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"Absorption of light, macro-algae and the atmosphere"

Final Report Summary - ALMA-MATER (Absorption of light, macro-algae and the atmosphere)

Seaweeds are among the most potent accumulators of iodine, however, the extent to which seaweeds release iodine to the atmosphere is still not fully understood [1-3]. The objective of ALMA MATER was to identify and further characterize potential sources of gaseous iodine and other halogenated compounds. These compounds are precursors of atomic iodine in the marine boundary layer. When brown algae (laminaria species) are put under stress at low tides, they release I2 in addition to a number of volatile iodocarbons (CH3I, C2H5I, CHBr3...). However, the gaseous emission of iodocarbons from seaweed has not been detected directly before by a direct in situ experiment. Therefore the hypothesis that seaweed are the main source of iodocarbons in the marine boundary layers was tested by studying the gaseous emissions of macroalgae under appopriate conditions (temperature, salinity, light levels) in the laboratory. Promising brown macroalgae (such as the Laminaria species digitata, saccharina or hyperborea) of the genus 'kelp' were harvested from the Irish coast and their gaseous emission was studied using a newly developed highly-sensitive cavity enhanced absorption spectroscopic (CEAS) technique, called Incoherent broadband (IBB) CEAS, pioneered at the host institute [4]. This experimental approach was combined with a Fourier Transform (FT) detection scheme, enabling broad spectral coverage for the potential detection of multiple species. In FT-IBBCEAS [5, 6], the transmitted light from the optical cavity is spectrally selected using an interferometer ensuring high spectral resolution, however at the expense of acquisition time. As part of the experiments, a new thermalized sample chamber was integrated into a modified FT-IBBCEAS setup. Efforts were also made to introduce various oxidative stress levels to the macro algae; e. g. irradiating the algae with UV light. In addition to experiments conducted with Laminaria species, measurements were also conducted with several other algae species like, Fucus and Ulva. However, the level of release of iodocarbons (specifically CH3I, CH2I2, CHBr3) from the algae was below the detection limit of the monitoring technique in the near IR. For given sampling times the spectra obtained were inconclusive as to the in situ elusion of iodocarbons, but also concerning the potential formation of HOI and HOBr.

Further experiments in collaboration with the National University of Ireland in Galway were undertaken to study the known emission of I2 from Laminaria Digitata over 6 hour tidal cycles in order to study the ability of the algae to recover from external oxidative stress factors. A separate IBBCEAS spectrometer (working in the visible region of the spectrum) incorporating a flow cell was setup for this purpose and tested. Despite the fact that iodine release from the algae was detected for all samples the dependence on the tidal cycle and the potential of the algae to recover from stress remained inconclusive. Other experiments including the role of H2O2 for the I2 release could also not unequivocally linked to mechanisms that cause the appearance of bursts of iodine release while algae are under oxidative stress. More experiments are planned for the future.

In order to exploit the successful high-end performance of the FT-IBBCEAS setup alternative spectroscopic studies with relevance for atmospheric chemistry were pursued successfully:

1. The near IR spectroscopy of nitrous acid (HONO) and its deuterated counterpart (DONO) and simultaneous detection of HNO3 and NO2 by FT-IBBCEAS was investigated. The relevance of these species is illustrated by a substantial number of spectroscopic studies [7-14]. In ALMA MATER the simultaneous measurement of absorption spectra of these species yielded for the first time information on the 2ν1+ν3 and the 3ν1 bands of the trans-isomers of DONO in the near infrared region between 5500 and 8000 cm-1. All bands were rotationally resolved, and spectroscopic constants could be established with good accuracy. New rotationally resolved bands of DNO3 were also detected. Additionally, several bands of HONO, HNO3 and NO2 have been measured simultaneously across the near IR spectral range, demonstrating the potential of the method to detect multiple trace gases simultaneously through their known line positions (a manuscript in preparation).

2. The very weak 2v1 + 3v3 absorption band of nitrogen dioxide, NO2, located at 7192.159 cm−1 was investigated using Fourier-transform incoherent broadband cavity-enhanced absorption spectroscopy (FT-IBBCEAS) in the 7080–7210 cm−1 spectral range [15]. The energy levels calculation was performed using a theoretical model which explicitly took Coriolis interactions coupling into account as well as spin rotational resonances within the (5, 1, 0), (2, 2,2) and (2, 0, 3) vibrational states. However, the spectral resolution was not sufficient to observe the spin splitting doublets in the spectrum, and the spin-rotation parameters were thus maintained at their ground state values in the theoretical model.

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