Inferring methane fluxes at a larch forest using Lagrangian, Eulerian, and hybrid inverse models

Abstract

Measuring methane (CH 4 ) flux at upland forests is challenging due to high levels of heterogeneity in upscaling chamber measurements and the detection limits of currently available micrometeorological methods. We estimated CH 4 fluxes in an upland forest from vertical concentration profiles using three different inverse multilayer models: the Lagrangian localized near field theory, Eulerian, and hybrid Lagrangian-Eulerian models. The approach could estimate spatially representative fluxes, and use of higher gradients within canopies than above them could minimize uncertainties associated with sensor noises. Comparing fluxes by the models and measurements by the micrometeorological hyperbolic relaxed eddy accumulation and chamber methods, daytime fluxes were reasonably reproduced, but nighttime fluxes were overestimated most likely due to an underestimation of stable conditions and storage effects. The models and measurements show that the forest acted as a CH 4 sink during the study period, and the soil acted as the dominant sink. The estimated sink increased with increasing soil temperatures and decreasing soil water content. The CH 4 sink estimated during the study period were 1.5 ± 0.2 nmol m À2 s À1 by the micrometeorological method, 2.4 ± 0.5 nmol m À2 s À1 by chambers, 2.8 ± 1.1 nmol m À2 s À1 by the Lagrangian model, 2.7 ± 1.0 nmol m À2 s À1 by the Eulerian model, and 3.7 ± 2.8 nmol m À2 s À1 by the hybrid model. The performance of the Lagrangian and hybrid models increased when the CH 4 sink/source was assumed to only exist in the soil.

Observation of vertical profiles of NO, O<SUB>3</SUB>, and VOCs to estimate their sources and sinks by inverse modeling in a Japanese larch forest

“…These discrepancies between the models and observation might be partly explained by an underestimation of the sources of isoprene, MACR, TMT and MVK at the forest floor in the calculation model. Ueyama et al 2014 reported that the wind speeds simulated by the closure model were approximately half the observed values. Since the modeled turbulence at the forest floor was underestimated, the modeled sink and source could be underestimated at the forest floor.…”

“…The models were validated at the same forest but during different period, in terms of CO 2 and CH 4 fluxes Ueyama et al, 2014 , where the models reasonably inferred diurnal variation, day-by-day variation, and magnitude of the fluxes above the forest during the daytime period.…”

“…To infer vertical distributions of sink and source strength, we used two different inverse models: Lagrangian localized near-field theory Raupach, 1989 and Eulerian closure model Katul and Albertson, 1999 . The details of the models and parameterizations were reported in Ueyama et al 2014 ; only a brief description is provided here.…”

“…Inverse multilayer models are a useful way of evaluating trace gas sink and source distribution within a canopy Katul and Albertson, 1999;Leuning et al, 2000;Siqueria et al, 2000;Ueyama et al, 2014 . Through mathematical simulation of flow in and above the canopy, the methods yield vertical gas flux distributions based on concentration profile measurements.…”

“…As far as we know, there have been no studies estimating the vertical sink and source distribution of NO and VOCs using inverse multilayer models for applying whole canopies, which consist of the canopy layer, trunk space, and forest floor. We examined the temporal variation in the NO, O 3 , and VOCs profiles within and above the canopy, then applied inverse multilayer models Ueyama et al, 2014 to identify sink and source distributions of NO, O 3 , and VOC within the forest canopy. We then inferred the production and loss processes occurring within the canopy from the obtained sink and source distributions.…”