PhD Thesis
Fluxes of the NO-O<SUB>3</SUB>-NO<SUB>2</SUB> triad above a spruce forest canopy in south-eastern Germany
Anywhere Tsokankunku (03/2008-11/2014)
Support: Thomas Foken
The work was supported by the Max Planck-Institute of Chemistry Mainz (Prof. Dr. F.X. Meixner, Dr. I. Trebs)
Nitrogen monoxide (NO) and nitrogen dioxide (NO2) (denoted together as NOx) are important compounds for the regulation of the tropospheric photochemical oxidant ozone (O3). Although they account for less than one millionth of the total constituents of the atmosphere, they play an important role in the composition and chemical processes of the atmospheric boundary layer. They affect the distribution of tropospheric O3 and its natural background levels, which plays an active role in air pollution.Together the three reactive trace gases NO, NO2 and O3 undergo a series of interconnected reactions and are often referred to as the NO-O3-NO2 triad. This triad has attracted attention from scientists in the recent past because of the role that it plays in causing problems such as acid rain, eutrophication of water bodies, and air pollution.
Within the global tropospheric O3 budget the low NOx concentration levels over large rural forest ecosystems (0.2-10 ppb) are assumed to be important potential counterparts to the high NOx concentration levels over industrialized urban areas (10-1000 ppb). Being able to directly and simultaneously measure the concentrations of the triad and their corresponding biosphere-atmosphere exchange fluxes can give valuable information for policy makers and environmentalists about the NOx-O3 budget of forest ecosystems and the troposphere.
The eddy covariance (EC) method is a well-known method for obtaining fast reliable fluxes within a large footprint and it has been successfully applied to measure CO2 and water vapour fluxes for over fifty years. However, up to now, not many EC measurements of NO and NO2 exchange fluxes above rural forest ecosystems have been conducted successfully because of a lack of analysers which meet the precision requirements for measuring the low concentrations of NO and NO2 characteristic at these sites. The limit of detection of most available analysers is within the range of the measured concentration, making it impossible to resolve concentrations. Moreover, a lack of fast analysers and suitable photolytic converters until the last decade has also hindered progress in the turbulent measurement of these trace gases to obtain fluxes. Another problem is that the NO-O3-NO2 triad are reactive trace gas compounds and the measurements of their fluxes tend to get affected by chemical transformations during turbulent transport.
To solve these problems, a unique approach was taken using a novel, fast (5 Hz), high precision, highly specific, 2-channel NO analyser to conduct EC flux measurements of NO and NO2. This analyser was deployed during the summer of 2008 in a spruce forest stand at the FLUXNET research site Waldstein-Weidenbrunnen (DE-Bay) in the rural Fichtelgebirge mountain range in north-eastern Bavaria, Germany, within the scope of the DFG-funded project EGER (Exchange Processes in Mountainous Regions). EC flux measurements of NO, NO2 and also O3 were conducted on a walk-up tower 32 m above ground level (9 m above the forest canopy) . For the first time in a spruce forest in central Europe, simultaneous EC fluxes of the NO-O3-NO2 triad were measured. CO2 and H2O EC fluxes were also measured as they were important for the verification of the EC method in use and for an insight into the biological response of the forest.
Vertical concentration profiles of NO, NO2, O3, CO2 and H2O above (25m, 32m) and below the canopy (0.005 m, 1.000 m) were measured by an independently operated but simultaneously operational profile system. Profiles of Summary iv meteorological data were also available. From these profile measurements, concentration differences between 25 and 32 m above the forest were evaluated, from which fluxes using the aerodynamic gradient method (AGM) and the Modified Bowen Ratio (MBRM) were calculated for comparison with the EC fluxes.
A period of four ‘golden days’ of excellent meteorological conditions was chosen for data evaluation. Since I was dealing with very low concentrations of trace gases, data quality assessment and control was crucial and was applied using reliable state-of-the-art tools. Spectral analysis showed minimal flux losses at high frequency. However, cut-off frequencies for NO (1.11 Hz), and NO2 (1.17 Hz), were higher than for O3 (0.79 Hz) and the nonreactive trace gases – an indication of high frequency damping due to the long tubes from the tower to the NO/NO2 analyser on the ground. Flux losses for the period 1100-1330 CET were acceptably low (NO: 2.1 %, NO2:7.5 %, O3: 1.8%). This result was a good indication of the good response and high quality of the measurement sensors. The losses were corrected for during data post-processing.
During the golden days, fluxes of reactive and nonreactive trace gases showed distinct diurnal patterns. Between 0600 and 1200 CET, there was a distinct rise in the concentration and fluxes of all trace gases. This was attributed to external sources of the trace gases being advected to the site from the surrounding country roads. The NO flux was unexpectedly directed downwards throughout the day, ranging from -1.75 to 0.00 nmol m-2 s-1. NO2 flux was positive throughout, ranging from 0.0 to a maximum of 1.5 nmol m-2 s-1 at 1400 CET. The O3 flux was directed into the canopy throughout the day, ranging from a minimum of 0 to a maximum of 20 nmol m-2 s-1 around 1100 CET. The strong peak of NO during the early to mid-morning period and the persistence of the NO2 flux throughout the day was a strong indicator of the existence of an external source of NOx being advected towards the canopy from a nearby country road.
The relation between turbulent and chemical time scales (Damköhler numbers) and segregation intensities (indicators of the extent of mixing between chemical species by turbulence) suggest that there was a minimal influence of chemistry on above canopy fluxes of the NO-O3-NO2 triad during the day. Although minimal, the chemistry might have been enough to create the NO sink evident in the above canopy fluxes. There is evidence of night-time conversion of soil biogenic NO to NO2 as well as the possible formation of nitrous acid (HONO) and nitric acid (HNO3) below the canopy through heterogeneous processes in the presence of dew and fog. Vertical gradients and estimated fluxes below the forest canopy showed that the O3 sink is so strong that none of the NO from the soil manages to reach the top of the canopy. The entire NO observed above the canopy is most likely due to horizontal advection and photochemical production.
This dissertation has proven that reliable, fast turbulent measurements of NO and NO2 can be simultaneously made above a forest canopy. It has also demonstrated the importance of quality control and precision on reactive trace gas flux measurements. For central European forests with low soil NO emissions under moderate pollution, the forest canopy acts as a net source of NO2 and a net sink of NO and O3. Issues such as advection make the evolution of the NO-O3-NO2 triad in the trunk space a complicated issue to fully understand without additional, rigorous on-site measurements and analysis. Additional work should look at the possibility of partitioning the in-canopy and above canopy flux of the triad as well as reformulating the Damköhler number to include an advection term.