Atmospheric sulfur compounds in the troposphere and stratosphere measured with an atomic emission detector
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Abstract
Carbonyl sulfide (OCS) plays a crucial role in the Earth’s sulfur budget. Globally OCS is the predominant reduced sulfur species in the Earth’s atmosphere with typical tropospheric mixing ratios of around 500 ppt. During volcanically quiescent periods OCS controls the atmospheric sulfur budget within the troposphere and stratosphere, and the upward transport governs the background stratospheric aerosol loading due to its long atmospheric lifetime. The stratospheric sulfate aerosol layer (Junge layer) affects the global radiative balance, as sulfate aerosol particles scatter a fraction of incoming solar energy back to space. Sulfate particles can also act as cloud condensation nuclei (CCN) and ice nuclei (IN), thus further increasing the albedo of the Earth. Furthermore, OCS also acts as a climate forcing gas, absorbing longwave outgoing infrared radiation.
Research into OCS has a long history, but nevertheless the atmospheric OCS budget remains unbalanced. Therefore, improving knowledge of OCS sources, and sink processes is essential for improving current models and thereby for accurate future climate forecasts.
Throughout this PhD work a novel analytical system was developed to measure volatile organic compounds (VOCs), with primary focus on organosulfur species. The system consists of a gas phase cryogenic pre-concentration system (Entech), gas chromatographic (GC) separation and 3rd generation atomic emission detection (AEDIII), hence termed Entech-GC-AEDIII. The setup and performance of this newly established system is demonstrated. The Entech-GC-AEDIII enables various VOC measurement, including organosulfur species, non-methane hydrocarbons (NMHC), halogenated compounds, volatile nitrogen compounds, monoterpenes etc. This is the first instrument report of a gas phase air sample analysis method with an AED instrument.
Whole air samples (WAS) were collected globally from the upper troposphere / lowermost stratosphere (UT/LMS) region onboard a Lufthansa Airbus A340-600 IAGOS-CARIBIC passenger aircraft into flasks by a fully automated system. The post-flight flask analyses were conducted between December 2015 and December 2018 by the automated Entech-GC-AEDIII system in a laboratory. From the OCS measurements a global OCS lifetime of 2.1 ± 1.3 years, and a significantly longer stratospheric lifetime of 47 ± 16 years were determined. Furthermore, a flux of 118 ± 39 Gg (S) yr-1 of OCS from the troposphere into the stratosphere was estimated, and the stratospheric sink estimate yielded 44 – 90 Gg (S) yr-1 of OCS. The 43% smaller sink serves as a 51 Gg (S) yr-1 estimate of the OCS fraction which is transported back from the stratosphere to the troposphere.
The global 3D ECHAM5 / MESSy Atmospheric Chemistry (EMAC) model was used to run the numerical calculations and sampled at the CARIBIC flight paths. A comparison between CARIBIC observations and EMAC model simulations led to a conclusion that the EMAC model substantially overestimates OCS MRs in the upper atmosphere.
A first of its kind measurement campaign with the new Entech-GC-AEDIII detector was conducted in a Finnish boreal forest at the Hyytiälä measurement station in September 2016. The boreal forests comprise 33% of the Earth’s forest cover, making it the second largest biome in the world. Therefore, it is an essential component of the atmospheric biosphere – geosphere interface. The OCS measurements demonstrated the boreal forest as a strong vegetative sink for OCS, which could be one of the reasons for the discrepancy between the EMAC model and CARIBIC observations in the tropopause region. Furthermore, the nighttime uptake of OCS was analyzed, concluding the light independence of OCS fixing carbonic anhydrase (CA) enzyme.