Enhanced bromine levels in Alpine ice cores

Bromine is a minor compound in our atmosphere, but it is an effective agent in partially destroying ozone in the lower layers of the atmosphere. In this way, along with iodine and chlorine, it partially compensates for the ozone pollution caused by the increasing emissions of nitrogen oxides and hydrocarbons from engines. Simulations of tropospheric ozone and its evolution since the pre-industrial period therefore require knowledge of past bromine emissions. Measurements of bromine compounds in the atmosphere, when they exist, document at best the last decades.

An international study of Mont Blanc ice has identified an anthropogenic bromine trend and examined its origin. The Mont Blanc ice shows first of all a bromine pollution associated with lead pollution. This results from the addition of dibromoethane (BrCH2CH2Br) to leaded petrol to prevent the lead oxides produced from fouling the engines. This addition leads to an emission of bromine in aerosol form (PbBrCl), which is clearly visible in the Mont Blanc ice, with a marked increase from 1955 to 1975, then a rapid decrease following the disappearance of the additives in the petrol. But this is not the only cause of the increase in bromine in the Mont Blanc ice, which also indicates a more recent increase unrelated to leaded petrol. Indeed, in the 1980s, other anthropogenic sources of bromine became dominant : emissions of bromomethane (CH3Br), used as a soil sterilisation agent, and halons such as halon 1301 (CF3Br), a fire extinguishing agent also used as a refrigerant gas.

These two components of anthropogenic bromine have very different environmental effects. Like all lead-containing particulate compounds, PbBrCl emitted from the combustion of leaded gasoline is classified as probably carcinogenic to humans but is not very reactive with ozone. In contrast, CH3Br and halons have a significant potential to destroy stratospheric ozone. Their degradation in the stratosphere forms highly reactive bromine compounds which, when redistributed in the troposphere, contribute to the destruction of tropospheric ozone. The 40% increase in these non-leaded gasoline compounds in the 1980s tells us that the Mont Blanc ice is in good agreement with simulations of reactive bromine resulting from anthropogenic bromocarbon emissions, which are used in models of tropospheric ozone evolution.

Article initialy published by INSU.

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►Legrand, M., McConnell, J. R., Preunkert, S., Chellman, N. J. and Arienzo, M. : Causes of enhanced bromine levels in Alpine ice cores during the 20 century : Implications for bromine in the free European troposphere. – Journal of Geophysical Research. DOI : https://doi.org/10.1029/2020JD034246

Local scientific contact

Susanne Preunkert, IGE / OSUG