Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

PM2.5 in the UK
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17. Dry deposition is the direct deposition of aerosol to vegetation driven by
turbulent fluxes. This may be as ‘dry’ particles or as cloud droplets formed
by activation of the aerosols. Aerosol particles act as cloud condensation
nuclei and are efficiently incorporated into cloud water at the point of droplet
formation (Pruppacher and Klett, 2010). In upland forests the concentrations of
sulphur and nitrogen in cloud water may be much higher than in precipitation.
Concentrations of particulate sulphur in the range 1-3 µg m -3 incorporated into
cloud water may result in damage to foliage through the direct deposition of
cloud droplets (Cape, 1993). However, there appear to be no direct effects of
dry particles on vegetation except where leaf surfaces are covered, e.g. by dust
from industrial or agricultural activity.
18. Estimates of the direct dry deposition of particulate nitrogen and sulphate in
the UK have been made using the Concentration Based Estimated Deposition
(CBED) technique (Smith et al., 2000; RoTAP, 2012), which combines
measurements from the Acid Gases and Aerosols Monitoring Network
(AGANet) with vegetation-specific deposition velocities. Annual dry deposition
of particulate sulphate, nitrate and ammonium to the UK was estimated at
3 Gg sulphur (S), 7 Gg N-NOy (nitrogen as oxides of nitrogen) and 8 Gg N-NHx
(nitrogen as ammonia and ammonium) respectively, averaged over the period
2006-2010. This represents only a modest contribution to the total deposition
of 39 Gg S, 77 Gg N-NOy and 76 Gg N-NHx in the UK averaged over the period
2006-2010.
19. Particulate matter is efficiently removed from the atmosphere by the
mechanism of wet deposition. Wet deposition is the removal of matter from
the atmosphere by precipitation. The growth of cloud droplets leads to the
formation of raindrops which will deposit particulate matter contained in
solution to the earth’s surface. Due to the solubility of gases (SO2, nitric acid
(HNO3), NH3, etc.) in rainwater, it is not possible to distinguish the relative
contributions of gases and particulates by measurement of concentrations
in precipitation. Atmospheric transport models are able to demonstrate that
wash-out of particulate matter is the dominant mechanism for wet deposition
of sulphur and nitrogen, in particular in remote upland regions with sensitive
ecosystems. Fine PM is an air pollutant which is associated with long-range
transboundary transport. The EMEP model generates source–receptor matrices
of the contribution to deposition of sulphur and nitrogen in each European
country from emissions from all other countries. These modelling results show
that emissions of primary pollutants from non-UK sources (including other
countries and international shipping) contribute 43% of total sulphur deposition
and 46% of nitrogen deposition in the UK (Nyri et al., 2010) in the form of
long-range particulate transport. The particulate phase of nitrogen and sulphur
pollutants therefore represents an important link between primary gaseous
emissions, long-range transport and eventual deposition in precipitation to
ecosystems in the UK.
20. The indirect effects of PM on ecosystems through wet and dry deposition can
be assessed by the impact of total deposition on soil processes, and therefore
on ecosystems, expressed as a “critical load”. A critical load is defined as “A
quantitative estimate of an exposure to one or more pollutants below which
significant harmful effects on specified sensitive elements of the environment
do not occur according to present knowledge” (UBA, 2004), and is the main