Critical factors determining the variation in SOA yields from terpene ozonolysis: A combined experimental and computational study

Abstract

A substantial fraction of the total ultrafine particulate mass is comprised of organic compounds. Of this fraction, a significant subfraction is secondary organic aerosol (SOA), meaning that the compounds are a by-product of chemistry in the atmosphere. However, our understanding of the kinetics and mechanisms leading to and following SOA formation is in its infancy. We lack a clear description of critical phenomena; we often don't know the key, rate limiting steps in SOA formation mechanisms. We know almost nothing about aerosol yields past the first generation of oxidation products. Most importantly, we know very little about the derivatives in these mechanisms; we do not understand how changing conditions, be they precursor levels, oxidant concentrations, co-reagent concentrations (i.e., the VOC/NOx ratio) or temperature will influence the yields of SOA. In this paper we explore the connections between fundamental details of physical chemistry and the multitude of steps associated with SOA formation, including the initial gas-phase reaction mechanisms leading to condensible products, the phase partitioning itself, and the continued oxidation of the condensed-phase organic products. We show that SOA yields in the alpha-pinene + ozone are highly sensitive to NOx, and that SOA yields from beta-caryophylene + ozone appear to increase with continued ozone exposure, even as aerosol hygroscopicity increases as well. We suggest that SOA yields are likely to increase substantially through several generations of oxidative processing of the semi-volatile products.

Relating CCN activity, volatility, and droplet growth kinetics of β-caryophyllene secondary organic aerosol

“…CCN activity is presented in terms of an activated fraction, the ratio of CCN/CN. The gradual increase in CCN activity suggests active chemical aging of the aerosol; this is in agreement with Donahue et al (2005), although it is unclear if it is from oxidation in the gas or aerosol phase. Although both CCN instruments show an increase of activity with aging, the magnitudes and trends of the activated fractions are remarkably different.…”

The Oxidation of Oleate in Submicron Aqueous Salt Aerosols: Evidence of a Surface Process

“…For the semivolatile ozonolysis products of R-pinene, Donahue et al and Offenberg et al estimate ∆H vap eff ≈ 30-44 kJ mol -1 from the temperature dependence of aerosol mass, whereas the typical single-component ∆H vap of suspected products ranged from 100 to 120 kJ mol -1 . 93,94 Summary and Atmospheric Implications. The sum of our results implies that the oxidation of oleate present in submicron aqueous salt particles at ∼3.2 wt % occurs at the particle surface.…”

A two-dimensional volatility basis set – Part 2: Diagnostics of organic-aerosol evolution

“…Our challenge is to measure and model both the mean trajectories (shown with arrows) but more importantly the full breadth of these trajectories as well as the kinetics -the rate at which organics progress through the trajectories. Both trajectories show increasing fragmentation as OS C rises, consistent with Kroll et al (2011), and both trajectories (especially the C 18 ) will involve thousands of individual compounds as the mechanism pushes into later generations (Donahue et al, 2005;Aumont et al, 2005). Especially in the non-traditional trajectory, heterogeneous chemistry will clearly be of major importance, and in both cases accretion reactions, with the possibility of adding to the product carbon numbers, add yet another layer of complexity to the challenge.…”

“…Unsaturated compounds (mostly terpenoids including isoprene) react within an hour or two, but even large saturated organics have gas-phase lifetimes against OH reaction of five to eight hours under typical conditions, meaning that one or two generations of oxidation will typically occur in a day (Donahue et al, 2005). OH and other radicals also collide with particles, heterogeneously oxidizing the condensed-phase compounds; because of mass-transfer limitations the timescale is longer, but it is still only a few days (Moise and Rudich, 2001;Smith et al, 2003;Molina et al, 2004;Robinson et al, 2006;George et al, 2008;Kroll et al, 2009).…”