Surface Water Structure and Hygroscopic Properties of Light Absorbing Secondary Organic Polymers of Atmospheric Relevance

Abstract

Hygroscopic properties and chemical reactivity of secondary organic aerosols (SOA) influence their overall contribution to the indirect effect on the climate. In this study, we investigate the hygroscopic properties of organic and organometallic polymeric particles, namely polycatechol, polyguaiacol, Fe-polyfumarte, and Fe-polymuconate. These particles efficiently form in iron-catalyzed reactions with aromatic and aliphatic dicarboxylic acid compounds detected in field-collected SOA. The structure of surface water was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and the uptake of gas water was quantified using quartz crystal microbalance (QCM) as a function of relative humidity. Spectroscopic data show that water bonding with organic functional groups acting as hydrogen bond acceptors causes shifts in their vibrational modes. Analysis of the hydroxyl group stretching region revealed weak and strong hydrogen bonding networks that suggest cluster formation reflecting water–water and water–organics interactions, respectively. A modified Type II multilayer Brunauer–Emmett–Teller adsorption model described the adsorption isotherm on the nonporous materials, polycatechol, polyguaiacol, and Fe-polymuconate. However, water adsorption on porous Fe-polyfumarate was best described using a Type V adsorption model, namely the Langmuir–Sips model that accounts for condensation in pores. The data revealed that organometallic polymers are more hygroscopic than organic polymers. The implications of these investigations are discussed in the context of the chemical reactivity of these particles relative to known SOA.

Aging of Atmospheric Brown Carbon Aerosol

“…[Reprinted with permission from Yu et al 72 Copyright 2014] Dark iron-catalysed reactions of phenolic compounds can also lead to the formation of aromatic oligomers. [166][167][168] Although formed through a different mechanism, the resulting oligomers are expected to be similar to those formed through photoreactions (Figure 6). 166,168 In addition, oligomeric products form through reactions of iron with two common aromatic oxidation products, fumaric acid and muconic acid.…”

“…166,168 In addition, oligomeric products form through reactions of iron with two common aromatic oxidation products, fumaric acid and muconic acid. [167][168][169] While iron is present in dust particles and is emitted from anthropogenic sources such as fly ash, 170 the importance and prevalence of these iron reactions with phenolic compounds has yet to be demonstrated. In the specific case of syringol, oligomeric compounds may also form through dark reactions in the presence of chloride salts.…”

“…213,[216][217][218] Heterogeneous reactions with gaseous ozone and OH and NO3 radicals can lead to oxidative aging, resulting in changes in both composition and absorptivity. 116,168,214,219,220 These reactions can be limited by diffusion for (semi-)solid BrC particles, which will be more important at low RH as particle viscosity increases with decreasing RH. [221][222][223][224][225][226] Moreover, SOA including secondary BrC can be internally mixed with aggregates of BC, leading to drastic changes in morphology, absorption efficiency, and hygroscopicity of BC, 99,121,131,227 though less pronounced than those induced by secondary inorganic species.…”

“…266 The latter, highly functionalized products contribute to increased absorptivity at visible wavelengths 266,269 as well as increased hygroscopicity, due to the presence of many hydrogen bond donor and acceptor groups. 270 Furthermore, heterogeneous ozone oxidation of particulate trimethylamine, a precursor of secondary imine BrC, has been shown to lead to a range of oxygen-and nitrogen-containing products distributed across both the gas and particle phases, including several with known health effects, like nitromethane. 268 In the atmosphere, this pathway could compete with the formation of secondary imine BrC but also could itself lead to changes in the optical properties of particles with amines.…”

Aging of atmospheric aerosols and the role of iron in catalyzing brown carbon formation

“…320 The weak and strong hydrogen bonding networks of adsorbed water on these organic powdered materials suggest cluster formation reflecting water-water and water-organics interactions. Also, water bonding with organic functional groups acting as hydrogen bond acceptors caused shifts in their vibration modes 307 and lead to partial dissolution due to the formation of hydrates as observed in the case of tannic acid. 319 Water molecules in contact with organic self-assembled monolayers (SAMs) at low RH form clusters, which have fewer hydrogen bonds and gave rise to spectral components at 3200 cm -1 , whereas molecules in the interior of water clusters have 3 and 4 hydrogen bonds similar to that of bulk water.…”