Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?

Despite several investigations on the atmospheric fate of cyclic volatile methyl siloxanes (VMS), the oxidation chemistry of these purely anthropogenic, high production volume compounds is poorly understood. This led to uncertainties in the environmental impact and fate of the oxidation products. According to laboratory measurements, the main VMS oxidation product is the siloxanol (a -CH3 replaced with an -OH); however, none of the mechanisms proposed to date satisfactorily explain its formation. Motivated by our previous experimental observations of VMS oxidation products, we use theoretical quantum chemical calculations to (1) explore a previously unconsidered reaction pathway to form the siloxanol from a reaction of a siloxy radical with gas-phase water, (2) investigate differences in reaction rates of radical intermediates in hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) oxidation, and (3) attempt to explain the experimentally observed products. Our results suggest that while the proposed reaction of the siloxy radical with water to form the siloxanol can occur, it is too slow to compete with other unimolecular reactions and thus cannot explain the observed siloxanol formation. We also find that the reaction between the initial D3 peroxy radical (RO2•) with HO2• is slower than previously anticipated (calculated as 3 × 10-13 cm3 molecule-1 s-1 for D3 and 2 × 10-11 cm3 molecule-1 s-1 for D4 compared to the general rate of ∼1 × 10-11 cm3 molecule-1 s-1). Finally, we compare the anticipated fates of the RO2• under a variety of conditions and find that a reaction with NO (assuming a general RO2• + NO bimolecular rate constant of 9 × 10-12 cm3 molecule-1 s-1) will likely be the dominant fate in urban conditions, while isomerization can be important in cleaner environments.

Airborne observations over the North Atlantic Ocean reveal the importance of gas-phase urea in the atmosphere.

Reduced nitrogen (N) is central to global biogeochemistry, yet there are large uncertainties surrounding its sources and rate of cycling. Here, we present observations of gas-phase urea (CO(NH2)2) in the atmosphere from airborne high-resolution mass spectrometer measurements over the North Atlantic Ocean. We show that urea is ubiquitous in the lower troposphere in the summer, autumn, and winter but was not detected in the spring. The observations suggest that the ocean is the primary emission source, but further studies are required to understand the responsible mechanisms. Urea is also observed aloft due to long-range transport of biomass-burning plumes. These observations alongside global model simulations point to urea being an important, and currently unaccounted for, component of reduced-N to the remote marine atmosphere. Airborne transfer of urea between nutrient-rich and -poor parts of the ocean can occur readily and could impact ecosystems and oceanic uptake of carbon dioxide, with potentially important climate implications.

Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products.

Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals that have come under scrutiny due to their widespread use and environmental persistence. Significant data on environmental concentrations and persistence of these chemicals exists, but their oxidation mechanism is poorly understood, preventing a comprehensive understanding of the environmental fate and impact of cVMS. We performed experiments in an environmental chamber to characterize the first-generation oxidation products of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under different peroxy radical fates (unimolecular reaction or bimolecular reaction with either NO or HO2) that approximate a range of atmospheric compositions. While the identity of the oxidation products from D3 changed as a function of the peroxy radical fate, the identity and yield of D4 and D5 oxidation products remained largely constant. We compare our results against the output from a kinetic model of cVMS oxidation chemistry. The reaction mechanism used in the model is developed using a combination of previously proposed cVMS oxidation reactions and standard atmospheric oxidation radical chemistry. We find that the model is unable to reproduce our measurements, particularly in the case of D4 and D5. The products that are poorly represented in the model help to identify possible branching points in the mechanism, which require further investigation. Additionally, we estimated the physical properties of the cVMS oxidation products using structure-activity relationships and found that they should not be significantly partitioned to organic or aqueous aerosol. The results suggest that cVMS first-generation oxidation products are also long-lived in the atmosphere and that environmental monitoring of these compounds is necessary to understand the environmental chemistry and loading of cVMS.

Atmospheric Chemistry of Volatile Methyl Siloxanes: Kinetics and Products of Oxidation by OH Radicals and Cl Atoms.

Volatile methyl siloxanes (VMS) are ubiquitous anthropogenic pollutants that have recently come under scrutiny for their potential toxicity and environmental persistence. In this work, we determined the rate constants for oxidation by OH radicals and Cl atoms at 297 ± 3 K and atmospheric pressure in Boulder, CO (∼860 mbar) of hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5). Measured rate constants with OH radicals were (1.20 ± 0.09) × 10-12, (1.7 ± 0.1) × 10-12, (2.5 ± 0.2) × 10-12, (3.4 ± 0.5) × 10-12, (0.86 ± 0.09) × 10-12, (1.3 ± 0.1) × 10-12, and (2.1 ± 0.1) × 10-12 cm3 molec-1 s-1, for L2, L3, L4, L5, D3, D4, and D5, respectively. The rate constants for reactions with Cl atoms with the same compounds were (1.44 ± 0.05) × 10-10, (1.85 ± 0.05) × 10-10, (2.2 ± 0.1) × 10-10, (2.9 ± 0.1) × 10-10, (0.56 ± 0.05) × 10-10, (1.16 ± 0.08) × 10-10, and (1.8 ± 0.1) × 10-10 cm3 molec-1 s-1, respectively. Substituent factors of F(-Si(CH3)2OR) and F(-SiCH3(OR)2) are proposed for use in AOPWIN, a common model for OH radical rate constant estimations. Cl atoms can remove percentage levels of VMS globally with potentially increased importance in urban areas.

The Impact of Molecular Oxygen on Anion Composition in a Hazy Archean Earth Atmosphere.

Atmospheric organic hazes are common in planetary bodies in our solar system and likely exoplanet atmospheres as well. In addition, geochemical data support the existence of an organic haze in the early Earth's atmosphere. Much of what is known about organic haze formation derives from studies of Saturn's moon Titan. It is believed that on Titan ions play an important role in haze formation. It is possible, by using Titan as an analog for the Archean Earth, to consider that an Archean haze could have formed by similar processes. Here, we examine the anion chemistry that occurs during laboratory simulations of early Earth haze formation and measure the composition of gaseous anions as a function of O2 mixing ratio. Gaseous anion composition and relative abundances are measured by an atmospheric pressure interface time-of-flight mass spectrometer and are compared to previous photochemical haze mass loading measurements. Numerous anions are observed spanning from mass-to-charge ratio 26 to 246, with a majority of the identified anions containing carbon, hydrogen, nitrogen, and/or oxygen. A shift in the anion composition occurs with increasing the precursor O2 mixing ratio. With 0-20 ppmv O2 in CH4/CO2/N2 mixtures, ions contain mostly organic nitrogen, with CNO- being the most intense ion peak. As the precursor O2 is increased to 200 and 2000 ppmv, inorganic nitrogen ions become the dominant chemical group, with NO3- having the most intense ion signal. The clear shift in the ionic composition could be indicative of a modification to the gas-phase chemistry that occurs in the transition from an anoxic atmosphere to an oxygen-containing atmosphere, with potential astrobiological significance.

Laser Ablation-Aerosol Mass Spectrometry-Chemical Ionization Mass Spectrometry for Ambient Surface Imaging.

Mass spectrometry imaging is becoming an increasingly common analytical technique due to its ability to provide spatially resolved chemical information. Here, we report a novel imaging approach combining laser ablation with two mass spectrometric techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, separately and in parallel. Both mass spectrometric methods provide the fast response, rapid data acquisition, low detection limits, and high-resolution peak separation desirable for imaging complex samples. Additionally, the two techniques provide complementary information with aerosol mass spectrometry providing near universal detection of all aerosol molecules and chemical ionization mass spectrometry with a heated inlet providing molecular-level detail of both gases and aerosols. The two techniques operate with atmospheric pressure interfaces and require no matrix addition for ionization, allowing for samples to be investigated in their native state under ambient pressure conditions. We demonstrate the ability of laser ablation-aerosol mass spectrometry-chemical ionization mass spectrometry (LA-AMS-CIMS) to create 2D images of both standard compounds and complex mixtures. The results suggest that LA-AMS-CIMS, particularly when combined with advanced data analysis methods, could have broad applications in mass spectrometry imaging applications.

An Atmospheric Constraint on the NO2 Dependence of Daytime Near-Surface Nitrous Acid (HONO).

Recent observations suggest a large and unknown daytime source of nitrous acid (HONO) to the atmosphere. Multiple mechanisms have been proposed, many of which involve chemistry that reduces nitrogen dioxide (NO2) on some time scale. To examine the NO2 dependence of the daytime HONO source, we compare weekday and weekend measurements of NO2 and HONO in two U.S. cities. We find that daytime HONO does not increase proportionally to increases in same-day NO2, i.e., the local NO2 concentration at that time and several hours earlier. We discuss various published HONO formation pathways in the context of this constraint.

Changes to the chemical composition of soot from heterogeneous oxidation reactions.

The atmospheric aging of soot particles, in which various atmospheric processes alter the particles' chemical and physical properties, is poorly understood and consequently is not well-represented in models. In this work, soot aging via heterogeneous oxidation by OH and ozone is investigated using an aerosol flow reactor coupled to a new high-resolution aerosol mass spectrometric technique that utilizes infrared vaporization and single-photon vacuum ultraviolet ionization. This analytical technique simultaneously measures the elemental and organic carbon components of soot, allowing for the composition of both fractions to be monitored. At oxidant exposures relevant to the particles' atmospheric lifetimes (the equivalent of several days of oxidation), the elemental carbon portion of the soot, which makes up the majority of the particle mass, undergoes no discernible changes in mass or composition. In contrast, the organic carbon (which in the case of methane flame soot is dominated by aliphatic species) is highly reactive, undergoing first the addition of oxygen-containing functional groups and ultimately the loss of organic carbon mass from fragmentation reactions that form volatile products. These changes occur on time scales comparable to those of other nonoxidative aging processes such as condensation, suggesting that further research into the combined effects of heterogeneous and condensational aging is needed to improve our ability to accurately predict the climate and health impacts of soot particles.

Chemical compositions of black carbon particle cores and coatings via soot particle aerosol mass spectrometry with photoionization and electron ionization.

Black carbon is an important constituent of atmospheric aerosol particle matter (PM) with significant effects on the global radiation budget and on human health. The soot particle aerosol mass spectrometer (SP-AMS) has been developed and deployed for real-time ambient measurements of refractory carbon particles. In the SP-AMS, black carbon or metallic particles are vaporized through absorption of 1064 nm light from a CW Nd:YAG laser. This scheme allows for continuous "soft" vaporization of both core and coating materials. The main focus of this work is to characterize the extent to which this vaporization scheme provides enhanced chemical composition information about aerosol particles. This information is difficult to extract from standard SP-AMS mass spectra because they are complicated by extensive fragmentation from the harsh 70 eV EI ionization scheme that is typically used in these instruments. Thus, in this work synchotron-generated vacuum ultraviolet (VUV) light in the 8-14 eV range is used to measure VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra of commercially available carbon black, fullerene black, and laboratory generated flame soots were obtained. Small carbon cluster cations (C(+)-C5(+)) were found to dominate the VUV-SP-AMS spectra of all the samples, indicating that the corresponding neutral clusters are key products of the SP vaporization process. Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and SP-AMS spectra are used to confirm spectral features that could be used to distinguish between different types of refractory carbon particles. VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing cores are also examined and found to display less thermally induced decomposition and fragmentation than spectra obtained with thermal vaporization at 200 °C (the minimum temperature needed to quantitatively vaporize ambient oxidized organic aerosol with a continuously heated surface). The particle cores tested in these studies include black carbon, silver, gold, and platinum nanoparticles. These results demonstrate that SP vaporization is capable of providing enhanced organic chemical composition information for a wide range of organic coating materials and IR absorbing particle cores. The potential of using this technique to study organic species of interest in seeded laboratory chamber or flow reactor studies is discussed.

Observational insights into aerosol formation from isoprene.

Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F(MAE formation)). The strong temperature dependence of F(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m(-3)) and MAE-derived organosulfates (MAE-OS; ~1 ng m(-3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m(-3)) relative to MAE-OS (<0.0005 ng m(-3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10-100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.