A case study of eddy covariance flux of N<sub>2</sub>O measured within forest ecosystems: quality control and flux error analysis

Abstract

Abstract. Eddy covariance (EC) flux measurements of nitrous oxide (N 2 O) obtained by using a 3-D sonic anemometer and a tunable diode laser gas analyzer for N 2 O were investigated. Two datasets (Sorø, Denmark and Kalevansuo, Finland) from different measurement campaigns including subcanopy flux measurements of energy and carbon dioxide are discussed with a focus on selected quality control aspects and flux error analysis. Although fast response trace gas analyzers based on spectroscopic techniques are increasingly used in ecosystem research, their suitability for reliable estimates of EC fluxes is still limited, and some assumptions have to be made for filtering and processing data. The N 2 O concentration signal was frequently dominated by offset drifts (fringe effect), which can give an artificial extra contribution to the fluxes when the resulting concentration fluctuations are correlated with the fluctuations of the vertical wind velocity. Based on Allan variance analysis of the N 2 O signal, we found that a recursive running mean filter with a time constant equal to 50 s was suitable to damp the influence of the periodic drift. Although the net N 2 O fluxes over the whole campaign periods were quite small at both sites (∼ 5 µg N m −2 h −1 for Kalevansuo and ∼ 10 µg N m −2 h −1 for Sorø), the calculated sub-canopy EC fluxes were in good agreement with those estimated by automatic soil chambers. However, EC N 2 O flux measurements show larger random uncertainty than the sensible heat fluxes, and classification according to statistical significance of single flux values indicates that downward N 2 O fluxes have larger random error.

Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event

“…Currently the height of the tree stand is 15-18 m, average basal area is 18 m 2 ha −1 , and average stem densities are 900, 750, and 40 stems per ha for the dominant Scots pine (Pinus sylvestris L.) trees and the smaller understorey downy birch (Betula pubescens) and Norway spruce (Picea abies L.) trees, respectively. The total LAI in the site is approximately 2 m 2 m −2 (Mammarella et al, 2010 The depth of the well decomposed Sphagnum peat at the site is approximately 2.5 m with peat a pH of 5.0 and C/N ratio of 41 in the litter layer and 45 in the top 10 cm of the peat soil.…”

“…Map of the measurement site showing the locations of above canopy eddy covariance (EC) mast (ECabove), sub-canopy EC mast (ECsub), manual chambers (MC, square) and automatic chambers (AC, circle). Dotted line next to one of the manual chamber groups show the place of soil gas concentration pits, and grey line around the sub-canopy EC mast show the footprint area from which 85% (at 30 m) of the sub-canopy N 2 O fluxes originate (see Mammarella et al, 2010). similar than in the peatland generally, however, tall dwarf shrubs were not present.…”

“…Our second aim was to evaluate the suitability of sub-canopy EC-based N 2 O measurements as a sophisticated alternative to traditionally used chamber methods in this environment. The quality control and flux error analysis of the EC N 2 O measurements at the site are presented in this issue in Mammarella et al (2010).…”

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

“…A 2-D rotation of sonic anemometer wind components was performed, and 30 min covariances between the scalars and vertical wind velocity were calculated using linear detrending method. Short-term drift in the QCLS highfrequency concentration data was negligible and there was no need to apply more sophisticated approach for detrending the data, e.g., high-pass recursive filters (Mammarella et al, 2010). The time lag between the concentration and wind measurements induced by the sampling line was determined by maximizing the covariance.…”

Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes

“…This analyzer was operated in a dual-ramp mode measuring both N 2 O and CO 2 throughout the measurement campaign, following a typical airsampling configuration for EC measurements ( Fig. 1d; see also Mammarella et al, 2010;Mishurov and Kiely, 2010;Molodovskaya, 2012;Rannik et al, 2015). Air was sampled through a 6 mL rain-shielded intake (part number 17882, Campbell Scientific, Inc., Logan, Utah, USA) and a dryer assembly (PD1000, Campbell Scientific, Inc., Logan, Utah, USA), which included a 2 µm polypropylene filter element in a low-volume, high-flow filter holder, and a multi-tube dryer (200-tube, 1.2 m Nafion ™ dryer element; Perma Pure, Lakewood, New Jersey, USA).…”