Large amplitude spatial and temporal gradients in atmospheric boundary layer CO₂mole fractions detected with a tower‐based network in the U.S. upper Midwest

Abstract

.[1] This study presents observations of atmospheric boundary layer CO 2 mole fraction from a nine-tower regional network deployed during the North American Carbon Program's Mid-Continent Intensive (MCI) during [2007][2008][2009]. The MCI region is largely agricultural, with well-documented carbon exchange available via agricultural inventories. By combining vegetation maps and tower footprints, we show the fractional influence of corn, soy, grass, and forest biomes varies widely across the MCI. Differences in the magnitude of CO 2 flux from each of these biomes lead to large spatial gradients in the monthly averaged CO 2 mole fraction observed in the MCI. In other words, the monthly averaged gradients are tied to regional patterns in net ecosystem exchange (NEE). The daily scale gradients are more weakly connected to regional NEE, instead being governed by local weather and large-scale weather patterns. With this network of tower-based mole fraction measurements, we detect climate-driven interannual changes in crop growth that are confirmed by satellite and inventory methods. These observations show that regional-scale CO 2 mole fraction networks yield large, coherent signals governed largely by regional sources and sinks of CO 2 .

Can seasonal and interannual variation in landscape CO<sub>2</sub> fluxes be detected by atmospheric observations of CO<sub>2</sub> concentrations made at a tall tower?

“…1). Our results are consistent with other findings of both modelling and observational studies in this regard Vermeulen et al, 2011;Lauvaux et al, 2012;Miles et al, 2012). Forests dominate the fractional activity after cropland harvest due to their continuing biological activity in the late summer (Figs.…”

“…However, the correlation between surface NEE and observed profiles of atmospheric CO 2 concentration declines with increasing distance from the observation tower Miles et al, 2012). A reduction in correlation between tall tower observations and ecosystem activity is consistent with signal dilution due to atmospheric transport Miles et al, 2012). The dominant influence on observations of atmospheric CO 2 concentrations are from the near field, although the total footprint can cover a large area.…”

Carbon and energy fluxes in cropland ecosystems: a model-data comparison

“…Eddy covariance measurements over growing maize show net ecosystem exchange (NEE, expressed as ecosystem respiration minus photosynthesis) of CO 2 as high as 75 lMol m -2 s -1 at midday (Lokupitiya et al 2009), more than three times the typical rate over the Amazon rainforest (Saleska et al 2003). As a result, seasonal drawdown of about 35 ppm of CO 2 in the atmospheric boundary layer was observed by a network of instrumented towers in the central US Corn Belt; it was the strongest seasonal CO 2 cycle ever observed (Miles et al 2012). Atmospheric inverse models using these data found that uptake rates had to be substantially increased relative to Bayesian prior estimates from ecosystem models (Schuh et al 2013;Ogle et al 2015).…”

The CarboCount CH sites: characterization of a dense greenhouse gas observation network

“…The Mid-Continent Intensive (MCI) measurement campaign attempted to oversample the US upper midwest over a longer period of time Miles et al, 2012), but even with an average distance of approximately 188 km between eight towers, Lauvaux et al (2012) found that the spatial density of the measurement network was insufficient to consistently retrieve the spatial distribution of carbon fluxes.…”