Chemistry Reference
In-Depth Information
as well as from nature. Anthropogenic combustion sources as well as (small) stable
emissions have been inventoried. Half of the observed concentrations of OC
[
21
,
22
] can be explained. Hence, a significant (unknown) contribution is from
anthropogenic sources like secondary formation, wildfires, harvesting, abrasion
processes and biological material (fungal spores and plant debris). As mentioned
previously,
14
C analysis suggests that some 70% of the OC mass in the Netherlands
is of living material where biogenic SOA, agriculture and other landscaping
activities may have contributed. The complicating factor in the estimation of the
natural part of OM is the uncertainty related to its secondary component (oxidation
of VOCs yields products with low vapour pressures that may condense on existing
aerosol) as the major formation routes are not well known. As a consequence, the
ratio of natural to anthropogenic SOA is under debate. It is postulated that biogenic
sources are a major contributor to atmospheric SOA (e.g. [
22
]). Assuming that the
unexplained OC using present-day emission inventories is for the largest part SOA,
an upper limit of around 50% for natural OM is obtained. As a substantial part of the
SOA may very well be anthropogenic, it is postulated that half of the SOA as
natural, leading to a lower limit of 25% for natural OM.
Sources of mineral dust in total PM10 are wind erosion of bare soils, agricultural
land management, resuspension of road dust from paved an unpaved roads, road
wear, handling of materials and building and construction activities. Only wind
erosion may contribute to the natural fraction. Saharan dust is regularly transported
to countries around the Mediterranean Sea. In north-western Europe, dust transport
from the Sahara occurs once or twice a year and is therefore not very significant.
Korcz et al. [
23
] and Schaap et al. [
24
] show that windblown dust at the European
continent is a rather small source compared to traffic resuspension and agricultural
land management. More importantly, the windblown dust source strength from
soils other than arable land is low. Windblown dust emissions are strongly related to
anthropogenic changes in surface vegetation cover and are regarded as mostly
anthropogenic. Consequently, the total mineral dust concentration in air is expected
to be anthropogenic for a large part. In the approximation, 10% is assumed as a
conservative estimate for the natural contribution to MD.
The emission of sea salt is mainly dependent on wind speed. It is considered the
second largest contributor in the global aerosol budget, as a vast area of the earth
consists of sea. The aerosols consist mainly of sodium chloride. Other constituents
of atmospheric sea salt reflect the composition of sea water (magnesium, sulphate,
calcium and potassium). Sea salt is the only pure natural aerosol component.
The anthropogenic contribution of the unknown part is by definition unknown.
Water accounts to some extent for the unknown fraction. As most of that water may
be associated with SIA, it is likely that a significant part of the unknown fraction is
anthropogenic. It is assumed that the anthropogenic and natural parts of the
unknown fraction resemble those of the defined mass.
The natural fraction in the total mass can now be obtained by adding the relative
natural mass contributions per constituent. For instance, 10% of PM10 is associated
with sea salt adding to 10% of natural PM10 (see Table
2
). Note that this fraction
represents the source attribution and therewith fresh sea salt as discussed in the
