Jan Kaiser

Professor of Biogeochemistry

I study 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, often involving the use of stable isotopes. This approach has been applied to N2O, nitrate, O2, N2 and CO2. I've 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 isotopologues in troposphere, stratosphere, polar firn air and ice. This work allowed reconstructing 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 becomes more enriched in 15N than the terminal one, but that during photo-oxidation the terminal atoms becomes more enriched. This pattern allows distinguishing between stratospheric photolysis and photo-oxidation. I am also interested in oxygen isotope anomalies and polyisotopologues (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 isotope measurements in CO2 revealed systematic uncertainties in carbon budget calculations.

United Kingdom