A53E-01 INVITED 13:40h
Concentrations, Time Variations, Size Distributions, and Mass Spectra of Primary and Oxygenated Organic Aerosols at Multiple Urban, Rural, and Remote Locations
We have recently developed a new procedure to estimate the mass concentrations and size distributions, and extract the mass spectra (MS) of primary and oxygenated organic aerosols (POA and OOA respectively) based on custom principal component analysis of Aerodyne AMS data (Zhang et al., this conference). Good correlation between AMS organic mass concentrations and OC from Thermal-Optical measurements has been observed at several locations (e.g. r2 = 0.88 in Pittsburgh). POA and OOA account for almost all the organic aerosol mass at most locations. OOA may comprise secondary organic aerosol (SOA) and also products of the oxidation of primary aerosol. In this presentation we will compare the mass concentrations and fractions, time variations, extracted MS, and size distributions, of POA and OOA in various urban, rural, and remote locations throughout the world. Urban locations include Pittsburgh, Mexico City, New York City, Houston, Boulder, Manchester and Edinburgh, UK, and Vancouver, Canada. Rural and remote locations include Storm Peak (Colorado), Duke Forest (North Carolina), Nova Scotia (Canada), Jungfraujoch (Switzerland), Trinidad Head (California), Jeju Island (Korea), Mace Head (Ireland), and Hyytiala (Finland). Primary aerosols represent a significant fraction of the organic aerosol in cities, although OOA is often larger, especially in the summer. Freshly-emitted combustion POA appear as a distinct mode at small vacuum aerodynamic diameters at all urban locations due to their fractal morphology, which is confirmed by "chase" AMS sampling behind individual vehicles. POA diurnal profiles are to a first order determined by the interplay of emissions and boundary layer height. OOA is generally concentrated in the accumulation mode, even in cities, indicating that most SOA condensation occurs on regional rather than urban scales. At times OOA may appear in the ultrafine mode, likely due to condensation on traffic particles or on growing nucleation particles. Organic aerosols at rural and remote locations are almost always dominated by OOA. There is evidence for additional components at some locations. The MS and size distributions of primary and oxygenated aerosols extracted with this procedure at various locations will be compared. The diurnal profiles and size distributions of OOA suggest that at least a significant fraction of this component is SOA, which is often internally mixed with ammonium sulfate. The extracted spectra of primary aerosols in urban areas are remarkably similar to that of directly sampled vehicle exhaust, while that of OOA is qualitatively similar across locations, and also shows similarity with the spectra of fulvic acid- a humic-like substance that has been previously used as an analogue to represent polyacid components found in highly processed and oxidized atmospheric organic aerosols.
http://cires.colorado.edu/jimenez/ams.html
A53E-02 13:55h
Size distribution and chemical composition of water soluble organic compounds in biomass burning and regional haze particles and their changes upon aging.
Particles from biomass burning and regional haze were sampled in Rondonia, Brazil, during the dry (intensive biomass burning), transition and wet periods as part of the LBA/SMOCC field campaign. Water soluble species, including organic acids, inorganic ions and anhydrous sugars, in bulk and size-resolved samples were determined by ion chromatography. A new analytical method for the detection of levoglucosan, 2-methylerythritol and related compounds, based on ion chromatography, will be presented. It is shown that low molecular weight (LMW) organic acids account for a significant fraction of the water soluble organic carbon (WSOC) in biomass burning aerosols. C2-C6 dicarboxylic acids reached up to 3.7% and one-ring aromatic acids reached up to 2% of fine fraction WSOC during the burning period. The measurements suggest that low molecular weight organic acids react with excess ammonia meaning that LMW organic acids should be incorporated in thermodynamic models used to predict inorganic aerosol composition. During the burning season, most of the ionic mass correlates with K+, a known biomass burning tracer, suggesting that many of the organic acids are directly emitted by vegetation fires. The distribution of the WSOC in different aerosol sizes shifts to larger sizes as the aerosol ages and mixes with other aerosol types. Finally, this study confirms that dicarboxylic acids are mostly confined to the particulate phase, and no evidence for semi-volatile behavior was observed.
A53E-03 14:10h
Investigation Into the Hydrophilic Versus Hydrophobic Fraction of Ambient PM2.5 Organic Particles Soluble in Water
On-line water-soluble organic carbon (WSOC) measurements are made by continuously collecting ambient particles into a purified flow of water using the Particle-Into-Liquid Sampler (PILS) [Orsini et al., 2003], filtering the liquid, and quantifying the dissolved carbon using a Total Organic Carbon (TOC) analyzer. Extension of this method has allowed for quantifying broad chemical classifications of the WSOC by passing the liquid sample through various XAD resins. The resins selectively remove the most hydrophobic WSOC compounds. The fractions of hydrophobic and hydrophilic carbonaceous compounds of the WSOC are quantified online by switching the XAD column on and offline to measure the hydrophilic fraction and hydrophobic fraction by difference. When combined with online measurements of the total organic carbon (OC), provided by a Sunset Labs Carbon Analyzer (Thermal/Optical Transmittance method), hydrophobic and hydrophilic fractions of OC can be measured. Ambient results from deploying this technique in urban Atlanta and St. Louis suggest that hydrophobic and hydrophilic fractions of WSOC are typically near 50/50. However, during strong photochemical events the hydrophilic fraction dominates, reaching 65% of the WSOC, and the WSOC comprises about 75% of the OC. Investigation of the seasonal and temporal trends in the hydrophobic and hydrophilic fractions of WSOC, and more detailed chemical speciation will be discussed. Orsini, D., Y. Ma, A. Sullivan, B. Sierau, K. Baumann, and R. Weber (2003) Refinements to the particle-into-liquid sampler (PILS) for ground and airborne measurements of water soluble aerosol composition, Atmos. Environ. 37: 1243-1259.
A53E-04 14:25h
Speciated Organic Composition of Atmospheric Aerosols: A New, In-Situ Instrument
Identification and quantification of the organic composition of ambient atmospheric aerosols is key to tracking sources of aerosols which impact human health, atmospheric visibility, and global climate. Organic matter is a major constituent of airborne particles, comprising 20-50% of the mass of airborne particles below 2.5 µm in diameter. The composition is complex, with hundreds of compounds identified through chromatographic mass spectrometry techniques. While the identified compounds only comprise a fraction of the total organic mass, those that are quantified serve as valuable tracers for sources. For example, hopanes, which are remnants of the biological material from which petroleum originated, serve as a unique tracer for fossil fuel combustion. Levoglucosan is a product of the breakdown of cellulose, and is a unique tracer for wood combustion. A substantial limitation in the use of organic marker compounds for source identification is the difficulty, and cost of the analyses. Needed are time-resolved, cost-effective measurements of specific organic marker compounds. Reported here are initial results from a new, in-situ instrument, the Thermal desorption Aerosol GC-MS/FID (TAG). This is an automated instrument for the time-resolved identification and quantitation of selected organic marker compounds in airborne particles over the size range from 0.1 to 2.5 µm. Atmospheric aerosol samples are collected into a thermal desorption cell by means of humidification and impaction. The sample is transferred onto a GC column by thermal desorption, with subsequent GC-MS/FID analysis. The collection and analysis steps are automated, yielding around the clock speciation. A droplet injector is being developed to provide internal standards for each sample. An advantage of our approach is that it builds on the extensive body of knowledge on the quantification of organic material, and on the identification of the origins of organic aerosols available from past research using filter-based GC/MS analyses. Initial results of ambient aerosol measurements made at Chebogue Point, Nova Scotia, Canada from July-August 2004 during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT 2004) study will be presented.
A53E-05 14:40h
Field and Laboratory Experiments Examining the Stability of Organic Molecular Markers Used for Source Apportionment
Individual organic compounds such as levoglucosan and hopanes are often used as tracers for different sources of primary organic aerosol. An important question is the stability of these reduced organic compounds, particularly in the eastern US and other areas where long-range transport is an important contributor to ambient aerosol concentrations. This paper presents evidence of photochemical aging of molecular markers using both in situ observations and laboratory experiments. First, a large in situ dataset of speciated condensed phase compounds is examined for evidence of aging of important tracers including cholesterol and hopanes. The challenge is to separate mixing and aging effects; we apply a dual-ratio chemical coordinate analysis technique used in the past to constrain photochemical age based on gas-phase hydrocarbon data to the condensed-phase organics data. The ambient data are consistent with significant aging, but a `smoking gun' is difficult to obtain due to the convolved mixing and aging effects, coupled with incomplete or inadequate source profile data. Second, smog chamber experiments have been performed to measure the oxidation of different condensed phase species in complex aerosol mixtures. The speciated smog chamber data are interpreted using a variant of the gas-phase relative kinetics technique. This approach allows us to determine relative oxidation rates for a large number of condensed-phase organic compounds in both model systems and real emissions. Experiments performed with model meat smoke mixtures varying in complexity from simple three component systems to actual meat smoke indicate rapid oxidation of oleic acid and much slower oxidation of cholesterol. We shall draw parallels between ongoing laboratory studies of model and real systems and the corresponding in situ measurements.
A53E-06 14:55h
Uptake of Organics by Water Aerosols Measured in a Low Pressure Aerosol Reactor
Uptake of trace gases by water aerosols (0.1-20 $\mu$m in diameter) was studied using a unique, newly-developed low pressure aerosol reactor that provides simultaneous measurements of both aerosol surface area density and gas phase concentration. This apparatus uses a spray atomizer to generate aerosols at pressures down to about 30 Torr and a fast flow reactor for kinetic studies. The number density and size distribution of the aerosol particles are determined from their infrared spectra. Mass spectrometry is used to monitor the concentrations of gas phase components. Decay kinetics are determined by monitoring the disappearance rates of the gas phase species as a function of the aerosol surface area. This technique has been used to investigate the uptake of acetone at room temperature and a pressure of 36.4 Torr. The direct uptake measurements are combined with calculations of the diffusion of acetone in the liquid droplets to determine $\alpha$, the mass accommodation coefficient. A value of $\alpha$ = 3.6(-2/+3.1) x 10$^{-3}$. was found in this way. The capabilities and advantages of the method will be discussed.
A53E-07 15:10h
Uptake of aldehydes by sulfuric acid aerosols through acid-catalyzed reactions
Atmospheric aerosols are complex mixtures of organic and inorganic compounds. While it is well understood how inorganic species form aerosols, the incorporation of organic material into atmospheric aerosols can occur by many different avenues, many of which are not well understood. In this study, we use an Aerodyne Aerosol Mass Spectrometer (AMS) and FTIR spectroscopy to probe the uptake of gas phase aldehydes by sulfuric acid and ammonium sulfate aerosols. Preliminary studies have focused on the uptake and reaction of hexanal with ammonium sulfate and sulfuric acid. Inorganic aerosols are produced via atomization and then combined with a flow of hexanal in a mixing cell. Additional mixing cells are added in order to vary residence time. The flow is then directed into either the FTIR flow tubes or the AMS for analysis. For ammonium sulfate aerosols that were exposed to hexanal vapor, no organics were observed to partition into the particle phase using either the FTIR or AMS. Pure hexanal also did not condense to form organic particles. In contrast, particulate organics were readily observed in the AMS when the same pressure of hexanal was exposed to sulfuric acid particles with a composition of 75 wt% sulfuric acid. The mass spectra for hexanal/sulfuric acid particles showed many peaks above m/z of 100 (that of hexanal) suggesting that acid-catalyzed reactions took place. In addition, the sulfuric acid particles showed substantial growth in the time of flight spectrum of the AMS when exposed to hexanal. Ongoing studies will probe the reaction as a function of residence time, sulfuric acid concentration, aldehyde composition and aldehyde concentration.
A53E-08 15:25h
Oxidative Processing of Organic Aerosol: Observations of Surface and Bulk Reactions
Aerosol particles in the atmosphere may contain significant quantities of organic species and these are continuously processed through oxidative reactions with traces gas species. In particular, reactions with O$_{3}$ and OH are believed to increase the degree of oxidation of such particles, though the rates and mechanisms are not well characterized. A more detailed understanding of such issues is required to predict how these reactions affect particles' abilities to participate in further reactions, take up water, act as cloud-condensation nuclei and grow. Using our Aerosol CIMS technique we have studied the oxidative transformation of organic aerosol by species such as O3 and OH in the laboratory. By measuring the rates of change of the organic constituent we can differentiate between reactions occurring at the surface of the particle and those occurring deeper in the particle. These findings suggest that the organic molecules orient and pack differently depending on their chemical functionality. Implications for the kinetics and mechanisms of reactions of tropospheric particles with different types of morphologies will be discussed.