Chemistry Reference
In-Depth Information
Of particular interest are the differences between days of the week. The so-called
“weekend effect” cycle in aerosol-weather interactions is based on observations of
day of the week-related variations of meteorological data [
25
]. These effects have
been connected to similar variations in particle mass concentrations and optical
thickness detected at measurements sites in urban and suburban and remote
locations [
26
]. The key point of the “weekend effect” is that the weekday-related
changes in anthropogenic emissions of aerosol particles could change the regional
meteorology in such an extent that the precipitation and air temperature could be
significantly affected. This could then have implications on, e.g., weather predic-
tion. The main mechanism behind this proposed weekend effect could be (semi)-
direct or indirect aerosol effects. As this could be a direct anthropogenic influence
to the short-term weather systems, such existence of such phenomena would
influence many atmospheric fields.
As discussed earlier, the indirect effects of aerosols are controlled by the number
of CCN, not by aerosol particle mass or optical properties used in many weekend-
effect publications. The datasets of CCN-sized aerosol number concentrations
in EUSAAR/ACTRIS and GUAN stations do not support indirect effects as a
major contributor to the (possible) weekend effect. Figure
10
shows distributions
(25-50-75th percentiles) of each weekday in a long time series of two stations. The
statistical tests and wavelet frequency analysis could not detect any consistent
statistically (
p
0.05) significant differences in CCN-sized number concentrations
in annual or seasonal datasets from the stations. This means that the concentrations
are not generally different in different weekdays, making a strong continent-wide
weekend effect unlikely to occur from aerosol indirect effects. The main reason for
the differences between number and mass-based weekday variation is probably a
combination of different sources and the fact that mass-based measurements mea-
sure particles with much lower lifetime [
11
]. As lower lifetime particles can be
removed efficiently from the atmosphere during the lower emission period, the
weekday signal for such aerosol properties is larger than for most CCN-sized
particles.
Even though there is no significant change in the CCN number concentrations
between days of the week, this is not yet the complete picture of potential aerosol-
cloud weekend effects. There are many other potential aerosol processes, such as
semi-direct effect, which could have significant effect on the local meteorology.
Also, the CCN number concentrations have a weak weekday variation within the
cities, which could then lead to meteorological weekend effect directly above the
urban environment. However, as the urban or semi-urban areas cover spatially quite
small area of the Europe (around 5%) [
27
], the weekend effect is then of much more
a local effect, if it exists at all.
The difference between mass and number-based metrics in weekly variation is a
good example on the short-time scale differences between the properties. One
should be careful on generalizing correlations from long timescales to small
timescales, especially near the lifetime of the particles in question.
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