A34B-01 16:00h
Aerosol Mass Spectrometry: Morphology and Chemistry of Combustion Particles in the Lab and the Atmosphere
Atmospheric aerosols play important roles in climate, visibility and local air quality. Of particular interest is the contribution of combustion emissions. Here we present results from laboratory and ambient sampling studies that combine aerosol measurements made with an Aerosol Mass Spectrometer (AMS) and a Scanning Mobility Particle Sizer (SMPS). Simultaneous sizing of aerodynamic and mobility diameters enables determination of particle density and shape factors. AMS analysis of organic carbon combined with external measurement of black carbon (BC) provides chemical mass balance. Lab and atmospheric measurements indicate that fresh combustion particles are fractal, becoming more spherical as processing coats particles with condensable vapors. The laboratory experiments sample particles produced in a pre-mixed propane flame. Particles with mobility diameter selected by a Differential Mobility Analyzer (DMA) are then aerodynamically sized and chemically analyzed with the AMS. At low fuel equivalence ratio (<4) fractal (mostly BC) particles are observed while higher fuel ratios nearly spherical particles coated with PAHs are produced. Field results are based on sampling of individual on-road diesel vehicles (New York City-PMTACS 2001; Mexico City-MCMA 2002/2003) and continuous ambient sampling (Pittsburgh-PAQS 2002; Mexico City-MCMA 2003). On-road vehicle particle emissions look very much like the flame produced fractal particles while the ambient measurements show evidence both of fresh and larger, processed particles. Recent lab studies are simulating processing via vapor condensation on fractal particles while modeling studies are comparing simultaneous AMS and SMPS ambient measurements.
A34B-02 16:15h
Characterization of a Single Particle Soot Photometer (SP2)
We have characterized a new type of sensor that detects individual soot particles -- the Single Particle Soot Photometer (SP2) developed by Droplet Measurement Technologies, Inc. The SP2 performs a two-color measurement of the blackbody emission from light-absorbing refractory aerosols as they are heated by an intra-cavity infrared laser beam. The light scattered from sampled particles is also recorded, as in a conventional optical particle counter. These measurements allow individual light-absorbing refractory particles to be differentiated from non-absorbing aerosols. The incandescence temperature derived from the two-color measurement constrains the refractory composition of the particle. We have quantified the SP2 response to a variety of elemental-carbon and non-carbon particles available commercially and have also sampled ambient aerosols near the laboratory. The laboratory results demonstrate the ability of the SP2 to identify elemental-carbon containing aerosols, its insensitivity to ambient pressure changes, and its good alignment stability. The instrument, which has been configured for use on the NASA WB-57F aircraft, will have its first flights from Houston in October 2004. It will sample soot particles in the upper troposphere and lower stratosphere. A two-channel SP2 instrument configuration will be described for use in 2005.
A34B-03 16:30h
Black Carbon Properties in the Marine and Ohio Valley Boundary Layers: Contrast and Similarities Related to Sources, Cloud Processing and Aging
The properties of black carbon (BC) on a particle-by-particle basis, have been measured from aircraft with the Single Particle Soot Photometer (SP2) in the marine boundary layer off the California coast in July, 2003 and in the boundary layer over the central Ohio Valley in the summer of 2004. The SP2 technique allows the BC particle mass to be determined directly by incandescent emissions and the diameter to be estimated from light scattering. The relative shapes and sizes of the two signals provides information about the fraction of BC in a mixed component particle and as well as the relative differences in composition from particle to particle. The comparison of BC properties from the marine and continental boundary layers highlights the importance of aging and cloud processing in the evolution of BC.
A34B-04 16:45h
A critical review of smoke particle evolutionary processes
Biomass burning, one of the largest sources of accumulation mode particles globally, has been closely studied for its radiative, geochemical, and dynamic impacts. These studies have taken many forms including laboratory burns, in situ experiments, remote sensing, and modeling. While the differing perspectives of these studies have ultimately improved our qualitative understanding of biomass burning issues, the varied nature of the work make inter-comparisons and resolutions of some specific issues difficult. Often, published results contradict each other greatly. In this paper we present results from over 4 years of collaborative research between NRL, Forschengszentrum in Juelich, and NASA GSFC trying to reconcile the literature with respect to particle evolution in smoke plumes. It is evident from these studies that the physical and chemical properties of smoke particles rapidly change with age, with regional hazes typically having volume median diameters ~0.05 mm larger than fresh smoke with significant enhancements in organic acids and various inorganic species such as sulfate. However, quantitative estimates of such growth rates and chemical evolutions are at odds with one another and properties such as hygroscopicity are highly uncertain. Much of this can be resolved between measurement bias and various environmental factors. We propagate the error and discuss the effects of these uncertainties in climate model and remote sensing parameterizations.
http://www.nrlmry.navy.mil/flambe/
A34B-05 17:00h
Modeling the Aging Process of Soot
Soot particles contribute both to the direct and indirect climate effect. While freshly emitted soot is initially hydrophobic and externally mixed, it can be transferred into an internal mixture by coagulation, condensation or photochemical processes. These aging processes affect the hygroscopic qualities and hence the growth behaviour, the optical properties and eventually the lifetime of the soot particles. However, our understanding of these processes is still fragmentary. The aging process of soot is therefore one of the key uncertainties concerning the burden and effect of black carbon. In this study we carry out 3D simulations with the coupled mesoscale model KAMM/DRAIS which provides a highly resolved boundary layer and allows for an explicit treatment of the aging process of soot by coagulation and condensation. The particle phase is treated with the aerosol model MADEsoot that calculates both the composition and the size distribution of the aerosol particles. Based on the results of our simulations, we derive the time scale on which soot is transferred from an external to an internal mixture. We focus on continental conditions in an industrialised environment and investigate two different meteorological scenarios, that is a summer and a winter episode. Generally, the aging time scales that we derive are smaller than most of the values that are currently used in global climate models and show a considerable variability in space and time. During daytime in summer, condensation of sulphuric acid is the governing process for the aging of soot. In wintertime, the formation of ammonium nitrate gains in importance. Overall, the time scales for daytime in summer and winter are comparable, about 2 h above 250 m and 8 h below During night time, condensation stops being the important process. Instead, coagulation becomes more significant, acting very slowly. This leads to a time scale during night of 10 to 40 h.
A34B-06 17:15h
Electron Tomography of Soot Aggregates: Quantitative Measurements of Individual Atmospheric Particles
Soot (also called black carbon) is a widespread type of aerosol particle that contributes to uncertainties in estimates of climate forcing. It typically occurs in complex clusters having highly variable physical properties, compositions, and surface coatings, and thus presents significant challenges for assessing the optical parameters used in modeling. Morphological variations arise from differences in fuel sources, combustion conditions, and atmospheric aging. Transmission electron microscopy (TEM) images have been widely used for recording the 2D shapes of soot clusters, and these have commonly been interpreted through fractal analysis. Fractal dimensions, D${f}$ are generally inferred from the analysis of 2D TEM images, although problems arise because of finite size effects, primary particle overlap, screening effects, and cluster anisotropy. Electron tomography (ET) is the first experimental approach that provides direct 3D information about soot aggregates. Soot surface character, porosity, and morphology can now all be determined quantitatively. Also, ET creates a 3D intensity map of the object, from which quantitative volume and surface areas can be derived. Using ET, we analyzed soot aggregates from diesel exhaust and a biomass-burning plume. Through comparisons of surface area, volume, and D${f}$ measurements derived from the tomograms, we describe differences in morphology between these two types of soot.
A34B-07 17:30h
Individual Particle Analysis of Biomass Burning Aerosols During the Yosemite Aerosol Characterization Study
The Yosemite Aerosol Characterization Study of summer 2002 occurred during an active fire season in the western U.S., and provided an opportunity to investigate many unresolved issues related to the effects of carbonaceous aerosols on visibility. Single particle analyses were performed over field collected aerosol samples using computer-controlled scanning electron microscopy with energy-dispersed detection of X-rays (CCSEM/EDX). Individual particle mixing characteristics based on composition data and morphology information suggest the major types of particles were organic/inorganic inclusions with volatile coatings, and tar balls. Tar balls were associated with long-range transport of biomass smoke to the park. These particles were associated with high particle light scattering coefficients during the peak of a smoke/haze event, as determined by other aerosol measurements concurrently performed during the study. The correlation between the frequency of the occurrence of tar balls and black carbon concentrations from a dual- wavelength aethalometer (370 and 880 nm), as well as the insignificant amounts of soot particles observed with the SEM, suggest that tar balls are absorbing in the UV and near-IR range of the solar spectrum. These results have important implications for visibility and climate considerations because absorption of solar radiation from biomass burning aerosols is typically assumed to be due to elemental carbon only.
A34B-08 17:45h
Carbon 14 Analysis of Carbonacous Aerosols in Oslo, Norway
Oslo experiences severe atmospheric inversions during the winter months, trapping high concentrations of particulate matter. The city is situated at the north end of the Oslofjord with surrounding hills of 300-500 m. The organic fraction is of concern due to the associated health effects. The objective of the present study was to determine the relative contributions to fine particulate matter in Oslo where the two main sources are wood burning and traffic. Sampling was done in collaboration with the Norwegian Institute of Air Research (NILU) at Sofienberg Park, an urban residential site. Sampling was done with a LVS1.0 quartz filter sampler with a collection time of 48 hours. Night and day samples were obtained (6 pm to 6 am) and (6 am to 6 pm), respectively. Accelerator mass spectrometry at LLNL was used to quantify the ratio between biogenic and fossil fuel contributions by carbon 14 analysis. The total carbon aerosol was twice as high at night versus day during the coldest periods. The biogenic contribution was 70% of the aerosol during nighttime. The total carbon concentration showed a very high correlation (r > 0.93) with nighttime low temperature. Factors contributing to the source apportionment and temperature relationship will be discussed.