Modeling Peroxy Radical Chemistry Using the Volatility Basis Set to Understand Trends in New Particle Formation
Creators: name: Schervish, Meredith
Publisher: name: Carnegie Mellon University
Year: 2020
Subjects: subject: 40101 Atmospheric Aerosols
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences

description: Aerosols influence climate through their direct effect, reflecting sunlight back to space, and their indirect effect, acting as cloud condensation nuclei; changes to these effects since the industrial revolution constitute the largest uncertainty in our understanding of anthropogenic climate impacts. One of the largest sources of uncertainty in anthropogenic aerosol forcing is the pre-industrial baseline and our understanding of pre-industrial aerosol is dependent on our understanding of the contribution of biogenic vapors to aerosols. There is evidence that pure biogenic nucleation occurs at very low levels of sulfuric acid and recently the role of autoxidation and rapid dimerization to produce highly-oxygenated organic molecules (HOMs) have emerged as pathways to produce (ultra and extremely) low volatility products. As the ability of a species to contribute nucleation and growth is dependent on that species’ volatility, we can use the Volatility Basis Set (VBS) to easily interpret how different conditions will affect nucleation and growth. To this end we will focus mainly on how different conditions affect dimers, especially the dimers that generate products in the Ultra Low Volatility Organic Compound (ULVOC) class of the VBS, and HOMs in theExtremely Low Volatility Organic Compound (ELVOC) and Low Volatility Organic Compound (LVOC) classes.We show that autoxidation is suppressed at low temperatures, leading to less oxidized productsand fewer dimers as autoxidation is strongly temperature-dependent and the dimerization branching ratio is modeled as being dependent on the extent of oxidation of the reacting peroxy radicals. Because of the competing effect of lowering temperature reducing the volatility of substances,the effect on nucleation and growth is much less pronounced. We also show this is true using a dynamic condensation model to predict growth rates of sub-30 nm particles based on gas-phase measurements. We also show a similar trend of suppressing autoxidized products and dimers with increasing NO<sub>x</sub> as NO competes for the peroxy radicals. Unlike with temperature, there is no compensating effect and thus NO<sub>x</sub> suppresses both nucleation and growth, especially growth at the smallest sizes as the nucleating ULVOCs are the most effected by NOx. Using CO as a proxy for oxidized volatile organic compounds (OVOCs) that promote the conversion OH to HO<sub>2</sub>, we show that depending on how fast association rate coefficients are, nucleation can be reduced by up to 4 orders of magnitude from an experiment with no CO to atmospheric conditions with OVOCs equivalent to 7000 ppb of CO. Because association rate coefficients are unknown for most peroxyradicals, it is incredibly important to understand what conditions chamber experiments are under in order to accurately interpret the atmospheric relevance of chamber results. Lastly, we show that the presence of smaller, more volatile peroxy radicals, in our case C<sub>5</sub> peroxy radicals derived from isoprene oxidation, can suppress the formation of C<sub>20</sub> ULVOC dimers.<br>