Chemistry Reference
In-Depth Information
2.4 Emission of NH
3
from Other Agricultural Sources
The remaining agricultural sources relate to emission from vegetation. Legumes and
plants taking up excess fertiliser are emitting NH
3
[
20
]. This emission depends on
the enrichment of the apoplast with NH
4
+
and the so-called compensation point [
30
].
The compensation point is a function of the plant status with respect to growth,
stress, etc. The emission is still not well described with respect to magnitude, as well
as temporal and spatial variation. Emissions from crops are often observed after
fertilisation with either manure (e.g. [
58
]) or mineral fertiliser (e.g. [
59
]). Grazing
(e.g. [
60
]) as well as cutting (e.g. [
61
]) of grass is known to release NH
3
. The
management of the crops can heavily influence the loss of NH
3
to the atmosphere.
However, little is known about these processes, and only few studies of N-exchange
between atmosphere and vegetation cover the entire season exploring the full cycle
of growth and decay [
22
]. Overall, the emission from vegetation is highly coupled to
both crop management and the atmospheric concentrations of NH
3
. Several mecha-
nistic descriptions of the compensation point have been derived [
31
,
62
], and these
rely highly on detailed information on agricultural production methods which for
several decades have been difficult to obtain and generalise (e.g. [
6
,
63
]).
2.5 Emissions from Nonagricultural Sources, e.g. Urban Areas
and Traffic
Emissions from nonagricultural sources are in general not well described but
include sweat from humans, excreta from pets and wild animals, exhaust from
gasoline cars with catalytic converters, stationary combustion sources and industry
and evaporation from waste deposits [
24
]. In Europe, the largest national nonagri-
cultural NH
3
emission has been reported for the UK with a fraction of about 15%
[
24
], a figure that may be compared with about 2.8% from traffic and 0.8% from
manufacturing processes and sewage sludge that has been reported for Denmark
[
64
]. A number of European studies have shown elevated NH
3
concentrations near
roads and in urban areas (e.g. [
65
]). Whether these elevated concentrations are due
to emission from traffic (e.g. Table
1
), humans or sewage or there are other
significant sources in the urban area has not been shown. Another possible expla-
nation for elevated concentrations at these locations could be enhanced evaporation
of NH
3
from NH
4
+
-containing salts, as many of the very high observed values are
reported for Southern and Central Europe such as Barcelona [
27
], Rome [
66
]or
Croatia [
67
]. Experimental studies have shown that over the sea, the atmospheric
fluxes of NH
3
may also be upward or downward [
33
,
68
,
69
] depending on the
meteorological conditions and the relationship between the pH and the NH
4
+
concentration in the upper surface waters, on the one side, and the NH
3
concentrations in ambient air just above the water surface, on the other side.
Similarly studies over
forests also indicate bidirectional flux patterns NH
3
