A CO-based method to determine the regional biospheric signal in atmospheric CO₂

Abstract

Regional-scale inverse modeling of atmospheric carbon dioxide (CO 2 ) holds promise to determine the net CO 2 fluxes between the land biosphere and the atmosphere. This approach requires not only high fidelity of atmospheric transport and mixing, but also an accurate estimation of the contribution of the anthropogenic and background CO 2 signals to isolate the biospheric CO 2 signal from the atmospheric CO 2 variations. Thus, uncertainties in any of these three components directly impact the quality of the biospheric flux inversion. Here, we present and evaluate a carbon monoxide (CO)-based method to reduce these uncertainties solely on the basis of co-located observations. To this end, we use simultaneous observations of CO 2 and CO from a background observation site to determine the background mole fractions for both gases, and the regional anthropogenic component of CO together with an estimate of the anthropogenic CO/CO 2 mole fraction ratio to determine the anthropogenic CO 2 component. We apply this method to two sites of the CarboCount CH observation network on the Swiss Plateau, Beromünster and Lägern-Hochwacht, and use the high-altitude site Jungfraujoch as background for the year 2013. Since such a background site is not always available, we also explore the possibility to use observations from the sites themselves to derive the background. We contrast the method with the standard approach of isolating the biospheric CO 2 component by subtracting the anthropogenic and background components simulated by an atmospheric transport model. These tests reveal superior results from the observation-based method with retrieved wintertime biospheric signals being small and having little variance. Both observation-and model-based methods have difficulty to explain observations from late-winter and springtime pollution events in 2013, when anomalously cold temperatures and northeasterly winds tended to bring highly CO-enriched air masses to Switzerland. The uncertainty of anthropogenic CO/CO 2 emission ratios is currently the most important factor limiting the method. Further, our results highlight that care needs to be taken when the background component is determined from the site's observations. Nonetheless, we find that future atmospheric carbon monitoring efforts would profit greatly from at least measuring CO alongside CO 2 .

Estimation of the fossil fuel component in atmospheric CO<sub>2</sub> based on radiocarbon measurements at the Beromünster tall tower, Switzerland

“…However, this can be due to enhanced CO emissions transported from other European cities towards Beromünster. Oney et al (2017) observed particularly large CO / CO 2 ratios at Beromünster during several pollution events in late winter and early spring 2013 which were associated with air mass transport from eastern Europe, where poorly controlled combustion of biofuels and coal likely results in high ratios.…”

“…Periods with winds from the east, colloquially known as Bise, are well known to be associated with very stable boundary layers and correspondingly strong accumulation of air pollutants during the cold months of the year between autumn and spring. Air masses reaching Beromünster from eastern Europe have recently been reported to contain unusually high levels of CO during late winter and early spring periods, coinciding with this sampling period (Oney et al, 2017).…”

“…2e). Based on measurements at Beromünster and other cities of the CarboCount CH network in 2013, Oney et al (2017) reported that for a daily mean temperature of greater than 20 • C the biosphere over the Swiss plateau tends to become a net CO 2 source. The observed positive spikes in CO 2 (Fig.…”

“…The CO measurements at Jungfraujoch were conducted using a CRDS analyzer (Picarro Inc., G-2401) with a measurement precision of ±2.5 ppb for 1 min aggregates (Zellweger et al, 2012). As CO is usually co-emitted with CO 2 during incomplete combustion of fossil and other fuels, we have also computed a tracer ratio designated as CO / CO 2 from the enhancements in the in situ measured CO and CO 2 mixing ratios over the Jungfraujoch background (Oney et al, 2017). CO 2bg and CO bg values were obtained by applying the robust extraction of baseline signal (REBS) statistical method (Ruckstuhl et al, 2012) to the continuous CO 2 and CO measurements at the high-altitude site Jungfraujoch (Schibig et al, 2016) with a bandwidth of 60 days.…”

“…Despite its advantage of providing a proxy for continuous CO 2ff data, the method introduces additional uncertainties due to diurnal and seasonal variability in the CO sink, and the presence of multiple non-fossil CO sources such as oxidation of hydrocarbons or wood and biofuel combustion (Gamnitzer et al, 2006). Spatial variations in the CO : CO 2 ratio across Europe due to different source compositions and environmental regulations, which affect the measured ratios due to changes in air mass origin (Oney et al, 2017), are the main reason for the temporally changing CO : CO 2ff ratio for a given measurement site. Additionally, variability in the CO / CO 2 emission ratios of the sources can contribute to its spatial and temporal variability (Vogel et al, 2010;Turnbull et al, 2015).…”