Untersuchung der Photooxidantien Wasserstoffperoxid, Methylhydroperoxid und Formaldehyd in der Troposphäre der Antarktis = Investigation of the photooxidants hydrogen peroxide, methylhydroperoxide and formaldehyde in the troposphere of Antarctica

Abstract

Photooxidants play an important role for the oxidation capacity of the troposphere. The presented work deals with seasonal changes of atmospheric photooxidants like hydrogen peroxide (H2O2), methylhydroperoxide (MHP) and formaldehyde (HCHO). The principal purpose was to follow and quantify the mixing-ratios of these photochemical species for the first time over a complete annual cycle in Antarctica. These investigations contribute to a better understanding of the photochemistry of these trace gases in the clean polar troposphere. During two field campaigns at the German Antarctic station Neumayer (overwintering campaign from January 1997 until March 1998 and a shorter campaign from January to March 1999) continuous measurements of atmospheric mixing-ratios were performed. Two commercially available analysers were modified to withstand Antarctic field conditions and integrated into the air-chemistry observatory. A continuous wetchemical technique was used for peroxides and formaldehyde sampling and analysis. Species were quantified by fluorescence detection. In order to compare the results and measurements with well-known reaction mechanisms a photochemical box model was applied. The obtained time series show a pronounced seasonal variation. In summer average values of 0.197 ppbv H2O2, 0.191 ppbv MHP and 0.360 ppbv HCHO were observed. For the first time atmospheric concentrations of H2O2, MHP and HCHO were documented during polar night in winter. At this time 0.054 ppbv H2O2, 0.089 ppbv MHP and 0.150 ppbv HCHO were detected. These mixing-ratios are significantly higher than expected due to missing actinic radiation essential for photochemical production. Trajectory analyses and calculations with a three-dimensional model showed that during winter the mixing-ratios of photooxidants are mainly affected by long-range transport in the free troposphere. An annual phenomenon in Antarctic spring is the stratospheric ozone depletion which is the cause of increased UV-B radiation as measurements at Neumayer station show. In consequence of the enhanced actinic radiation, an increase of photochemical activity was expected. However, peroxide mixing-ratios were lower during this time, whereas formaldehyde shows higher mixing-ratios compared to autumn when the sunshine duration is similar. Apparently, the increase of UV-B radiation during the “ozone hole period” does not show any significant impact on the photooxidants. From the obtained year-round time series periods could be extracted, in which photooxidants showed a pronounced diurnal variation. In contrast to recent publications maximum hydrogen peroxide mixing-ratios occurred at night and highest formaldehyde mixing-ratios in the early afternoon. Model calculations provide evidence that photochemistry alone can not explain the observed diurnal cycles. Interactions with the snow surface, like adsorption of photooxidants on snow crystals or the incorporation into snow crystals by co-condensation may probably be responsible for the observed diurnal cycles. The findings of this work contribute to a better understanding of the photochemistry of the investigated trace gases in the clean troposphere of Antarctica and provide a useful data record for further photochemical modeling.