Reactive Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles

The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γIEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γIEPOX for trans-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γIEPOX is more than 500 times greater on ammonium bisulfate (γ ∼ 0.05) than on ammonium sulfate (γ ≤ 1 × 10–4). We could accurately predict γIEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry’s law coefficient (1.7 × 108 M/atm). Suppression of γIEPOX was observed on particles containing both ammonium bisulfate and poly­(ethylene glycol) (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation

Reactive Uptake of an Isoprene-Derived Epoxydiol to Submicron Aerosol Particles

The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γ_IEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γ_IEPOX for <i>trans</i>-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γ_IEPOX is more than 500 times greater on ammonium bisulfate (γ ∼ 0.05) than on ammonium sulfate (γ ≤ 1 × 10^–4). We could accurately predict γ_IEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry’s law coefficient (1.7 × 10⁸ M/atm). Suppression of γ_IEPOX was observed on particles containing both ammonium bisulfate and poly­(ethylene glycol) (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0) but is greater than 25 h on less acidic particles (pH > 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation

Heterogeneous Reactions of Isoprene-Derived Epoxides: Reaction Probabilities and Molar Secondary Organic Aerosol Yield Estimates

A combination of flow reactor studies and chamber modeling is used to constrain two uncertain parameters central to the formation of secondary organic aerosol (SOA) from isoprene-derived epoxides: (1) the rate of heterogeneous uptake of epoxide to the particle phase and (2) the molar fraction of epoxide reactively taken up that contributes to SOA, the SOA yield (ϕ_SOA). Flow reactor measurements of the <i>trans</i>-β-isoprene epoxydiol (<i>trans</i>-β-IEPOX) and methacrylic acid epoxide (MAE) aerosol reaction probability (γ) were performed on atomized aerosols with compositions similar to those used in chamber studies. Observed γ ranges for <i>trans</i>-β-IEPOX and MAE were 6.5 × 10^–4−0.021 and 4.9–5.2 × 10^–4, respectively. Through the use of a time-dependent chemical box model initialized with chamber conditions and γ measurements, ϕ_SOA values for <i>trans</i>-β-IEPOX and MAE on different aerosol compositions were estimated between 0.03–0.21 and 0.07–0.25, respectively, with the MAE ϕ_SOA showing more uncertainty

Trends in the oxidation and relative volatility of chamber-generated secondary organic aerosol

<p>The relationship between the oxidation state and relative volatility of secondary organic aerosol (SOA) from the oxidation of a wide range of hydrocarbons is investigated using a fast-stepping, scanning thermodenuder interfaced with a high-resolution time-of-flight aerosol mass spectrometer (AMS). SOA oxidation state varied widely across the investigated range of parent hydrocarbons but was relatively stable for replicate experiments using a single hydrocarbon precursor. On average, unit mass resolution indicators of SOA oxidation (e.g., AMS <i>f</i>₄₃ and <i>f</i>₄₄) are consistent with previously reported values. Linear regression of H:C vs. O:C obtained from parameterization of <i>f</i>₄₃ and <i>f</i>₄₄ and elemental analysis of high-resolution spectra in Van Krevelen space both yield a slope of ∼−0.5 across different SOA types. A similar slope was obtained for a distinct subset of toluene/NO<i>_x</i> reactions in which the integrated oxidant exposure was varied to alter oxidation. The relative volatility of different SOA types displays similar variability and is strongly correlated with SOA oxidation state (_C). On average, relatively low oxidation and volatility were observed for aliphatic alkene (including terpenes) and <i>n-</i>alkane SOA while the opposite is true for mono- and polycyclic aromatic hydrocarbon SOA. Effective enthalpy for total chamber aerosol obtained from volatility differential mobility analysis is also highly correlated with _C indicating a primary role for oxidation levels in determining the volatility of chamber SOA. Effective enthalpies for chamber SOA are substantially lower than those of neat organic standards but are on the order of those obtained for partially oligomerized glyoxal and methyl glyoxal.</p> <p>© 2018 American Association for Aerosol Research</p

Predicting Thermal Behavior of Secondary Organic Aerosols

Volume concentrations of secondary organic aerosol (SOA) are measured in 139 steady-state, single precursor hydrocarbon oxidation experiments after passing through a temperature controlled inlet. The response to change in temperature is well predicted through a feedforward Artificial Neural Network. The most parsimonious model, as indicated by Akaike’s Information Criterion, Corrected (AIC,C), utilizes 11 input variables, a single hidden layer of 4 tanh activation function nodes, and a single linear output function. This model predicts thermal behavior of single precursor aerosols to less than ±5%, which is within the measurement uncertainty, while limiting the problem of overfitting. Prediction of thermal behavior of SOA can be achieved by a concise number of descriptors of the precursor hydrocarbon including the number of internal and external double bonds, number of methyl- and ethyl- functional groups, molecular weight, and number of ring structures, in addition to the volume of SOA formed, and an indicator of which of four oxidant precursors was used to initiate reactions (NO_<i>x</i> photo-oxidation, photolysis of H₂O₂, ozonolysis, or thermal decomposition of N₂O₅). Additional input variables, such as chamber volumetric residence time, relative humidity, initial concentration of oxides of nitrogen, reacted hydrocarbon concentration, and further descriptors of the precursor hydrocarbon, including carbon number, number of oxygen atoms, and number of aromatic ring structures, lead to over fit models, and are unnecessary for an efficient, accurate predictive model of thermal behavior of SOA. This work indicates that predictive statistical modeling methods may be complementary to descriptive techniques for use in parametrization of air quality models

Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT): Overview of a wintertime air chemistry field study in the front range urban corridor of Colorado

The Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) field experiment took place during late winter, 2011, at a site 33 km north of Denver, Colorado. The study included fixed-height measurements of aerosols, soluble trace gases, and volatile organic compounds near surface level, as well as vertically resolved measurements of nitrogen oxides, aerosol composition, soluble gas-phase acids, and halogen species from 3 to 270 m above ground level. There were 1928 individual profiles during the three-week campaign to characterize trace gas and aerosol distributions in the lower levels of the boundary layer. Nitrate and ammonium dominated the ionic composition of aerosols and originated primarily from local or regional sources. Sulfate and organic matter were also significant and were associated primarily with longer-range transport to the region. Aerosol chloride was associated primarily with supermicron size fractions and was always present in excess of gas-phase chlorine compounds. The nighttime radical reservoirs, nitryl chloride, ClNO2, and nitrous acid, HONO, were both consistently present in nighttime urban air. Nitryl chloride was especially pronounced in plumes from large point sources sampled aloft at night. Nitrous acid was typically most concentrated near the ground surface and was the dominant contributor (80%) to diurnally averaged primary OH radical production in near-surface air. Large observed mixing ratios of light alkanes, both in near-surface air and aloft, were attributable to local emissions from oil and gas activities

On the Role of Particle Inorganic Mixing State in the Reactive Uptake of N₂O₅ to Ambient Aerosol Particles

The rates of heterogeneous reactions of trace gases with aerosol particles are complex functions of particle chemical composition, morphology, and phase state. Currently, the majority of model parametrizations of heterogeneous reaction kinetics focus on the population average of aerosol particle mass, assuming that individual particles have the same chemical composition as the average state. Here we assess the impact of particle mixing state on heterogeneous reaction kinetics using the N₂O₅ reactive uptake coefficient, γ­(N₂O₅), and dependence on the particulate chloride-to-nitrate ratio (<i>n</i>Cl^–/<i>n</i>NO₃^–). We describe the first simultaneous ambient observations of single particle chemical composition and in situ determinations of γ­(N₂O₅). When accounting for particulate <i>n</i>Cl^–/<i>n</i>NO₃^– mixing state, model parametrizations of γ­(N₂O₅) continue to overpredict γ­(N₂O₅) by more than a factor of 2 in polluted coastal regions, suggesting that chemical composition and physical phase state of particulate organics likely control γ­(N₂O₅) in these air masses. In contrast, direct measurement of γ­(N₂O₅) in air masses of marine origin are well captured by model parametrizations and reveal limited suppression of γ­(N₂O₅), indicating that the organic mass fraction of fresh sea spray aerosol at this location does not suppress γ­(N₂O₅). We provide an observation-based framework for assessing the impact of particle mixing state on gas–particle interactions