was carried out.
3. Results
Mud rains occurred in 22 dust outbreaks during the period 2001-2005. The majority
of them (17) were between April and July. In all cases calcium concentration was
higher then annual volume weighted average (VWA) and pH was greater than 5.
Trend analyzes of calcium concentrations VWA showed increasing trends for
both, all precipitation samples and samples without the mud rain.
Compared to Site 2, the mud rains at Site 1 were more frequent, and they were
accompanied with higher calcium concentrations and pH values.
Considering entire investigated period, the most intensive episode with highest
AI occurred in April 2002
. Figure 1
shows Saharan dust transport over Mediterranean
towards Croatia on 12 April 2002. The backwards trajectories
(Fig. 1c, d)
for Site
1 and 2 also point to Saharan area as a source region.
4. Conclusions
The influence of mud rains on VWA concentrations of calcium in precipitation in
Croatia was negligible. This is in accordance with previous research where
significant increasing trends of the annual VWA of calcium in precipitation at
Croatian sampling sites was associated with the post-war recovery. The spatial
changes of mud rain chemistry in Croatia were influenced by topography and
distance from Sahara.
