Accounting for spectroscopic effects in laser‐based open‐path eddy covariance flux measurements

Abstract

A significant portion of the production and consumption of trace gases (e.g. CO 2 , CH 4 , N 2 O, NH 3 , etc.) by world ecosystems occurs in areas without sufficient infrastructure or easily available grid power to run traditional closed-path flux stations.Open-path analyzer design allows such measurements with power consumption 10-150 times below present closed-path technologies, helping to considerably expand the global coverage and improve the estimates of gas emissions and budgets, informing the remote sensing and modeling communities and policy decisions, all the way to IPCC reports. Broad-band nondispersive infrared devices have been used for openpath CO 2 and H 2 O measurements since the late 1970s, but since recently, a growing number of new narrow-band laser-based instruments are being rapidly developed.The new design comes with its own challenges, specifically: (a) mirror contamination, and (b) uncontrolled air temperature, pressure and humidity, affecting both the gas density and the laser spectroscopy of the measurements. While the contamination can be addressed via automated cleaning, and density effects can be addressed via the Webb-Pearman-Leuning approach, the spectroscopic effects of the in situ temperature, pressure and humidity fluctuations on laser-measured densities remain a standing methodological question. Here we propose a concept accounting for such effects in the same manner as Webb et al. proposed to account for respective density effects. Derivations are provided for a general case of flux of any gas, examined using a specific example of CH 4 fluxes from a commercially available analyzer, and then tested using "zero-flux" experiment. The proposed approach helps reduce errors in open-path, enclosed, and temperature-or pressure-uncontrolled closed-path laserbased flux measurements due to the spectroscopic effects from few percents to multiple folds, leading to methodological advancement and geographical expansion of the use of such systems providing reliable and consistent results for process-level studies, remote sensing and Earth modeling applications, and GHG policy decision-making.

Impact of forest plantation on methane emissions from tropical peatland

“…Frequency response loss corrections were applied to compensate the flux losses at different frequencies (Massman, , ; Moncrieff, Clement, Finnigan, & Meyers, ). Fluctuations in CH 4 density due to temperature (thermal expansion) and water vapor (dilution) were corrected using the Webb–Pearman–Leuning correction (Webb, Pearman, & Leuning, ) and spectroscopic effects taken into account by EddyPro (Burba, Anderson, & Komissarov, ). Differences between deployment‐specific variables, that is, sensor separation distance and instrument placement, were considered while processing the data.…”

Importance of the Webb, Pearman, and Leuning (WPL) correction for the measurement of small CO<sub>2</sub> fluxes

“…According to the original work on the WPL correction (Webb et al, 1980), it should be noted that in some conditions the correction is expected to be on the same order of magnitude as the flux. This fact has only been given the necessary attention in a few works on the error analysis of CO 2 fluxes or other trace gas fluxes (Burba et al, 2019), to which the above statements apply analogously, measured with open-path gas analysers. From the investigation of individual events of CO 2 fluxes presented in this study, we have formulated the following conclusions.…”

A new open‐path eddy covariance method for nitrous oxide and other trace gases that minimizes temperature corrections

“…Temperature measurements from the CSAT3 were used for temperature‐related corrections, and H 2 O measurements from the LI‐7500 were used for H 2 O‐related corrections. Pressure‐related corrections were neglected in this study because of lacking high‐frequency pressure measurements as in most EC flux studies (Burba et al, 2019). Using an NDIR CO 2 sensor, Zhang et al (2011) showed that the pressure‐related density correction accounted for 20% of the annual net ecosystem production of a forest in China.…”

“…A laser-based, open-path sensor design provides high precision measurements and overcomes the power challenge but is intrinsically sensitive to ambient condition changes. McDermitt et al (2011) developed a commercial open-path CH 4 sensor for EC measurements with a precision of five parts per billion volume (ppbv) at 10 Hz and low power consumption (<10 W), which has significantly improved spatial coverages of CH 4 flux measurements (Burba et al, 2019). However, when exposed to the ambient environment, open-path measurements are impacted by variations in atmospheric conditions, especially temperature fluctuations.…”