Chemistry Reference
In-Depth Information
5 Discussion and Conclusions
This chapter provides a survey of the episodic peak ozone levels across Europe over
the 1980-2009 period as recorded by the EMEP ozone monitoring network.
The database of annual highest 8-h average ozone levels contains over 2,291 entries
for 178 monitoring stations. All monitoring stations in almost all years show
exceedance of the WHO air quality guideline of 50 ppb daily maximum 8-h average
ozone level.
The spatial distribution of the decadal average episodic peak ozone levels shows
a maximum in central Europe covering much of Belgium, Germany, Switzerland,
Slovenia, and northern Italy. Decadal average annual maximum 8-h average ozone
levels in this maximum region approach and exceed 90 ppb, just under twice the
WHO air quality guideline. Surrounding this maximum, there is a belt of countries
including the UK, Netherlands, Czech Republic, France, Austria, Poland and Spain
where decadal average annual maximum 8-h average ozone levels fall in the range
80-95 ppb. Outside of this belt, there are some monitoring stations on the fringes of
Europe where decadal average annual maximum 8-h average ozone levels still
exceed 60 ppb and are hence elevated above the WHO air quality guideline.
The origin of these episodic peak ozone levels is unquestionably photochemical
ozone formation from man-made NO
x
and VOC precursors. Efficient long-range
transboundary transport ensures that these episodic peak levels are observed at even
the most remote monitoring stations in the EMEP monitoring network. Equally well,
efficient transboundary transport ensures that responses to reductions in man-made
NO
x
and VOC precursor emissions should be felt across Europe in terms of
reductions in episodic peak ozone levels.
These episodic peak ozone levels have unquestionably responded to European
air quality policy initiatives that have secured reductions in man-made NO
x
and
VOC precursor emissions. There are two monitoring stations with long-running
time series covering the three decades from the 1980s to the 2000s. These stations
show highly statistically significant downwards trends of between
1.2
0.7 ppb
per year and
1.4 ppb per year. Examination of residuals shows no evidence of any
let up in these downwards trends during the 1990s and 2000s. These downwards
trends are fully consistent with expectations based on computer modelling studies
[
23
] based on published European emission inventories for man-made NO
x
and
VOCs over the 1990-2009 period. This modelling focuses on the GB0036 Harwell
monitoring station in the southern United Kingdom. However, the results should be
similar for other monitoring stations in northwest Europe.
There are 40 monitoring stations that have suitable monitoring records for trend
analyses covering the 1990-2009 period and a further 29 over the slightly reduced
1993-2009 period. Statistically significant (
a
0.1) downwards trends were found
at 23 out of the 69 stations. Downwards trends were larger for those stations with
higher initial episodic peak ozone levels in 1990 and decline to zero for those
stations with the lowest initial levels. This behaviour is best explained by the
resultant of two opposing influences. There is a downwards trend associated with
