Composition of Br-containing aerosols and gases related to boundary layer ozone destruction in the arctic

Abstract

During the third Arctic Gas and Aerosol Sampling Program (March 1989), aircraft measurements of atmospheric gases and aerosols were performed in the European Arctic for the purpose of investigating the phenomenon of boundary layer O₃ destruction. Eight high-volume aerosol filter samples were collected in tropospheric air over the pack ice. In these sampling periods, continuous O₃ measurements were made and trace gases were collected in flasks. For all samples, total elemental bromine collected on the filters in excess of the estimated sea salt component (XSFBr) was found to anticorrelate stronly (r = −0.90) with the mean ozone concentration observed during the sampling period. These findings are similar to earlier observations at Alert and Barrow.

Air samples collected during these periods were analysed for Br-containing gases and hydrocarbons. None of these compounds were well correlated with either O₃ or XSFBr concentration over the course of the experiment. This is probably because variable conditions of local meteorology, atmospheric structure and geographic location influenced the degree to which O₃ was depleted, by affecting the size of the reaction reservoir and the source(s) of the reactants. Samples collected in the surface (∼ 50 m deep) isothermal or slightly stable layer (SSL) over pack ice and with light winds from the direction of the central Arctic showed the highest O₃ depletions. When winds were from the direction of open water, significantly higher O₃ and lower XSFBr values were observed. When the SSL was not present, samples collected below the strong inversion showed less O₃ destruction and lower XSFBr concentrations than similar low altitude samples collected within the SSL. This is consistent with the notion of a larger reservoir volume available for reaction. Gas and aerosol chemistry results were compared for two samples collected close spatially and temporally over ice north of Spitsbergen. Our data indicate that (1) CHBr₃ may be the key organobromine species supplying Br atoms and BrO radicals in a heterogeneous photochemical reaction cycle causing the photolytic destruction of O₃ in the springtime Arctic surface layers, and (2) ambient hydrocarbons (especially C₂H₂) are depleted during O₃ destruction, and may be important in the overall reaction mechanism. This catalytic O₃ depletion process was observed to occur to an extent causing near-total O₃ destruction in the SSL over a 1–2 d period. Thus, relatively rapid photochemical reactions between atmospheric Br, hydrocarbons and aerosols are suggested as driving the photolytic O₃ destruction process.