Large Enhancement in the Heterogeneous Oxidation Rate of Organic Aerosols by Hydroxyl Radicals in the Presence of Nitric Oxide

Abstract

In the troposphere, the heterogeneous lifetime of an organic molecule in an aerosol exposed to hydroxyl radicals (OH) is thought to be weeks, which is orders of magnitude slower than the analogous gas phase reactions (hours). Here, we report an unexpectedly large acceleration in the effective heterogeneous OH reaction rate in the presence of NO. This 10−50 fold acceleration originates from free radical chain reactions, propagated by alkoxy radicals that form inside the aerosol by the reaction of NO with peroxy radicals, which do not appear to produce chain terminating products (e.g., alkyl nitrates), unlike gas phase mechanisms. A kinetic model, constrained by experiments, suggests that in polluted regions heterogeneous oxidation plays a much more prominent role in the daily chemical evolution of organic aerosol than previously believed. O rganic material comprises a significant fraction of submicron tropospheric aerosol (20−90%).1,2 Organic aerosol (OA), once formed or emitted into the atmosphere, is transformed by photochemical reactions, heterogeneous oxidation, aqueous phase chemistry, and the condensation of low-volatility organic species from the gas phase. These chemical transformations alter key microphysical OA properties (e.g., particle size, optical properties, volatility, toxicity, hygroscopicity) that in turn have large scale impacts on cloud droplet formation, human health, and radiative forcing.3 For chemical transport models to accurately predict the impact that OA has on air quality and climate relies on accurate descriptions of multiphase chemistry and their associated kinetic time scales.4 Secondary organic aerosol (SOA) formation is fast and occurs within hours by reactions of O 3 and OH with gas phase anthropogenic and biogenic SOA precursors. Heterogeneous aerosol oxidation is generally considered to be at least ∼10 times slower (i.e., weeks), occurring on a similar time scale as dry and wet deposition. Here, new experimental evidence is reported for radical chain reactions initiated by a heterogeneous reaction in the presence of two common anthropogenic pollutants. This free radical chain reaction leads to large effective reaction rates and much shorter kinetic lifetimes (hours) than previously thought possible for heterogeneous oxidation.Scheme 1 shows a canonical reaction mechanism used to explain the heterogeneous OH oxidation of hydrocarbons (RH) in the presence of O 2 . OH reacts with RH by H atom abstraction forming an alkyl radical (R) and H 2 O. The heterogeneous rate of the reaction is quantified by a reactive uptake coefficient (γ), which is determined by measuring the loss of either gas-phase OH or particle phase RH. Kinetic measurements of OH loss by definition yield γ OH ≤ 1. This is not necessarily the case when the reaction is measured by the decay of RH because, in addition to OH, other free radical intermediates (i.e., RO, Scheme 1) can consume the hydrocarbon leading to effective uptake coefficients (γ eff ) larger than 1. γ eff > 1 simply means that the reactiv...

Sulfur Dioxide Accelerates the Heterogeneous Oxidation Rate of Organic Aerosol by Hydroxyl Radicals

“…This reaction scheme is exactly analogous to the mechanism recently reported by us for heterogeneous reactions of OH with OA in the presence of NO. 6 The rate coefficients in the model are from previous literature and are shown in Table 1. 6,15,24,25 The branching ratio for the RO2 self-reaction (R4) and (R3) to form either a carbonyl-alcohol pair or two RO· are fixed at 90:10, respectively, and are based upon a previous study.…”

“…6 The rate coefficients in the model are from previous literature and are shown in Table 1. 6,15,24,25 The branching ratio for the RO2 self-reaction (R4) and (R3) to form either a carbonyl-alcohol pair or two RO· are fixed at 90:10, respectively, and are based upon a previous study. 26 As shown recently in the gas phase by Crounse et al, 27 hydroperoxides (ROOH) could be formed at low…”

“…This is also consistent with our recent results that found no evidence for particle phase alkyl nitrate formation from the RO2 + NO reaction. 6 for the reaction of methyl peroxy radicals with SO2. This difference in rate coefficient for the initial adduct formation may not be that surprising given the large differences in molecular structure of the peroxy radicals (methyl vs. C30) and environments (i.e.…”

Organic peroxy radical chemistry in oxidation flow reactors and environmental chambers and their atmospheric relevance

“…We detail the RO 2 production and loss pathways of interest in this study, the approximations used to simplify this complex problem and the steps to investigate it methodically. We briefly introduce the base OFR design and the model, which are described in detail elsewhere (Kang et al, 2007;Peng et al, 2015Peng et al, , 2018. The concept of the base OFR design simulated in this study, the potential aerosol mass (PAM) reactor, was first introduced by Kang et al (2007) The geometry of the most popular PAM OFR is a cylinder of ∼ 13 L volume.…”

“…These, together with the ability to rapidly achieve high photochemical ages, are advantageous for field applications. These advantages of OFRs have led a number of atmospheric chemistry research groups (Lambe and Jimenez, 2018) to deploy them in field (Hu et al, 2016;Ortega et al, 2016;Palm et al, 2016Palm et al, , 2017, source Tkacik et al, 2014;Karjalainen et al, 2016;Link et al, 2016) and laboratory studies (Kang et al, 2011;Lambe et al, 2013;Richards-Henderson et al, 2016;Lim et al, 2017).…”