Low modeled ozone production suggests underestimation of precursor emissions (especially NOx) in Europe
Authors: Oikonomakis Emmanouil , Aksoyoglu Sebnem , Ciarelli Giancarlo , Baltensperger Urs , Prévôt André Stephan Henry 
Published: February 2018

Abstract. High surface ozone concentrations,
which usually occur when photochemical ozone production takes place, pose a
great risk to human health and vegetation. Air quality models are often used
by policy makers as tools for the development of ozone mitigation strategies.
However, the modeled ozone production is often not or not enough evaluated in
many ozone modeling studies. The focus of this work is to evaluate the
modeled ozone production in Europe indirectly, with the use of the
ozone–temperature correlation for the summer of 2010 and to analyze its
sensitivity to precursor emissions and meteorology by using the regional air
quality model, the Comprehensive Air Quality Model with Extensions
(CAMx). The results show that the model significantly
underestimates the observed high afternoon surface ozone mixing ratios (≥ 60 ppb) by 10–20 ppb and overestimates the lower ones (< 40 ppb) by
5–15 ppb, resulting in a misleading good agreement with the observations
for average ozone. The model also underestimates the ozone–temperature
regression slope by about a factor of 2 for most of the measurement stations.
To investigate the impact of emissions, four scenarios were tested:
(i) increased volatile organic compound (VOC) emissions by a factor of 1.5 and 2 for the anthropogenic
and biogenic VOC emissions, respectively, (ii) increased nitrogen oxide (NOx) emissions
by a factor of 2, (iii) a combination of the first two scenarios and
(iv) increased traffic-only NOx emissions by a factor of 4. For
southern, eastern, and central (except the Benelux area) Europe, doubling
NOx emissions seems to be the most efficient scenario to reduce the
underestimation of the observed high ozone mixing ratios without significant
degradation of the model performance for the lower ozone mixing ratios. The
model performance for ozone–temperature correlation is also better when
NOx emissions are doubled. In the Benelux area, however, the third
scenario (where both NOx and VOC emissions are increased) leads to a
better model performance. Although increasing only the traffic NOx
emissions by a factor of 4 gave very similar results to the doubling of all
NOx emissions, the first scenario is more consistent with the
uncertainties reported by other studies than the latter, suggesting that high
uncertainties in NOx emissions might originate mainly from the
road-transport sector rather than from other sectors. The impact of meteorology
was examined with three sensitivity tests: (i) increased surface temperature
by 4 ∘C, (ii) reduced wind speed by 50 % and (iii) doubled wind
speed. The first two scenarios led to a consistent increase in all surface
ozone mixing ratios, thus improving the model performance for the high ozone
values but significantly degrading it for the low ozone values, while the
third scenario had exactly the opposite effects. Overall, the modeled ozone
is predicted to be more sensitive to its precursor emissions (especially
traffic NOx) and therefore their uncertainties, which seem to be
responsible for the model underestimation of the observed high ozone mixing
ratios and ozone production.