Abstract. In the framework of the third phase of the Air Quality Model Evaluation
International Initiative (AQMEII3), and as contribution to the second phase
of the Hemispheric Transport of Air Pollution (HTAP2) activities for Europe
and North America, the impacts of a 20 % decrease of global and regional
anthropogenic emissions on surface air pollutant levels in 2010 are simulated
by an international community of regional-scale air quality modeling groups,
using different state-of-the-art chemistry and transport models (CTMs). The
emission perturbations at the global level, as well as over the HTAP2-defined
regions of Europe, North America and East Asia, are first simulated by the
global Composition Integrated Forecasting System (C-IFS) model from European
Centre for Medium-Range Weather Forecasts (ECMWF), which provides boundary
conditions to the various regional CTMs participating in AQMEII3. On top of
the perturbed boundary conditions, the regional CTMs used the same set of
perturbed emissions within the regional domain for the different perturbation
scenarios that introduce a 20 % reduction of anthropogenic emissions
globally as well as over the HTAP2-defined regions of Europe, North America
and East Asia. Results show that the largest impacts over both domains are simulated in
response to the global emission perturbation, mainly due to the impact of
domestic emission reductions. The responses of NO2, SO2
and PM concentrations to a 20 % anthropogenic emission reduction
are almost linear (∼ 20 % decrease) within the global perturbation
scenario with, however, large differences in the geographical distribution of
the effect. NO2, CO and SO2 levels are strongly affected
over the emission hot spots. O3 levels generally decrease in all
scenarios by up to ∼ 1 % over Europe, with increases over the hot
spot regions, in particular in the Benelux region, by an increase up to
∼ 6 % due to the reduced effect of NOx titration.
O3 daily maximum of 8 h running average decreases in all scenarios
over Europe, by up to ∼ 1 %. Over the North American domain, the
central-to-eastern part and the western coast of the US experience the
largest response to emission perturbations. Similar but slightly smaller
responses are found when domestic emissions are reduced. The impact of
intercontinental transport is relatively small over both domains, however,
still noticeable particularly close to the boundaries. The impact is
noticeable up to a few percent, for the western parts of the North American
domain in response to the emission reductions over East Asia. O3
daily maximum of 8 h running average decreases in all scenarios over north
Europe by up to ∼ 5 %. Much larger reductions are calculated over
North America compared to Europe. In addition, values of the Response to Extra-Regional Emission Reductions
(RERER) metric have been calculated in order to quantify the differences in
the strengths of non-local source contributions to different species among
the different models. We found large RERER values for O3
(∼ 0.8) over both Europe and North America, indicating a large
contribution from non-local sources, while for other pollutants including
particles, low RERER values reflect a predominant control by local sources. A
distinct seasonal variation in the local vs. non-local contributions has been
found for both O3 and PM2.5, particularly reflecting the
springtime long-range transport to both continents.
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