This course introduces the mechanisms controlling atmospheric composition in the lower atmosphere in both remote and polluted environments. A particular focus is placed on the understanding of the oxidation capacity of the troposphere and the composition and properties of atmospheric aerosols. The main processes involved in the development of air pollution episodes at urban and regional scales, as well as the tools used in the scientific community and air quality management services for the monitoring and forecasting of air quality, are then described. The specific structure of the boundary layer and the associated chemical and dynamical processes are detailed, including emissions, deposition, and chemical evolution.
All aspects are introduced theoretically before providing a specific description of the practical application in modelling platforms. These models are presented in the context of the current air quality policies in Europe and key locks are presented to understand realistic reduction choices being discussed for air quality improvement and climate change mitigation strategies. Various current applications are described as extreme case analysis, scenario studies until operational forecast, pollution health impacts evaluation, chemistry-climate analyses.

Each course will be divided into a lecture and an associated tutorial with applied examples. In addition, students will work in groups on different practical projects with 2 sessions as numerical practical class. Example of project subjects:
• Chemical regimes: understanding the strategies to implement for the regulation of photo-oxidant pollution episodes (ozone) in the Paris area (modelling project);
• Atmospheric observation of air quality: what complementarities between observing systems? (data analysis project);

Course outline

1) Tropospheric composition (Lectures 1 – 2)
• General chemical composition of the troposphere gaseous and particles species to study relative amounts, life time, trends;
• Gaseous atmospheric chemistry
o The radical cycle
o Photochemical equilibrium
o NOx and VOCs chemistry
• Chemical regimes

2) Introduction to aerosols (Lecture 3)
• Characteristics of a population of aerosol: chemical composition and size distribution;
• Formation of atmospheric aerosols.

3) Modelling emissions, deposition, chemistry and transport (Lecture 4-5)
• General concepts and modelling choices: from box models to 3D Earth system models;
• Example of modelling platforms
• For the main natural and anthropogenic emissions sources: Description of the process, species emitted and inventory construction;
• Wet and Dry deposition.

4) Polluted boundary layer and air quality management (Lecture 6)
• Meteorological characteristics of the boundary layer
• Dispersion of pollutants
• Development of pollution episodes
• Impacts of air pollution
o Impact on environment: feedbacks between vegetation and surface atmospheric concentrations;
o Impact on health in urbanized environments

5) Interactions between air quality and climate (Lecture 7)
• Radiative forcing of trace species;
• Atmospheric pollution in a changing climate:
• Impact on meteorological conditions
• Impact on emissions
• Impact on chemistry
Implementing strategies for future mitigation assessments