A51H-01 INVITED
High Resolution PTR-TOFMS: A New Instrument for Organic Compound Measurements
Over the last decade proton transfer reaction mass spectrometry (PTR-MS) has become very popular in many scientific fields. PTR-MS allows for the quantitative detection of volatile organic compounds (VOCs) at pptv-level virtually in real time. Monitoring of VOCs with a time resolution of typically a second per compound has, for instance, enabled the tracking of pollution plumes by air-borne measurements, thus revealing the photo- chemical fate of pollutants. It has also been employed in direct eddy covariant flux measurements. This rapidity, however, has been achieved at the cost of the number of compounds to be analyzed and compound selectivity. Conventional PTR-MS can, for example, not distinguish between hydrocarbons and their oxygenated isobaric species, e.g. between naphthalene and octanal or between isoprene and furan. In a mass range up to 200 Dalton, such a task would require a mass resolving power of 5500. The use of a time of flight (TOF) instead of a quadrupole mass analyzer in PTR-MS provides a sufficient high mass resolution to identify the atomic composition of product ions by their exact mass and their characteristic isotope patterns. In addition PTR-TOF-MS can record full mass spectra within a fraction of a second which is a dramatically increase in duty cycle. At the University of Innsbruck a high resolution PTR-TOFMS has recently been developed, coupling a PTR-ion source and a high resolution TOFMS. We achieved a mass resolving power of 6000 (FWHM), and a detection limit of tens to a few hundreds of pptv if integrating mass spectra for one minute. First results and future directions will be discussed in this paper.
A51H-02
Measurement of aerosol organic compounds during TexAQS 2006 using a novel collection/thermal-desorption PTR-ITMS instrument
Knowledge of the organic species present in atmospheric aerosols is needed in order to understand their effect on aerosol microphysical and optical properties, to resolve outstanding questions about important organic aerosol sources and formation mechanisms, and to elucidate the role of aerosols in the chemistry of the atmosphere through their interaction with gas-phase compounds. The measurement of aerosol organic compounds poses a significant experimental challenge due to the complexity and large number of organic species and the low concentration at which individual species are present. A new instrument that utilizes proton- transfer-reaction mass spectrometry (PTR-MS) to probe the organic composition of atmospheric aerosols has been developed to investigate semi-volatile and condensed-phase organic species in the atmosphere. Aerosols are collected by impaction and then thermally desorbed into a carrier gas that transports the organic analyte molecules into a drift tube where they are ionized by reaction with H3O+ ions. Analyte ions are detected using an ion trap mass spectrometer. The instrument was deployed for the first time during summer 2006 in the Texas Air Quality Study (TexAQS 2006) aboard NOAA R.V. Ronald H. Brown. Signals significantly above detection limit were observed at a number of masses during periods of elevated photochemical activity when aerosol loading increased in the 0.5-1 μm size range and aerosol mass spectrometer (AMS) measurements indicated increased organic mass. Different masses exhibited different temporal behaviors, indicating varying composition of the aerosol organic fraction even during periods when the AMS organic mass (OA) loading was relatively constant. Plumes of aerosol-phase pyridine were observed during sampling near the entrance to the Houston Ship Channel, indicating a relatively local source and rapid partitioning to the aerosol phase. These field results and results of laboratory instrument performance experiments will be presented.
A51H-03
SPLAT II: An Ultra-Sensitive, High Precision Instrument for the In-Situ Characterization of the Size, Composition, Density, Shape, Hygroscopicity, and Fractal Dimension of Fine and Ultrafine Particle
Single particle mass spectrometers (SPMS) have been developed and deployed to characterize, in real-time the size and chemical compositions of individual ambient particles. SPMS have undeniably proven to be extremely valuable, improving our understanding of the properties and evolution of atmospheric aerosols. At the same time these instruments have also persistently been subject to criticism on a number of fronts. Recently we have completed the construction of our second generation single particle mass spectrometer - SPLAT II, in which we have made significant improvements that directly address the shortcomings that are often associated with SPMS. We will present the results of instrument characterization experiments demonstrating that considerable improvements in instrument performance have been accomplished. SPLAT II offers significantly improved detection sensitivity of small particles, such that 30% of all 100 nm particles that enter the instrument are detected and characterized. This represents an improvement of nearly 2 orders of magnitude over the previous instrument. The efficiency of detection of particles larger than 150 nm is 100%. SPLAT II can measure and record the vacuum aerodynamic diameters of up to 300 particles per second and record 100 individual particle mass spectra per second - an increase of a factor of 10 in temporal resolution over the previous instrument. SPLAT II uses an IR laser to evaporate the semivolatile fraction of the particle and a time delayed UV laser to generate ions. The IR and UV lasers are configured to assure that the individual particle mass spectra represent the complete composition of each particle. The semivolatile fraction is ionized in the gas phase, while the nonvolatile fraction is concurrently ablated. This approach yields extremely reproducible, high quality mass spectral signatures while providing complete particle characterization. The two step – IR/UV - approach is particularly suitable for the characterization of organic molecules in fine and ultrafine particles. SPLAT II uses a data acquisition system with a 14 bit dynamic range- an increase of a factor of 128 in dynamic range over the previous instrument. This allows the instrument to record mass spectra of internally mixed particles that contain a range of substances with very different ionization probabilities. SPLAT II offers ultra-sensitive detection capabilities, making it possible to combine it with a DMA or a tandem DMA. The SPLAT II/DMA can simultaneously measure, in-situ individual particle size, composition, density, effective density, dynamic shape factor, fractal dimension, and hygroscopicity at a rate that yields statistically robust data. The SPLAT II/DMA system can also provide quantitative individual particle composition. To take a full advantage of the vast amounts of detailed data produced by SPLAT II we have developed two software packages: SpectraMiner and ClusterSculptor. SpectraMiner is a statistical analysis and data mining and visualization program that offers the user a wide range of visually driven controls to explore the data. ClusterSculptor is a software package that is coupled to SpectraMiner and is designed to overcome the limitation of statistics by offering the user the ability to insert his/hers scientific knowledge into the data organization phase.
A51H-04
A Rapid In-Situ Technique for Aerosol Chemical Composition as a Function of the Hygroscopic Growth: Results from Urban, Remote and Polar Field Sites
Aerosols, both natural and anthropogenic, are important factors with respect to the radiation budget of the atmosphere. One important property is the ability of an aerosol particle to take up water which can have an impact on aerosol optical properties and cloud formation. To date little is known about how aerosol water uptake depends on the chemical composition of the aerosol. In this study an in-situ measurement setup to determine the chemical composition of atmospheric aerosols as a function of hygroscopicity is presented. This has been done by connecting a custom-built Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) and an Aerosol Time-of-Flight Mass Spectrometers (ATOFMS), commercially available from TSI (Model 3800). Thus, single particle bipolar mass spectra from aerosols leaving the HTDMA could be obtained as a function of hygroscopic growth factor. For these studies the HTDMA was deployed at a relative humidity of 82% and particles with a dry diameter of 260 nm were selected. This novel setup was laboratory-tested with mixed hydrophobic and hygroscopic aerosols. Subsequently, several sets of field experiments were performed during the last year. Two datasets were obtained during wintertime 2007 in Switzerland: One in the urban Zurich environment and the other at the remote high alpine station Jungfraujoch (JFJ). Further data was obtained in July 2007 in Abisko, Sweden. Located 200km north of the Arctic Circle, air masses from different directions were distinguished and examined. In Zurich a large data set was obtained within less than two days of measurements due to a high aerosol loading. At the JFJ, due to low particle concentrations, a longer sampling period was required. At the Abisko station particle concentration (i.e., data acquisition rate) strongly depended on the airmass origin. Both in Zurich and at the JFJ two different growth factor modes were observed. First results from these two locations show that most aerosols were generally internally mixed. A large contribution of organics and biomass combustion was found in the non-hygroscopic growth mode particles for both locations. Refractory material (e.g. metals, mineral dust, fly ash elements) was also highly enhanced in the non-hygroscopic particles. Sulfate was found to be a constituent in almost all particles independent of their growth factor. The Abisko dataset is currently being evaluated.
A51H-05
A new instrument for measuring the chemical composition of soot containing particles
Black carbon containing aerosols play important roles in governing the optical properties of atmospheric aerosol. Absorption of incident solar radiation by elemental carbon containing aerosol leads to atmospheric heating, potentially affecting cloud formation, for example. An outstanding question is the role of coatings on black carbon cores and how such coatings may alter the optical properties of the particles. Instruments such as the Photoacoustic Spectrometer (PASS) and the Single Particle Soot Photometer (SP2) are well suited to characterize total black carbon and the size of black carbon cores in ambient particles (via absorption and incandescence measurements, respectfully). However, these instruments lack the capability to measure the chemical composition of species adsorbed on the black carbon cores. We present a new instrument for measuring the chemical composition of adsorbed inorganic and organic material coating particles containing black carbon. This new instrument couples SP2 laser heating with the aerosol mass spectrometer (AMS). Black carbon containing particles intersect an intense, continuous intracavity laser beam, absorbing the 1064 nm radiation. Vaporized components are then ionized by electron impact ionization and detected using a time of flight mass spectrometric (TOFMS) approach. We report results which demonstrate the utility of this technique for a variety of particles with and without black carbon cores, with and without various coatings. These measurements clearly show that the method is sensitive only for particles containing an absorbing component. The results also show that the mass spectrometric signals vary linearly with the amount of the condensed species. This allows for the quantification and identification of the nonrefractory chemical composition of absorbing particles with the goal of tracking and characterizing primary combustion particles as they are processed and transported in the ambient atmosphere. Signals for vaporized carbon atom clusters are also observed; on-going work is aimed at interpreting the significance of these signals.
A51H-06
Measurement of Oxygen-to-Carbon Ratios of Organic Aerosols and Implications for the Atmospheric Evolution of SOA
We present a new elemental analysis (EA) technique for organic aerosols (OA) that allows fast on-line analysis (10 s) and reduces the required sample size to ~ 1 ng, about 6 orders of magnitude less than standard techniques. The composition of the analyzed samples is approximated by the average elemental composition of the ions from high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) spectra, with a calibration to correct for biases introduced by the fragmentation process. EA of organic species can be performed on organic/inorganic mixtures. Elemental ratios for the total organic mass, such as oxygen/carbon (O/C), hydrogen/carbon (H/C), and nitrogen/carbon (N/C), in addition to the organic mass to organic carbon ratio (OM/OC), can be determined. Additionally, we provide approximations that can be used with unit mass resolution (UMR) AMS datasets collected with the quadrupole (Q-AMS) and C-ToF-AMS. We apply this method to urban and regional organic aerosols measured from ground sites and aircraft during MILAGRO and SOAR-1, to chamber secondary OA (SOA), and to source measurements of several primary OA sources. A clear diurnal cycle is observed in urban areas for O/C with a maximum in the afternoon due to SOA formation and a minimum during the morning rush hour due to primary OA (POA) emissions. The average O/C atomic ratio of the total urban OA varies between about 0.2 and 0.7. H/C has the opposite diurnal cycle due to higher H content in POA than SOA. N/C is low around 0.05 and relatively constant, with some upward excursions in the early morning due to amine-containing plumes. EA can also be applied to OA components extracted with Positive Matrix Factorization (PMF). Hydrocarbon-like OA (HOA, a POA surrogate) has low O/C ratios ~ 0.1, while fresh oxygenated OA Type II (OOA-II, a surrogate for fresh SOA) has O/C ~ 0.6, and regional aged OOA (OOA-I) reaches O/C ~ 1. Chamber SOA has a similar O/C (and mass spectra) as ambient fresh SOA. We clearly observe the evolution of OOA-II into OOA-I during photochemistry in several case studies. Time series and thermal denuder data also indicate that OOA-I is less volatile than OOA-II, consistent with the higher O/C of the former, and potentially with oligomer formation during aging. Our results indicate that fresh urban SOA is similar to chamber SOA, but that a key parameter missing from chamber studies is increased and realistic aging. Finally, the identification of OOA-II and OOA-I with similar properties in multiple environments highlight the global importance of these results. http://cires.colorado.edu/jimenez/ams.html
A51H-07
Broadband Cavity Enhanced Absorption Spectroscopy: an in-situ DOAS for Atmospheric Measurements
From the processing of natural and anthropogenic emissions to the nucleation of new aerosol particles, tropospheric chemistry is driven by species present at trace concentrations. Field based observations provide the means to study complex trace gas processes in-situ, and advancements in understanding are often driven by developments in instrumentation that push forward the limits of sensitivity, target selectivity and deployment flexibility. In this work we present an ultra-sensitive optical instrument for quantification of a wide range of atmospheric trace gas components. The instrument is based upon a broadband variant of cavity enhanced absorption spectroscopy (BBCEAS) using high intensity light emitting diode sources, a short (1.1 m) high finesse optical cavity and wavelength resolved CCD detection. The BBCEAS approach enables measurements akin to those of traditional DOAS to be made in-situ and at high temporal resolution. This is important for the study of short lived species which show high spatial and temporal variability. In addition, the instrument offers deployment flexibility for field studies, being well suited for use on a range of platforms including aircraft, ships and vehicles. In this paper the instrument design and principles of operation will be described in detail, with particular attention being given to description of the calibration system. The performance of the instrument will be presented using results from a number of recent field and laboratory based studies. Most notably these include ppt level measurements of NO3 and N2O5 in coastal environments, ppt level detection of IO formed in reactor experiments from the oxidation of molecular iodine emitted from marine algae and lab based experiments showing sub ppb level detection of nitrous acid.