10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Fate of Highly Oxygenated Organic Molecules in Wet Acidic Aerosols
LIINE HEIKKINEN, Matthieu Riva, Otso Peräkyla, Qiaozhi Zha, Simon Schallhart, Matti Rissanen, Tuukka Petäjä, Mikael Ehn, University of Helsinki
Abstract Number: 654 Working Group: Aerosol Chemistry
Abstract Organic aerosol (OA) makes up a large mass fraction of tropospheric fine particulate matter, worldwide. OA commonly originates from gas-to-particle transition processes and in this case can be referred to secondary organic aerosol (SOA). Biogenic volatile organic compounds (BVOCs) are an important source of SOA. Among the BVOCs, monoterpenes (MT) represent a major fraction of organic compounds produced over forested areas. In the boreal forest, α-pinene (AP) contributes significantly to the SOA formation through the generation of highly oxygenated molecules (HOMs). Indeed, such compounds are expected to have low vapor pressures and to significantly contribute to the formation of new particles in the atmosphere. In the presence of NOx, the formation of multifunctional organic nitrates (ONs) has been reported as well as their subsequent presence in the particle phase. While the importance of HOMs and multifunctional ONs in SOA formation has been demonstrated, their fate within the particle phase has received much less attention and remain uncertain/unknown. It is worth noting that such compounds are expected to contain a wide variety of functional groups and thus are presumably highly reactive within the condensed phase.
In the present study, we investigated the fate of particulate HOMs both in the boreal forest in Southern Finland (SMEAR II station) and in a newly constructed atmospheric simulation chamber (COALA chamber). The laboratory experiments focused on the ozonolysis of AP under atmospherically relevant conditions. Seed aerosols with varying acidities and liquid water contents were used in order to replicate the conditions observed in the boreal forest. The particles sampled in both field and laboratory studies were chemically characterized utilizing different mass spectrometric techniques, such as a Filter Inlet for Gases and AEROsols coupled with an iodide adduct Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) and a High Resolution Aerosol Mass Spectrometer (HR-AMS). Both instruments have an extended time-of-flight chamber enabling a high mass resolution and thus a more detailed understanding of the mass spectra. Our results from the field reveal that the HOMs, especially the dimers, undergo further particle phase reactions in the presence of sulfate aerosols, producing high molecular weight oligomeric products. These results were later confirmed by targeted laboratory experiments in the COALA chamber and stress the importance of acid-catalyzed reactions. Our study highlights that aerosol acidity leads to fast particle phase processing influencing the physicochemical properties of MT-derived SOA. Finally, the chamber studies reveal the decomposition of the particulate multifunctional ONs formed from the ozonolysis of AP in wet acidic aerosols, which could impact the atmospheric NOx cycling.