The influence of nitrogen oxides on the activation of bromide and chloride in salt aerosol

Abstract. Experiments on salt aerosol with different salt contents were performed in a Teflon chamber under tropospheric light conditions with various initial contents of nitrogen oxides (NOx = NO + NO2). A strong activation of halogens was found at high NOx mixing ratios, even in samples with lower bromide contents such as road salts. The ozone depletion by reactive halogen species released from the aerosol, was found to be a function of the initial NOx mixing ratio. Besides bromine, large amounts of chlorine have been released in our smog chamber. Time profiles of the halogen species Cl2, Br2, ClNO2, BrNO2 and BrO, ClO, OClO and Cl atoms were simultaneously measured by various techniques (chemical ionization mass spectrometry, differential optical absorption spectrometry coupled with a multi-reflection cell and gas chromatography of hydrocarbon tracers for Cl and OH, employing cryogenic preconcentration and flame ionization detection). Measurements are compared to calculations by the CAABA/MECCA 0-D box model, which was adapted to the chamber conditions and took the aerosol liquid water content and composition into account. The model results agree reasonably with the observations and provide important information about the prerequisites for halogen release, such as the time profiles of the aerosol bromide and chloride contents as well as the aerosol pH.</jats:p

Chamber-based insights into the factors controlling epoxydiol (IEPOX) secondary organic aerosol (SOA) yield, composition, and volatility

We present measurements utilizing the Filter Inlet for Gases and Aerosols (FIGAERO) applied to chamber measurements of isoprene-derived epoxydiol (IEPOX) reactive uptake to aqueous acidic particles and associated secondary organic aerosol (SOA) formation. Similar to recent field observations with the same instrument, we detect two molecular components desorbing from the IEPOX SOA in high abundance: C5H12O4 and C5H10O3. The thermal desorption signal of the former, presumably 2-methyltetrols, exhibits two distinct maxima, suggesting it arises from at least two different SOA components with significantly different effective volatilities. Isothermal evaporation experiments illustrate that the most abundant component giving rise to C5H12O4 is semi-volatile, undergoing nearly complete evaporation within 1 h while the second, less volatile component remains unperturbed and even increases in abundance. We thus confirm, using controlled laboratory studies, recent analyses of ambient SOA measurements showing that IEPOX SOA is of very low volatility and commonly measured IEPOX SOA tracers such as C5H12O4 and C5H10O3, presumably 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diols, result predominantly from thermal decomposition in the FIGAERO-CIMS. We infer that other measurement techniques using thermal desorption or prolonged heating for analysis of SOA components may also lead to reported 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diol structures. We further show that IEPOX SOA volatility continues to evolve via acidity-enhanced accretion chemistry on the timescale of hours, potentially involving both 2-methyltetrols and organosulfates.Peer reviewe

Constraints on the Reactivity and Components of Nocturnal Nitrogen Oxides

Thesis (Ph.D.)--University of Washington, 2013NO and NO2 (NOx) are fundamentally important species to tropospheric chemistry. NOx abundances are tied to ozone production and thus determine the oxidizing capacity of the troposphere. Nocturnal reactions of NOx are often considered a major loss pathway for NOx and ozone. Recent measurements have shown that nitryl chloride (ClNO2) is produced at night by reactions of dinitrogen pentoxide (N2O5) on chloride containing particles. ClNO2 is photolyzed during the morning hours after sunrise to liberate highly reactive chlorine atoms. This chemistry takes place primarily in polluted environments where the concentrations of N2O5 precursors, NOx, and ozone, are high, though it can likely occur in remote regions at lower intensities. The following describes estimates and ambient measurements of the reactive processes central to ClNO2 formation and field measurements illustrating the potential importance of ClNO2 as a NOx reservoir and as a chlorine atom source. The nocturnal reactions of N2O5 to form ClNO2 were traditionally thought of as marine phenomena given the more obvious source of particle-phase chloride offered by sea spray emissions. However, long term chemical measurement databases and aerosol thermodynamic models are employed to show that this chemistry is likely widespread as is suggested by recent field measurements of ClNO2 in Boulder, CO, a site far removed from local sea salt aerosol sources. Direct measurements of N2O5 reaction probability on ambient aerosol particles were made in La Jolla, CA, using a custom flow reactor alongside measurements of aerosol particle size distributions and non-refractory composition. The largest apparent driver of day-to-day variability in the measured reaction probabilities at this site was the particle nitrate loading. The relative change as a function of particle nitrate illustrates the atmospheric importance of the so-called "nitrate effect" on N2O5 heterogeneous reactions that lead to the formation of ClNO2. The magnitude and sources of chlorine atoms in marine air remain highly uncertain but have potentially important consequences for air quality in polluted coastal regions. Continuous measurements of ambient nitryl chloride and molecular chlorine concentrations were made in southern California. In the Los Angeles region, ClNO2 was more ubiquitous than Cl2 during most nights of the study period. These observations are used to estimate the relative importance of chlorine atom sources in the polluted marine boundary layer. In contrast to the emphasis in previous studies, ClNO2 and hydrochloric acid are likely the dominant primary chlorine atom sources for the Los Angeles basin. As part of a wintertime field study in Weld County, CO, vertically resolved ClNO2 and Cl2 measurements taken on a 300 meter tall tower are reported. Gas and particle phase measurements aboard a moveable tower carriage allowed for a detailed description of the chemical state of the nocturnal atmosphere as a function of height. These observations show significant vertical structure in ClNO2 and Cl2 mixing ratios that undergo dynamic changes over the course of a night. From these measurements ClNO2 yields from N2O5-aerosol reactions are inferred. The derived yields in these plumes suggest efficient ClNO2 production within distinct combustion plumes originating from the Denver-Boulder urban corridor. Finally, the effects of ClNO2 production, photolysis, and subsequent chlorine atom reactions on chemical species relevant to air quality are examined. ClNO2 formation is incorporated into an existing Master Chemical Mechanism box model framework constrained by a large number of measurements taken during field studies in a polluted coastal environment. These results are compared to model runs excluding ClNO2 formation to assess the effects of ClNO2 on tropospheric oxidants, ozone, and nitrogen oxide partitioning

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