Unexplained enhancements of CH₃Br in the Arctic and sub‐Arctic lower troposphere during TOPSE spring 2000

Elevated concentrations of methyl bromide (CH 3 Br) were observed in the Arctic atmospheric boundary layer (BL) during periods of widespread BL ozone (O 3 ) depletion episodes (ODEs: O 3 mixing ratios < 20 × 10 −9 or parts per billion by volume, ppbv) particularly during major ODEs (MODES: O 3 < 4 ppbv). No other organic gases measured during TOPSE (Tropospheric Ozone Production about the Spring Equinox) exhibited anti‐correlations with O 3 during these ODEs. Methyl bromide has both natural and anthropogenic sources and contributes ∼ half of the bromine (Br) to the stratosphere, where it can catalytically destroy O 3 . Several known CH 3 Br sources are evaluated, but the current knowledge cannot explain the observed enhancements. If the mechanism is direct gas‐phase photochemical production, a significant portion of the unknown CH 3 Br source may be found.