Depletion of lower tropospheric ozone during Arctic spring: The Polar Sunrise Experiment 1988

Abstract

Results are presented from the 2‐month Polar Sunrise Experiment 1988, which was undertaken to further investigate the cause of ozone destruction during spring in the lower Arctic atmosphere. A strong anticorrelation between the decrease in ozone and concurrent increase in bromine compounds collected on filters observed during a 21‐day period in 1986 was confirmed. It is shown that the reason for this observation is a meteorological modulation which alternately brings to the sampling location lower boundary layer air, depleted in ozone and enriched in filterable bromine, and free tropospheric air with abundant ozone and few bromine compounds. Several other compounds that may have a bearing on the chemical reactions leading to ozone depletion were measured. Bromoform, a potential source for Br atoms, was found to be present at levels between 1 and 10 parts per trillion by volume (pptv); good evidence was obtained that, as with filterable bromine, it had increased in ozone depleted boundary layer air. The mean NO2 level was 85 pptv with an estimated uncertainty of a factor less than 2. However, this measurement of NO2 may have been the sum of NO2+N2O5. The possibility that Br atoms are formed from an N2O5+NaBr interaction is therefore not ruled out. There was a good indication that apparent NO2 was depleted during episodes of low ozone. An upper limit for the mixing ratio of formaldehyde was found to be 39 pptv, while acetaldehyde was observed at mixing ratios of ca. 65 pptv. Ethylene and acetylene were also found to be depleted concurrently with O3. These compounds do react with Br radicals in air (as do the aldehydes) and it is speculated that they may create a link with HOx. chemistry. Data on several other chemical compounds are also presented. Their participation in the O3 depletion chemistry is less clear. The implications of the measurements with respect to the hypothesis that the ozone depletion are due to a BrOx‐O3 destruction cycle are explored.