Efficient Isoprene Secondary Organic Aerosol Formation from a Non- IEPOX Pathway Jiumeng Liu, †,◆ Emma L. D’Ambro, ‡,◆ Ben H. Lee, § Felipe D. Lopez-Hilfiker, § Rahul A. Zaveri, † Jean C. Rivera-Rios, ∥ Frank N. Keutsch, ∥ Siddharth Iyer, ⊥ Theo Kurten, ⊥ Zhenfa Zhang, # Avram Gold, # Jason D. Surratt, # John E. Shilling,* ,†,∇ and Joel A. Thornton* ,‡,§,○ † Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory Richland, Washington 99352, United States ‡ Department of Chemistry and § Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, United States ∥ Paulson School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States ⊥ Department of Chemistry, University of Helsinki, Helsinki FI-00014, Finland # Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27599, United States * S Supporting Information ABSTRACT: With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth’s climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO 2 radicals is key to understanding the efficient SOA formation and the NO x -dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NO x -dependent SOA yields may be important in models. ■ INTRODUCTION Global forests emit 500 to 750 Tg of isoprene per year, the largest flux of biogenic or anthropogenic nonmethane hydrocarbons to the atmosphere. 1 Isoprene-derived secondary organic aerosol (iSOA) is predicted to comprise a significant fraction of the organic aerosol (OA) burden over large regions of the globe. 2−5 Isoprene also influences the oxidative capacity of the tropo- sphere, particularly in pristine forested regions, 6,7 due to its high reactivity toward atmospheric radicals and its effect on the fate of reactive nitrogen oxides, with consequences for the abundance and lifetime of greenhouse gases such as methane and tropospheric ozone, and likely indirect effects on particle formation, growth, 8 and SOA yield from other VOCs. 9 There is thus great interest in resolving the atmospheric fate of isoprene-derived carbon. Recently, there has been significant interest in determining whether anthropogenic pollutants, 8,10 such as NO x and sulfur oxides, may enhance iSOA formation and thereby increase the climate effects of aerosol particles, which directly interact with solar radiation and alter the reflectivity and life cycle of clouds. The magnitude of this anthropogenic forcing of a natural aerosol formation process has been difficult to quantify with certainty, 11 in part because the iSOA formation potential under preindustrial “pristine” conditions is poorly known. The net anthropogenic aerosol forcing of climate is determined by referring to year 1750 conditions, when NO x and sulfur emissions were approximately 4 and 6 times lower, respectively, than they are today. 12 Thus, assessing the anthropogenic aerosol forcing is inherently tied to accurately quantifying natural aerosol sources under preindus- trial conditions. 10 This need, in turn, requires a mechanistic-level understanding of the iSOA formation pathways. Received: April 15, 2016 Revised: August 6, 2016 Accepted: August 22, 2016 Published: August 22, 2016 Article pubs.acs.org/est © 2016 American Chemical Society 9872 DOI: 10.1021/acs.est.6b01872 Environ. Sci. Technol. 2016, 50, 9872−9880