Pascal Wintjen

Pascal Wintjen successfully defended his doctoral thesis entitled “High-frequency flux measurements of reactive nitrogen as a validation tool for dry deposition modeling: Technical improvements and application over different land-use types” at the Free University of Berlin on February 3, 2023. The dissertation was conducted as part of the BMBF-funded Junior Research Group NITROSPHERE and the UBA project FORESTFLUX. At Thünen Institute of Climate-Smart Agriculture, Pascal was supervised by Dr. Christian Brümmer and at the FU Berlin by Prof. Dr. Martijn Schaap.

Reactive nitrogen compounds (N_r) such as ammonia, nitrogen oxides and nitric acid as well as particulate ammonium and nitrate are nutrients for the productivity of ecosystems and play an important role in atmospheric chemistry, thereby having a strong influence on air quality. Since the beginning of the 20th century, N_r concentrations in the atmosphere have increased sharply due to industrial and agricultural emissions, resulting in increased deposition of reactive nitrogen in both natural and managed ecosystems. High atmospheric nitrogen deposition can have negative effects on semi-natural ecosystems. Depending on the type of nitrogen excess, acidification or eutrophication, for example, have been observed, which can significantly affect the CO₂ uptake capacity of plants. Therefore, an accurate assessment of the nitrogen exchange between the biosphere and the atmosphere is essential in order to identify endangered ecosystems and to evaluate the efficiency of environmental protection measures. So-called critical loads indicate the limit of ecosystems' ability to cope with the input of air pollutants and have proven to be a suitable concept for identifying regions at risk from long-term and excessive nitrogen deposition.

While the determination of the exchange of non-reactive greenhouse gases such as CO₂, methane or nitrous oxide is largely established from a metrological point of view, the measurement of reactive substances is a major challenge due to their chemical properties. In his doctoral thesis, Pascal used a micrometeorological method (eddy covariance) in which a converter transforms all reactive compounds into nitrogen monoxide (NO) at high temporal resolution. The converter system is called TRANC and stands for Total Reactive Atmospheric Nitrogen Converter. Pascal further developed the methodology so that a separation into oxidized and reduced N compounds could be carried out. This is of crucial importance for the source allocation of the substances in order to be able to classify the origin of the emissions (agriculture vs. industry/transport).

Pascal's doctorate resulted in a total of three peer-reviewed publications. In the first paper, he compares different correction methods for the attenuation of high-frequency components in eddy covariance-based exchange fluxes and gives recommendations for the application of reactive substances. For example, while only a small proportion of the flux is attenuated in CO₂ measurements, corrections of over 30% may be necessary in N_r measurements. This work ensures an improved estimate of the nitrogen budget and avoids a significant error in the N balance.

The second and third studies of his work included the application of the TRANC methodology in a forest ecosystem and the use of the field data collected to check and compare different model systems for estimating nitrogen input. For two and a half years, Pascal supervised the measurements at a spruce-dominated site within the Bavarian Forest National Park, a region that experiences a relatively low N deposition of about 5 kg N per hectare annually due to its distance from agricultural or industrial emission sources. The campaign contributed significantly to a better understanding of forest ecosystems and to a robust classification of different methods for determining atmospheric N deposition. In particular, the coupling of the measurement data with existing deposition models (e.g. depac, LOTOS-EUROS) represented an unprecedented level of detail in the data analysis. It was shown that classic models without an ammonia compensation point considerably overestimate N deposition. In large-scale models that are coupled to chemical transport models, the accurate use of input emissions is of crucial importance. The campaign data and models could also be used to test common gap-filling methods within eddy-covariance measurements. Pascal's doctoral thesis is an important contribution to the understanding of the interactions between land ecosystems and the atmosphere and, in addition to methodological developments in measurement technology and data evaluation, provides an integrative analysis in the field of reactive nitrogen compounds that will be used in various scientific and practical applications.