by Dr. Birgit Quack, GEOMAR Helmholtz Centre for Ocean Research Kiel

There are two regions of our atmosphere where the trace gas ozone is especially important: in the stratosphere and in the surface boundary layer. In the upper region of our atmosphere, the stratosphere ozone is produced from the sunlit decay of oxygen and acts as an important UV radiation protector for life on earth. It is rapidly destroyed by radical reactions of halogens, such as chlorine, bromine, and iodine, which occur in various forms in our atmosphere. A severe ozone hole occurred in the 1980s and could be related to long-lived fluorochlorocarbons, which were banned by the Montreal Protocol in the 1990s. Because of this, the ozone hole over Antarctica is closing and ozone in the upper stratosphere is generally increasing again. However, in the lower stratosphere of the tropics, it is still declining. Surface ozone, on the other hand, is increasing due to air pollution, especially when fossil fuel is combusted.

Under windy.com, anyone who has internet access can find a global map for ozone and local values for every place in the world (Figure 1). Surface ozone, which is also a greenhouse gas, is produced naturally to a background of 10-40 µg/m3. As it is a strong oxidant, it can irritate eyes and lungs in higher concentrations. Therefore, legal thresholds and warning systems are established for cities where ambient concentrations increase in summer >100µg/m3 and can harm sensitive people.

Figure 1 shows the ozone surface ozone concentrations in the area where our cruise SO305 operated in the Bay of Bengal. It is very high over the entire India during the day, and it is also apparent that the ozone concentrations sharply decline at the coastlines and towards the open oceans. This is mainly due to an active halogen chemistry, which destroys ozone over the oceans, caused by natural halogenated volatile compounds. They are formed in the oceans by sunlight, phytoplankton, and chemical reactions and are partly released into the atmosphere. Those compounds comprise bromoform (CHBr₃), dibromo¬methane (CH₂Br₂), methyl iodide (CH₃I), diiodomethane (CH₂I₂) iodochloromethane (CH₂CII), dichlorobromomethane (CHBrCl₂) and all have short atmospheric lifetimes of minutes to six months. The anthropogenic industrial solvent dichloromethane (CH₂Cl₂) and chloroform (CHCl₃) from freshwater chlorination are also short-lived and contribute to ozone depletion in the atmosphere as well. The compounds lifted up into the stratosphere by deep convection in the tropics release their halogens in the lower stratosphere while moving to the North and South poles. Thus, the tropical processes exert their effects globally.
During the last month, we were on board RV Sonne in the tropical Bay of Bengal, discovered in 2001 as a major source of some natural halogenated volatile compounds to the atmosphere. We measured the compounds in water on board, with a Gas-chromatographic Mass-spectrometric (GC/MS) Purge-system (Figure 2), in order to understand their source strengths and learn about their biogeochemical cycling in the oxygen minimum zone of the Bay of Bengal.

Jule, doing her master thesis on the topic, Julia, joining R/V Sonne for the second time as a student helper, and I, traveling the oceans for 30 years for halocarbons, brought our analytical system on board, which ran smoothly since the beginning of the cruise 24/7 in shifts of each 8 hours, where the instrument gets a new sample every hour. We have taken water from the deep ocean and the surface waters in brown glass bottles and pumped air into stainless steel canisters (Figure 3). The gases in the water samples were extracted with helium, frozen in liquid nitrogen, separated on a gas-chromatographic column, and detected with a Mass-spectrometer. The air samples will be analysed using a similar method for approximately 50 compounds by Elliot Atlas at the National Center for Atmospheric Research in Boulder, Colorado, US. From the concentrations in the ocean and atmosphere, we calculate the air-sea gas exchange of the compounds, try to understand their distribution in the region related to physical and biogeochemical parameters, and estimate how the oceanic source and sink may develop in the future and what this means for ozone as a greenhouse gas and as UV-shield.

During every ship expedition in my 30 years of travelling the oceans, a team of scientists and crew on board different research vessels explored the ocean as an ancient interplay of water, chemicals, trillions of bacteria, and higher organisms, influenced by gravity, the rotation of the earth as well as by sunlight and the moon. The biota responds to the distribution of nutritious chemicals, while the waters interact closely with the atmosphere, from the sea surface to the stratosphere. Every time, a bit of the myriad of global and regional secret interactions, which determine the basis of our life, was discovered and published. The old interplay is now influenced by human activities, which I hope do not tip the evolving equilibrium so that future generations have the chance to further unravel it to sustainably live with it.

I was able to conduct my last cruise on RV Sonne, a starship of the German research fleet, with a professional crew, which enabled again very good data sets, by the smooth operation of the ship until Singapore harbour. During the next years, I will likely mainly explore my 30-year data sets with AI for the benefit of extracting the most knowledge about halocarbons out of them.  Jule and Julia will hopefully find more opportunities to conduct their career with ocean voyages of discovery, as those not only promote knowledge, but also evolve social skills of team effort, discipline, endurance, considerateness and tolerance. I will miss the daily challenges of a successful research cruise.

Birgit Quack, Singapore 18.05.2024