A11B-0104
Optical properties of organic multicomponent aerosol particles using CW-Cavity Ring Down Spectrometry
We will present laboratory studies using cavity ring down (CRD) aerosol spectrometer to derive the extinction and complex refractive index of aerosols containing a significant organic component. By precisely measuring extinction as a function of particle size, the real and imaginary refractive indices are obtained and the single scattering albedo may be calculated. Specifically, we will present the absorption of aerosols containing HULIS and inorganic salts at 532 and 390 nm, test various optical mixing rules and will present results on the extinction of core-shell particles. In addition, we will also present a new portable continuous wave spectrometer (CW-CRD-AS) and will discuss its advantages compared to pulsed systems.
A11B-0105
Optical Properties of Internally Mixed Aerosols Composed of Dicarboxylic Acids and Ammonium Sulfate
We have investigated the optical properties of internally mixed aerosol particles composed of dicarboxylic acids and ammonium sulfate as a function of the fraction of the organic component. The extinction coefficient of size-selected aerosol particles at 532 nm is measured using cavity ring-down spectroscopy. Using the extinction coefficient and the concentration, we calculate the extinction cross section. We compare the experimental extinction cross sections to those predicted by Mie scattering calculations to determine the real refractive index of these non-absorbing systems. For the system of succinic acid and ammonium sulfate, we find that the real refractive index does not follow a linear trend with the fraction of organic component. In particular, additions of small amounts of ammonium sulfate to succinic acid result in large changes to the refractive index. We will compare our results to those predicted from several optical mixing rules. Deviations from these mixing rules may arise due to the structure and shape of the particles. As a consequence, we will address differences between our results and the predictions of mixing rules by exploring the aerodynamic diameter, non-spherical Mie code calculations, and Atomic force microscopy. Impacts of these deviations on direct radiative forcing calculations will be discussed.
A11B-0106
Sunlight Initiated Photochemical Reactions: Relevance to Atmospheric Chemistry and Climate
Despite the importance of organics in the chemistry of the atmosphere, their chemical composition and atmospheric transformation remain uncertain. This presentation targets a novel photoprocessing mechanism for oxygenated and low-volatility organic acids and alcohols whose overall budgets are poorly constrained, as are the processes governing their concentrations. Most atmospheric photochemical reactions are initiated through electronic transitions of molecules in the UV spectral region. Results of an investigation of sunlight initiated chemical reactions occurring at low energy by overtone induced chemistry of oxygenated organics will be presented. The targets for this study are atmospherically relevant acids and alcohols where excitation of OH vibrational overtones in the near-IR and visible, near the solar maximum photon output leads to dehydration, decaboxylation or decarbonylation. Relatively low energy concerted photochemical reactions produce high-energy intermediates and products. These photochemical reactions are in some cases water catalyzed and expected to occur on atmospheric aerosols. This new chemistry opens the possibility of photochemical processing for acids and alcohols in the atmosphere, in or on aerosols. This chemistry affects atmospheric aerosol processing and is therefore important in climate.
A11B-0107
Ozone Oxidation of Self-Assembled Monolayers on SiOx-Coated Zinc Selenide Surfaces
Airborne particles are important for visibility, human health, climate, and atmospheric reactions. Atmospheric particles contain a significant fraction of organics and such compounds present on airborne particles are susceptible to oxidation by atmospheric oxidants, such as OH, ozone, halogen atoms, and nitrogen trioxide. Oxidized organics associated with airborne particles are thought to be polar, hygroscopic species with enhanced cloud-nucleating properties. Oxide layers on silicon, or SiO2-coated substrates, act as models of environmentally relevant surfaces such as dust particles upon which organics adsorb. We have shown previously that ozone oxidation of unsaturated self-assembled monolayers (SAMs) on silicon attenuated total reflectance (ATR) crystals leads to the formation of carbonyl groups and micron-sized, hydrophobic organic aggregates surrounded by carbon depleted substrate that do not have increased water uptake as previously assumed. Reported here are further ATR-FTIR studies of the oxidation of alkene SAMs on ZnSe and SiO2-coated ZnSe. These substrates have the advantage that they transmit below 1500 cm-1, allowing detection of additional product species. These experiments show that the loss of C=C and formation of carbonyl groups is also accompanied by formation of a peak at 1110 cm-1, attributed to the secondary ozonide. Details concerning the products and mechanism of ozonolysis of alkene SAMs on surfaces based on these new data are presented and the implications for the oxidation of alkenes on airborne dust particles are discussed.
A11B-0108
Oxidation From the "Bottom Up" of Organics on Sodium Nitrite/Sodium Chloride Mixtures
Sea salt and other particles in air are believed to be coated at least in part with organics. Oxidation of the coating can change the properties of the particles, including their water uptake and growth which affects visibility, their radiative properties and potentially health effects. We report here a new mechanism of oxidation of adsorbed organics by OH generated by the photolysis of nitrite ions in the underlying substrate. The UV photolysis in air of a mixture of NaNO2 and NaCl coated with 1-oleoyl-2-palmitoyl-sn- glycero-3-phosphocholine (OPPC) was followed using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) at 296 K, both with and without added water vapor. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was used to confirm the identification of the products. Organic nitrates and carbonyl compounds were observed as products and in the absence of added water vapor, carboxylate ions were also formed. These experiments show that the generation of O- and OH in the photolysis of nitrite ions in and on solids leads to oxidation of organics adsorbed on the salt surface. Similar chemistry is likely to occur for nitrate ions which also photolyze to generate O- and OH. This suggests a new mechanism of oxidation of organics on surfaces in air that have co-adsorbed nitrite or nitrate ions.
A11B-0109
Identification of Components in Organic Films by Coupled Liquid Chromatography-Mass Spectrometry
Recent studies have shown that viscous organic films of unknown composition develop on the surface of solutions containing propanal, glyoxal and high concentrations of sulfuric acid. These films, the products of significant contributors to atmospheric chemistry and particle composition, are theorized to have major implications in real world settings. Surface films that form on aerosol particles can alter cloud formation, diurnal patterns, and the optical properties of the atmosphere. In order to predict these chemical and physical properties, the components of both the films and the solutions that form them are studied via coupled Liquid Chromatography-Mass Spectrometry (LC-MS). Spectra reveal the presence of a varying range of product masses (75 m/z to ~ 700 m/z) within these film-making solutions. This suggests the formation of oligomers and/or complex products from the simple starting components. As such, much of the chemistry involved in this process results from a series of acid-catalyzed mechanisms. This presentation will focus on detailed interpretation of the LC-MS results with the goal of determining the building-blocks for the oligomers and identifying as many of the molecules present in the films and the reaction solutions as possible.
A11B-0110
Organic Film Formation on Acidic Solutions: Growth Conditions and Film Persistence
Organic aerosols are found throughout the Earth's atmosphere, and they may coagulate with strongly acidic particles in the upper troposphere and lower stratosphere. Many organic compounds are known to be highly reactive in strong acid, thus we are exploring the fate of representative organic molecules in aqueous sulfuric acid solutions. Under a variety of conditions, we find that surface films can form and be degraded over time scales of several weeks. Films formed on organic aerosols found in the atmosphere can impact the particle's climate properties by changing the way it interacts with radiation and by influencing cloud nucleation. We observed organic solutions under a variety of conditions: varying organic components, temperature, acidity, exposure to visible light, and amount of exposed surface. Color changes, UV-vis spectra, and the quantity of film present were monitored over periods of weeks to years. Methylglyoxal, glyoxal, and propanal were examined. We observe that propanal solutions always form films, and some films begin to degrade after weeks. Films tend to form most rapidly at sulfuric acid concentrations of approximately 50 weight percent, and formation is inhibited, or at least dramatically slowed, for temperatures in the range of -20 C. The surface to volume ratio of the solution may also affect the film formation rate. Additionally, we find that the addition of methylglyoxal can inhibit the formation of films, while the addition of glyoxal to a propanal solution can accelerate the film formation and degradation process.
A11B-0111
Measuring and Modeling the Surface Tensions of Organic Aqueous Solutions With Atmospheric Significance
Organic compounds account for a substantial fraction of dry submicron atmospheric aerosol mass. Additionally, single particle composition data suggest that individual aerosols are often mixtures of organic and inorganic components. This study measured the surface tensions of atmospherically relevant pure and mixed organic / inorganic aqueous solutions. Measurements were performed using the Wilhelmy plate method at 25°C and 5°C. Small water-soluble organic compounds previously identified in aerosols (i.e. sugars, dicarboxylic acids) were found to alter the surface tension of water to a limited extent. Humic and fulvic acids were used as analog species for the unidentifiable humic-like substances (HULIS) found in atmospheric particles. Natural humic substances were considerably more effective at reducing the surface tension of water than small water-soluble species. However, humic matter reduced the surface tension of water to a lesser degree than measurements reported for atmospheric HULIS. The addition of inorganic species was found to significantly affect the surface activity of natural humic materials. Surface tension data were fit to the Szyszkowski equation to extract Langmuir adsorption parameters (maximum surface excess, Γmax, and the adsorption constant, β) for the aqueous systems. Adsorption parameters were used to model the surface tensions of multi-component solutions.
A11B-0112
Volatility of Oxalic Acid Compounds: Atmospheric Particulate Matter?
Organic species are ubiquitous in the atmosphere and there is mounting evidence that they adhere to and significantly affect atmospheric aerosol particles. An important species in this regard is oxalic acid and its presence in atmospheric particles is qualitatively consistent with its general nonvolatile nature. Numerous field studies have reported significant amounts of oxalic acid/oxalate in particles, and some have reported determinations of gas-phase concentrations. Oxalic acid can be produced via both gas- and liquid-phase oxidation of C2 and C3 organic compounds and total oxalate mass appears to be dominated by liquid-phase processes in many studies. However, nascent particle formation might be significantly impacted by gas- phase uptake of oxalic acid, thus thermodynamic information on oxalic acid compounds is important. A recent study has shown that the presence of oxalic acid, presumably as the di-hydrate, can affect cirrus clouds. Several recent studies have attempted to evaluate the volatility of oxalic acid in neat and aqueous forms but work to date on the oxalic acid dihydrate is insufficient to evaluate its stability in the atmosphere. Here we report the detection of oxalic acid vapor with PTrMS and the use of this technique to measure the partial pressure of oxalic acid, pOA, over the dihydrate. The variation of pOA with temperature and relative humidity will be discussed. Ammonium oxalate has been suggested to form as particulate matter in the atmosphere from ammonia and oxalic acid vapors. We report a preliminary study of the oxalic acid vapor pressure over this type of solid that indicates the pOA is very low. The stability of these compounds of oxalic acid as particulate matter in the atmosphere will be discussed. Acknowledgements. Special thanks to the Eppley Foundation for Research for supporting this work.
A11B-0113
A Method for Determining Hygroscopic Growth Factor for Organic Aerosols From Vapor Pressure Experiments
Currently, the tandem differential mobility analyzer (TDMA) is one of the most commonly used instruments to study the hygroscopic behavior of aerosols. The hygroscopic growth factor (HGF), defined as the ratio of the diameter of a spherical particle when it is exposed to humid conditions to that at dry conditions, is typically used to quantify particle water uptake. We present a new formulation to express the HGF of an aerosol particle as a function of water activity (aW) in the aqueous phase. Our approach is based on the fact that water activity limits the growth of a particle that can be attributed to water uptake. We have assembled a vapor pressure apparatus to measure aW of aqueous solutions as a function of solution concentration and temperature. For the pertinent solutions, we report coefficients resulting from a least square fitting of the water activity data as a function of molality for temperatures from 0 to 30°C. We compared the results obtained using our measured water activities in the HGF formulation with previous studies published, where TDMA is used to directly measure the HGF, for solutes commonly found in atmospheric aerosols. Our results indicate agreement with TDMA studies for common inorganic salts and water-miscible organic particles that are known to deliquesce into aqueous drops at high relative humidity (RH). However, we find a difference for organic particles that show no deliquescence behavior at low RH. For example, one TDMA study measured a HGF of 1.18 for 100 nm phthalic acid particles at 90% RH (aW= 0.9) and 30°C. Our data showed that even an aqueous solution saturated in phthalic acid did not lower the vapor pressure of pure water at 30°C. We propose that the adsorption of a negligible mass of water by a porous particle can lead to an apparent growth in particle size by changing the particle morphology.
A11B-0114
Investigation of thermodynamic properties of ambient and laboratory-generated multi- component organic aerosols
Ambient aerosol, a significant portion of which is composed of a complex mixture of semi-volatile organic compounds, has substantial impact on human welfare via adverse health effects and global climate change. Prediction of ambient semi-volatile organic aerosol remains highly problematic and air quality models often do not agree with observations. One of the pieces of knowledge needed for better predictions of ambient semi- volatile organic aerosol is understanding of the partitioning of semi-volatile compounds between the gas and the aerosol phases. In this study the equilibrium thermodynamic properties of a number of multi-component aerosols were investigated under controlled laboratory conditions, as well as in the field. In the laboratory experiments, model mixtures of organic compounds as well as modified ambient aerosols were tested. The ambient aerosols were modified in a controlled way by adding known amounts of different organic substances of known thermodynamic properties. The equilibrium gas / aerosol partitioning in a temperature range relevant to ambient conditions was investigated using the Integrated Volume Method (IVM). The field measurements of ambient aerosol equilibrium properties were carried out during June 2007 - January 2008 at the FACTS research facility in Duke Forest (Chapel Hill, NC). The results can be used to derive equilibrium vapor pressures and activity coefficients of test compounds and to verify and improve the parameterizations used in group contribution models, such as UNFAC.
A11B-0115
Hydration-Influenced Sorption of Organic Compounds by Model and Atmospheric Humic- Like Substances (HULIS)
Atmospheric humic-like substances (HULIS) constitute a major fraction of the water soluble organic carbon portion of aerosol particles. We investigated sorption and desorption of water and two model organic contaminants (toluene and benzyl alcohol) on HULIS and a standard humic substance (Suwannee River fulvic acid; SRFA) under conditions of varying relative humidity using a quartz crystal microbalance. Simultaneous sorption of water and benzyl alcohol (capable of specific interactions) on HULIS and SRFA shows significant, humidity-dependent, cooperative sorption of benzyl alcohol at intermediate levels of water activity, as well as a dependence of sorption distribution coefficient on the wetting-drying pathway. In contrast, sorption of toluene (capable of only non-specific interactions) was humidity-independent. Atmospheric HULIS is thus found to have many sorption features in common with terrestrial and aquatic humic substances and soil organic matter. These features are consistent with the Link Solvation Model (LSM) concept, whereby water assists in cooperative sorption of specifically-interacting compounds with the organic matter sorbent, and subsequent changes in sorbent structure result in sorption hysteresis. Sorption of compounds capable of only non-specific interactions is unaffected by hydration status. Such sorption features can lead to considerable uncertainty in predicting and modeling transport of organic pollutants in the atmosphere.
A11B-0116
Detection of Sulfate Esters as a Function of Particle Composition using Single Particle Mass Spectrometry
Secondary organic aerosols (SOA) are of increasing importance due to their effects on atmospheric chemistry. The formation of SOA is acid catalyzed, and since sulfuric acid is readily formed in the atmosphere, it is important to find useful methods to measure SOA particles under atmospheric conditions. It is challenging to differentiate particles containing organosulfates from those containing organics plus sulfuric acid. The Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) can measure single particles in real-time, and has been used in a number of studies of SOA. In order to interpret the ATOFMS results from SOA particles, known particle types need to be analyzed under real-world conditions to understand how ATOFMS signal depends on the particle composition. This work focuses on sulfate esters, one chemical component of SOA that has been proposed to form by acid-catalysis. We investigate the characteristic peaks seen in the single-particle mass spectra when sulfate-ester containing particles are ionized in the ATOFMS to determine whether the intensity of the peaks changes as a function of particle composition. Solutions of a sulfate ester with different concentrations of XSO4 (X = H2, (H)(NH4), or (NH4)2) were nebulized and sampled into an ATOFMS. The measured spectra were analyzed for significant ion peaks and ion intensities. The peak intensities of the detected ions were found to depend on the composition of the particle, with certain characteristic sulfate-ester peaks increasing in intensity as the concentration of XSO4 increases. We will present these trends for a number of model compounds as a function of particle composition.
A11B-0117
Development and Modification of a GC-IRMS System for Ambient Atmospheric Studies of Low-Molecular Weight Oxygenated Volatile Organic Compounds
Determining δ13C values for reduced hydrocarbons in atmospheric samples is emerging as an important area of interest in isotopic analytical chemistry. The importance of stable isotopic data stems from its usefulness to differentiate between multiple sources and allows for an assessment of changing source structure and source strength in a constantly changing environment. Though much stable isotopic work is available on CH4 and other VOCs, particularly NMHCs, few studies have focused on oxygenated volatile organic compounds (OVOCs) such as methanol, ethanol, acetone, and propanal. Both anthropogenic and biogenic sources exist for these OVOCs and their role in atmospheric chemistry is important. The OVOCs of interest here are found in very low concentrations in ambient air (low ppbv to high pptv) and thus provide unique challenges for analysis by GC-C-IRMS. To address the challenges of measuring OVOCs, a Hewlett Packard 6890 gas chromatograph interfaced with a Europa Scientific Geo 20-20 IRMS was modified to accept ambient atmospheric samples. To sharpen peak shape all dead volume within the system was minimized; starting with the addition of a fused silica combustion tube (0.25 mm i.d.) containing Cu, Pt, or Ni wires (0.1 mm dia.). To assist water removal from the sample stream before delivery to the IRMS a small volume nafion dryer (0.20 mm i.d.) and a water-trap submersed in a dry-ice / acetone slurry were tested individually. Deactivated fused silica (0.1 mm i.d.) joins the custom designed open split to the ion source and effectively decreases dead volume while maintaining chromatographic separation and desired source pressure. To decrease the variability of the instrumentation, and to increase the total amount of carbon at the ion source, total carrier gas flow is reduced to 0.7 mL/min. Reference gas addition is manually facilitated by a six port rotary valve upstream of the open split and delivers diluted CO2 reference gas (0.1% CO2 in He) directly to the ion source while maintaining continuous flow conditions from the gas chromatograph. Experimental results of initial biogenic source sampling will be presented and future directions will be discussed.
A11B-0118
Unknown chlorinated and brominated organic compounds in the troposphere and stratosphere
We present observational evidence for the presence of unknown organic halogenated organic trace gasses in the atmosphere, which could contribute to stratospheric ozone depletion. The three Chlorofluorocarbons trifluorochloroethene, 3-chloropentafluoropropene and 4,4-dichlorohexafluoro-1-butene were detected in tropospheric air samples by means of Gas Chromatography with Electron Capture and Mass Spectrometric detection (GC-ECD-MS). Indications for the presence of additional unidentified Chlorofluoroalkenes were found and it is uncertain up to now, whether any of these substances are able to reach the stratosphere. Moreover, results from balloon-borne observations performed above Brazil in 2005 in the main stratospheric entrance region - the tropical tropopause layer (TTL) - are presented. The collected whole air samples from altitudes between 15 and 34 kilometres were analysed via GC-MS by running the MS in Negative Ion Chemical Ionization (NICI) mode. Indications were found for additional organic brominated substances to be present in the tropical upper troposphere and stratosphere. The project 'CLEARFOGG - Checking Layers of the Earths Atmosphere For halogenated Ozone-depleting and Greenhouse Gases' aims to bring these issues forward. It will start in November 2008 and its outline is presented, too.
A11B-0119
Atmospheric Concentrations of Persistent Organic Pollutants in the Southern Ocean
Long-range transport of persistent organic pollutant (POPs) is a global concern. Remote regions such as the
Southern Ocean are greatly under-sampled though critical components in understanding POPs cycling.
Over 20 high-volume air samples were collected in the Southern Ocean aboard the RV Brown during the
GASEX III experiment between Mar 05 to April 9 2008. The relatively stationary platform (51S,38W) enabled
the collection of a unique atmospheric time series at this open ocean station. Air sampling was also
conducted across transects from Punto Arenas, Chile and to Montevideo, Uruguay. Samples were collected
using glass sleeves packed with poly-urethane foam plugs and C-18 resin in order to collect target organic
pollutants (per-fluorinated compounds, currently and historically used pesticides) in this under-sampled
region. Here we present POPs concentrations and trends over the sampled period and compare variations
with air parcel back trajectories to establish potential origins of their long-range transport.
http://copal.uconn.edu/main.html
A11B-0120
Use of Chiral Signatures of Organochlorine Pesticides in Asian, Trans-Pacific, and Western U.S. Air Masses to Identify Source Regions
Chiral signatures of organochlorine pesticides were measured in air masses on Okinawa Japan and three remote locations in the Pacific Northwestern U.S.: Cheeka Peak Observatory (CPO), a coastal site on the Olympic Peninsula of Washington at 500 m; Mary's Peak Observatory (MPO), a site at 1250 m in Oregon's Coast range; and Mt. Bachelor Observatory (MBO), a site at 2300 m in Oregon's Cascade range. The chiral signature of composite soil samples collected from agricultural areas in China and South Korea were also measured. Racemic alpha-HCH was measured in Asian air masses and soil from China and South Korea. Non-racemic (enantiomer fraction (EF) = 0.528 ± 0.0048) alpha-HCH was measured in regional air masses at CPO, a marine boundary layer site, and may reflect volatilization from the Pacific Ocean and regional soils. However, during trans-Pacific transport events at CPO, the EFs were significantly (p-value <0.001) more racemic (EF = 0.513 ± 0.0003). Racemic alpha-HCH was consistently measured in trans- Pacific air masses at MPO and MBO. The alpha-HCH EFs in CPO, MPO, and MBO air masses were positively correlated (p-value = 0.0017) with the amount of time the air mass spent above the boundary layer along the 10-day back air mass trajectory prior to being sampled. This suggests that the alpha-HCH in the free troposphere is racemic. The racemic signatures of cis and trans chlordane in air masses at all four air sampling sites suggest that Asian and U.S. urban areas continue to be sources of chlordanes that have not yet undergone biotransformation.