8. 1 Residence times of aerosols
The radioisotopes Pb and Be are often used to determine the atmospheric
residence times of aerosols.
1. Lead-210 is produced by radioactive decay of Rn emitted from soils, and
condenses immediately on preexisting aerosol particles. The 222Rn emission flux
is 1.0 atoms cm s from land (30% of Earth’s surface) and zero from the oceans.
The only sink of 222Rn is radioactive decay (half-life 3.8 days), producing 210Pb.
Removal of 210Pb is by radioactive decay (half-life 23 years) and by aerosol
deposition. The total mass of Pb in the troposphere is estimated from
observations to be 380 g. Derive the residence time against deposition of
210Pb-carrying aerosols in the troposphere. You should find a value of 8 days.
2. Beryllium-7 is produced by cosmic rays in the stratosphere and upper
troposphere. Similarly to 210Pb, it condenses immediately on preexisting aerosol
particles. The global source of 7Be is 150 g yr ; 70% of that source is in the
stratosphere and 30% is in the troposphere. Removal of Be is by radioactive
decay (half-life of 53 days) and by aerosol deposition (in the troposphere only).
We assume that the troposphere and stratosphere are well-mixed reservoirs, that
Be is at steady state in each of these reservoirs, and that the transfer rate
constant from the stratosphere to the troposphere is k = 0.8 yr-1The total mass
of Be in the troposphere is estimated from observations to be about 3 g. Derive
the residence time against deposition of Be-carrying aerosols in the troposphere.
You should find a value of 24 days.
3. Why are the lifetimes of Pb-carrying aerosols and Be-carrying aerosols in
the troposphere so different?
4. Since most of the Rn emitted at the surface decays in the troposphere, one
might expect 21