Shipboard measurements of trace gases, noble gases and dimethyl sulfide, in the Antarctic seawater made by membrane inlet mass spectrometry

Abstract

Research background and objectives The light noble gases, helium (He) and neon (Ne), dissolved in seawater, can be useful tracers of freshwater input from glacial melting because the dissolution of air bubbles trapped in glacial ice results in an approximately ten-fold supersaturation. However, the conventional method to determine noble gases is dependent on shore-based mass spectrometric system and very time-consuming processes. The other volatile trace gas, Dimethyl sulfide (DMS), as important precursor of cloud condensation nuclei (CCN), directly affects the radioactive budget and resultant climate change. The oceanic emission is a main natural source of atmospheric DMS. Antarctic polynya, especially, can be a key source region of DMS due to high productivity. However, its observations are evidently lacked due to the limited accessibility of polar ocean. Membrane inlet mass spectrometry (MIMS) technique directly samples analyte gases from the aqueous phase gases in seawater through a semi-permeable membrane. Since this method does not require headspace equilibration, MIMS enables us to make a near-real time, high frequency continuous observation of dissolved gases. During the Southern Ocean cruise (Jan. ~ Feb. 2016), we applied the MIMS for onboard measurements of various dissolved gases in Antarctic seawater on the Korean icebreaker R/V Araon. We have attempted i) to make onboard analysis of 5 noble gases (4He, 22Ne, 38Ar, 84Kr, and 132,136Xe) in seawater, for the first time, to get a broader understandings of glacial meltwater distribution in the Southern Ocean. With discrete measurements of noble gases, we have also made ii) continuous DMS observations. Here, we will present the detailed methods and preliminary results of noble gases and DMS measurments as a beginning of our effort to understand the climate-related glacial melting and environment changes in polar oceans.

Method in brief The details of noble gas analysis method are as follows: we made the quicker and more efficient noble gas extraction system. This, so-called noble gas MIMS (NG-MIMS) system has a simplified version of traditional gas extraction lines. This system consists of a membrane inlet (0.2 mm silicon wall), water vapor trap immersed into cooled MeOH (~ -65 oC), CO2 trap (Carbosorb), 400 oC of hot getter (Zr-alloy), and 250 oC of Sorb AC. Two syringe pumps were used to deliver samples and standards. The high vacuum of the whole line (~10-7 torr) allows the dissolved gases to pass through the membrane from the water. To test the linearity of this method, we prepared a range of binary mixtures of air-equilibrated seawater (AEW) and degassed (boiled) water by changing the each flow rates. The AEW was prepared by gently bubbling of ambient air through seawater kept in a water bath at 2 oC. The 2 end-members (degassed water and AEW) and their mixtures showed good linearity (n=7 trials) with R2 value of 0.97 ？ 0.99 for 4He, 38Ar, and 132Kr.