AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Photodegradation of Alpha-pinene Secondary Organic Aerosol
Veronika Pospisilova, DAVID BELL, Imad El Haddad, Lamkaddam Houssni, Amelie Bertrand, Josef Dommen, Andre S.H. Prévôt, Urs Baltensperger, Jay G. Slowik, Paul Scherrer Institute
Abstract Number: 550 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosol (SOA) species create a large fraction of atmospheric aerosol mass. These particle-phase organics can be affected by various processes such as evaporation, particle-phase reactions or photochemical aging, leading to a change of particle properties. Small organic molecules, likely products of photodegradation, have been identified in the gas phase upon SOA irradiation and decrease in organic mass together with changes in the bulk chemical composition upon UV light exposure was reported. However, little is known about the effect of UV radiation on the chemistry of oxidized SOA species as direct molecular level observation of condensed-phase organics and the associated changes in the physico-chemical properties (e.g. volatility) are lacking. Therefore, the particle-phase photochemical changes are likely underestimated.
To investigate the effect of solar radiation on the molecular composition of SOA formed from α-pinene ozonolysis we performed a series of experiments simulating the photochemistry using a set of 40 UV lights (90–100 W) and measuring the particle-phase composition with an extractive electrospray time-of-flight mass spectrometer (EESI-TOF). EESI-TOF provides real time measurement of aerosol species unaffected by thermal decomposition or ionization-induced fragmentation. All experiments were performed in batch-mode in Teflon chamber with 40-50% relative humidity. In addition, isothermal evaporation measurements were conducted of SOA before and after UV lights.
We show that the behavior of individual α-pinene SOA species in the particle phase is altered upon the UV irradiation. We observe significant decay of certain molecules after the UV lights are turned on dependent on the particle mass loading and presence of isomers of different photo-stability. We determined the particle-phase photolytical rates to be on the order of 0.5 - 2*10-3 s-1on average, corresponding to lifetimes of ~8-30min. Further, significant decrease in the effective volatility of photochemical aged SOA is observed.