On concentration of $^{42}$Ar in the Earth's atmosphere

Data from the DBA liquid argon ionization chamber experiment have been used to obtain an estimate on the concentration of $^{42}$Ar in the Earth's atmosphere, $6.8^{+1.7}_{-3.2}\cdot10^{-21}$ atoms of $^{42}$Ar per atom of $^{40}$Ar corresponding to the $^{42}$Ar activity of $1.2^{+0.3}_{-0.5}$ $\mu$Bq per cubic meter of air.