Reactive Uptake of Monoterpene derived Epoxides, an Aldehyde and an Alcohol on Acidic Sulfate Particles
The formation of organosulfates from reactive uptake of α-pinene oxide, β-pinene oxide, campholenic aldehyde and carveol is investigated in a laboratory chamber study. Organosulfates were analyzed using UPLC/ESI-QTOFMS (electrospray ionization quadrupole time-of-flight mass spectrometry). Organosulfates were only found in the samples from α-pinene oxide, β-pinene oxide and campholenic aldehyde in the presence of acidic sulfate seed particles. In particular, significantly higher concentrations of organosulfates were found in the sample from β-pinene oxide reactive uptake than α-pinene oxide. Campholenic aldehyde and carveol showed a very little or no organosulfate, indicating that epoxides most likely serve as precursors for the formation of organosulfates in the atmosphere. An organosulfate with a sulfate group at a tertiary carbon atom comprises 64% of the detected β-pinene organosulfates whereas an organosulfate with sulfate at a secondary carbon atom comprises 80% of the detected α-pinene organosulfates, indicating different behaviors of exocyclic and endocyclic monoterpene oxides in the formation of organosulfates. This study provides evidence for the importance of the reactive uptake of epoxides in the formation of atmospherically relevant organosulfates.
Analysis of reversibility and reaction products of glyoxal uptake onto ammonium sulfate aerosol
We present chamber aerosol and bulk-phase laboratory studies aimed at improving understanding of the processes controlling glyoxal uptake onto aerosol. Both reversible and irreversible condensed-phase processes were found and the chemistry controlling both types of processes and their reaction products will be discussed. In the chamber studies overall uptake into aqueous ammonium sulfate seed aerosol was found to be reversible with a substantial contribution from glyoxal oligomers. The effective Henry's law constant was found to be about 2 orders of magnitude larger than for the glyoxal-water system. Possible reasons for this difference, including the effect of inorganic compounds on the equilibrium concentrations of glyoxal oligomers, will be discussed. Organosulfates did not form under dark conditions in the presence of either neutral or acidified sulfate seed aerosol, but under irradiated conditions an organosulfate was observed. This organosulfate has also been observed in chamber studies of isoprene oxidation and in ambient aerosol. We present studies to identify specific species via comparison of chamber data with synthesized organosulfate standards. The influence of irradiation on organosulfate formation will also be discussed. Reaction of glyoxal with neutral and acidified ammonium sulfate seed aerosol as well as bulk samples was observed to irreversibly form carbon-nitrogen containing compounds under irradiated and dark conditions and identified products and their properties will be presented. Ammonium sulfate aerosol systems have reduced complexity compared to atmospheric aerosol, which can have numerous organic components. However, our studies show that even in such a simplified model system complex chemistry occurs. The results of our studies increase understanding of aerosol-phase processes and highlight areas that require further study.
Contributions of Organic Vapors to Aerosol Aging and Growth
Atmospheric aerosols impair visibility and human health, interfere with radiative transfer, and alter cloud formation. The major contributors include sulfate and organic aerosols from anthropogenic and biogenic activities, which are produced through a multitude of complex multiphase atmospheric processes by photochemical oxidation of emitted sulfur dioxide and volatile organic compounds (VOCs) into less volatile forms and gas-to-particle conversion. Condensation of organic vapors onto the pre-existing atmospheric aerosols, followed by chemical reactions within the particles medium, is believed to be one of the major pathways that contribute to particle growth. Recent research has focused on the total mass increase on pre- existing seed particles, but the chemistry that determines the efficiency of organic uptake remains to be elucidated. This talk will focus on the growth of nano- to sub-micron sulfuric acid droplets exposed organic vapors. Experiments performed at different relative humidity and using different organic vapors (i.e., small alpha-dicarbonyls and large aldehydes) will be presented. The chemical mechanisms and size dependence of the particle growth will be demonstrated. Implications of the present results to aging and growth of aerosols under ambient conditions will be discussed.
Diels-Alder Chemistry in the Acid-catalyzed Oligomerization of Methacrolein in Simulated Atmospheric Aerosol: Insights using Desorption Electrospray Ionization Mass Spectrometry
Through the analysis of acid-catalyzed methacrolein (MACR) oligomer by Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) and MS/MS, direct and indirect evidence for the occurrence and potential importance of the Diels-Alder reaction has been gained. A literature evaluation shows that many atmospheric oxygenated volatile organic compounds (OVOCs) undergo the Diels-Alder reaction with themselves and each other. When taken in combination with the large annual production of these species, this reaction likely accounts for part of production of low volatility species by condensed-phase oligomerization reactions. Acid- catalyzed disproportionation has been observed and a mechanism suggested, as has acid-catalyzed polymerization of MACR and MACR dimer, and hydrolysis of aldehyde and olefinic groups. Though counter- intuitive, organosulfate formation was not observed due to acid cleaving of the sulfate group and competition with the hydrolyzed product's stability. While the "aged" appearance of aerosol has been considered to be produced by photochemical means, oxidation, oxidative dehydrogenation, and disproportionation take place in this system in the absence of photochemistry, which may significantly increase the degree of oxidation observed in SOA under acidic conditions. The flexibility of DESI-MS has also been tested against ambient samples and has yielded the first known spectra of high MW species in cloud water. ESI yielded better signal and allowed the detection of organosulfates by neutral loss experiments. This represents the first known soft ionization spectrum of cloud water and is direct and unambiguous evidence that oligomeric material is present in cloud water.
Uptake of Ambient Organic Gases to Acidic Sulfate Aerosols
The formation of secondary organic aerosols (SOA) in the atmosphere has been an area of significant interest due to its climatic relevance, its effects on air quality and human health. Due largely to the underestimation of SOA by regional and global models, there has been an increasing number of studies focusing on alternate pathways leading to SOA. In this regard, recent work has shown that heterogeneous and liquid phase reactions, often leading to oligomeric material, may be a route to SOA via products of biogenic and anthropogenic origin. Although oligomer formation in chamber studies has been frequently observed, the applicability of these experiments to ambient conditions, and thus the overall importance of oligomerization reactions remain unclear. In the present study, ambient air is drawn into a Teflon smog chamber and exposed to acidic sulfate aerosols which have been formed in situ via the reaction of SO3 with water vapor. The aerosol composition is measured with a High Resolution Aerodyne Aerosol Mass Spectrometer (HR-ToF-AMS), and particle size distributions are monitored with a scanning mobility particle sizer (SMPS). The use of ambient air and relatively low inorganic particle loading potentially provides clearer insight into the importance of heterogeneous reactions. Results of experiments, with a range of sulfate loadings show that there are several competing processes occurring on different timescales. A significant uptake of ambient organic gases to the particles is observed immediately followed by a slow shift towards higher m/z over a period of several hours indicating that higher molecular weight products (possibly oligomers) are being formed through a reactive process. The results suggest that heterogeneous reactions can occur with ambient organic gases, even in the presence of ammonia, which may have significant implications to the ambient atmosphere where particles may be neutralized after their formation.
Products of the radical initiated oxidation of model solid and liquid organic acid particles in simulated "clean" and "polluted" environments.
Using a flow tube reactor coupled to a chemical ionization mass spectrometer, the Cl-initiated oxidation of solid and supercooled liquid organic acid particles were investigated at 293 K. In creating aerosols of species which are able to be supercooled or solid at room temperature, it is possible to distinguish the effect of phase on particle reactivity and product formation. In a clean atmosphere, where there are negligible concentrations of NOx, the primary fate of peroxy radicals (formed from H-abstraction by Cl and OH radicals in the presence of O2) are their reactions to form ketone and alcohol products. These products are then able to undergo further oxidation to form multiply oxidized products. The formation of low-molecular weight volatile species may also be important in the oxidative aging of organic aerosols, however neither the mechanism of their formation nor their formation yields are well understood. We have shown that, for equivalent Cl exposures, more multiply-oxidized species as well as more low-molecular-weight species were created from the oxidation of solid particles than from liquid particles. The findings from these studies suggest that slower diffusion of the oxidation products in solid particles confines them to the surface where they continue to react with Cl radicals producing more-highly- functionalized products which may decompose more readily. By introducing nitric oxide to the flow tube reaction system, we show that in a polluted atmosphere, where NOx is present in significant concentrations, organic nitrate formation may become important on the surface of solid particles but not liquid particles as the RO2 are confined to the surface of solid particles (causing a enhanced localized concentration of RO2) where they may then react with ambient nitric oxide through the reaction RO2 + NO → RO2NO* → RONO2. These experiments of these model systems indicate that particle phase could be important in determining how organic aerosols evolve chemically through radical-initiated oxidation in the atmosphere.