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

(c) The importance of secondary organic aerosol in assessments of air
quality–climate interactions has become more apparent since the 2007
AQEG report. However, there are now open questions concerning
biogenic versus anthropogenic sources. For example, there is some
evidence that a major fraction of organic aerosol (OA) may be derived
from anthropogenically modified biogenic volatile organic compounds
(i.e. biogenic secondary OA is formed preferentially in polluted
environments). Understanding such links is highly relevant to policy
measures because it means that changes in gas phase pollutants
(NOx, ozone, etc.) might impact PM and aerosol forcing. The impact of
climate on biogenic emissions is still an open question.
(d) It is important to review our understanding of how the aerosol system
will respond to changes in anthropogenic emissions and to integrate
our knowledge about PM and climate-relevant particles. It is important
to understand how PM concentrations and climate-relevant particles
might respond differently to changing emissions. The future trajectory of
anthropogenic secondary organic aerosol (SOA) is very uncertain because
of changes in the nature of the emissions as well as the response of SOA
to atmospheric chemistry changes (NOx, ozone, etc.). Changes in some
emissions are likely to have a large effect on particle mass but a small
effect on number (e.g. non-nucleating semi-volatile species or large
primary particles), while for some emissions the opposite may be true
(e.g. emissions of numerous small particles from combustion that may
affect climate but not PM). These climate and air quality issues should be
considered in an integrated way.
(e) Climate change will exert a potentially important upward pressure on PM
for many reasons that are not well understood. The effects of climate
change on PM and climate-relevant particle number concentrations
should be assessed, recognising that these parameters may behave
differently. Also, it would be useful to understand the importance of:
(i) changes in atmospheric circulation patterns which will affect the
occurrence of blocking anticyclonic weather (and the build-up of PM
extremes);
(ii) temperature increases causing greater biogenic VOC emissions and
consequently increased biogenic SOA;
Introduction
(iii) changes in removal processes, primarily wet scavenging. This is likely to
dominate future changes in PM, but maybe only on a long-term average
basis with little effect on PM extremes; and
(iv) changes in atmospheric chemistry affecting oxidants, nitrate formation
and SOA chemistry.
17