Fig. 1: Two zodiacs meet at the equator... . (photo: Martin Hieronymi)
ANT-XXVI/1, Weekly Report No. 3
3 November - 9 November 2009
We continue our cruise southwards along 23° longitude and we will almost reach 20° S by the end of this reporting week. From there Polarstern will take a south-westerly course towards the Vema channel. At first we pass the thermal equator with the Inner-Tropical Convergence Zone (ITCZ) above it, which is still fully north of the geographical equator. The highlight of this week is our rendezvous with the research vessel Meteor directly at the equator.
Like us, the Meteor steams along 23° W southwards to mount moorings until 4°S in order to monitor the tropical current system. Chief scientist of this Meteor cruise is Professor Peter Brandt from IFM-GEOMAR. On Thursday, the 5th of November at about 7:00 Meteor shows up at the horizon. At 9:00 our rendezvous starts. Both ships send rubber boats back and forth to exchange visitors (see Fig. 1).
Fig. 2: Polarstern and Meteor at the equator on the 5th of November 2009 (photo: M. Ettlin)
With Meteor and Polarstern the two largest German research vessels meet. We ferry across and get a warm welcome from Peter Brandt and his colleagues. A guided tour through the ship is much appreciated and we return a Polarstern-tour directly after that. We haven't seen a boat for several days and the view of both research vessels from the zodiac is impressive (see Fig. 2). Unfortunately, there is not enough time for a joint lunch. After the meeting we quickly perform a CTD to determine the biological activities in the equatorial upwelling water. Than, Polarstern steams on in southerly direction. Two midday-stations for radiation measurements are cancelled because of strong cloudiness. Other than that our measurements are running as planned.
This week Frank Laturnus and Johannes Lampel report about the air chemistry observations on board Polarstern:
During the cruise of Polarstern from Bremerhaven to Punta Arenas, Chile, the concentration of volatile organohalogens in ambient air and in seawater is monitored constantly. Volatile organohalogens are known to contribute to several atmospheric reactions including the warming of the troposphere and the destruction of the stratospheric ozone layer. Several industrial and natural sources of volatile organohalogens have been identified, and extrapolation of the global emission of volatile organohalogens from natural sources into the atmosphere revealed sources strengths comparable to the industrial input. However, exact data on the natural part are still scarce.
Fig. 3: Tanja Teschner at the gas chromatograph performing chemical analysis of air samples (photo: Frank Laturnus)
Our project on board Polarstern follows two objectives. First, specification of the volatile halogenated compounds occurring in seawater and ambient air, determination of their concentrations and calculation of the fluxes to determine sources and sinks. Second, to determine the carbon stabile isotopic signatures of volatile organohalogens and their mixing ratios, both for marine and coastal terrestrial sources, and to identify the origin of the volatile organohalogens. The signature of carbon stabile isotopes may allow distinguishing between different sources, tracing transport processes, and estimating life time cycles of selected volatile organohalogens.
Six times a day, we are measuring the concentration of volatile organohalogens in ambient air samples and in surface seawater. Ambient air is collected on the upper deck of the Polarstern, 30 m above seawater level against wind direction. The air is preconcentrated on a trap by liquid nitrogen and analysed for volatile organohalogens on board by gas chromatography (see Figure 3). Simultaneously to the air sample, seawater is collected 10 m below surface by the ships own seawater collection system. The volatile organohalogens in the seawater sample are removed by helium and preconcentrated by liquid nitrogen before analysed by gas chromatography. Additionally with the seawater, the concentration of chlorophyll will be determined. Chlorophyll is an indication for the occurrence of microalgae known to emit volatile organohalogens. Besides the direct analysis of ambient air and seawater for volatile organohalogens, up to 400L of ambient air is collected on special designed absorption tubes for the determination of the carbon stable isotopes signature. The analyses will be done by stable isotope mass spectrometry at our home laboratory in Hamburg.
Fig. 4: MAXDOAS telescope with Sahara dust (photo: J. Lampel)
Halogen compounds are also measured using the MAXDOAS set-up (Fig. 4) from the University of Heidelberg on the observation deck. That what can be measured are not halocarbons, but instead their products due to photolysis which are sometimes also directly emitted. Those reactive halogen species are short-lived, therefore samples cannot be taken, instead the measurement has to be done directly.
The MAXDOAS measurements are conducted aboard for about 10 years with different instruments and are also used for satellite data validation. The current spectrometer is capable of detecting continuously low BrO and IO background concentrations. The spectral range has been extended to retrieve reliably aerosol and trace gas profiles. On the current cruise concentrations of nitrogendioxide, formaldehyde, glyoxal, bromine oxide and iodine oxide have been above detection limit.
Fig. 5: The baptism team after the post-equator baptism (photo: A. Baecker)
Now for the weather: At the beginning of this reporting week we reach the southern boundaries of the ITCZ and leave the regions of high convection that are associated with that, behind. Indeed, during the noon-station on November 4 we can still see the partly precipitating high reaching thunderstorm clouds to the north of us and the shallow fair weather clouds of the southern subtropical Atlantic high pressure region to the south. It remains mostly sunny. Every now and then streets of convective clouds and shallow marine strato-cumulus clouds show up and disturb our aerosol and subsurface light observations.
However, these cases are the more interesting for our cloud measurements and – modelling activities. There is something for everyone. Associated with the subtropics a strong south-east trade wind is blowing with wind force 5. Wind sea and swell add up to about 3 m wave heights, good conditions for our zodiac missions.
A late equator baptism took place (Fig. 5)
Best regards on behalf of Neptune, crew and scientists
Andreas Macke