Depositional Fluxes Of 7Be Research Articles

Atmospheric deposition of cosmogenic 7Be and 137Cs from fallout of the Chernobyl accident

Atmospheric (tropospheric) depositional fluxes of the naturally occurring 7Be of cosmogenic origin and 137Cs from fallout of the Chernobyl accident were measured over a 7-year period (January 1987–December 1993) at Thessaloniki, Greece (40° 38′N, 22° 58′E). The annual total deposition fluxes of 7Be varied between 854 Bq/m 2 (1987) and 1242 Bq/m 2 (1992), showing a minimum in the years 1988–1989. The annual total deposition fluxes of 137Cs varied between 183 Bq/m 2 (1987) and 16.4 Bq/m 2 (1992), showing a significant decrease as expected for natural removal and radioactive decay and no new releases from nuclear facilities or weapons testing. The annual average total deposition velocity for 7Be was from 0.3 cm/s (1988) up to 0.8 cm/s (1991), while for 137Cs the corresponding values were much higher, hence 137Cs was associated with larger atmospheric particles. High 7Be concentrations in air were related to the very little solar activity (1987–1988 and 1993–1994), while low 7Be concentrations in air related to the high solar activity (1989–1991). Maximum 137Cs concentrations in air were registered during the spring 1991 and 1992, reflecting some stratospheric inputs. An unusual highly elevated value of 137Cs concentration in air, reaching 0.25 mBg/m 3, was observed during the summer 1990.

A search for the seasonal variability on the depositional fluxes of 7Be and 210Pb

Investigations of the atmospheric fallout of 7Be and 210Pb in many places around the globe indicate that there are seasonal variations on the depositional fluxes of these nuclides: high in winter in a few places, while in most places the high occurs in spring. However, these earlier studies did not address quantitatively the corresponding seasonal variations of the amount of precipitation during these periods. Data on the depositional fluxes of 7Be and 210Pb in Galveston, Texas, during the years 1989–1991 indicate that the seasonal variation is not uniform from year to year, and the amount of precipitation primarily controls the depositional fluxes. All the recently published data on the depositional fluxes of these radionuclides to date are synthesized. Also, the seasonal variations of the depositional fluxes of 7Be and 210Pb due to the variations in the amounts of precipitation during these seasons are discussed. This study clearly shows that the depositional fluxes of 7Be and 210Pb during fall were minimum, and most of the time the fallout maxima were confined to either spring and/or summer. Interseasonal variations of the depositional fluxes of these nuclides do not remain the same. The data presented in this paper in conjunction with earlier published data seem to indicate that the observed seasonal increase in the depositional fluxes of 7Be during spring is not due to the troposphere‐stratosphere exchange of air masses during that season. The 7Be/ 210Pb activity ratio values remain the highest in summer and this is attributed to the increased rate of vertical transport of 7Be (from the upper troposphere to the middle and lower troposphere) and 210Pb (from the lower troposphere to upper troposphere) due to decreased stability of the troposphere during summer months.

Depositional fluxes and other physical characteristics of atmospheric beryllium-7 in the temperate zones (40°N) with a dry (precipitation-free) climate

Abstract Atmospheric depositional fluxes of 7Be were measured over a 3-year period (January 1987–April 1990) in Thessaloniki, Greece (40°38′N, 22°58′E). Total precipitation accumulation during 1987, 1988 and 1989 was 65.0, 46.2 and 32.6 cm, respectively, reflecting a relatively dry (precipitation-free) climate. The volume-weighted concentrations of 7Be in rainwater ranged between 0.1 and 5.2 Bq l −1 (average 0.6 Bq l −1). The annual total deposition fluxes of 7Be in 1987, 1988 and 1989 were 841, 510 and 483 Bq m−2, respectively, while the average concentrations of 7Be in the atmosphere in the corresponding periods were 6.3, 5.7 and 4.2 mBq m−3, showing a decreasing trend with time. On an annual average, the washout (scavenging) ratio for 7Be was 144 and the resuspension factor was 2.5 × 10−4 m−1, the latter is too high. The annual average total deposition velocity for 7Be was estimated to be 0.4 cm s−1. The air-to-vegetation transfer rate estimate for 7Be is 5150 m3 kg−1. Seasonal variations of 7Be concentration depend considerably on the local meteorological conditions, mainly the rainfalls.