Photo-oxidant pollution in source and transport regions: a comparative study at northern mid-latitudes, in the Mediterranean and in West Africa

Abstract

Ozone and its gaseous precursors, nitrogen oxides (NOx) and Volatile Organic Compounds (VOC), are still major air pollutants nowadays. Ozone concentrations are driven by the emissions of its precursors, its photochemical production and consumption, its atmospheric transport and deposition. Reducing effectively ozone levels to mitigate its adverse effects on air quality, climate, health and the environment requires a good representation of all these factors. However, the latters are still uncertain including those relating to VOC emissions, their chemical transformation and the complex and non-linear relationship between ozone and its precursors. In this context, my thesis aims at : (1) a better understanding of the emission sources of anthropogenic VOC at urban scale and (2) a better understanding of their fate and the one of photo-oxidants from observations collected in three contrasting regions: Northern mid –latitudes (Paris), the Mediterranean Basin and West Africa. In-situ surface and airborne observations from the MEGAPOLI, ChArMEx and DACCIWA international campaigns with the French ATR-42 from Safire have been analyzed by a source-receptor approach and the implementation of original and commonly used metrics. Urban sources of VOC were first assessed for the megacity of Istanbul through the implementation of the multivariate model PMF (Positive Matric Factorization) and the emissions ratios applied to COV data of the TRANSEMED campaign, a sister project of ChArMEx. The results show that the traffic is not the dominant source of VOC in Istanbul (16%) contrary to the other cities in the East Mediterranean. Moreover, global inventories largely underestimate the emissions of VOC. Secondly, the Ozone-NOy-VOC system has been characterized and compared within the boundary layer plumes of the three regions of interest from an evaluation based on the photostationnary state and the chemical regimes, the potential reactivity of VOC measured by the air