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

PM2.5 in the UK
16
in ozone in the background troposphere but increases in more polluted regions
where there are higher concentrations of NOx. There could also be an increase
in the flux of ozone from the stratosphere to the troposphere. Hot summers,
like that of 2003 when there was a substantial photochemical smog episode in
Europe, including the south-east of England, are likely to become ‘typical’ by
the 2040s, leading to a higher frequency of summer pollution episodes. There
was evidence that emissions of volatile organic compounds from vegetation
played a role in the 2003 episode; increases in temperature led to increases in
emissions of biogenic compounds such as isoprene.
Future challenges
27. Several challenges remain in understanding the impacts of PM on climate
change and of climate change on air quality:
(a) Modelling of air quality–climate interactions has tended to focus on
time-averaged responses rather than responses under meteorological
conditions that are more relevant for air quality episodes. For example,
global modelling studies have calculated the response of ozone to
various global warming scenarios averaged over seasons and at low
resolution. There is a need to examine air quality–climate interactions
on much finer time and space scales and during episodic conditions
relevant to winter and summer pollution events. For example, arguments
about the dominant effect of increasing precipitation on pollutants are
irrelevant for anticyclonic pollution episodes in which precipitation plays
no role.
(b) Our understanding of air quality–climate interactions involving aerosols
needs to be updated and reappraised to provide a UK or European
picture. Our current simplistic view of how decreases in PM would cause
positive radiative forcing (through direct and indirect effects) is a global
mean picture that may not be applicable in a UK context. Developments
in our understanding of aerosol microphysics from models, combined
with new intensive aerosol observations over Europe, have provided a
much better picture of how PM and aerosol microphysics are related.
There are several examples of how our understanding of aerosol air
quality–climate effects has changed, including: (i) changes in surface
aerosol (e.g. on a daily or seasonal timescale) do not always correspond
to changes at cloud level (which would be needed to drive an indirect
forcing). Nitrate aerosol over Europe has also been observed to be much
more prevalent at cloud level than at the surface, due to the effect of
temperature; (ii) there is evidence that carbonaceous combustion aerosol
may form a large fraction of cloud condensation nuclei, which might
make arguments about mitigation of black carbon too simplistic. Overall,
a more sophisticated approach to appraising climate versus air quality
impacts of aerosols needs to be considered, rather than just relying on
generic model results.