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

13. However, these conclusions relate to PM as measured by mass as opposed to
different sources or components of PM. Airborne particles differ greatly from
place to place in size and chemical composition. There is currently no clear
understanding of which particle properties, such as their size or the presence
of specific chemical substances, are most responsible for the toxic effects.
COMEAP (2009), mirroring the position of the World Health Organization
(WHO), states that:
‘Our view is that particulate matter represented by PM2.5 is a complex and
possibly interacting mixture of many components, including sulphate, and
though these components may differ from one another in terms of their
toxicity, such data as we have do not allow confident separation of their effects
on health. In the absence of clear evidence to the contrary we consider that
the recommended coefficient should apply equally to all components of PM2.5,
including particulate matter measured as sulphate and nitrate. This is not to say
that all components of PM2.5 do have the same toxicity – but, rather, that there
is not, at present, evidence to quantify different components differently, in a
way that would gain wide consensus.’
14. Additionally, the composition of PM will change over time as further mitigation
measures are introduced and as new technologies (and therefore sources)
emerge. An improved understanding of the behaviour and composition of PM
will in turn help improve the understanding of its impacts on health.
1.3.2 Ecosystem impacts of PM2.5 in the UK
Introduction
15. PM2.5 may have both direct and indirect effects on ecosystems. In terms of direct
effects, aerosols are hygroscopic, often deliquescent, and can exist in liquid form
on transpiring leaves. Burkhardt (2010) has proposed the concept of “hydraulic
activation of stomata’’, whereby aerosols deposited on leaf surfaces enable the
efficient bi-directional transport of water and solutes between leaf interior and
leaf surface. If air pollution led to large accumulations of particulates on leaves,
the drought tolerance of trees might be affected, leading to regional tree dieback.
Aerosols may also have an indirect effect by modifying plant exposure to
sunlight. Both theoretical and observational studies have demonstrated that
photosynthesis is more efficient under diffuse light conditions. Mercado
et al. (2009) used a global model to estimate that “global dimming” caused by
increased global aerosol in the atmosphere enhanced the land carbon sink by
one quarter between 1960 and 1999.
16. The largest effects of man-made aerosols on ecosystems are likely to be indirect,
through their role as long-range vectors of air pollutants. Ammonium sulphate
and ammonium nitrate aerosol are formed by the atmospheric oxidation and
reaction of precursor gases (SO2, NOx) with NH3 (Seinfield and Pandis, 1998)
and comprise a major component of fine particulate matter. In this form, PM2.5
makes an important contribution to sulphur and nitrogen deposition leading to
the acidification and eutrophication of natural ecosystems. Surface deposition of
PM2.5 may occur via two different pathways: dry deposition and wet deposition.
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