A study on aerosol residence time in El-Minia, Egypt

J Aem.~ol Set

"vbl.31, Suppl. I, pp. $47f~$471,2000

Pergamon www.elsevier.com/locate/jaerosci

Poster Session I. Radioactive aerosols

A STUDY ON AEROSOL RESIDENCE TIME IN EL-MINIA, EGYPT A. A. AHMED, A.MOHAMMED, A. A. ALl. A. EL-HUSSEIN and A. BARAKAT Physics Department, Faculty of Science, EL-Minia University., EL-Minia, Egypt. keywords: RESIDENCE TIME, AEROSOL, ACTIVITY CONCENTRATION

INTRODUCTION As long as the aerosol particles (ofa given size and chemistry) are injected into, or formed in the atmospheric air they can interact with physical (e.g. radiative) and chemical atmospheric properties (Bo L.B.Wiman. 1990). These interactions depend on the particle number concentration and their residence time. The residence time of aerosol particles in the atmosphere is a function of various removal processes. Also, there could be variation of the removal rates at different continental locations of the globe due to change in meteorological conditions (C.Papastefanou, 1991). Therefore, the aim of the present work is to study the residence time and its correlation to the meteorological parameters.

EXPERIMENTAL METHOD Aerosol samples were collected on a glass fiber filter (5 cm diameter) at a flow rate 25 m 3 / h. The collection was performed at a height of about 20 m on ground level. About 130 aerosol samples were collected from Jan.99 up to Dec.99. After air sampling, the filter was compressed into a plastic cover and the gamma activities of 2~°pb and 2~4pbwere detected by using a high-purity Germanium detector. For both radionuclides, the activities were detected for 24-hours. The time difference between the end of sampling to the start of counting was taken into consideration. The residence time has been calculated according to the formula which given by H. W. Gaggelex. 1995. Wr

=

~' -I Pb-210 (A(pb-210) / A(Pb214) )

Where Tr: The residence time. A: The activity concentration (Bq/m 3) : The decay constant of Pb-210 sec -~ RESULTS Fig (1) shows the relation between mean residence time (MRT) and relative humidity (R.H.) for each month during the year. It is clear from this Fig. that the residence time increases as the RH. decreases and vice verse. This may be attributed as increasing of RH. increases the probability of condensation and growth of the aerosol particles by coagulation, which in term increases the deposition velocity of the aerosol particles i.e. reduces the residence time values. The relation between (MRT) and the Temperature for each month during the year is represented in Fig. (2). It can be seen that there are inversely proportional between the residence time and temperature. This can be referred to the change in convection of the air parcels.

$470

Abstracts of the 2000 EuropeanAerosol Conference

$471

Because the measurements were carried out under different meteorological conditions, considerable fluctuations of the MRT were observed. The obtained values within the whole year fall in the range 4.3 - 12.8 days with an arithmetic mean of 9.83 _+ 3.1 days. Because of the absence of wet deposition, this value of MRT for the whole year is quality higher than the published data 8.2 days (Papastefanou. 1991) and 6 days (Gaggeler H, 1995). Our values suggest that, for the same location the aerosol particles may display different values of the residence time. :~Tr

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Figure (2): The relation between the MRT and Temperature. REFERENCES

Bo IB.Wiman, H. Michael Unsworth, E. Steven lindberg, Bo Bergkvist, Ruprecht Jaenicke and Hans-Christen Hanssan, (1990). Perspectives on Aerosol Deposition to Natural Surface: Interactions Between Aerosol Residence Times. Removal Processes. The Biosphere and Global Environmental Change, J.Aerosol Science 21, 313. Gaggeler H.W.. D.T. Jost, U. Baltensperger and M.Schwikowski (1995)• Radon and Thorn decay Product and "l°pb measurements at Jungfrujoch, Switzerland, Atmospheric Environment 29, 607. Papastefanou C. and EA. Bondietti (1991). Mean Residence Times of the Atmospheric aerosol in the boundary, layer as determined from Z~°Bi / 21°Pb activity' Ratios, J. Aerosol Science22. 927.