The relationship between emissions and deposition of air pollutants,
both spatially and in time forms an important focus for
science and for policy makers. In practice, this relationship
may become nonlinear if the underlying processes change
with time, or in space. Nonlinearities may also appear due to
errors in emission or deposition data, and careful scrutiny of
both data sources and their relationship provides a means of
picking up such deficiencies. Nonlinearities in source receptor relationships for sulfur and nitrogen compounds in Europe have been identified in measurement data for the UK. In the case of sulfur, the dry deposition process has been shown to be strongly influenced by ambient concentrations of NH3,leading to substantial increases in deposition rate as SO2 concentrations decline and the ratio SO2/NH3 decreases. The field evidence extends to measurements over three different surfaces in three countries across Europe.
A mechanistic
understanding of the cause of this nonlinearity has been provided.
Apparent nonlinearities also exist in the sulfur deposition
field through the influence of shipping emissions. The
effect is clear at west coast locations, where during a period
in which land-based sulfur emissions declined by 50%, no
significant decline in concentrations of SO4
2- in precipitation
were observed. The sites affected are primarily the coastal
regions of southwestern UK, where shipping sources contribute
a substantial fraction of the deposited sulfur, but the
effect is not detectable elsewhere. Full quantification of the
spatially disaggregated emission and their changes in time
will eliminate this apparent nonlinearity in the source—receptor
data. For oxidized nitrogen emission and deposition
in the UK, there is strong evidence of nonlinearity in the
source—receptor relationship. The concentrations and deposition
of NO 3 - in precipitation have declined little following
a reduction in emissions of 45% during the period 1987 to
2001. The data imply a significant decrease in the average
transport distance for oxidized nitrogen and most probably
an increase in the average oxidation rate. However, the
net effect of changes in aerosol chemistry due to changes
in sulfur emissions and less competition for the main
oxidants as a consequence of reductions in sulfur emission
have not been separated. A quantitative explanation
of the cause of this nonlinearity is lacking and the effects
are therefore identified as an important uncertainty for the
development of further protocols to control acidification,
eutrophication and photochemical oxidants in Europe
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