Vertical distribution of BrO in the boundary layer at the Dead Sea

Abstract

Environmental context Reactive halogen species affect chemical processes in the troposphere in many ways. The reactive bromine species bromine monoxide (BrO) is found in high concentrations at the Dead Sea, but processes for its formation and its spatial distribution are largely unknown. Information on the vertical distribution of BrO at the Dead Sea obtained in this work may give insight into the processes leading to BrO release and its consequences. Abstract We present results of multi-axis differential optical absorption spectroscopy (MAX‐DOAS) and long‐path DOAS (LP‐DOAS) measurements from two measurement campaigns at the Dead Sea in 2002 and 2012. The special patterns of its dynamics and topography in combination with the high salt and especially bromide content of its water lead to the particular large atmospheric abundances of more than 100 ppt BrO close to the ground and in several hundred meters above ground level. We conclude that vertical transport barriers induced by the special dynamics in the Dead Sea Valley lead to an accumulation of aerosol and reactive bromine species. This occurs in situations of weak synoptic winds and of mountain induced thermal circulations. Thus BrO release strongly depends on the topography and local and meso-scale meteorology. In case of strong zonal winds, the Dead Sea valley is flushed and high BrO levels cannot accumulate. NO2 levels below 1–2 ppb seem to be a prerequisite for a high BrO production. We assume that at least a part of the missing NO2 might be converted to BrONO2 leading to a deposition of nitrate within the aerosol and acting as a reservoir for reactive bromine. From these measurements, it was possible for the first time to simultaneously retrieve vertical profiles of aerosols, BrO and NO2 and gain also information on the distribution at the Dead Sea, allowing for a thorough characterization of the chemical processes leading to halogen release in the context of the special atmospheric dynamics in the Dead Sea Valley.