Prof Dr Jan Kaiser

Biography

I study global chemical, physical and biological processes in the Earth’s atmosphere and oceans, and their variations through time under natural and anthropogenic influences, by integrating fieldwork, laboratory studies and modelling into a coherent framework, often involving the use of stable isotopes.

This approach has been applied to nitrous oxide (N₂O), nitrate, oxygen, nitrogen, argon and carbon dioxide (CO₂). I have led studies on the isotopic composition of atmospheric N₂O, emphasizing position-dependent ¹⁵N/¹⁴N isotope ratio measurements and the fate of N₂O in stratospheric photolysis and photo-oxidation. I developed analytical methods to accurately measure N₂O isotopes in troposphere, stratosphere, polar firn air and ice. This work allowed reconstruction of past changes and demonstrated increasing agricultural influences on atmospheric N₂O. In laboratory experiments, I studied temperature- and wavelength-dependent isotope effects in N₂O photolysis. I discovered that during photolysis the central nitrogen position always becomes more enriched in ¹⁵N than the terminal one, but that during photo-oxidation the opposite position-dependent fractionation occurs. This pattern allowed me to distinguish photolysis and photo-oxidation in the stratosphere.

I am also interested in rare isotope signatures, such as oxygen isotope anomalies (¹⁷O-excess, Δ(¹⁷O)) and multiply-substituted isotopologues (clumped isotopes) such as ¹⁵N¹⁵NO. The small ¹⁷O-excess of atmospheric N₂O motivated me to consider how best to characterise such anomalies. My review of data reduction practices for CO₂ isotope measurements revealed systematic uncertainties in global carbon budget calculations.

The N₂O work led on to the first online gas chromatography-isotope ratio mass spectrometric method to measure Δ(¹⁷O) of nitrate, using bacterial conversion to N₂O, which drastically reduced sample sizes and enabled seawater analyses. Δ(¹⁷O) combined with nitrogen isotope ratios were used to understand the annual cycle of nitrate in coastal Antarctica, uncovering stratospheric influences and re-evaporation from surface snow. Low sample size requirements make this technique suitable for ice core studies of past changes in the atmospheric oxidation capacity.

I have also developed a sea-going membrane inlet mass spectrometer (MIMS). The instrument continuously measures dissolved oxygen, nitrogen, argon and CO₂. Oxygen/argon ratios allow separation of physical and biological effects on gas concentrations and provide net community production estimates on unprecedentedly fine scales.

More recent work included studies of N₂O and nitrate in marine and terrestrial environments as well as measurements of the chlorine (³⁷Cl/³⁵Cl), sulfur (³⁴S/³²S, ³³S/³²S) and carbon (¹³C/¹²C) of volatile organic compounds such as chloromethane (CH₃Cl) and carbonyl sulfide (COS).