Emission factors

Upscaling of emission data is often accomplished using specific EFs. In many cases, EFs are derived from the ratio between N2O flux to the atmosphere (FN2O ) and N flux into the system (FNin). This approach can be seen as one of several concepts for deriving EFs and is thus referred to as conceptual EF 1 (CEF1) which is defined as follows:

CEF1=FN2O,oUt/FNin (8.1)

This concept is used by the IPCC methodology for calculation of national inventories (Mosier et al, 1998; Groffman et al, 2002; IPCC, 2006). Indirect emissions consist of emissions associated with depositions of agricultural N that has been transferred to the atmosphere (N2OA), or to human sewage (N2Os), and N that leaves crop fields in leaching and runoff (N2OL), all of which end up in aquatic systems.

N2Ol is calculated as follows:

where NLeach is the amount of N that leaves crop fields in leaching and runoff, EF5 is the N2O EF for N that leaves crop fields in leaching and runoff and is processed as it moves ultimately to the world ocean. This factor is partitioned as EF5g (groundwater), EF5r (rivers) and EF5e (estuaries), with a value of 0.0025 for each of the partial EFs and thus 0.0075 for the overall EF5 (IPCC 2006). An earlier EF5 estimate of 0.025 (Mosier et al, 1998; IPCC, 2001) has been revised since more recent results indicated that the previous EF5g (0.015) and EF5r (0.0075) were too high (Hiscock et al, 2002, 2003; Reay et al, 2004a, 2004b, 2005; Sawamoto et al, 2005; Clough et al, 2006).

Moreover, although the basic idea was to use Equation 8.1 for deriving EFs, EF5g was based on a different concept with the following ratio:

where cN2Oaq and cNO3aq are the concentrations of N2O and NO3- measured in agricultural drainage water or groundwater. This concept assumes that these aquatic systems act solely as a domain of transport without any processing of NO3~ and N2O. The conceptual basis of Equation 8.3 has been questioned (Groffman et al, 2000; Nevison, 2000; Well et al, 2005a) because reduction of NO3~ as well as production and reduction of N2O occur in denitrifying systems and N2O loss to the atmosphere can occur before the water reaches the sampling spots. Thus cNO3aq is often much lower than the influx of N (Nin) and cN2Oaq can both increase and decrease during transport, which implies that CEF2 (Equation 8.3) is likely to yield lower values than CEF1 (Equation 8.1) (Figure 8.2). Consequently, CEF1 would yield more realistic estimates of EF5g. However, determining CEF1 implies more difficult measurements as both Fin, i.e. NO3~-leaching, and FN2O , i.e. the total advective and diffusive N2O-flux from the water leaving crop fields, have to be quantified (Figure 8.1).

Another concept for EFs is to compare N2O flux with N loss via denitrifi-cation (Well et al, 2005a):

It is known that in surface soils, the N2O-to-(N2+N2O) ratio of emitted N gases, i.e. CEF3, can vary between 0 and 1 (Granli and B0ckman, 1994). Applying CEF3 to aquatic systems means to compare the beneficial and harmful effects of denitrification, i.e. improving water quality and polluting the atmosphere, respectively. Thus CEF3 could be used to classify the environmental tolerance of denitrification in various systems. Determining CEF3 for groundwater, rivers and estuaries would help to answer the question of how N2O emission resulting from NO3~ flow across these systems is influenced, if measures are taken to enhance upstream denitrification in order to reduce NO3~ discharge to the oceans, for example by riparian and wetland restoration projects (Groffman et al, 2000).

Figure 8.2 (a) Modelled NO3~, N2O and N2 during ongoing progress of denitrification; and (b) resulting EFs CEF1, CEF2 and CEF3

Figure 8.2 (a) Modelled NO3~, N2O and N2 during ongoing progress of denitrification; and (b) resulting EFs CEF1, CEF2 and CEF3

Note: Reaction progress is the ratio between denitrified and initial NO3~. Concentrations of NO3, N2O and N2 are modelled assuming a two-step reaction (NO3~ to N2O; N2O to N2) and using first-order kinetics. For the rate coefficients of the NO3~-to-N2O step and N2O-to-N2 steps, a 1:20 ratio was set to fit maximum N2O concentration approximately to observations (see Table 8.1). N2O emission to the atmosphere in relation to NO3~ reduction during each time step is assumed to be 0 (closed systems) or >0 (0.02 or 0.1, semi-open systems). Source: Adapted from Well et al (2005b)

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