A33B-1177
Characterization of Positive Matrix Factorization of Aerosol Mass Spectrometer Data With Real and Synthetic Data
Mass spectrometric measurements of ambient aerosols yield organic spectra that are a mix of nucleated particles, freshly emitted particles from many sources, and particles which have undergone some amount of processing (condensation, oxidative reaction, cloud processing, etc.). Further understanding of the important sources and processes for organic aerosols requires deconvolution of the organic fraction of ambient aerosols. A well-known source apportionment technique, Positive Matrix Factorization (PMF), has been applied to Aerodyne aerosol mass spectrometer (Q-AMS) datasets acquired in Pittsburgh (2002). Extensive sensitivity analyses of the Pittsburgh case are performed with synthetic datasets to characterize the behaviour of PMF with AMS datasets PMF is a least-squares fitting method for source apportionment commonly applied to speciated aerosol datasets (Paatero, Chemomet. Intell. Lab. Sys. 1997, 37, 23-35). The organic portion of the Pittsburgh dataset (Zhang et al. ( ES&T 2005, 39, 4938-4952) was analyzed with PMF. The factors from the two- and three-factor solutions are used directly and modified to create synthetic datasets to explore the ability of PMF to retrieve factors with strongly (weakly) correlated mass spectra or (and) time series, to retrieve factors with a small fraction of the mass, and the effect of rotations of the solution on these retrievals. When PMF solves for more factors than were used in the input, major components are "split" into multiple factors and/or "mixed" to create new factors that have high correlation with actual AMS database spectra from ambient studies, chamber studies, and pure compounds (http://cires.colorado.edu/jimenez-group/AMSsd/spectra.html).
A33B-1178
Atmospheric Measurements Aboard C-130 During the Pacific Atmospheric Sulfur Experiment
The Pacific Atmospheric Sulfur Experiment (PASE) is a study with a primary goal aimed at understanding the sulfur cycle in a remote marine atmosphere. The study will be conducted in August and September months of 2007 at Christmas Island on board the NSF/NCAR C-130 aircraft. It will foremost focus on measurements of DMS (dimethyl sulfide) and its contribution to formation of H2SO4 (sulfuric acid) and MSA (methane-sulfonic acid) by reaction with OH (hydroxyl). PASE will also concentrate on subsequent production of aerosols and cloud condensation nuclei from H2SO4, MSA, and NH3 concentrations in a cloud free convective boundary layer (CBL) and in outflow of marine cumulus. This study explains the measurement technique for OH, H2SO4, MSA, HO2, HO2+RO2 (peroxy radicals), and NH3 (ammonia) using the SICIMS (Selected ion chemical ionization mass spectrometer). It also presents sample measurements from research flights of OH, H2SO4, MSA, HO2, and HO2+RO2. In addition, this paper discusses the measurement technique utilized aboard the C-130 in testing SO2, DMS, DMSO (dimethyl sulfoxide), DMSO2 (dimethyl sulfone), O3 (ozone), aerosols, and cloud condensation nuclei. It includes several adaptations to technique and instrumentation from previous studies conducted: the First Aerosol Characterization Experiment (ACE-1) in 1998, the Pacific Exploratory Missions A and B (PEM- Tropics) in 1996, and the Intercontinental Transport Experiment (INTEX-B) in 2006.
A33B-1179
Direct Measurement of Aerosol Absorption Using Photothermal Interferometry
Efforts to bound the contribution of light absorption in aerosol radiative forcing is still very much an active area of research in large part because aerosol extinction is dominated by light scattering. In response to this and other technical issues, the aerosol community has actively pursued the development of new instruments to measure aerosol absorption (e.g., photoacoustic spectroscopy (PAS) and multi-angle absorption photometer (MAAP)). In this poster, we introduce the technique of photothermal interferometry (PTI), which combines the direct measurement capabilities of photothermal spectroscopy (PTS) with high-sensitivity detection of the localized heating brought about by the PT process through interferometry. At its most fundamental level, the PTI technique measures the optical pathlength change that one arm of an interferometer (referred to as the ‘probe' arm) experiences relative to the other arm of the interferometer (called the ‘reference' arm). When the two arms are recombined at a beamsplitter, an interference pattern is created. If the optical pathlength in one arm of the interferometer changes, a commensurate shift in the interference pattern will take place. For the specific application of measuring light absorption, the heating of air surrounding the light- absorbing aerosol following laser illumination induces the optical pathlength change. This localized heating creates a refractive index gradient causing the probe arm of the interferometer to take a slightly different optical pathlength relative to the unperturbed reference arm. This effect is analogous to solar heating of a road causing mirages. As discussed above, this altered optical pathlength results in a shift in the interference pattern that is then detected as a change in the signal intensity by a single element detector. The current optical arrangement utilizes a folded Jamin interferometer design (Sedlacek, 2006) that provides a platform that is robust with respect to sensitivity-reducing vibrations enabling this technique to be used in field campaigns. A series of calibration and intercomparison experiments have recently been carried out in our laboratory to evaluate the performance of the PTI technique towards aerosol absorption measurement and monitoring. Since PTI is a calorimetric technique, calibration can be performed using an absorbing gas of known concentration and known absorption cross-section. Following this calibration, a series of intercomparison experiments using laboratory-generated nigrosin aerosols and a 3-? Particle Soot Absorption Photometer (PSAP) were carried out where correlation between the PTI and PSAP was measured to be 0.96±0.02. (Sedlacek and Lee, 2007) Extension of this intercomparison to the measurement of ambient aerosols reveals continued agreement between the two instruments except for periods of high relative humidity whereupon the PSAP reported a larger absorption coefficient. (Sedlacek and Lee, 2007) A discussion of the PTI technique, along with the results of this intercomparison and some preliminary results examining absorption enhancement brought about by coating black-dyed PSL particles with dibutyl phthalate will be presented. References: Sedlacek, A. J., and Lee, J., (2007) Photothermal interferometric aerosol absorption spectroscopy, Aerosol Sci. Tech. (in press). Sedlacek, A. J. (2006). Real-time detection of ambient aerosols using photothermal interferometry: Folded Jamin interferometer, Rev. Sci. Instrum. 77:064903.
A33B-1180
Cavity Attenuation Phase Shift Detection of Trace Atmospheric Species: Nitrogen Dioxide
We present details of an apparatus capable of measuring optical extinction (i.e., scattering and/or absorption) which employs one variant of cavity enhanced detection, specifically cavity attenuated phase shift spectroscopy (CAPS). It uses a near confocal arrangement of two high reflectivity (R~0.9999) mirrors in tandem with an enclosed cell 26 cm in length, a light emitting diode (LED) and a vacuum photodiode detector. Square wave modulated light from the LED passes through the sample cell and is detected as a distorted waveform which is characterized by a phase shift with respect to the initial modulation. The amount of that phase shift is a function of fixed instrument properties - cell length, mirror reflectivity, and modulation frequency – and of the presence of a scatterer or absorber (air, particles, trace gases, etc.) within the cell. The monitor is enclosed within a standard rack-mounted instrumentation box, weighs 10 kg and uses 70 W of electrical power including a vacuum pump. The instrument has demonstrated a detection limit of 0.02 Mm-1 (1σ) in ten seconds integration time and a baseline drift of less than ± 0.1 Mm-1 over a 24 hour period. We also present an intercomparison of measurements of ambient nitrogen dioxide concentrations using this sensor, a quantum cascade laser-based infrared absorption spectrometer and a standard hot metal/chemiluminescence-based NOx analyzer. Measurements using the quantum cascade-laser and CAPS monitor agree well within the inter- calibration error. The chemiluminescence analyzer data falls outside the expected error band, a fact that is ascribed to the presence of known chemical interferences for the chemiluminescence-based analyzer.
A33B-1181
Quantifying the influence of particulate sulfate in a variety of marine environments: from coastal California to the tropical Atlantic
We present measurements of oxygen isotopes in aerosol nitrate and sulfate in a variety of marine environments. Our measurements of aerosol sulfate in Southern California, when measured as a function of size, reveal significant differences in the isotopic composition of aerosol sulfate found in sub-micron and super-micron sized aerosols. In general we found that sulfate in aerosols less than 1 micron have lower Δ17O values than that found in aerosols greater than 1 micron in size. Isotopic measurements of particulate sulfate from ship stacks reveals that these have a specific and unique isotopic composition in δ18O and Δ17O space. We present evidence for the influence of particulate sulfate sources in both Southern California and on the islands of Bermuda and Izana off the coast of Africa and examine probable sources of particulate sulfate in these locations.
A33B-1182
In situ observations of light-absorbing non-refractory aerosol
Light-absorbing non-refractory aerosol particles may contribute significantly to total aerosol absorption of solar radiation and have been observed with a new method associated with the NOAA Single-Particle Soot Photometer (SP2). The SP2 was designed to measure incandescence from refractory carbon in individual particles sampled from ambient or laboratory air samples. Although light-absorbing non-refractory particles do not incandesce, they lose mass while transiting the SP2 laser beam. This loss of mass is detected by the asymmetry produced in the particle scattering signal. Air sampled on board the NOAA WP-3D research aircraft during the TexAQS/GoMACCS campaign were examined for evidence of light-absorbing non-refractory particles. These particles are found in some plumes sampled during the campaign and are compared to the abundance of refractory black carbon particles.
A33B-1183
Volatility of Primary and Secondary Organic Aerosols in the Field Contradicts Current Model Representations
A novel combination of a temperature-stepping thermodenuder (TD) in front of an Aerosol Mass Spectrometer (AMS) has allowed rapid quantification of chemically-resolved volatility in the field for the first time. The TD consists of a tube heated to a uniform temperature, followed by a carbon charcoal denuder to adsorb volatilized gases, leaving a core of less volatile material for detection by the AMS. The TD temperature can be changed quickly (1 to 10 min) to allow rapid measurements of volatility spectra. The volatility of organic aerosols (OA) influences the concentration and lifetime of OA, secondary OA (SOA) formation, and OA measurements. The TD- AMS system was used to characterize urban OA, primary OA (POA) from biomass burning, and chamber- generated SOA. Almost all atmospheric models represent POA as non-volatile and SOA as semi-volatile. Our results indicate nearly the opposite: urban POA is semi-volatile, while urban SOA is much less volatile. Biomass burning POA exhibited a wide range of volatilities, but was often similar in volatility to urban OA. Chamber- generated SOA was significantly more volatile than urban OOA, challenging the extrapolation of laboratory volatility measurements to the atmosphere. Our results indicate that the representation of OA volatility in most current models is highly inaccurate and should be revised.
A33B-1184
The application of ion drift - chemical ionization tandem mass spectrometry (ID-CIMS/MS) for fast detection and quantification of organic and inorganic compounds
Chemical ionization mass spectrometry (CIMS) has been widely employed in atmospheric chemistry research, including laboratory kinetic investigations and field trace gas measurements. In the CIMS method, a neutral species is ionized by a reagent ion to yield a product ion, which is analyzed by a quadrupole mass analyzer for species identification and abundance determination. One type of CIMS applications involves proton-transfer reaction mass spectrometry (PTR-MS), which has been developed for on-line quantification of volatile organic compounds (VOCs) in air. The CIMS method has also been widely used in atmospheric monitoring of inorganic species. In this talk, we describe the application of ion drift ?chemical ionization tandem mass spectrometry (ID- CIMS/MS) for fast detection and quantification of organic and inorganic compounds. Demonstrations of the ability of ID-CIMS/MS to differentiate and quantify VOCs and inorganic compounds in a laboratory setting are presented. We will illustrate that the ID-CIMS/MS technique facilitates detection and quantification of organic and inorganic species in laboratory kinetic investigations and field measurements.
A33B-1185
A Micro-Orifice Volatilization Impactor coupled to a Chemical Ionization Mass Spectrometer for the detection of organic acids in atmospheric aerosol particles
Significant uncertainties related to sources and removal processes of particulate organic matter persist due, in part, to a poor understanding of the molecular-level composition. To address these issues, we are developing a novel technique that couples a micro-orifice volatilization impactor (MOVI) to a chemical ionization mass spectrometer (CIMS) for fast, in situ measurements of specific organic acids expected to be in atmospheric particles. The MOVI-CIMS process has three steps: 1) aerosol collection by inertial impaction, 2) volatilization and sample transfer, and 3) chemical ionization and detection using a quadrupole mass spectrometer. We present results from laboratory characterization of two MOVI designs, one operating at low pressure (60 Torr) and the other at near ambient pressure. The low-pressure impactor has a theoretical cut point of 40nm while the atmospheric pressure impactor (API) has a theoretical cut point of 280nm with a pressure drop of less than 5%. We compare the advantages and disadvantages of these two designs in terms of typical atmospheric particle size distributions. Experimental tests of their theoretical cut-points are used to assess the importance of jet-to- plate distance and particle bounce. In addition, we demonstrate the utility of the MOVI-CIMS technique by employing it in studies of heterogeneous oxidation of particle organics and of secondary organic aerosol formation from biogenic hydrocarbon oxidation. Based on typical signal-to-noise ratio, the MOVI-CIMS demonstrates a detection limit of ~50 ng for monocarboxylic acids when using the LPI version and the iodide ion as a chemical ionization reagent. Preliminary results suggest even lower detection limits are possible with other reagent ions.
A33B-1186
Characterization of the desorption step in a new two-laser desorption/ionization single- aerosol ion-trap mass-spectrometer
While state of the art aerosol mass spectrometers have dramatically expanded our knowledge of atmospheric aerosol composition and chemistry over the last decade, the analysis of complex organic mixtures still remains a challenge. We have developed a single particle mass spectrometer that combines a series of previously tried, promising approaches into one single instrument. Soft desorption (by using a dedicated pulsed CO2 laser, as shown by Prather, Baer & coworkers) and soft ionization (pulsed two photon UV ionization, as demonstrated by Prather, Zelenyuk, Zimmerman & coworkers) or tunable one photon VUV lasers (Baer, Leone & coworkers) to ensure a minimum of fragmentation. By ionizing the aerosol plume in the center of an ion trap both high sensitivity and the ability to elucidate structure by MS/MS (Reilly & coworkers) can be achieved. Coupled with a highly efficient aerosol focusing and timing interface (> 80% efficiency, < 2% timing precision, based on Prather & coworkers), this instrument shows promise to help elucidate the composition of accumulation mode organic particles for compounds up to 1500 Da Since "soft" methods, both in the desorption and ionization case are prone to massive biases, extensive testing has been done on examining the linear response of both for an array of aerosol sizes, test molecules, matrices and mixtures. In order to separate the effects of each step, desorption was studied using mixtures of PAHs that were ionized by two-photon UV, a well characterized and very sensitive ionization step.
A33B-1187
Development and use of Negative Ion Proton Transfer Reaction Chemical Ionization Mass Spectrometry (NI-PTR-CIMS) for the Measurement of Trace Organic Acids in the Atmosphere
Negative-Ion Proton-Transfer Reaction Chemical Ionization Mass Spectrometry (NI-PTR-CIMS) has been developed for real-time measurement of gas-phase organic acids in the atmosphere. The method is based on the non-dissociative proton transfer reactions of CH3COO- with most of the common organic acids. The potential to detect organic acids (monocarboxylic, dicarboxylic, inorganic, and rare acids) has been shown by a number of laboratory investigations. The sensitivity of the instrument to several organic acids has been observed to be on the order of several ion counts per pptv. Detection limits well below the ppbv level are expected for a 1-second integration time. Various instrumental features including zeroing, time response, calibration and validation of the ion chemistry have been investigated and will be discussed. Intercomparison of formic acid measurements with Proton Transfer Reaction Mass Spectrometry (PTR-MS) and Quantum Cascade Laser IR absorption (QCL) serve to validate this measurement technique. Further development of this instrument will provide a fast, sensitive, and selective measurement technique for gas-phase acids. NI-PTR-CIMS has great potential for use in atmospheric chemistry field studies.
A33B-1188
O2+ as Reagent ion in the PTR-MS Instrument: Detection of Gas-phase Ammonia
Oxygen was used as a source gas in a conventional Innsbruck PTR-MS instrument to produce O2+ ions as chemical ionization (CI) reagents instead of H3O+ ions. The use of O2+ ions as CI reagents allows for fast, highly sensitive and specific measurements of gas-phase ammonia (NH3) via the electron transfer reaction O2+ + NH3 -> NH3+ + O2. The instrument was tested to be linear in the 2 to 2000 ppbv range. Instrument sensitivity was observed to be humidity-independent and amounted to ~40 cps/ppbv. The instrumental background was determined by sampling NH3-free air from a heated platinum/palladium catalyst. A humidity-dependent increase of the instrumental background from 70 pptv at dry conditions to 470 pptv at humid conditions was observed. The corresponding 2σ-detection limits at 1 s signal integration time were 90 pptv for dry conditions and 230 pptv for humid conditions, respectively. The observed background may be intrinsically formed in the ion source but it may also be the result of incomplete NH3 oxidation in the catalyst used for zeroing. The reported background levels and detection limits are thus to be considered as upper limits. The 1/e response time of the instrument was in the range of 3 to 5 s. The PTR-MS instrument was successfully deployed in the field to monitor changes in gas-phase NH3 concentrations in the few seconds to tens of seconds time range. Laboratory and field intercomparison measurements between the PTR-MS instrument and a commercial NH3 analyzer (AiRRmonia) were in good agreement. The use of O2+ ions as CI reagents will significantly improve the analytical capabilities of the Innsbruck PTR-MS instrument.
A33B-1189
Studies of the kinetic isotope effect during SOA formation from beta pinene oxidation
We investigated the stable carbon isotope ratios of selected VOCs in both the gas phase and particulate phase during beta pinene-ozone reaction. The experiments were performed in a large indoor chamber. Gas phase samples were collected in Silcosteel canisters (SilcoCan), with a time interval of 30min. Aerosol samples were collected on quartz fiber filters (Whatman) at a flow rate of 25l/min for 1h. All filters were pretreated at a temperature of 600 °C overnight before sampling. A customized thermal desorption system combined with cryofocusing (from Gerstel) was used for desorbing the compounds. 2-D gas chromatography coupled with isotope ratio mass spectrometry (GC-IRMS) system was used for investigations of the compound specific stable carbon isotope ratios. In addition the stable carbon isotope ratio of the total carbon from the filter samples was measured. We observed a progressing enrichment of the beta pinene as it diminishes in concentration. One of the major oxidation products of beta pinene, nopinone, on the other hand, did not show a significant change in its isotope composition both in the gas and aerosol phase. The total carbon isotope ratio from the aerosol samples showed enrichment in the heavy isotope than the initial beta pinene. The combined measurement of gas and aerosol phase isotope ratios gives additional constraints to carbon balances in SOA formation.
A33B-1190
Quantifying Tropospheric Peroxy Radicals using Chemical Ionization Mass Spectroscopy with Chemical Conversion
We report the development of a mass spectrometric technique for the measurement of tropospheric peroxy radicals, particularly from aircraft platforms. This method makes use of chemical reactions involving added nitric oxide and sulfur dioxide reagent gases and dilution with nitrogen or oxygen (oxygen dilution modulation). This chemistry specifically converts HO2 radicals to gas-phase sulfuric acid in one mode (high reagent concentrations and dilution with nitrogen) and converts HO2 and most RO2 radicals in the other mode (low reagent concentrations and dilution with oxygen). We have performed extensive laboratory assessments of the instrument response over a range of environmental conditions to a wide variety of radical species. This instrument has been deployed in several recent campaigns (MIRAGE, INTEX-B, PASE). Data from those campaigns will be used to illustrate the performance of the instrument.
A33B-1191
A new Tunable VUV Tabletop Light Source for Selective, High Sensitivity, Near-Threshold Ionization of Organics in the Range 7.3 to 10.2eV
Over the past two decades aerosol mass spectrometry has proven to be an invaluable tool in atmospheric research. However, one enduring challenge has been in developing instruments specifically for studying organic aerosols. The large number of different organic compounds that can be present in an aerosol, and the relative structural complexity of these molecules, makes it difficult to construct instruments that are both broadly applicable and sufficiently sensitive. In addition, since many organic molecules are relatively fragile, aerosol mass spectra can be complicated by large numbers of fragment ions making the parent molecule difficult to identify. In the following we describe a laser-based vacuum ultraviolet (VUV) light source which is coupled to an ion trap mass spectrometer and an aerosol inlet. The VUV light source allows for near-threshold, single photon ionization of many organic molecules and the ion trap allows for subsequent MS/MS analysis of a given compound. Near-threshold VUV laser photoionization is extremely useful for ionization of organic molecules because it produces ions with very little internal energy, minimizing fragmentation, and reducing the complexity of the mass spectra obtained. Furthermore, it is broadly applicable to many classes of organic compounds. The VUV source delivers high intensity pulses (109 -1012 photons/pulse, 5 ns FWMH) of light with a continuously tunable wavelength from 122 nm (10.2 eV) to 170nm (7.3 eV). These pulses are highly monochromatic (>107 discrimination of input laser pulses) and have a very low bandwidth (<1 cm- 1). Since this source is designed to be used in a single aerosol mass spectrometer, the setup allows for tight (<1 mm2) and precise focusing (<50 μm resolution) of the VUV light, as well as the ability to generate pulses on demand. When coupled with an ion trap mass spectrometer, the light source has proven to be very efficient for detection of a wide array of environmentally relevant organic molecules in the gas phase, with little to no fragmentation in most cases. Photoionization efficiency curves were collected by scanning the VUV light source and recording the ion signal as a function of photon energy. This allowed for fast determination of first ionization energies, which is a useful piece of information when trying to identify a particular compound. This also allowed for determination of the onset of fragmentation and the degree of fragmentation as a function of photon energy. The continuous scanning of the VUV source can also be used to separate the components of organic mixtures by ionization energy.
A33B-1192
Continuous Monitoring of Nitrate and Sulfate in Aerosols with Microchip Electrophoresis
Routine monitoring of aerosol composition is important since aerosols can negatively affect both the environment and health. Water-soluble inorganic ions are commonly monitored using the particle-into-liquid-sampler coupled to ion chromatography (PILS-IC). However, a less-expensive, faster, and more portable analysis system is desirable. Here, we present the coupling of microchip capillary electrophoresis (MCE) to a water-based condensation particle counter (WCPC) for rapid and continuous monitoring of chloride, nitrate, and sulfate in atmospheric aerosols. To achieve a working system, several obstacles were overcome. A working interface between the electrophoresis microchip and the WCPC sampler was developed. This interface was designed to remove insoluble particles from the analysis stream and to prevent the sampling-induced pressure gradient from altering flow in the microfluidic device. The electrophoresis separation chemistry was optimized for the small chip size, to be free from potential interfering compounds, and to operate continuously for several hours. In-field performance of the integrated system was tested with ambient aerosols. Anion analyses can be performed in less than two minutes with aerosol detection limits similar to the PILS-IC, but with greater portability and reduced cost. Coupling microfluidic devices to aerosol sampling technology proves successful for inorganic anion analysis and shows potential for faster and more sensitive measurements as well as monitoring of other water- soluble aerosol components such as organic acids, cations, and carbohydrates. The reduced cost and size relative to current technology indicate that greater deployment of monitoring stations or the advent of portable analyzers may be feasible.
A33B-1193
Attenuation of Fine Particle Concentration Fluctuations in Turbulent and Isokinetically Sampled Laminar Flows
Rapid measurement of gas and particulate concentrations have been used with synchronous wind velocity measurements to determine atmosphere-land fluxes using the eddy correlation method. For valid flux calculations, the concentration measurements must be made with a frequency of at least 1 Hz, and atmospheric samples are commonly drawn through a sampling line to a concentration sensor located at the base of a tower. Isokinetic subsampling from the center of a laminar flow is a widely-used technique to minimize loss of particles to sampling line walls. Here we evaluate frequency attenuation of fine particle concentration fluctuations in this design. Step changes in concentrations were introduced into stable laminar flow and particle concentrations were measured at 10 Hz using an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS). The experimental response times and transfer functions for particle concentration fluctuations, averaged over the isokinetic region, were calculated and the results are compared with the transfer functions for the cup-mixing average concentration in laminar and turbulent flows. For a 10 m long sampling line with an inner diameter of 0.5 in, the frequency response was greater than three orders of magnitude higher in the isokinetic region of a laminar flow (isokinetic radius ratio = 0.06, Re = 1300) compared to the analogous sampling line operated under turbulent flow (Re = 10000). A theoretical transfer function for fine particle concentration fluctuations, integrated over the isokinetic region, was developed to evaluate the experimental results.
A33B-1194
Water Condensation Growth Cells for Ultrafine Particle Collection Onto Concentrated Spots
A laminar flow, condensation method, analogous to that employed in the water-based condensation particle counters, is utilized to provide concentrated, low-pressure drop collection of fine and ultrafine particles. With the laminar flow water condensation approach, the aerosol flow is first chilled by a cold walled conditioner, and then introduced into a hot wet-walled condenser. Because water vapor diffuses more rapidly then heat, the air vapor is supersaturated resulting in particles large enough to be collected by impaction. Several types of collectors have been designed and tested. A compact system utilizing a single TED as a heat pump to provide a ~ 25 ° C temperature difference provides collection at 0.4 L/min with a lower cutpoint of 10 nm, a pressure drop of 1 kPa, and a power consumption of 1 Watt. A larger, parallel plate system samples at 10 L/min, and yields a cutpoint of 20 nm. The design of these systems was guided by numeric modeling of the saturation ratios, particle activation and growth. The model includes the heat release from condensation, and the associated warming of the flow that reduces the supersaturation and particle growth at high particle number concentrations. By controlling the system geometry (either plate separation or tube diameter), we are able to activate at small particle sizes while minimizing concentration effects. Our method of particle collection provides a number of other advantages. Particle bounce off the impaction surface can be eliminated by controlling the temperature of the impaction surface so as to maintain a thin film of water on the surface. Particles can also be collected into a small liquid vial containing less then 1 ml of fluid, which eliminates the need for particle extraction from filters or resuspension from surface, it minimizes the total volume of the sample, and it allows for continuous automated collection and analysis.
A33B-1195
Optical Properties of Core-Shell Aerosols: Oleic Acid and Water
Most tropospheric aerosols are comprised of several different chemical compounds that can at times be distributed across multiple phases within the particles themselves. Knowledge of the composition and structure of such particles is important for a number of reasons including insight into their roles in atmospheric chemistry and their interaction with electromagnetic radiation. Although different analytical methods can be used to elucidate composition and structure of particulate matter, this work focuses on the use of aerosol extinction spectroscopy to determine these properties for particles made from two immiscible compounds namely oleic acid and water. Reported here are results and limitations of such analyses on model bi-component aerosols, and the feasibility of extending these methods to more complex systems.
A33B-1196
Fast Measurements of Atmospheric Aerosol Size Distributions Using a Fast Integrated Mobility Spectrometer in Aircraft
Measurements of aerosol size distributions with a time resolution of 1 Hz were carried out using a recently developed Fast Integrated Mobility Spectrometer (FIMS). The FIMS is capable of measuring sub-micrometer aerosol size distributions with high time resolution, size resolution, and counting statistics. These attributes make the FIMS an ideal instrument both for aircraft-based measurements and for studies of rapidly changing aerosol populations. The FIMS consists of a classifier, condenser, and detector. Inside the classifier, under the influence of an electric field, charged particles are separated into different flow streams based on their electrical mobility. The classified particles are then carried by a butanol-saturated sheath flow into the condenser, where a supersaturation of butanol is generated through electrical cooling and the classified particles grow into super-micrometer droplets. At the exit of the condenser, a laser sheet illuminates the grown droplets, and their images are captured by a high-speed CCD camera. The images provide not only the particle concentration, but also the particle position, which directly relates to the particle electrical mobility. By simultaneously measuring particles of different sizes/mobilities, the FIMS can provide full size spectrums of submicron aerosol at a time resolution of 1 Hz, nearly 100 times faster than traditional SMPS systems. Since the individual particle and its position are detected optically using the high resolution CCD camera, the FIMS also offers high size resolution and counting statistics. The performance of the FIMS, including its time resolution and sizing accuracy, was characterized under a variety of conditions using both atmospheric aerosols and laboratory-generated aerosols. The FIMS was recently deployed in a research aircraft in the Cumulus Humilis Aerosol Processing Study (CHAPS 2007). Comparisons are made between atmospheric size distributions measured by the FIMS and a customized SMPS for data from the CHAPS program.
A33B-1197
OH reactivity measurement at Tomakomai Experimental Forest, Japan in late summer
OH reactivity in the atmosphere provides useful information as an index for evaluation of air quality. There exist at least a couple of hundreds of VOCs in the atmosphere which react with OH radicals to contribute for oxidant formation. OH reactivity can be measured by laser pump and probe technique developed by our laboratory. We found considerable amount of missing sink of OH in semi-urban atmosphere in Tokyo priory to Tomakomai expedition. The contribution of missing sink is predominant in spring and summer time. Possible missing sinks could be either biogenic VOC or secondary organic compounds which we did not measure. To make it clear we carried out OH reactivity measurement at Tomakomai Experimental Forest whether unknown OH sinks exist or not. We measured around 80 chemical species by GC-MS, FIF, and PTR-MS techniques in addition to careful observation of NOx by laser induced fluorescence. We successfully measured OH reactivity in early September 2006 for two days showing isoprene like missing sink. To account for the missing sink we need double amount of isoprene. Either secondary oxidation products from isoprene or other biogenic VOCs which has similar emission character such as isoprene.
A33B-1198
Aerosol Interfaces Examined with Ambient Pressure Photoemission Spectroscopy
Heterogeneous chemistry occurring at the liquid/vapor and solid/vapor interfaces plays a significant role in environmental and atmospheric chemistry. Despite the importance of understanding differences in chemical reactivity between surface and bulk solutions, there is a considerable lack of quantitative and chemically specific techniques that can operate under environmental and atmospherically relevant conditions. The significance of these interfacial types of measurements is becoming increasingly apparent. For example, recent studies have shown that ions can segregate to the surface in liquid salt solutions, changing the chemical reactivity of the liquid at its interface. Recent development of the ambient pressure photoelectron spectroscopy (APPES) facilitates chemical identification of molecules adsorbed onto liquid and solid surfaces at atmospherically relevant pressures. The specific advantage of this technique is that measurements can be made at pressures greater than 5 Torr, i.e. above the equilibrium vapor pressure of water at its triple point. Here, we describe the development of a novel synchrotron-based instrument that combines APPES with real-time droplet and nanoparticle surface analysis. Three experiments that address chemistry of model systems are presented: (a) the adsorption and chemical reaction of an atmospherically relevant polycyclic aromatic hydrocarbon (PAH) compound on model surfaces (b) a combined droplet train/APPES setup for the investigation of the liquid/vapor interface and (c) a particle flow reactor for the investigation of heterogeneous chemistry on liquid and solid nanoparticle surfaces. Initial results of the surface composition of methanol/water mixtures in the droplet train and oxidation of the PAH coronene by ozone on a model surface will be presented.
A33B-1199
Evaluating and Improving Measurements of Black Carbon Aerosol
Evaluation and optimization of black carbon (BC) measurement methods are difficult because neither a BC standard or a BC reference measurement method exist. In this study, we generate BC particles with an inverted diffusion flame [Kirchstetter and Novakov, Atmos. Environ., 2007]. This remarkably stable flame generates nearly constant concentrations of BC particles that contain essentially no organic carbon, so the quantification of BC mass using the thermal-optical analysis (TOA) methods is straightforward. In this case, the TOA measurement of BC can be used to evaluate the accuracy of other BC measurement methods. We applied this diffusion flame to evaluate BC measurements made with the widely-used aethalometer. Laboratory measurements of the flame-generated BC, as well as field measurements of diesel vehicle soot, illustrate that the aethalometer's response diminishes as its sampling filter becomes darkened with soot. When sampling BC at constant concentration, the aethalometer reports decreasing BC concentrations because it treats the BC attenuation coefficient as a constant whereas it actually decreases with increasing BC mass on the filter. Experiments with mixed-composition aerosol evaluated the aethalometer's response to aerosol mixtures characterized by increasing light-scattering material relative to light-absorbing BC. The aethalometer exhibits the greatest particle-loading effect when sampling aerosols with the lowest single scattering albedo (SSA between 0.15 and 0.62), and exhibits a much smaller particle-loading effect when sampling aerosols with SSA as high as 0.86. This indicate that the aethalometer responds to light-scattering aerosol in addition to light-absorbing BC aerosol, and that the response to light-scattering increases the apparent absorption and thus minimizes (i.e., offsets) the particle-loading effect. Moreover, it suggests that in many ambient environments, the particle loading effect may not be an issue. A modified aethalometer calibration is presented for situations where the aerosol SSA is known to be below about 0.6, such as near BC emission sources. When it can be verified that the aerosol SSA is greater than about 0.85, time-resolved BC data need no correction for the particle loading effect provided 5%-10% uncertainty is acceptable. These conclusions pertain only to temporally resolved BC measurements; time-averaged measurements of BC are not influenced by the particle-loading sampling artifact.
A33B-1200
Determination of Bimolecular Rate Constants for Reactions of Hydroxyl Radical with Large Organic Compounds in Atmospheric Water Drops
The hydroxyl radical (OH) is a potent oxidant in the aquatic environment. The lifetimes of many organic compounds are determined by reactions with hydroxyl radicals, and the lifetime of OH is often determined by reactions with organic compounds. Many studies have determined bimolecular rate constants for reactions between hydroxyl radical and relatively small inorganic and organic compounds, but only a few studies have determined rate constants for the reactions of OH with very large organic molecules such as atmospheric humic- like substances (HULIS). We are working to develop a competition kinetics technique to determine effective bimolecular rate constants with HULIS and other molecules. In this technique, we illuminate aqueous solutions containing three components: (1) the organic compound of interest, (2) benzoic acid (BA) as a competitive scavenger of OH, and (3) hydrogen peroxide as a source of OH. By changing the BA concentrations while maintaining the concentration of the organic compound, we can determine the lifetime of OH in the system in the absence of BA. We then use this lifetime to determine the bimolecular rate constant for the reaction of OH with the organic compound. Preliminary results, using organic compounds whose rate constants have been previously reported, indicate that the technique gives good results. We will report on these verification tests as well as on studies of rate constants of OH with humic substances as a function of pH.
A33B-1201
High Resolution Mass Spectrometry of Organic Nitrogen Species in Atmospheric Fog and Cloud Waters
Past studies have shown that organic nitrogen (ON) species are ubiquitous in atmospheric particles and water droplets and they are significant components of both wet and dry depositions. However, very little is known about the characteristics of this class of compounds and the roles that they play in atmospheric chemistry. To fill in this gap, we have developed a method that allows us to bulk-characterize and quantify organic nitrogen species in atmospheric aqueous phases using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). We evaluated this method by analyzing a suite of ON compounds including amino acids, amines, proteins, amides, and nitriles. The mass spectra of these compounds show similar structures to those in the NIST database, though with more fragmentation due to the higher vaporization/ionization temperature (~ 600 oC). The elemental compositions determined from the high resolution mass spectra agree well with the theoretical values. With this method, we analyzed a number of fog waters collected from the Central Valley of California and cloud waters from the Whiteface Mountain of New York. A large fraction of water soluble materials in both fog and cloud waters was identified to be organic, of which a significant portion contains nitrogen. On average, ON accounts for ~ 20% and 5%, respectively, of the total nitrogen (= NH4+ + NO3- + NO2- + ON) in the Central Valley fog and Whiteface Mountain cloud waters. Water soluble organic matter (WSOM) in the Central Valley fog and Whiteface Mountain cloud waters show highly oxygenated properties with mass spectra resemble those of highly aged organic aerosols sampled in rural areas and humic/fulvic acids. Finally, we will attempt to extend pertinent data analysis techniques to in-situ AMS data for ON characterization in ambient aerosols.