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(Gurney et al. 2002). In another approach, which is more direct, biomass inven-
tories (for example, forest and cropland inventories) are used for estimating up-
take by monitoring changes in biomass stocks. A third approach involves spa-
tially explicit modeling of ecosystem processes on the basis of weather, soil,
land use, and land cover (Schwalm et al. 2010). Each of those approaches has
limitations and uncertainties, and derived estimates show only moderate agree-
ment (Hayes et al. 2012). Hayes et al. (2012) demonstrate the value of the inven-
tory approach, which relies on stock estimates obtained from EPA reports (for
example, EPA 2011b), for subcontinental-scale estimates of carbon fluxes.
Integrated modeling of greenhouse-gas sources and sinks
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will continue to
develop rapidly given continuing advances in remote sensing of ecosystem
properties and understanding of the carbon cycle. To meet its regulatory man-
date and to support policies that address climate change, EPA could benefit from
increased science and engineering capacity in ecosystem ecology and Earth-
system science.
Air-Quality Monitoring
Advances in atmospheric remote sensing have created a new paradigm for
air-quality monitoring and prediction from regional to global scales (NRC
2007b). Research and applications have focused on fine particulate aerosols,
tropospheric ozone, nitrogen dioxide, formaldehyde, sulfur dioxide, and carbon
monoxide, but have also included other compounds, such as benzene, ethylben-
zene, and 1,3-butadiene (NRC 2007b, Fishman et al. 2008, Hystad et al. 2011).
Active sensors, such as satellite and aircraft-mounted light detection and ranging
systems (LiDAR) (for example, the cloud-aerosol LiDAR with orthogonal po-
larization), can provide information on the vertical distribution of clouds and
aerosols on the basis of the magnitude and spectral variation in backscatter of
the vertical beam. However, most remote sensing of air quality has relied on
passive sensors, for example, measurements of pollution in the troposphere, the
moderate-resolution imaging spectroradiometer and multi-angle imaging spec-
troradiometer on the National Aeronautics and Space Administration's (NASA)
Terra platform, and the ozone-monitoring instrument and tropospheric emission
spectrometer on NASA's AURA platform (Martin 2008). Those collect radio-
metric data on solar backscatter or thermal infrared emissions that are then used
in retrieval algorithms that incorporate other geophysical information and radia-
tive-transfer models. The reliability of results depends on the surface reflectivity
or emissivity, clouds, the viewing geometry, and the retrieval wavelength
(Martin 2008). Estimating ground-level concentrations, which are of greatest
relevance to EPA, requires additional information on the vertical structure of the
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The ocean is the largest sink, inasmuch as carbon dioxide is dissolved in seawater
and is in equilibrium with the atmosphere (in freshwater bodies, it can change the water
pH to some extent).
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