Seasonal variations of atmospheric dimethylsulfide, dimethylsulfoxide, sulfur dioxide, methanesulfonate, and non‐sea‐salt sulfate aerosols at Dumont d'Urville (coastal Antarctica) (December 1998 to July 1999)

Abstract

A study of oxidation products of dimethylsulfide (DMS) was conducted at Dumont d'Urville (DDU), coastal Antarctica, from summer 1998/1999 to midwinter 1999. The study involved multiple daily measurements of DMS and dimethylsulfoxide (DMSO) using a gas chromatograph at the site. Methanesulfonate (MSA), and non‐sea‐salt sulfate (nssSO42−) aerosols were studied on a daily basis. A few studies of size‐segregated aerosol chemistry indicate that both MSA and nssSO42− are present in a dominant submicronic mode at 0.3 μm. MSA and nssSO42− levels exhibit a well‐marked seasonal cycle characterized by summer maxima (0.6±0.3 and 3.8±1.4 nmol m−3 of MSA and nssSO42− in January 1999, respectively) and winter minima (0.01±0.004 and 0.21±0.22 nmol m−3 of MSA and nssSO42− in July, respectively). In contrast, weak seasonal cycles of DMS (from 13.1±6.1 nmol m−3 in January to 3.9±1.3 nmol m−3 in July) and DMSO (from 0.15±0.1 nmol m−3 in January to 0.06±0.01 nmol m−3 in July) are observed there. A few SO2 samplings indicate a seasonal cycle with 2.8±0.9 nmol m−3 in January and levels close to the detection limit (0.25 nmol m−3) in winter. The major finding of this study is the presence of large amounts of DMS and DMSO in winter, whereas MSA levels are strongly decreased. These winter DMS levels may be due to small DMS emissions from open water present in sea ice located offshore DDU and/or advection from further north in conjunction with a long lifetime of DMS. The hypothesis of a heterogeneous uptake of DMSO onto aerosols followed by a rapid oxidation into MSA could explain the seasonal DMSO and MSA changes. With respect to the summer situation, in winter, DMSO levels of a tenths of nmol m−3 would result from transport of air masses located further north associated with a lifetime of DMSO of 2 days (instead of a few hours in summer) and a local production of DMSO from DMS oxidation. Such a winter DMSO production in spite of decreased DMS/OH addition pathway (50 times slower than in summer) results from decreased heterogeneous uptake (30 times slower) partly driven by reduction of available aerosol surface by a factor of 15. Finally, when katabatic regime took place bringing air from inland Antarctica, it is shown that the free troposphere above the Antarctic plateau in summer is enriched in DMSO and MSA with respect to DMS and nssSO42−, respectively. That supports the assumption of a different chemistry of DMS taking place in the free troposphere over Antarctica due to dry air conditions and absence of high aerosol levels.