Revealing nighttime atmospheric chemistry and its impact on air quality in urban environments – NIGHT-SIGN

Air pollution has been recognized as a major worldwide public health problem by the World Health Organization. Pollution mitigation policies have proved effective in reducing exposition to primary pollutants but are insufficiently effective in reducing secondary pollutants. This underlines the need for deep understanding of complex atmospheric processes. Previous research aimed at characterizing chemical processes occurring during daytime while numerous studies have highlighted the need of fully considering nighttime chemistry to tackle air pollution issues. During nighttime, atmospheric chemistry is mainly driven by the nitrate (NO3) radical which initiates the oxidation of volatile organic compounds (VOCs). This chemistry contributes to the formation of secondary organic aerosols which affect air quality and climate, but also of organic nitrates which act as reservoirs for reactive nitrogen and affect NOx and ozone budgets at regional and global scales. Despite these impacts, investigation of urban nighttime chemistry remains very scarce. The NIGHT-SIGN project proposes an innovative approach which combines laboratory studies, field measurements and modelling activities to highlight nighttime chemistry and its impact on air quality in urban areas. It aims at answering two questions: Are there any specific signatures of NO3-VOC chemistry that could be used to highlight its contribution and impact on air quality in urban environments? What is the current impact of NO3-VOC chemistry on urban air quality, and what will it be in the future in the context of NOx emission reductions? Experiments will be conducted in simulation chamber to investigate NO3-VOC reactions and identify specific signatures which will then be searched in urban environment thanks to intensive measurement campaign near Paris city. Chemical-transport model will be used to assess the impact of nocturnal chemistry on air quality and its evolution in the context of NOx emission reductions.