Trace gas fluxes with DMBR method

During the LIBRETTO campaign the surface layer fluxes of non-reactive (CO2, H2O) and also of reactive trace gases (NO, NO2, O3) were computed by using a distributed modified Bowen ratio method (DMBR) (Mayer et al., 2008a, Appendix, D). Because of the spatial separation between the measurements included in this method, the horizontal homogeneity of the experimental site with respect to the sensible heat flux H was an absolute precondition for the application of the DMBR method. The fulfilment of this precondition was tested by comparing vertical temperature difference at the place of the measurements of H and at the place of the measurements of trace gas concentration differences. Figure 4 shows the comparison, confirming the homogeneity by a good agreement between the temperature differences at both locations.

For passive trace gases, the corresponding fluxes, as computed from the measured vertical difference of mixing ratios, can be regarded as valid without further considerations. During 20 days of the LIBRETTO campaign, the median fluxes of CO2

(Figure 5a) were of comparable magnitude as reported from other sites (Frank and Dugas, 2001; Frank, 2002). The flux of H2O showed a very clear diurnal cycle with low or almost slightly negative values during night times (Figure 5b).

Figure 4: Comparison of measured temperature differences at the profile station with the temperature differences at the EC station. The dashed line gives the 1:1 ratio, the solid grey line indicates the linear regression. Figure taken from Mayer et al. (2008a, Appendix D).

Results 16

The flux of O3 was found to be always directed towards the surface (Figure 5c). This was expected, as no source of O3 is known at the surface, while O3 is destroyed by dry deposition onto surfaces (soil, plants) and during daytime additionally by stomatal uptake. An additional O3 sink at the surface results from NO emission from the soil. The effect of the additional sink of O3 during daytime is clearly visible in the median diurnal flux of O3, showing strongest downward fluxes in the early afternoon. The higher fluxes started with sunrise around 06:00 h and ended with sunset around 18:00 h.

Figure 5: Median (lines) diurnal course of the fluxes of (a) CO2, (b) H2O, (c) O3, (d) NO, (e) NO₂ and (f) sensible heat from 11 August 2006 to 30 August 2006. Colored areas comprise the respective inter quartile ranges. The bars at the bottom of each graph show the number of values available for the corresponding median and quartiles. Figure taken from Mayer et al. (2008a, Appendix D).

Results 17 In contrast to O3, NO has a source at the ground. It is produced by microorganisms in the soil, leading to a net production and thus a positive flux (Figure 5d). Considering only meteorological parameters, microbial NO production is, besides soil moisture, primarily dependent on soil temperature (Q10 law). Thus, highest production rates and therefore fluxes were expected around noon or early afternoon, when highest soil temperatures were observed. While positive fluxes were observed throughout the day, a diurnal cycle of the flux was barely visible with values around 0.1 nmol m^-2 s^-1. Nevertheless, a small decrease in the NO flux was observed shortly before sunset.

During the night, the NO flux slowly increased back to its previous level.

The median NO2 flux remained most of the time negative (Figure 5e), indicating a net deposition. A maximum deposition flux was observed at 08:30 h. This could be attributed to advection events. If days with advection affecting the site were excluded in the analysis, the NO2 flux did not show the negative excursion. In the early afternoon, small positive NO2 fluxes were observed. During the night, the NO2 flux remained slightly negative. Besides the trace gases, H is also shown (Figure 5f). It exhibited very small, negative values during night time (not more than -5 W m^-2). With sunrise, H increased rapidly, reaching maximum values around noon. In the afternoon, H decreased again and dropped below zero around 18:00 h, indicating the onset of surface cooling.

In contrast to passive trace gases, the calculation of fluxes of reactive trace gases must take into account possible influences due to chemical reactions. The intensity of chemical alteration of the mixing ratios during the vertical transport between the DMBR

Figure 6: Median diurnal cycle of the dimensionless Damköhler numbers (DA) for the period of 11 August 2006 – 30 August 2006. The individual Damköhler numbers for (a) O₃ and (b) NO are shown. The solid line indicates the median, the dark shaded areas cover the interquartile range, and the light shaded areas comprise the range from the 5^th percentile to the 95^th percentile. Figure taken from Mayer et al. (2008a, Appendix D).

Results 18

measuring heights is reflected by the Damköhler number (Figure 6). The Damköhler number represents the ratio of turbulent transport time scale to the timescales of relevant chemical reactions during the transport. It is shown by Mayer et al. (2008a), that O3 at a site with low NO mixing ratio (typically less than 1 ppb during the LIBRETTO campaign) can be assumed as quasi passive (Figure 6a). However, some measurements during night are nevertheless affected by chemical reactions. This happens, if turbulence ceases, and the transport time increases dramatically. Such situations are often observed in the first part of the night, when turbulence is suppressed due to very strong stabilization. Also NO2 was found to act quasi passively. For NO, the situation is different. NO is destroyed by reaction with O3. Low NO values together with relative high O3 mixing ratios (typically more than 20 ppb during the LIBRETTO campaign) lead to fast destruction of NO. This is clearly visible in the Damköhler number for NO (Figure 6b). The median Damköhler number was about 0.25, indicating that more than 50 % of the NO concentration difference data were severely affected by chemical reaction. To derive correct flux data from the concentration differences, only NO data with corresponding Damköhler number < 0.25 were used. This assured derived fluxes to be only negligibly affected by chemical reactions, because turbulent transport is assured to be at least four times faster than the chemistry