PhD Thesis
Transport processes of reactive trace gases in the atmospheric boundary layer
Jens-Christopher Mayer (04/2005-02/2009)
Support: Thomas Foken
Transport of trace gases within the atmospheric boundary layer plays a key role in feedback processes between the earth’s surface and the atmosphere and consequently in ecosystem budgets of carbon and nitrogen (among many more). For a correct quantification of the exchange between surface and atmosphere, it is crucial to understand the transport processes involved and to determine limitations of the presently available measurement techniques in order to apply the right technique with respect to the currently active transport processes. This dissertation focuses on three topics: (a) The analysis of effects of vertical transport mechanisms on surface measurements of trace gases, (b) the appropriate choice of an experimental setup to assess specific measurement errors of moving measurement systems and (c) the application of a series of measurement techniques for surface fluxes of reactive trace gases to determine their degree of agreement and to assess potential source of deviations. To study the impact of vertical transport mechanisms on surface trace gas measurements, this thesis presents a comprehensive set of measurements at the surface and within the atmospheric boundary layer (by tethered balloon). It enables the attribution of a recurrent negative excursion of ozone mixing ratios in the morning hours at a mountain summit to a very efficient vertical transport by free convection. It has been shown that, due to the rapid vertical transport, a layer of approximately 20 m thickness developed at the equilibrium height of the free convection, being located within the residual layer. It had a chemical composition similar to the air close to the ground while being surrounded by residual layer air masses. Hence, very strong gradients of the chemical composition were found within the residual layer. Evidence was found, that such a transport occurs rather frequently at this location, affecting at least 18 % of the days between April and September. To assess measurement errors introduced by the application of scanning methods as compared to gradient approaches, a higher temporal resolution of the vertical profiles was needed. Because of limitation inherent to a tethered balloon, an elevator based profiling system was installed, providing a temporal resolution of 10 minutes with a maximum ceiling of 100 m. Prior to the investigation of transport processes, the proper functioning of correction algorithms for the so-called dynamical error was investigated under real atmospheric conditions. This dynamical error is inherent to all moving measurement systems and arises from the non-zero response time of the deployed sensors. It has been shown that existing algorithms as well as one developed by the authors reliably balance the dynamical error. Furthermore it has been demonstrated, that the elevator data correlate with reference data at fixed levels with coefficients of determination being always greater than 0.992 at every level (10, 20, 40, 60, 80, 98 m). To evaluate the applicability of different flux measurement techniques for the determination of surface fluxes of reactive trace gases, three different approaches were compared. In order to determine surface fluxes of trace gases, a new modification of the modified Bowen ratio method was used. In this modification, the measurements of sensible heat flux and of the gradients were horizontally separated. This allowed the simultaneous measurement of the fluxes of various trace gases without creating errors due to flow distortion by bulky inlet systems. Surface emission fluxes of nitric oxide were found to be in the range 0.02 – 0.15 nmol m-2 s-1 (night/day), nitrogen dioxide fluxes varied around 0.1 nmol m-2 s-1 (deposition) with slightly positive values in the early afternoon, indicating emission. Ozone deposition fluxes ranged from close to zero to about 6 nmol m 2 s-1. A laboratory parameterization of biogenic soil emission fluxes of nitric oxide from incubated soil samples yielded values from 0.025 nmol m-2 s-1 to 0.12 nmol m-2 s-1 for environmental conditions encountered during the field campaign. This was in excellent agreement with the fluxes from field observations. Besides the comparison of field fluxes with laboratory data, a case study (1 night) comparison of carbon dioxide and ozone fluxes between two field methods was done. Results from the modified Bowen ratio method have been compared to fluxes derived from the integral boundary layer budget method. Both methods yielded similar mean carbon dioxide fluxes during the night. In contrast, ozone fluxes deviated between both methods. This deviation was attributed to chemical in-situ loss of ozone during night time within the profile being integrated by the budget method.