Mapping CO2 Concentrations and Fluxes

Integrating spatial data and measurements of carbon dioxide.

Carbon dioxide is a strong attenuator of infrared radiation and is believed to be important in trapping heat in the lower atmosphere, contributing to global climate change. Understanding how sources and sinks for CO2 vary in both time and space can be important in evaluating the potential impacts of different land covers and management practices on the environment and human health. One approach to characterizing this variability is to integrate spatial data with concurrent observations of CO2 concentrations and/or fluxes.

Mapping CO2 concentrations:

The CO2 concentration of the well-mixed atmosphere is around 390 ppm. The concentration of CO2 at any given location close to the ground may deviate substantially from this, however, owing to the strength of local sources and sinks for CO2.

Urban environments have been shown to have ambient CO2 concentrations much higher than surrounding rural areas, resulting in domes of CO2 enriched air. These CO2 domes tend to show variation on time scales as long as seasons or as short as days, and can track changes in community behavior. It has been suggested that while CO2 at typical ambient concentrations poses little or no direct health risk to humans, it may still serve as a valuable air quality indicator in urban environments. Increases in CO2 concentration are correlated with increases in ground level ozone and particulate matter, and may actually enhance ozone production.

Characterization of CO2 domes has historically been done by sampling CO2 concentrations at heights of a few meters or less above the ground using a portable gas analyzer along transects across the area of interest. By sampling CO2 concentrations concurrent with spatial data, a clear picture of how ambient CO2 concentration varies across the landscape can be built.

Integrated spatial and CO2 concentration data can be useful in a much broader range of applications. It has been suggested that monitoring local variations in CO2 concentration along with seismic activity may improve predictions of volcanic activity. Water vapor and CO2 are the two most abundant gases produced by volcanic activity, and have been shown to be released in higher quantities as volcanic activity increases in an area.

For carbon capture and sequestration (CCS) applications, integrated spatial and CO2 concentration data can provide a means of identifying potential leaks of CO2. Where CO2 is sequestered in geological formations, the risk of CO2 escaping poses a potential risk to human health and necessitates constant monitoring. By sampling CO2 concentrations close to ground level, areas of abnormally high concentration can be identified for further investigation.

Characterizing spatial variability in soil CO2 flux measurements:

forest-edge

Soil CO2 fluxes measured in a forest edge/mowed field transition.
Fluxes (µmol m-2 s-1) were measured on August 18, 2011 in the late afternoon, and are plotted in false color. Isotherms for soil temperature (°C) measured at 5 cm depth are included. Soil temperature correlates positively with soil CO2 flux over much of the study area, with the exception of the region where the highest temperatures were observed. In this small area vegetation is sparse and the soil is predominantly exposed.

The pool of carbon stored in soils is large, roughly twice the carbon present in the atmosphere and three times more than that stored in land plants. Not surprisingly, the movement of carbon from reserves in soil to the atmosphere as CO2 flux is also quite large.

The movement of CO2 out of soil is diffusive in nature and primarily driven by a soil-to-atmosphere CO2 concentration gradient. The larger the gradient and the smaller the resistance to diffusion of CO2 in the soil, the greater the flux of CO2 out of the soil. The resistance to diffusion through the soil is generally a function of the soil texture and moisture content, while the magnitude of the concentration gradient is driven predominantly by the production of CO2 in the soil by biological processes and the resistance.

Both soil organic content and the biological processes driving soil CO2 production can be highly variable over a wide range of spatial scales. Across regional scales, differences in land cover and use can drive differences in soil CO2 flux. On more local scales, small variations in vegetation, available soil moisture and temperature can drive differences in soil CO2 flux for sites only a few meters apart.

One approach to characterizing the spatial variability in soil CO2 flux measurements is to conduct point, or survey, measurements across the area of interest. By combining soil CO2 flux and ancillary environmental measurements with spatial coordinate data, a detailed picture of how the flux varies across the study area can be built.

Want to know more about the processes and mechanisms behind soil CO2 flux? View the soil CO2 flux webinar series.


Combining ground level CO2 concentration (GLCC) mapping and soil CO2 flux measurements:
A tool for Carbon Capture and Sequestration (CCS) applications

log-of-flux

False color images of soil CO2 fluxes measured at the ZERT site on July 19 prior to the subsurface CO2 release, and on July 21 during the release.
The color scale represents the log of the measured flux (log µmol m-2 s-1). On July 19, fluxes ranged from 5 to 15 µmol m-2 s-1. On July 21, peak fluxes were around 2000 µmol m-2 s-1. The X and Y axes are in meters from the center point of the injection pipe, following the same system as Lewicki et al., 2009.

The Zero Emission Research and Technology Center (ZERT) near Bozeman, Montana, focuses on the basic science of geological carbon sequestration. The site represents a combined effort of the US Department of Energy, Montana State University, and West Virginia University, and offers researchers the opportunity to explore various technologies related to geological carbon sequestration. Controlled releases of CO2 from an underground well are periodically performed at the site, and are used to test various monitoring and validation technologies which could potentially be used to verify that carbon injected into geological formations remains sequestered there.

Using integrated spatial and ground level CO2 concentration data, potential leak sites can be rapidly identified as areas of unusually high CO2 concentrations relative to background levels. GLCC mapping is qualitative in nature, but provides a means of screening potentially large areas for sites that warrant further investigation. When combined with soil CO2 flux measurements, this not only allows identification of potential leaks from sequestration sites, but direct quantification of leak rates.

mapping-a-b

The data shown here give an example of the combined power of mapping ground level CO2 concentrations and soil CO2 flux.
Ground Level CO2 Concentrations measured at the ZERT site on July 19 prior to the subsurface CO2 release (panel A), and on July 22 during the release (panel B). CO2 concentrations are adjusted to ground elevation equal to 360 ppm. In panel B the false color image of soil CO2 flux from Figure 3 has been overlaid to show the agreement between spikes in GLCC (maximum of 506 ppm) and peak soil flux measurements. A reduced transect is shown in panel B to better illustrate the agreement. The path around the outside of the plot (as shown in panel A) yielded GLCC near ambient (mean 392.12 ppm, st. dev. 5.04 ppm).

References:

  • Griffin, TP, JA Diaz, CR Arkin, C Soto, CH Curley, and O Gomez. 2008. Three-Dimensional Concentration Mapping of Gases using a Portable Mass Spectrometer System. Journal of the American Society for Mass Spectrometry 19:1411-1418.
  • Idso, CD, SB Idso, and RC Balling Jr. 2000. An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA. Atmospheric Environment. 35:995-1000.
  • Idso, CD, SB Idso, and RC Balling Jr. 2002. Seasonal and diurnal variations of near-surface atmospheric CO2 concentration within a residential sector of the urban CO2 dome of Phoenix, AZ, USA. Atmospheric Environment. 36:1655-1660.
  • Jacobson, MZ. 2010. Enhancement of local air pollution by urban CO2 domes. Environmental Science Technology. 44:2497-2502.
  • Lewicki, JL, GE Hilley, L Dobeck and L Spangler. 2009. Dynamics of CO2 fluxes and concentrations during a shallow subsurface CO2 release. Environ Earth Sci. Special Issue.

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GLCC

Integrating Spatial Data with CO2 Measurements

Integrating Spatial Data
with CO2 Measurements


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