Available via license: CC BY-NC-ND 4.0
Content may be subject to copyright.
HNPS Proceedings
Vol. 24, 2016
Time lag between the tropopause height and the
levels of 7Be concentrations in surface air
Ioannidou E. Department of Physics,
Aristotle University of Athens
Leppänen A.-P. Radiation and Nuclear
Safety Authority
Melas D. Department of Physics,
Aristotle University of
Thessaloniki
Ioannidou A. Department of Physics,
Aristotle University of
Thessaloniki
http://dx.doi.org/10.12681/hnps.1851
Copyright © 2016 E. Ioannidou, A.-P. Leppänen, D. Melas, A.
Ioannidou
To cite this article:
Ioannidou, Leppänen, Melas, & Ioannidou (2016). Time lag between the tropopause height and the levels of 7Be
concentrations in surface air. HNPS Proceedings, 24, 102-107.
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
Time lag between the tropopause height and the levels of 7Be
concentrations in surface air
E. Ioannidou1,*, A. –P. Leppänen2, D. Melas3, A. Ioannidou1
1 Physics Department, Nuclear Physics Lab., Aristotle University of Thessaloniki, Thessaloniki
54124, Greece
2 Regional Laboratory in Northern Finland, Radiation and Nuclear Safety Authority – STUK, 96400
Rovaniemi, Finland
3 Physics Department, Atmospheric Physics Lab., Aristotle University of Thessaloniki, Thessaloniki
54124, Greece
___________________________________________________________________________
Abstract The concentration of 7Be at near surface air has been determined over 2009, which
was a year of a deep solar minimum, at three different locations in Finland: Ivalo (68°64’N,
27°57’E), Rovaniemi (66°51’N, 25°68’E) and Kotka (60°48’N, 26°92’E). In geomagnetic
latitudes over λ = 60° N, the elevation of tropopause during the warm summer months and the
vertical exchange of air masses within the troposphere cause greater mixture of the air masses
resulting in higher concentration levels for 7Be in surface air. However, different climatic
phenomena, such as air masses from the East, make the correlation between the monthly
activity concentrations of 7Be and the tropopause height fairly weak. For Ivalo and Rovaniemi
it was found that changes in the daily surface concentrations of 7Be lag the changes in the
elevation of the tropopause by four days. In Kotka, the correlation is weakest.
Keywords 7Be, atmosphere, tropopause, Solar cycle, correlation coefficient
___________________________________________________________________________
INTRODUCTION
Beryllium – 7 (t1/2 = 53.3 d) is a cosmic – ray produced radionuclide, which is formed in the
upper troposphere and lower stratosphere by spallation reactions of light atmospheric nuclei.
Its flux to the Earth’s surface varies with the 11 – year solar cycle and has a latitudinal
dependence with higher values around the magnetic poles and lower values in the equatorial
region.
Besides the latitude, the cosmic ray flux and therefore the production rate of cosmogenic
nuclides depends on the altitude. The production rate begins to increase at the top of the
atmosphere, reaches a maximum at about 20 km in the stratosphere and finally decreases
gradually down to the Earth’s surface [Masarik and Beer, 1999]. The combined effects of
high 7Be production rates in the stratosphere (about 70%; [Lal and Peters, 1967]) and the
relatively rapid removal of aerosol – associated species from the troposphere, produce
stratospheric 7Be concentrations about an order of magnitude higher than those just below the
tropopause [Bhandari et.al., 1966]. Because of the thermal structure of the stratosphere and
its separation from the troposphere by the tropopause, the residence time of aerosols in the
stratosphere is substantially longer (about 1 – 2 years) than in the troposphere, where is in the
* Ioannidou Eleftheria, email: eleioann@physics.auth.gr
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
order of week [Papastefanou and Ioannidou, 1995]. Stratosphere serves as a reservoir of 7Be
–rich air injected into the troposphere via the global – scale Brewer – Dobson circulation
[Holton et al., 1995] or during stratosphere to troposphere exchange events [Feely et al.,
(1989), Zanis et al., (1999)]. The concentrations of 7Be in the troposphere and near the
ground level, show variations which are connected with exchange of air between the
stratosphere and the troposphere in situation of tropopause folding events.
The tropopause marks the boundary between the troposphere and stratosphere, and a
fundamental characteristic of the tropopause is a change in static stability (temperature lapse
rate) across the interface. The WMO [World Meteorological Organization, 1957] definition
of the tropopause is based on lapse rate criteria (decrease of temperature with height becomes
less than 2° C/km), although the tropopause can also be defined by more general stability
criteria, quantified by potential vorticity (PV) [Hoerling et al., 1991].
In the tropics the tropopause is relatively high (~16 km), reflecting a transition between
radiative – convective balance in the troposphere and radiative balance in the stratosphere
[Thuburn and Craig, 2002]. The tropopause in the extratropics is lower ( 8 – 12 km), with an
equilibrium structure determined by baroclinic wave dynamics [Held, (1982), Haynes et al.,
(2001), Schneider, (2004)]. The extratropical tropopause is characterized by large dynamic
variability, often with complex spatial structure (such as three – dimensional folds, e.g.
[Bithel et al., (1999), Nielsen-Gammon, (2001)]). There is a well – marked ‘’tropopause
gap’’ or break where the tropical and polar tropopauses overlap at 30° - 40° latitude
[Kochanski, 1955]. The break is in the region of the subtropical jet stream and is of major
importance for the transfer of air and tracers (humidity, ozone, radioactivity) between
stratosphere and troposphere. The height of the tropopause varies seasonally and also daily
with the weather systems, being higher and colder over anticyclones than over depressions.
The current study presents an analysis of 7Be data at geomagnetic latitudes over 60° N in
Finland, during the year 2009, a year of a deep solar minimum, and as a consequence a year
of maximum concentrations of 7Be in near surface air. During a year of solar minimum any
fluctuation on 7Be concentrations are unaffected by the solar modulation and the differences
in 7Be fluctuations due to meteorological and seasonal variations are becoming easily to be
revealed. The main objective of this study is to define the time – lag between the elevation of
tropopause and concentrations of 7Be in near surface air for three different regions in Finland.
INSTRUMENTATION AND METHODS
Atmospheric concentrations of Beryllium – 7 were measured by air sampling, using
Staplex high – volume air samplers with Staplex type TFAGF 810 glass – fiber filters
8’’x10’’ and having 99.28% collection efficiency for particles as small as 0.3 μm. This
design involves a regulated air – flow rate of 1.7 – 1.92 m3 min-1 (60 – 68 ft3 min-1). The
length of each collection period was one week.
After the collection procedure, the filters are folded and compressed by means of
hydraulic press at up to 3 tons to give a cylinder 5.8 cm diameter and 2 mm height. All the
samples were measured for 7Be activity (Eγ = 477 keV) using a high resolution (1.9 keV at
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
1.33 MeV) and high efficiency (42%) low – background HPGe detector. The 1σ counting
uncertainties for 7Be measurements were almost always smaller than 8%. Blank filters were
regularly checked.
Meteorological data concerning the temperature T (°C), Relative Humidity (RH%),
during sampling dates were obtained from a meteorology station at the roof of the Faculty of
Science building.
Apart for the meteorological parameters, a tropopause height time series of daily values
for the period of 7Be observations was obtained from the NCEP/NCAR Reanalysis data.
RESULTS AND DISCUSSION
The periodic pattern of mean weekly 7Be activity concentrations in near surface air over
year 2009 presents a strong seasonal variation with the highest values being observed in the
summer months and the lowest in the winter months. Also, high values were observed during
the spring period. For the area of Ivalo the largest concentration of 7Be was 5.428 mBq/m3
and was observed in January. For Rovaniemi the largest concentration was 4.3574 mBq/m3
and was observed in August and finally the highest concentration of 7Be in Kotka region was
6.93 mBq/m3 and was presented in June. Also, we applied the method of deviation from the
moving average, for the calculation of average weekly values of 7Be. (Fig. 1). The production
rate of 7Be depends on the flow of cosmic radiation. Furthermore, the concentrations of 7Be
are known that are formed by large – scale atmospheric phenomena (such as NAO and
ENSO).
Fig. 1. Weekly variation of 7Be activity concentrations for the three areas.
According to our knowledge, in mid latitudes there is a strong positive correlation
between the seasonal changes of the tropopause height and the concentrations of 7Be in
surface air and in case of 40°N has defined a time – lag between tropopause height and 7Be
surface concentrations of 3 – 4 days [Ioannidou et al., 2014]. For higher latitudes, the
elevation of tropopause in the summer months, which are warmer, along with the vertical
exchange of air masses in the troposphere, lead to higher concentrations of 7Be in surface air.
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
In this work we examine the influence of tropopause height on 7Be concentrations in
surface air at latitudes above 60°N. The concentration of 7Be at near surface air has been
determined over the year 2009, at three different locations in Finland: Ivalo (68°64’N,
27°57’E), Rovaniemi (66°51’N, 25°68’E) and Kotka (60°48’N, 26°92’E). Year 2009 was a
year of solar minimum, ie. a year of high production rate, and at the same a period when the
cosmogenic flux was stable. So it was the ideal period to study atmospheric changes and
reveal the differences in 7Be fluctuations due to any meteorological and seasonal variations.
For our analysis, the tropopause height was determined for a small shell that covers each
one region for year 2009 daily (Fig. 2, 3, 4).
The equation used in order to find the height of the tropopause is given here.
The number 1 refers to the one isobaric surface and (i) runs from 1 to 365 days of 2009.
The same is for the second surface. (Temp) and (Tropopres) refer to the temperature and
pressure of the air at the tropopause levels. (GH) and (Pr) refer to the geopotential height and
pressure of the closest isobaric levels. Holding the same column of data for 7Be we calculate
the correlation coefficient (R) for each new column of daily data of the tropopause height.
The new columns are created by going back in time with a step of one day in order to find
how many days we have to wait until the concentrations of 7Be responds to the elevation of
the tropopause height.
Fig. 2. Tropopause height for Ivalo.
Fig. 3. Tropopause height for Rovaniemi.
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
Fig. 4. Tropopause height for Kotka.
What is necessary to study and understand are the factors that affect the time delay of the
interdependence of the height of tropopause and 7Be concentrations, knowing in advance that
the 7Be located in the surface layer of the atmosphere did not immediately responds to
changes of the height of the tropopause. Analysis gave that at latitudes over 60°N the
correlation between the tropopause height and 7Be concentrations is weak.
For Ivalo and Rovaniemi it was found that changes in the daily surface concentrations of
7Be lag the changes in the elevation of the tropopause by four days (Fig. 5).
In Kotka station, the influence of tropopause height on the surface concentrations of 7Be
is the weakest (Fig. 5). In Kotka region it seems that the influence of air masses from the East
has greater influence on 7Be concentrations instead of the influence of the tropopause height.
0 2 4 6 8 10
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
Correlation Coefficient
Daily Steps
Ivalo
Rovaniemi
Kotka
Fig. 5. Day lag plot for the three locations in Finland.
CONCLUSIONS
One year of 7Be data obtained during a year of a deep solar minimum were analyzed
together with a set of meteorological parameters and tropopause height in order to define the
time – lag between the elevation of tropopause height and the 7Be concentrations in near
surface air.
7Be concentrations were found to have a distinct annual cycle with a clear maximum
during warm summer months.
In general the large fluctuations in the values of the correlation coefficients show a weak
correlation between the 7Be and tropopause height for latitudes above 60°N. The factors
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
affecting 7Be surface air concentrations in Finland are mainly of atmospheric origin and the
observed differences in 7Be concentrations in surface air are mainly caused by the different
climate/weather patterns during the time of observations [Leppänen and Paatero, 2013]. The
changes in air mass transport patterns associated with NAO (North Atlantic Oscillation) and
AMO (Atlantic Multidecadal Oscillation) were determined to be the main contributor to the
interannual variability of surface air 7Be activities in Finland [Leppänen et al., 2012].
This is the first approach of determining the data and further analysis is needed for more
accurate conclusions.
References
[1] Bhandari N. et al., Tellus 18, p. 391 (1966)
[2] Bithel M. et al., Journal of the Atmospheric Sciences 56, p. 673 (1999)
[3] Feely H. W. et al., Journal of Environmental Radioactivity 9, p. 223 (1989)
[4] Haynes P. et al., Geophysical Research Letters 28 (22), p. 4179 (2001)
[5] Held I., Journal of Atmospheric Sciences 39, p. 412 (1982)
[6] Hoerling M. et al., Journal of Monthly Weather Review 126, p. 3303 (1991)
[7] Holton J. R. et al., Review ofGeophysics 33, p. 403 (1995)
[8] Ioannidou A. et al., J. Environm. Radioactivity 129, p. 80-85 (2014)
[9] Kochanski A., Journal of Meteorology 12, p. 95 (1955)
[10] Lal D. et al., Handbuch der Physik 46, p. 551 (1967)
[11] Leppänen A-P. et al., Journal of Atmospheric and Solar-Terrestrial Physics 74, p. 164-180
(2012)
[12] Leppänen A-P. et al., Journal of Atmospheric and Solar-Terrestrial Physics 97, p. 1-10 (2013)
[13] Masarik J. et al., Journal of Geophysical Research 104 (D0) 12.099 (1999)
[14] Nielsen-Gammon J. W., Bulletin of the American Meteorological Society 82, p. 1151 (2001)
[15] Papastefanou et al., Journal of Environmental Radioactivity 26, p. 273 (1995)
[16] Schneider T., Journal of the Atmospheric Sciences 61, p. 1317 (2004)
[17] Thuburn J. et al., Journal of Geophysical Research 107 (D2) 4017 (2002)
[18] World Meteorological Organization (1957)
[19] Zanis P. et al., Tellus 51B 789 (1999)
Powered by TCPDF (www.tcpdf.org)
http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 08/03/2020 22:19:07 |
