Ozone source apportionment during peak summer events over southwestern Europe

Pay, María Teresa; Gangoiti, Gotzon; Guevara, Marc; Napelenok, Sergey; Querol, Xavier; Jorba, Oriol; Pérez García-Pando, Carlos

It is well established that in Europe, high inline-formulaO3 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 inline-formulaO3 formation within each country relative to the imported (regional and hemispheric) inline-formulaO3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the inline-formulaO3 target value set by the European air quality directive. inline-formulaO3 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 inline-formulaO3 and precursors from neighbouring regions, inline-formulaO3 regional and hemispheric transport and stratospheric inline-formulaO3 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 inline-formulaO3 events in Spain relative to the contribution of imported (regional and hemispheric) inline-formulaO3. We also assess the potential of our source apportionment method to improve inline-formulaO3 modelling. Our study applies and thoroughly evaluates a countrywide inline-formulaO3 source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 inline-formula× 4 kminline-formula2) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both inline-formulaO3 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 inline-formulaNOx local sources to high inline-formulaO3 concentrations compared with the contribution of imported inline-formulaO3. We show, for the first time, that imported inline-formulaO3 is the largest input to the ground-level inline-formulaO3 concentration in the IP, accounting for 46 %–68 % of the daily mean inline-formulaO3 concentration during exceedances of the European target value. The hourly imported inline-formulaO3 increases during typical northwestern advections (70 %–90 %, 60–80 inline-formulaµg minline-formula−3), and decreases during typical stagnant conditions (30 %–40 %, 30–60 inline-formulaµg minline-formula−3) due to the local NO titration. During stagnant conditions, the local anthropogenic precursors control the inline-formulaO3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level inline-formulaO3 concentrations are strongly affected by vertical mixing of inline-formulaO3-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 inline-formulaO3 at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high inline-formulaO3 concentrations for local inline-formulaO3 management, and show inline-formulaO3 source apportionment to be an essential analysis prior to the design of inline-formulaO3 mitigation plans in any non-attainment area. Achieving the European inline-formulaO3 objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies.

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Pay, María Teresa / Gangoiti, Gotzon / Guevara, Marc / et al: Ozone source apportionment during peak summer events over southwestern Europe. 2019. Copernicus Publications.

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