Simultaneous study of gas and particulate products formed from isoprene oxidation : laboratory and field measurements
Résumé
Isoprene is one of the most abundant non-methane hydrocarbons emitted by vegetation into the troposphere. For a long time, it has been generally accepted that isoprene oxidation did not contribute to global SOA burden because of the volatility of its main oxidation products (methacrolein, methyl vinyl ketone...) (Pandis, 1991). It is only recently that isoprene oxidation contribution to SOA formation has been proven (Edney, 2005; Kroll, 2006). There are still remaining uncertainties on the importance of isoprene contribution to atmospheric SOA and further studies combining ambient measurements of tracers, identified in laboratory experiments, coupled with atmospheric modeling are needed to better parameterize SOA yields and quantitatively model SOA production from isoprene oxidation (Carlton, 2009). Therefore, the first part of this work focuses on laboratory experiments to study isoprene oxidation under realistic conditions in the EUropean PHOtoREactor (EUPHORE) in Valencia during the period 13-22 September 2010 (Figure 1(a)): seed aerosols (10 µg/m3) were present in the chamber before isoprene and H2O2 introduction (70 ppb and 4 ppm respectively) under sunlight irradiation and a relative humidity close to 30 %. Moreover this study of the (isoprene + OH) reaction was performed under free-NOx condition. A method recently developed in our laboratory (Rossignol, 2012) was used to investigate the chemical composition of both gas and particulate phases of SOAs formed. This method consists in using stainless steel Tenax-TA adsorbent tubes previously coated with PFBHA (to specifically study carbonyl compounds) or MTBSTFA (to specifically study hydroxyl compounds) to collect the gas phase. Particulate sample are collected onto filters (quartz and Teflon-Quartz) subsequently exposed to PFBHA or MTBSTFA before analysis. These tubes and filters are subsequently analyzed by thermaldesorption coupled with gas chromatography and mass spectrometry (TD-GC-MS) analysis. Figure1: (a) EUPHORE smog chamber, (b) St.Michel l'Observatoire with OHP in the background About thirty compounds have been positively or tentatively identified in gas and/or particle phases. Some of these are monofunctionalized such as methacrylic acid or methacrolein, others are polyfunctionalized such as malic acid. These compounds have been quantified and for those identified in both gas and particulate phases, an experimental partitioning coefficient was calculated. In a second time, these results will be compared to real atmosphere samples during a field campaign which will take place in June 2012 at the Observatoire de Haute provence (OHP; situated in southeast France) (Figure 1(b)) as a part of the CANOPEE field campaign. This work falls within the framework of a research program about the study of combined model-measurement of intra canopy chemistry. Not only gas and particles samplings will be realized but particles formation and isoprene emission will be observed at the same time. Results obtained in both smog chamber and field campaign during isoprene oxidation will be presented and discussed