Speaker
Prof.
Alexandra IOANNIDOU
(Aristotle University of Thessaloniki, Physics Department, Nuclear Physics & Elementary Particle Physics Division, Thessaloniki 54124, Greece)
Description
The objective of this study is to define the time-lag between the elevation of tropopause and the concentration of 7Be in near surface air. The concentration of 7Be at surface air has been determined in the region of Thessaloniki, Greece at 40°N, over 52 weekly measurements covering the year 2009, a year of a deep solar minimum and of maximum concentration of 7Be, where any fluctuation due to meteorological and seasonal variations are easily revealed.
Sampling of 7Be aerosols was carried out by Staplex high-volume air sampler with glass-fiber filters, and a regulated airflow rate of 1.7-1.92 m3 min-1 (60-68 ft3 min-1). The length of each collection period was 24 h. All samples were measured for 7Be activity using an HPGe detector (42% relative efficiency). The tropopause height time series of daily values was obtained from the NCEP/NCAR Reanalysis data.
The positive correlations between the monthly activity concentrations of 7Be and the tropopause height (0.94, p<0.0001) as well as between 7Be concentrations and the temperature T (°C) (R=0.97, p<0.001), confirm that the increased rate of vertical transport within the troposphere, has as a result the descent to surface of air masses enriched in 7Be. However, the 7Be concentration levels in near surface air are not expected to respond immediately to the change of elevation of the tropopause. The calculation of time lag is the next step in understanding the atmospheric procedures behind the correlation. This step will also reveal information about radioactive aerosols movement in the atmosphere’s transfer cycles.
The time lag was found by holding the same column of data for 7Be and calculating 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 waiting for the best correlation in order to find how many days we have to wait until the concentration of 7Be responds to the elevation of the tropopause height.
The correlation coefficient (R) between 7Be and the tropopause height was successively calculated for different time lags starting with lag equal to zero. The analysis of the daily data revealed that the time delay between the elevation of tropopause and 7Be concentrations in surface air is about 3days. The most striking feature of the data is the four day plateau (including the day of the measurement and the three previous days) revealing persistence in the state of the atmosphere.
In order to test our results and confirm that the correlation depends on the successive waves of air masses of a four day period a cumulative index should be calculated.
The results improved as the correlation coefficient hits a peak in the first calculation at 0.47 and 0.46 in the second one. This indicates that the time lag is 2 to 3 days as the first two calculations refer to the correlation between the 7Be concentrations and 4day average (3 previous days and the day we measured 7Be) measurements of tropopause heights. .
Primary author
Prof.
Alexandra IOANNIDOU
(Aristotle University of Thessaloniki, Physics Department, Nuclear Physics & Elementary Particle Physics Division, Thessaloniki 54124, Greece)
Co-authors
Mr
ALEXANDROS VASILEIADIS
(Aristotle University of Thessaloniki, Physics Department, Nuclear Physics & Elementary Particle Physics Division, Thessaloniki 54124, Greece)
Prof.
DIMITRIOS MELAS
(Aristotle University of Thessaloniki, Physics Department, Applied and Environmental Physics Division, Laboratory of Atmospheric Physics, 54124 Thessaloniki, Greece)
