Atlas of Ozone chemical regimes in Europe
While concentrations of most air pollutants have been decreasing in Europe over the past 20 years, ozone (O3 ) is showing variable trends, with increasing average concentrations and decreasing peak ones. The complexity of O3 chemistry adds to the difficulty of understanding both the trends observed and how concentrations can be mitigated.
This article tries to answer the following questions: which emission sectors should be targeted, and what levels of reduction could be achieved? To address these reflections, an ‘Atlas of ozone chemical regimes in Europe’ has been compiled. For this Atlas, 22 European cities were selected and the surrogate model Air Control Toolbox (ACT) was used toevaluate the simulated changes in several ozone metrics as a result of reductions in road transport and industrial emissions. Ozone chemical regimes have been classified and put in perspective with meteorological and emissions data in and around each city location. The O3 sensitivity to road transport and industrial emissions differ from one city to another, but also for the same city when considering different ozone metrics and seasons (e.g. annual means versus SOMO35 or summer peaks). Counterproductive impacts yielding an increase in O3 when emissions are reduced are mainly encountered in regions or during periods where O3 concentrations are relatively low. In terms of meteorological factors, O3 chemical regimes are mostly impacted by the amount of solar radiation received, but wind speed also has a considerable impact. Most cases show a higher sensitivity to reductions in road transport emissions, or equal sensitivity to emission reductions from road transport and industry. Very few cases are most sensitive to emissions from the industrial sector. However, the response of annual or seasonal average O3 metrics to industrial and road transport emissions can be considered relatively low with a maximum reduction of 33% for a 100% reduction of both industrial and road transport emissions. This is because anthropogenic emissions can only mitigate ozone above a substantial natural tropospheric background. But it is precisely this incremental anthropogenic ozone that should be targeted by efficient policies in order to achieve significant improvements in attaining the European target values in all of the cities studied.