Ozone source apportionment during peak summer events over southwestern Europe
Authors: Pay María Teresa , Gangoiti Gotzon , Guevara Marc , Napelenok Sergey , Querol Xavier , Jorba Oriol , Pérez García-Pando Carlos 
Published: April 2019

Abstract. It is well established that in Europe, high O3
concentrations are most pronounced in southern/Mediterranean countries due to
the more favourable climatological conditions for its formation. However, the
contribution of the different sources of precursors to O3 formation
within each country relative to the imported (regional and hemispheric)
O3 is poorly quantified. This lack of quantitative knowledge
prevents local authorities from effectively designing plans that reduce the
exceedances of the O3 target value set by the European air quality
directive. O3 source attribution is a challenge because the
concentration at each location and time results not only from local biogenic
and anthropogenic precursors, but also from the transport of O3 and
precursors from neighbouring regions, O3 regional and hemispheric
transport and stratospheric O3 injections. The main goal of this
study is to provide a first quantitative estimation of the contribution of
the main anthropogenic activity sectors to peak O3 events in Spain
relative to the contribution of imported (regional and hemispheric)
O3. We also assess the potential of our source apportionment method
to improve O3 modelling. Our study applies and thoroughly evaluates
a countrywide O3 source apportionment method implemented in the
CALIOPE air quality forecast system for Spain at high resolution
(4 × 4 km2) over a 10-day period
characterized by typical summer conditions in the Iberian Peninsula (IP). The
method tags both O3 and its gas precursor emissions from source
sectors within one simulation, and each tagged species is subject to the
typical physico-chemical processes (advection, vertical mixing, deposition, emission and chemistry) as
the actual conditions remain unperturbed. We quantify the individual
contributions of the largest NOx local sources to high
O3 concentrations compared with the contribution of imported
O3. We show, for the first time, that imported O3 is the
largest input to the ground-level O3 concentration in the IP,
accounting for 46 %–68 % of the daily mean O3
concentration during exceedances of the European target value. The hourly
imported O3 increases during typical northwestern advections
(70 %–90 %, 60–80 µg m−3), and decreases during
typical stagnant conditions (30 %–40 %,
30–60 µg m−3) due to the local NO titration. During stagnant
conditions, the local anthropogenic precursors control the O3 peaks
in areas downwind of the main urban and industrial regions (up to 40 % in
hourly peaks). We also show that ground-level O3 concentrations are
strongly affected by vertical mixing of O3-rich layers present in
the free troposphere, which result from local/regional layering and
accumulation, and continental/hemispheric transport. Indeed, vertical mixing
largely explains the presence of imported O3 at ground level in the
IP. Our results demonstrate the need for detailed quantification of the local
and remote contributions to high O3 concentrations for local
O3 management, and show O3 source apportionment to
be an essential analysis prior to the design of O3
mitigation plans in any non-attainment area. Achieving the European O3 objectives in southern
Europe requires not only ad hoc local actions but also decided national and
European-wide strategies.