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Interfacial and multiphase chemical processes involving gases typically involve adsorption and desorption onto liquid or solid substrates. The desorption energy, which depends on the intermolecular forces between adsorbate and substrate, determines the residence time of chemical species at the interface. In this study, we demonstrate how variations in desorption energy and temperature influence the net uptake or release of gas species, which in turn affects the rates of surface and bulk reactions, surface-bulk exchange, and the equilibration time scales of gas-particle partitioning. We survey experimentally and theoretically derived desorption energies to develop a parameterization that enables the prediction of desorption energies based on the molecular weight, polarizability, and oxygen to carbon ratio of the desorbing chemical species independent of substrate-specific properties, which is possible because of the dominating role of the desorbing species' polarizability. The data and analyses compiled in this study provide new insights into the relationship between desorption energy and enthalpies of vaporization and solvation, reflecting the central role of desorption in the multiple steps of interfacial exchange and multiphase processes, including mass accommodation and heterogeneous chemical reactions. Practical implications are discussed for gas-particle partitioning, organic phase changes, secondary organic aerosol formation, and indoor surface chemistry. We conclude that future research in aerosol, atmospheric, and environmental physical chemistry, air quality, climate, and Earth system science as well as chemical engineering and materials science may benefit from further insight and constraints on the influence of desorption lifetimes and energies on multiphase processes and their temperature dependence. [ABSTRACT FROM AUTHOR]

Aerosols and films are found in indoor and outdoor environments. How they interact with pollutants, such as ozone, has a direct impact on our environment via cloud droplet formation and the chemical persistence of toxic aerosol constituents. The chemical reactivity of aerosol emissions is typically measured spectroscopically or by techniques such as mass spectrometry, directly monitoring the amount of material during a chemical reaction. We present a study which indirectly measures oxidation kinetics in a common cooking aerosol proxy using a low-cost Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). We validated this approach by comparison with kinetics measured both spectroscopically and with high-intensity synchrotron radiation. Using microscopy, we found that the film morphology changed and film rigidity increased during oxidation. There was evidence of surface crust formation on oxidised particles, though this was not consistent for all experiments. Crucially, our kinetic modelling of these experimental data confirmed that the oleic acid decay rate is in line with previous literature determinations, which demonstrates that performing such experiments on a QCM-D does not alter the underlying mechanism. There is clear potential to take this robust and low cost, but sensitive method to the field for in-situ monitoring of reactions outdoors and indoors. [ABSTRACT FROM AUTHOR]

Siloxanes are composed of silicon, oxygen, and alkyl groups and are emitted from consumer chemicals. Despite being entirely anthropogenic, siloxanes are being detected in remote regions and are ubiquitous in indoor and urban environments. Decamethylcyclopentasiloxane (D5) is one of the most common cyclic congeners, and smog chamber and oxidation flow reactor (OFR) experiments have found D5 + OH to form secondary organosiloxane aerosol (SOSiA). However, there is uncertainty about the reaction products, and the reported SOSiA mass yields (YSOSiA) appear inconsistent. To quantify small volatile oxidation products (VOP) and to consolidate the YSOSiA in the literature, we performed experiments using a Potential Aerosol Mass OFR while varying D5 concentration, humidity, and OH exposure (OHexp). We use a proton transfer reaction time-of-flight mass spectrometer to quantify D5, HCHO, and HCOOH, and detect other VOP, which we tentatively identify as siloxanols and siloxanyl formates. We determine molar yields of HCHO and HCOOH between 52 - 211 % and 45 - 127 %, respectively. With particle size distributions measured with a scanning mobility particle sizer, we find YSOSiA to be < 10 % at OHexp < 1.3 × 1011 s cm-3 and ~20 % at OHexp corresponding to that of the lifetime of D5 at atmospheric OH concentrations. We also find that YSOSiA is dependent on both organic aerosol mass loading and OHexp. We use a kinetic box model of SOSiA formation and aging (aging-VBS model) to reconcile the YSOSiA values found in this study and the literature. The model uses a volatility basis set (VBS) of the primary oxidation products as well as an aging rate coefficient in the gas phase, kage, gas, of 2.17 × 10-11 cm³ s-1, and an aging rate coefficient in the particle phase, kage, particle, which is ten times smaller. The combination of primary VBS and OH-dependent oxidative aging predicts SOSiA formation much better than a standard-VBS parameterization that does not consider aging (R2 = 0.970 vs. 0.847). The need for an ageing-dependent parameterization to accurately model SOSiA formation shows that concepts developed for secondary organic aerosol precursors, which are able to form low-volatile products at low OHexp, do not necessarily apply to D5 + OH. The resulting yields of HCHO and HCOOH and the parameterization of YSOSiA may be used in larger scale models to assess the implications of siloxanes on air quality. [ABSTRACT FROM AUTHOR]

Interaction of water with fine particulate matter leads to the formation of reactive species (RS) that may influence the aging, properties, and health effects of atmospheric aerosols. In this study, we explore the RS yields of fine PM from remote forest (Hyytiälä, Finland) and polluted urban air (Mainz, Germany and Beijing, China) and relate these yields to different chemical constituents and reaction mechanisms. Ultrahigh-resolution mass spectrometry was used to characterize organic aerosol composition, electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was used to determine the concentrations ·OH, O2·-, and carbon- or oxygen-centered organic radicals, and a fluorometric assay was used to quantify H2O2 concentration. The mass-specific yields of radicals were lower for sampling sites with higher concentration of ambient PM2.5 (particles with a diameter < 2.5 µm), whereas the H2O2 yields exhibited no clear trend. The abundances of water-soluble transition metals and aromatics in ambient PM2.5 were positively correlated with the relative fraction of ·OH to the totally detected radicals, but negatively correlated with the relative fraction of carbon-centered radicals. Moreover, we found that the relative fractions of different types of radicals formed by ambient PM2.5 were comparable to the surrogate mixtures comprising transition metals, organic hydroperoxide, H2O2, and humic or fulvic acids. Therein humic and fulvic acids exhibited strong radical scavenging effect to substantially decrease the radical yield of mixtures comprising cumene hydroperoxide and Fe2+. The interplay of transition metals (e.g., iron), highly oxidized compounds (e.g., organic hydroperoxides), and complexing agents (e.g., humic or fulvic acids), leads to non-linear concentration dependencies of production and yields of different types of RS. Our findings show that how the composition of PM2.5 influences the amount and nature of RS produced upon interaction with water, which may explain differences in the chemical reactivity and health effects of particulate matter in clean and polluted air. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic air pollutants. The dispersion of PAHs in the atmosphere is influenced by gas-particle partitioning and chemical loss. These processes are closely interlinked and may occur at vastly differing timescales, which complicates their mathematical description in chemical transport models. Here, we use a kinetic model that explicitly resolves mass transport and chemical reactions in the gas and particle phases to describe and explore the dynamic and non-equilibrium interplay of gas-particle partitioning and chemical losses of PAHs on soot particles. We define the equilibration timescale τeq of gas-particle partitioning as the e-folding time for relaxation of the system to the partitioning equilibrium. We find this metric to span seconds to hours depending on temperature, particle surface area and the type of PAH. The equilibration time can be approximated using a time-independent equation τeq &approx; 1/(kdes + kads), which depends on the desorption rate coefficient kdes and adsorption rate coefficient kads, both of which can be calculated from experimentally-accessible parameters. The model reveals two regimes in which different physical processes control the equilibration timescale: a desorption-controlled and an adsorption-controlled regime. In a case study with the PAH pyrene, we illustrate how chemical loss can perturb the equilibrium particulate fraction at typical atmospheric concentrations of O3 and OH. For the surface reaction with O3, the perturbation is significant and increases with the gas-phase concentration of O3. Conversely, perturbations are smaller for reaction with the OH radical, which reacts with PAHs on both the surface of particles and in the gas phase. Global and regional chemical transport models typically approximate gas-particle partitioning with instantaneous equilibration approaches. We highlight scenarios in which these approximations deviate from the explicit-coupled treatment of gas-particle partitioning and chemistry presented in this study. We find that the discrepancy between solutions depends on the operator-splitting time step and the choice of time step can help to minimize the discrepancy. The findings and techniques presented in this work are not only relevant for PAHs, but can also be applied to other semi-volatile substances that undergo chemical reactions and mass transport between the gas and particle phase. [ABSTRACT FROM AUTHOR]

Organic aerosol constitutes a major fraction of the global aerosol burden and is predominantly formed as secondary organic aerosol (SOA). Environmental chambers have been used extensively to study aerosol formation and evolution under controlled conditions similar to the atmosphere, but quantitative prediction of the outcome of these experiments is generally not achieved, which signifies our lack in understanding of these results and limits their portability to large scale models. In general, kinetic models employing state-of-the-art explicit chemical mechanisms fail to describe the mass concentration and composition of SOA obtained from chamber experiments. Specifically, chemical reactions involving nitrate radical (NO3) oxidation of volatile organic compounds (VOCs) are a source of major uncertainty for assessing the chemical and physical properties of oxidation products. Here, we introduce a kinetic model that treats gas-phase chemistry, gas-particle partitioning, particle-phase oligomerization, and chamber wall loss and use it to describe the oxidation of the monoterpenes α-pinene and limonene with NO3. The model can reproduce aerosol mass and nitration degrees in experiments using either pure precursors or their mixtures and infers volatility distributions of products, branching ratios of reactive intermediates as well as particle-phase reaction rates. The gas-phase chemistry in the model is based on the Master Chemical Mechanism (MCM), but trades speciation of single compounds for the overall ability of quantitatively describing SOA formation by using a lumped chemical mechanism. The complex branching into a multitude of individual products in MCM is replaced in this model with product volatility distributions, detailed peroxy (RO2) and alkoxy (RO) radical chemistry and amended by a particle-phase oligomerization scheme. The kinetic parameters obtained in this study are constrained by a set of SOA formation and evaporation experiments conducted in the Georgia Tech Environmental Chamber (GTEC) facility. For both precursors, we present volatility distributions of nitrated and non-nitrated reaction products that are obtained by fitting the kinetic model systematically to the experimental data using a global optimization method, the Monte Carlo Genetic Algorithm (MCGA). The results presented here provide new mechanistic insight into the processes leading to formation and evaporation of SOA. Most notably, much of the non-linear behavior of precursor mixtures can be understood by RO2 fate and reversible oligomerization reactions in the particle phase, but some effects could be accredited to kinetic limitations of mass transport in the particle phase. The methodologies described in this work provide a basis for quantitative analysis of multi-source data from environmental chamber experiments with manageable computational effort. [ABSTRACT FROM AUTHOR]

Organic compounds present at/near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH radical initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modelling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxyl functionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon-carbon bond scission) of tertiary alkoxy radicals resulting from the OH-abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high oxidation stages. [ABSTRACT FROM AUTHOR]

We present a Monte-Carlo Genetic Algorithm (MCGA) for efficient, automated and unbiased global optimization of model input parameters by simultaneous fitting to multiple experimental data sets. The algorithm was developed to address the inverse modelling problems associated with fitting large sets of model input parameters encountered in state-of-the-art kinetic models for heterogeneous and multiphase atmospheric chemistry. The MCGA approach utilizes a sequence of optimization methods to find the solution of an optimization problem and to explore the space of solutions with similar model output. It addresses a problem inherent to complex models whose extensive input parameter sets might not be uniquely determined from limited input data. Such ambiguity in the derived parameter values can be reliably detected using this new set of tools. The MCGA algorithm has been used successfully to constrain parameters such as reaction rate coefficients, diffusion coefficients and Henry's law solubility coefficients in kinetic models of gas uptake and chemical transformation of aerosol particles as well as multiphase chemistry at the atmosphere-biosphere interface. It should be portable to any numerical model with similar computational expense and extent of the fitting parameter space. [ABSTRACT FROM AUTHOR]

We report the first measurements of HO2 uptake coefficients, γ, for secondary organic aerosol particles (SOA) and for the well-studied model compound sucrose which was doped with copper. Above 65 % relative humidity (RH), γ for copper doped sucrose aerosol particles equalled the surface mass accommodation coefficient α = 0.22 ± 0.06 but decreased to γ = 0.012 ± 0.007 upon decreasing the RH to 17 %. The trend of γ with RH can be explained by an increase in aerosol viscosity, as demonstrated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). SOA from two different precursors, α-pinene and 1,3,5- trimethylbenzene (TMB), was investigated, yielding small uptake coefficients of γ < 0.001 and γ = 0.004 ± 0.002, respectively. It is postulated that the larger values measured for TMB derived SOA compared to α-pinene derived SOA are either due to differing viscosity, a different liquid water content of the aerosol particles or a HO2 + RO2 reaction occurring within the aerosol particles. [ABSTRACT FROM AUTHOR]

We report the first measurements of HO2 uptake coefficients, γ, for secondary organic aerosol particles (SOA) and for the well-studied model compound sucrose which was doped with copper. Above 65% relative humidity (RH), γ for copper doped sucrose aerosol particles equalled the surface mass accommodation coefficient β = 0.22 ± 0.06 but decreased to γ = 0.012 ± 0.007 upon decreasing the RH to 17%. The trend of γ with RH can be explained by an increase in aerosol viscosity, as demonstrated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). SOA from two different precursors, α-pinene and 1,3,5- trimethylbenzene (TMB), was investigated, yielding small uptake coefficients of γ < 0.001 and γ = 0.004 ± 0.002, respectively. It is postulated that the larger values measured for TMB derived SOA compared to α-pinene derived SOA are either due to differing viscosity, a different liquid water content of the aerosol particles or a HO2 + RO2 reaction occurring within the aerosol particles. [ABSTRACT FROM AUTHOR]

Organic oligomers and other organic high molecular mass compounds are potential sources of viscous atmospheric aerosol components, which may inhibit the mass transport of water and introduce kinetic limitations to water uptake. This, in turn, impacts aerosol optical properties and the efficiency with which these particles serve as cloud condensation nuclei (CCN). To investigate the influence of particle viscosity on water-uptake behavior, measurements of hygroscopic growth under subsaturated relative humidity (RH) conditions and of the CCN activity of laboratory surrogates for oligomers in atmospheric aerosols were conducted with the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe and a CCN counter, respectively. In order to explore variability in water-uptake behavior across aerosol systems with differing viscosities, experiments were conducted for particles comprised of polyethylene glycol (PEG) with average molecular masses of 200, 1,000, and 10,000 g/mol, as well as mixtures of these PEG oligomers with ammonium sulfate (AS). Experimental results were compared with calculations of hygroscopic growth at thermodynamic equilibrium conducted with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model, and the potential influence of kinetic limitations to observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. Under subsaturated RH conditions, we observed little variability in hygroscopic growth across PEG systems with different molecular masses; however, an increase in CCN activity with increasing PEG molecular mass was observed. This latter finding is attributed to an increase in the efficiency of PEG as a surfactant with increasing molecular mass. This effect is most pronounced for PEG-AS mixtures, and a modest enhancement in CCN activity was observed for the PEG10,000-AS mixture as compared to pure AS, as evidenced by a 8% reduction in critical activation diameter at a supersaturation of 0.6%. The supplementation of experimental observations with the AIOMFAC-based model results and estimations of water diffusivity in PEG suggests that apparent discontinuities in hygroscopicity above and below water saturation can be attributed, at least in part, to a combination of RH-dependent differences in the sensitivity of water uptake behavior to non-ideal interactions and to surface tension effects. There was no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components influenced hygroscopic growth. This work provides insight into the factors likely to be contributing to discontinuities in aerosol water-uptake behavior below and above water saturation (e.g., low hygroscopic growth at subsaturated RHs, but enhanced CCN activity for a given aerosol population) that have been observed previously in the ambient atmosphere. [ABSTRACT FROM AUTHOR]