Environmental Products
AAI has extended its QSC Water Quality application to estimate the emission rates of the greenhouse gases methane (CH4) and carbon dioxide (CO2) from water bodies. The core QSC process retrieves the water composition (column-integrated concentrations of suspended chlorophyll, suspended minerals, and colored dissolved organic matter) and water clarity properties (wavelength-dependent turbidity and subsurface sighting ranges, and Secchi Depth) of each water pixel in a multispectral image (see Environmental Water Quality). A detailed bio-geochemical model-based process then transforms the image-retrieved concentrations of suspended minerals, suspended chlorophyll, and colored dissolved organic carbon into image "maps" of predicted methane and carbon dioxide emission rates from the water body.
Atmospheric methane is nearly as important as carbon dioxide as a greenhouse gas. Although the abundance of methane in the atmosphere is typically significantly lower than that of carbon dioxide, its contribution to atmospheric radiative forcing (global warming) is frequently greater than half that of CO2. There are several anthropogenic and natural sources of methane, but water quality is a major factor controlling current production rates in areas with significant natural and agricultural wetland coverage, such as in East Asia, South America, and Africa. It has been estimated that natural wetlands (lakes, bogs, and other inland water bodies) produce approximately 23% of the total methane in East Asia, for example, while wetlands associated with agriculture (rice cultivation) in that region are responsible for about 33%. This indicates that potentially significant reductions in greenhouse gas emissions from these and the other regions can be realized through targeted improvements in water quality. It also means that progress against green house gas emission reduction targets can be measured through the periodic regional scale monitoring of the surrogate water composition parameters for the relevant water bodies from space.
Aquatic Methane and CO2 Production
Aquatic methane is produced primarily below the sediment / water interface of a water body. Very little, if any, is produced in the water column. Stratification within lakes, reservoirs, and other closed water bodies limits downward oxygen diffusion, and anoxic conditions can sometimes develop at depth in the water column. In general, such conditions are more likely to occur in the sediments. The methane is produced largely by the activity of methane-producing organisms, called methanogens, in the sub-surface sediments. There are two groups of methanogens: one uses H2 + CO2 to form CH4, and the second splits acetate to form CH4 and CO2. The compounds used by both groups are products of plant matter decomposition in the anoxic environment. Decomposition starts with depolymerization of plant matter, primarily dead algae, in which extracellular enzymes break down plant polymers into monomers (e.g., simple sugars and amino acids). Because oxygen diffusion into the soil is blocked, these monomers are then used by fermenting bacteria, which produce H2 and acetate (as well as other simple organic acids and alcohols) as waste products. These compounds are then available to the methanogens to produce methane.
Estimated methane emission rates across Hongze Lake on 20 April 2004. This image map of predicted methane emission rates was based on the concentrations of suspended chlorophyll, suspended sediments, and colored dissolved organic matter shown in the compositional maps illustrated in Image-Retrieved Environmental Water Quality Products. The mean ± 1 standard deviation emission rate per pixel is rCH4 = 4.52 ± 2.85 x 10-11 Kg CH4 s-1 m-2. The emission rates for individual pixels are color coded according to the accompanying key (units are x 10-11 Kg CH4 s-1 m-2).
The image-derived methane emission rates agree with the known "reference" field for natural wetlands (from MATCH Climate Model) in the Hongze Lake region.
Methane emission rates across Hongze Lake on 9 July 2004, a few months later than the April results shown above. The mean (± 1 standard deviation) methane emission rate per pixel is rCH4 = 2.95 ± 1.99 x 10-11 Kg CH4 s-1 m-2. The emission rates for individual pixels are color coded according to the accompanying key (units are x 10-11 Kg CH4 s-1 m-2). Note that the production of methane was somewhat lower in July than in April 2004, and that the spatial patterns shifted. The regions emitting relatively higher levels of methane, e.g., along the northern shoreline of the lake, were not the same as in April. The mean emission rate per pixel was still in good agreement with the "reference" value (see previous figure) used by climate models for the region containing Hongze Lake, however. The monthly production from Hongze is estimated to have been 0.98 x 105 Kg CH4 over the month of July.
The water quality results provide a means for identifying the non-point sources of pollution responsible for the methane production. Recurrent plumes of suspended sediment occur near the nutrient-rich aquaculture installation across the river from Gaolianglian. The sediments may transport nutrients from the aquaculture facility into the lake, stimulating a cycle of algae over-growth, decay, and methane production.
Environmental Products List