Analysis of emissions-driven changes in the oxidation capacity of the atmosphere in Europe

Anthropogenic emissions in Europe have been gradually reduced thanks to a combination of factors, including restrictive regulation and policy implementation, fuel switching, technological developments, and improved energy efficiencies. Many measures have been specifically introduced to meet the annual and hourly limit value of NO₂ for the protection of human health, mainly targeting traffic emissions. Due to NO_X reduction policies in Europe, NO₂ levels have generally declined, but O₃ concentrations have been found to increase. This phenomenon would cause changes in the oxidant capacity of the atmosphere, altering the concentration of tropospheric oxidants in urban areas. The Community Multiscale Air Quality (CMAQ) modelling system has been used to study concentration changes of NO₂, O₃ and the main radicals in Europe between 2007 and 2015 for two months representatives of winter and summer conditions (January and July). In addition to describing the general situation in Europe, variations in pollutants along with NO_X emission changes over 67 large European cities have been analysed by means of statistical methods. NO_X emissions and NO₂ concentrations decreased in both seasons during the period in all the selected cities. In most of them O₃ concentrations increased in winter but decreased in summer. The concentration of the OH radical, the main oxidant during the daytime, shows an increase in winter. This is also the case for the main cities in summer although we found a general decrease in continent for this season. The NO₃ radical, the main night-time oxidant, was found to increase in winter and decrease in summer. HNO₃ shows a concentration decline in both seasons. The studied cities are classified in five groups by means of k-mean clustering procedure. We identified five groups with specific patterns, suggesting that the oxidant capacity of the European urban atmospheres has reacted differently to NO_X emission abatement policies.