Aerosol deposition velocities on the Pacific and Atlantic oceans calculated from 7Be measurements

Abstract

The concentrations of 7Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface. 7Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation 7Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of 7Be surface water concentrations, 7Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of 7Be across the ocean surface was calculated to be 0.027 atoms cm −2 s −1 which is probably not significantly greater than the worldwide average 7Be flux across land and ocean surfaces of 0.022 atoms cm −2 s −1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s −1. This value may be 10–50% too low, since it was calculated using atmospheric 7Be concentrations which were measured at continental stations. Measurements of atmospheric 7Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.