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Accueil du site > Thèmes de recherche > Atmosphère & environnement > 2.3 Réactions contrôlant le pouvoir oxydant de la troposphère

2.3 Réactions contrôlant le pouvoir oxydant de la troposphère

These studies concern the formation of nitric acid and organic nitrates, RONO2 (R = alkyl group) in the minor channels (b) of the reactions between the peroxy radicals, HO2 or RO2, with NO : HO2 (or RO2) + NO → OH (or RO) + NO2 (a) HO2 (or RO2) + NO → HNO3 (or RONO2) (b) Nitric acid and organic nitrates play an important role in the tropospheric ozone budget, as “sink” or “reservoir” species for both NOx and HOx (OH, HO2). They, therefore, influence the oxidative capacity of the troposphere and the concentrations of reactive greenhouse gases (methane, ozone, hydrofluorocarbons…). The formation yields of HNO3 or RONO2 (or branching ratios kb/ka) are determined using a turbulent flow reactor coupled with a chemical ionisation mass spectrometer (CIMS). This technique allows measurements in the whole range of tropospheric temperatures and pressures (220-300 K, 70-760 Torr), and in the presence of water vapor. The combination of CIMS analysis in both positive and negative modes provides a sensitive and selective detection of the species, in particular HNO3 and RONO2 which are produced at very low yields (less than a few percents).

2.3.1 Formation of HNO3 in the HO2 + NO reaction
The formation of HNO3 in the HO2 + NO reaction has been observed for the first time and the HNO3 yield has been determined in the whole ranges of tropospheric temperatures and pressures. This yield ranges from 0.5% at the Earth surface to 0.9% in the tropopause region (ACL 82). The integration of these data in chemistry-transport atmospheric models indicates that they have a significant impact on the concentrations of species like HOx, NOx, HNO3 and O3, in particular in the tropical upper troposphere (ACL 85). Numerous and complex experiments due partly to CIMS detection complications in the presence of H2O, have led to observe a H2O enhancement of the HNO3 formation yield in the HO2 + NO reaction (for example an increase by a factor 8 at 50% relative humidity at 298 K and 200 Torr). This effect has been confirmed by a theoretical study, and it has been interpreted as a fast HNO3 formation in the reaction of NO with the HO2·H2O complex (ACL 94).

2.3.2 Organic nitrate formation in the RO2 + NO reactions
The studies on organic nitrate formation in the RO2 + NO reactions focused on the short chain alkyl nitrates C2H5ONO2 and i-C3H7ONO2. The related peroxy radicals RO2 are derived from the atmospheric oxidation of ethane and propane, respectively, and from larger hydrocarbons. The ongoing study concerns the formation of hydroxypropyl nitrate isomers in the reaction of NO with the hydroxypropyl peroxy radicals derived from the OH-initiated oxidation of propene. The formation yields of these different nitrates have been determined as a function of pressure and also as a function of temperature for C2H5ONO2 (ACL 107, 108). All these yields, which are lower than 3%, could be precisely determined by using a novel CIMS detection process of the nitrates (ionisation by the F- ion). The obtained data are available to be included in chemical mechanisms of atmospheric models, either explicitly or indirectly by contributing to improve the structure-activity relationships used to reduce these mechanisms. The direct integration of our data in chemistry-transport models should allow assessing the impact of the studied organic nitrates on the VOC/NOx/ozone chemistry in urban plumes and in the whole troposphere. These studies have been carried out within the EU FP6 SCOUT integrated project, the LEFE programme of CNRS-INSU and the ongoing Primequal programme of the French Ministry of Ecology (MEEDDM).