Validation of farm-scale methane emissions using nocturnal boundary layer budgets

J Stieger,N Buchmann,I Bamberger,W Eugster

doi:10.5194/acp-15-14055-2015

J Stieger, N Buchmann + Show 2 more

Open Access

https://doi.org/10.5194/acp-15-14055-2015

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Abstract

Abstract. This study provides the first experimental validation of Swiss agricultural methane emission estimates at the farm scale. We measured CH4 concentrations at a Swiss farmstead during two intensive field campaigns in August 2011 and July 2012 to (1) quantify the source strength of livestock methane emissions using a tethered balloon system and (2) to validate inventory emission estimates via nocturnal boundary layer (NBL) budgets. Field measurements were performed at a distance of 150 m from the nearest farm buildings with a tethered balloon system in combination with gradient measurements at eight heights on a 10 m tower to better resolve the near-surface concentrations. Vertical profiles of air temperature, relative humidity, CH4 concentration, wind speed, and wind direction showed that the NBL was strongly influenced by local transport processes and by the valley wind system. Methane concentrations showed a pronounced time course, with highest concentrations in the second half of the night. NBL budget flux estimates were obtained via a time–space kriging approach. Main uncertainties of NBL budget flux estimates were associated with nonstationary atmospheric conditions and the estimate of the inversion height zi (top of volume integration). The mean NBL budget fluxes of 1.60 ± 0.31 μg CH4 m-2 s-1 (1.40 ± 0.50 and 1.66 ± 0.20 μg CH4 m-2 s-1 in 2011 and 2012 respectively) were in good agreement with local inventory estimates based on current livestock number and default emission factors, with 1.29 ± 0.47 and 1.74 ± 0.63 μg CH4 m-2 s-1 for 2011 and 2012 respectively. This indicates that emission factors used for the national inventory reports are adequate, and we conclude that the NBL budget approach is a useful tool to validate emission inventory estimates.

Highlights

Efforts to mitigate climate change via commitments to reduce national emissions require independent techniques to validate or at least constrain emission estimates reported by each nation
The governmental inventory estimates are based on standard procedures recommended by the IPCC (Lassey, 2008; Nisbet and Weiss, 2010; Heimann, 2011; Wang et al, 2011), e.g. national livestock numbers multiplied with default emission factors (EFs) derived from the IPCC guidelines and guidance (Penman et al, 2000)
Direct inventory estimates based on actual local livestock numbers (CHAI: 1.29 ± 0.47 and 1.74 ± 0.63 μg CH4 m−2 s−1 in 2011 and 2012 respectively) and mean nocturnal boundary layer (NBL) budget fluxes agreed well, after appropriate selection of available sounding data was made (Sect. 2.3)

Summary

Introduction

Efforts to mitigate climate change via commitments to reduce national emissions require independent techniques to validate or at least constrain emission estimates reported by each nation. We aim at quantifying the farm-scale CH4 emissions from a farmstead in Switzerland in order to compare these flux estimates with best estimates used in the national inventory report (NIR) under the Kyoto protocol. The governmental inventory estimates are based on standard procedures recommended by the IPCC (Lassey, 2008; Nisbet and Weiss, 2010; Heimann, 2011; Wang et al, 2011), e.g. national livestock numbers multiplied with default emission factors (EFs) derived from the IPCC guidelines and guidance (Penman et al, 2000). Despite considerable uncertainty of these default values, this estimation methodology has become commonly used in order to ensure conformity and comparability of reported greenhouse gas emissions among different nations (Lassey, 2007). An experimental validation of such inventory estimates by independent means would be highly beneficial to improve their overall credibility, the assessment of agricultural CH4 emissions by direct atmospheric measurements is still lacking for Switzerland. Methane emissions in Switzerland are dominated by the agricultural sector (77.7 %), followed by emis-

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