Bromine free radicals are emerging as important players in the photochemistry of the lower atmosphere, at least on local scales. Earlier this month, Hebestre-it et al. described the discovery of air laden with bromine monoxide (BrO) wafting off the salt pans near the Dead Sea. And on page 338 of this issue, McElroy and colleagues report observations of BrO, obtained by remote measurements in the Arctic, that suggest bromine is present between the tropopause (at about 8 km height) and the top ofthe planetary boundary layer (～1 km). The planetary boundary layer is unlike the free troposphere that lies above it because the turbulence mixing timescale is very fast, and so exchange of gases with the surface occurs quickly (less than 1 day).
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