10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Impact of SO2 on Particle Formation and Growth from α-Pinene Ozonolysis
CHRIS STANGL, Justin Krasnomowitz, Murray Johnston, Shanhu Lee, University of Delaware
Abstract Number: 500 Working Group: Remote/Regional Atmospheric Aerosol
Abstract Secondary organic aerosol (SOA) plays a major role in atmospheric nanoparticle growth, and greatly impacts climate by enhancing cloud condensation nuclei (CCN) number. Biogenic emissions, in particular isoprene and monoterpenes, represent the major volatile precursors to organic aerosol formation, accounting for roughly 90% of the global SOA flux. Although the potential for these species to form SOA has been studied extensively, less remains known about how interactions with anthropogenic emissions affects this process. In this work, the effects of SO2 on nanoparticle formation and growth from α-pinene ozonolysis were studied in a flow tube reactor. Experiments were conducted in the presence of 60 nm monodisperse ammonium sulfate seed particles under a 4-minute residence time, and at atmospherically relevant levels of the gas-phase reactants. Under dry (RH < 10%) conditions, significant growth of the seed particles was observed from α-pinene ozonolysis, with very little new particle formation under the conditions used. Following the addition of SO2, new particle formation was observed due to the oxidation of SO2 to H2SO4 by OH radical and/or the stabilized Criegee Intermediate (sCI) formed during ozonolysis. Growth of the seed particles as well as the total amount of secondary aerosol produced, however, were found to be reduced in the presence of SO2, likely due to consumption of OH by SO2 that would otherwise oxidize additional α-pinene molecules, or reaction with sCI to inhibit the formation of highly oxidized molecules (HOMs) that could condense to the particle phase. Elemental composition analysis of the grown seed particles was conducted in the presence and absence of SO2 using the Nano Aerosol Mass Spectrometer (NAMS), and revealed a larger sulfur-to-carbon mole fraction ratio of the particles exposed to SO2, suggesting a significant sulfuric acid contribution to particle growth. Additional experiments were carried out with cyclohexane present in the flow tube to investigate the effect of OH removal on particle formation and growth. With cyclohexane, growth of the seed particles by α-pinene was decreased, indicating a significant OH contribution to HOM formation under the conditions examined. With both cyclohexane and SO2 present, growth was decreased even further, however particle formation was still observed to a similar extent as without cyclohexane, suggesting that the sCI plays an important role in SO2 oxidation. These results suggest that SO2 is able to inhibit SOA production by reducing the amount of HOMs formed, and this effect may lead to a reduction in CCN number in locations where forested air can become infused with anthropogenic pollutants, such as the southeastern U.S. Ongoing work entails investigating the effect of SO2 on particle formation and growth with other SOA precursors, seed particle compositions and higher relative humidities.