Oxidized VOCs are both significant sources and sinks of free radicals. Indeed, these compounds can be very reactive with the atmospheric oxidants (OH, NO3, O3) and important sources of radicals in their photolysis. This is the particular case of aldehydes. Generally, they are essential precursors to the formation of tropospheric ozone and other photo-oxidants species. A certain number of campaigns conducted in recent years show the limits of our knowledge on these issues, indicating such an overestimation by the models of the levels of radicals (especially of peroxyl radicals) and ozone production rates (greater than 10 ppbv/h and up to 50 ppbv/h) in studied urban plumes

Under these various findings, it appears essential to improve our understanding of the processes involved to assess the role of VOCs and their oxidation products on the balance sheet of the photo-oxidants species and in particular radical species. In this context, the work of LISA decomposes into two parts :

• The study of the speciation of secondary organic components and their evolution according to the photochemical age of the masses of air by field campaigns. To answer this question, LISA was particularly involved in international campaigns (MEGAPOLI(2009-2010), AMMA (2005-2007) (Figure1)) and is involved in the CHARMEX campaign planned for 2012-2013. One of the objectives of this latest campaign is to document the oxidation of organic carbon in air masses of different photochemical ages.

Figure 1: Changes in concentrations of isoprene with altitude and latitude, measured in Nigeria during the AMMA campaign (2006) by airborne measurements with the instrument AMOVOC.

 

• The study of the kinetics and mechanisms of anthropogenic and biogenic unsaturated VOCs oxidation by NO₃ radical in simulation chambers (Figure 2). Indeed, recent work has shown that the nitrate radical is a major sink of unsaturated and highly reactive VOCs like terpenes at night, as well as during the day. NO3 initiated oxidation leads to the formation of very different oxidation products of those formed in the oxidation by the radical OH and the impact on the balance sheets of photo-oxidants is itself very different. In addition to simulation chamber experiments, LISA is involved in the development of a predictive method of Structure-activity (SAR) to estimate the kinetics of the oxidation of VOCs by the NO₃ radical.

 

Figure 2: Mechanism of oxidation of vinyl ethers by the NO3 radical obtained from experiments in simulation chambers