Direct Observation of Secondary Organic Aerosol Formation during Cloud Condensation-Evaporation Cycles (SOAaq) in Simulation Chamber Experiments

Friday, 19 December 2014: 1:40 PM
Jean-Francois Doussin1, Lola Bregonzio-Rozier1, Chiara Giorio2,3, Frank Siekmann4, Aline Gratien1, Brice Temime-Roussel4, Sylvain Ravier4, Edouard Pangui1, Andrea Tapparo2, Markus Kalberer3 and Anne Monod4, (1)Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS UMR7583, Universités Paris-Est Créteil et Paris Diderot, Institut Pierre Simon Laplace (IPSL), Créteil, France, (2)Università degli Studi di Padova, Department of Chemistry, Padova, Italy, (3)University of Cambridge, Department of Chemistry, Cambridge, United Kingdom, (4)Aix Marseille University, Marseille Cedex 03, France
Biogenic volatile organic compounds (BVOCs) undergo many reactions in the atmosphere and form a wide range of oxidised and water-soluble compounds. These compounds can partition into atmospheric water droplets, and react within the aqueous phase producing higher molecular weight and/or less volatile compounds which can remain in the particle phase after water evaporation and thus increase the organic aerosol mass (Ervens et al., 2011; Altieri et al., 2008; Couvidat et al., 2013). While this hypothesis is frequently discussed in the literature, so far, almost no direct observations of such a process have been provided.
The aim of the present work is to study SOA formation from isoprene photooxidation during cloud condensation-evaporation cycles.
The experiments were performed during the CUMULUS project (CloUd MULtiphase chemistry of organic compoUndS in the troposphere), in the CESAM simulation chamber located at LISA. CESAM is a 4.2 m3 stainless steel chamber equipped with realistic irradiation sources and temperature and relative humidity (RH) controls (Wang et al., 2011). In each experiment, isoprene was allowed to oxidize during several hours in the presence on nitrogen oxides under dry conditions. Gas phase compounds were analyzed on-line by a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF-MS), a Fourier Transform Infrared Spectrometer (FTIR), NOx and O3 analyzers. SOA formation was monitored on-line with a Scanning Mobility Particle Sizer (SMPS) and an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The experimental protocol was optimised to generate cloud events in the simulation chamber, which allowed us to generate clouds lasting for ca. 10 minutes in the presence of light.
In all experiments, we observed that during cloud formation, water-soluble gas-phase oxidation products (e.g., methylglyoxal, hydroxyacetone, acetaldehyde, formic acid, acetic acid and glycolaldehyde) readily partitioned into cloud droplets and new SOA mass was promptly produced which partly persisted after cloud evaporation. Chemical composition, elemental ratios and density of SOA, measured with the HR-ToF-AMS, were compared before, during cloud formation and after cloud evaporation.