Dr Willem Asman concluded that the major global sources of atmospheric
NH3 are excreta from domestic
animals and fertilizers. A question raised was: how reliable are the
emission estimates and extrapolations?
The answer was that emission estimates are surrounded by uncertainty,
which is a major handicap to sound
modelling of NH3 dry deposition and, consequently, to obtaining
good estimates of critical load exceedences.
Major uncertainties in emission estimates seem to be related to the
use of
simple emission factors, many
of which are highly empirical or have been derived from measurements
carried out under conditions which
deviate considerably from those following modern practices of handling
and applying manure and fertilizers.
An example is provided by the commonly used emission factors for
synthetic fertilizers (see e.g. Bouwman
et al. (1997)), which are much higher than recent
micrometeorological assessments seem to suggest. Thus,
emission from urea, the most widespread fertilizer used in the
world (currently around 55% of world N
consumption) can be completely avoided if the fertilizer is
incorporated into the upper soil layers. Similarly,
a growing crop can reduce losses to well below 10% of the applied amount
of urea-N, i.e. to less than half
of the generally used emission factors of 15% for Europe and
25% for the tropics. The emission factor for
NPK-fertilizer is set at 4%, whereas that for pure calcium-ammonium-nitrate,
the same N compound as is present in NPK-fertilizers, is assumed to be
only 2%.