A33C-01 13:40h
Laboratory Studies of the Impact of Atmospheric Aging on the Physicochemical Properties of Mineral Dust Aerosol
Mineral dust aerosol can impact a wide range of global processes including the chemistry of the Earth's atmosphere, the Earth's climate and human health. Atmospheric processing of mineral dust through heterogeneous chemical reactions will alter how these particles impact global processes. Laboratory studies can provide a framework in which to understand the molecular-level details of the changes in the properties of mineral dust aerosol as these suspended particles are processed or aged in the atmosphere. A combined approach of applying state-of-the-art probes, including ones based on microscopy and spectroscopy, and kinetic measurements allows for an understanding of the complexity of how atmospheric aging can change the physicochemical properties of mineral dust aerosol.
A33C-02 13:55h
Evidence of Soluble Sulfate Coatings on Aged Mineral Dust Particles in the East Asian Region
Recently published results from the Intercontinental Transport and Chemical Transformation 2K2 experiment (Perry et al., 2004) demonstrated that the mass distribution of Asian mineral dust particles arriving at the west coast of the United States responds to changes in relative humidity. The most likely explanation for the hygroscopic behavior of the aged mineral dust is the formation of soluble coatings either within the East Asian region or during transit across the Pacific Ocean. This study will use ground-based measurements of the size-resolved aerosol elemental composition collected during the ACE-Asia experiment to determine the prevalence of soluble sulfate coatings on mineral dust within the East Asian region and along the west coast of the U.S. Measurements from Hefei, China clearly show that there are periods of coated and non-coated mineral dust. Meteorological analyses of these episodes revealed that the coarse sulfate mass was highly correlated with the transit time from the mineral dust source regions to the receptor site. Little or no coarse sulfate was observed when the transit time was less than 3 days. Simultaneous measurements from Cheju, Korea show that the prevalence and mass of the soluble sulfate coatings on the mineral dust increases as the air mass is advected downwind from China. By the time the air mass arrives at the U.S., all of the mineral dust particles have a sulfate coating. This study demonstrates that the formation of soluble sulfate coatings on the mineral dust begins within the East Asian region and continues to evolve throughout the long-range transport process.
A33C-03 14:10h
Interfacial Segregation of Halogen Ions in Alkali Halide Solutions - A Model for Sea Salt Aerosols.
The chemistry of sea salt particles in the marine troposphere has recently generated substantial interest in the atmospheric chemistry community. Reactions of gas phase atmospheric constituents at the surfaces of such sea salt aerosols and particles have been shown to extract halogens into the marine troposphere. Recent reaction studies have led to the suggestion that the surface composition of sea salt aerosols is enriched in chloride ion[1]. This suggestion has been supported by Molecular Dynamics (MD) simulations[2,3]of alkali halide solutions which have predicted the surface segregation of polarizable halogen ions. The proposed enhanced surface availability of halogen ions would mean that gas surface reactions involving these ions would be much more important than previously expected. We present here results from our studies of the surface composition of saturated salt solutions. Using a novel high pressure photoelectron spectroscopy (HPPES) system at beamline 9.3.2 of the ALS synchrotron radiation source, we have compared the surface composition of KBr and NaCl crystals cleaved in vacuum with the surface composition of the saturated solutions formed at the deliquescence point, in equilibrium with water vapor. Our results show that the anion/cation ratio in the saturated solution is significantly enhanced within one nanometer of the surface. This is consistent with the predictions from the MD simulations by Jungwirth et al1,[2, 3] regarding halogen surface segregation. While a number of research groups are carrying out experiments aimed at the verification of this theoretical prediction, the experiments described here are the first direct experimental measurement of the enhanced halogen concentration at the surface of a salt solution in equilibrium with water vapor. References : 1. E.M. Knipping, M. J. Lakin, K. L. Foster, P. Jungwirth, D. L. Tobias, R. B. Gerber, D. Dabdub, B. J. Finlayson-Pitts, Science, 288, pg 301-306, 2000. 2. Jungwirth, P.; Tobias, D. J. J. Physical Chemistry 2002, 106(2), 379. 3. Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361.
A33C-04 14:25h
Using Surface Stress to Model the Sea Salt Source Function: A New Approach
Currently, the sea salt source function ({$\mathrm{S}^{3}\mathrm{F}$}) most often relies on {$\mathrm{U}_{10}$}, the wind speed at 10 meters above the surface, to derive a relation between atmospheric conditions over the ocean and the number flux of sea salt particles into the marine boundary layer. However, {$\mathrm{U}_{10}$}\ lacks critical information about boundary layer stability, which should play a role in the {$\mathrm{S}^{3}\mathrm{F}$}. We present a new {$\mathrm{S}^{3}\mathrm{F}$}\ dependent on actual surface stresses to generate a size distributed number flux of sea salt particles in the range 0.1 to 100 $\mu$m. We use our new {$\mathrm{S}^{3}\mathrm{F}$}\ in a regional transport model and compare the results of our model to other source functions and available field data. We find that the mass fluxes of the functions tested vary over several orders of magnitude, but that this descrepancy stems from the generation of large particles unimportant to long-range transport. However, in our model runs, we find that column mass loading varies by roughly 10% between previous schemes and our new {$\mathrm{S}^{3}\mathrm{F}$}. As a result, model runs carried out with our new {$\mathrm{S}^{3}\mathrm{F}$}\ better agree with available field data on column optical thickness and surface mass loading.
A33C-05 14:40h
Sea Salt Aerosol Production: Parameterization and Uncertainty
Based on a critical review$^{1}$ of sea salt aerosol (SSA) properties and processes, field measurements of concentrations, and production fluxes estimated by a variety of methods, a parameterization is presented for the effective (10 m) size-dependent production flux of SSA particles for r$_{80}$ (radius at 80% relative humidity) 0.1-25 $\mu$m and U$_{10}$ (10 m wind speed) 5-20 ms$^{-1}$. This flux is given by a single lognormal of the form dF/dlogr$_{80}$/(m$^{-2}$s$^{-1}$)= 50[U$_{10}$/(ms$^{-1}$)]$^{2.5}$$\times$exp\{(-0.5[log(r$_{80}$/r$_{80}$\'{ })/log($\sigma$)]$^{2}$\}, with r$_{80}$\'{ } = 0.3 $\mu$m and $\sigma$ = 4, and with associated multiplicative uncertainty of a factor of 4-5. At typical wind speed 10 ms$^{-1}$, this formulation is less than that given by some commonly used parameterizations by up to a factor of 80. This production flux formulation is based on a size distribution of SSA concentration that encompasses the great majority of reported field measurements and can be represented over the same range of r$_{80}$ and U$_{10}$ by a single lognormal of the form dN/dlogr$_{80}$/(cm$^{-3}$)= 0.07[U$_{10}$/(ms$^{-1}$)]$^{2}$$\times$exp\{-0.5[log(r$_{80}$/r$_{80}$\'{ })/log($\sigma$)]$^{2}$\}, with r$_{80}$\'{ } = 0.3 $\mu$m and $\sigma$ = 2.8, and with associated multiplicative uncertainty of a factor of 3 reflecting variability resulting from factors other than wind speed. Based on this expression, the number concentration of SSA particles that function as CCN is a weak function of supersaturation, with more than 80% of SSA particles activating at supersaturations greater than 0.1%. The time characterizing uptake of reactive gases such as H$_{2}$SO$_{4}$ at wind speed 10 ms$^{-1}$ is 0.15-1.5 h, implying a possible strong influence of SSA on nucleation suppression and aerosol evolution. The dominant contribution to light scattering by SSA particles is from particles having r$_{80}$ 1-10 $\mu$m, implying that measurements that do not include SSA particles throughout this entire size range substantially underestimate the influence of SSA on light scattering and Earth's radiative balance. 1. Lewis, E. R. and Schwartz, S. E. {\it Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models - A Critical Review}. In press, American Geophysical Union Monograph Series, 2004.
A33C-06 14:55h
Sodium Nitrate Particles: Physical and Chemical Properties During Hydration and Dehydration. Implications for aged Sea Salt Aerosols.
Experiments probing the phase and behavior of NaNO3 particles at different relative humidities, important for elucidating the role these particles play in the chemistry and radiative properties of marine regions, are presented. Changes in NaNO3 particles during hydration were studied using environmental scanning electron microscopy (ESEM) and conventional SEM coupled with energy dispersive X-ray analysis (SEM/EDX). Mixtures of NaNO3 and NaCl, which are typical of partially processed sea salt particles, were also studied. Complementary studies using long path FTIR and FTIR microscopy were carried out to determine the extent of water association with NaNO3 aerosols, and for comparison, NaCl, MgCl2, and NH4NO3, as a function of relative humidity. The combination of these techniques shows that NaNO3 particles exist as unusual metastable, amorphous solids at low relative humidity that undergo continuous hygroscopic growth with increasing relative humidity.
A33C-07 15:10h
Characterization of Particulate Inorganic Ions At Selected IMPROVE Sites
Aerosol nitrate is a major contributor to fine particle mass and regional haze at many locations. In order to evaluate the quality of regional particulate nitrate measurements, six field experiments were carried out at selected IMPROVE (Interagency Monitoring of Protected Visual Environment) sites. Issues examined include recovery of nitrate from nylon collection filters, loss of particulate ammonium and nitrate from the nylon collection filters, and the chemical form and size distribution of aerosol nitrate. Five sites were included in the study: Bondville, IL, San Gorgonio Wilderness Area, CA, Grand Canyon National Park, AZ, Brigantine National Seashore, NJ, and Great Smoky Mountains National Park, TN. Field campaigns at each site were approximately one month in duration. Multiple configurations of annular denuder/filter pack systems were utilized to evaluate nylon filter extraction efficiencies (by water and by a basic sodium carbonate/bicarbonate solution) and ammonium nitrate loss daily. A Micro-Orifice Uniform Deposit Impactor (MOUDI) was used to measure ion size distributions in 48 hr samples. A Particle-Into-Liquid-Sampler coupled with two Ion Chromatographs (PILS-IC) was operated continuously with 15 minute time resolution to capture temporal variations in aerosol anion and cation concentrations. The nylon filters were found to exhibit 100% efficiency for retention of collected particulate nitrate. Deionized water was found to be as efficient as the basic solution for extracting particulate nitrate from nylon filters, even in cases where significant ammonium nitrate volatilization occurred leading to recapture of nitric acid by the filter. The deionized water filter extracts are superior for analysis of aerosol cations. Significant losses of ammonium from the nylon filters were observed, however, when ammonium nitrate was collected. Analysis of MOUDI and PILS-IC data reveal that particulate nitrate at some locations was predominantly in the form of submicron ammonium nitrate. In several studies, however, nitrate was present predominantly in coarse mode particles associated with sea salt or soil dust. Findings from this study will help investigators better understand observations of aerosol nitrate made by IMPROVE and other regional measurement networks.
A33C-08 15:25h
Physical State of Aqueous Sulfate-Nitrate-Ammonium-Proton Aerosol Particles
The crystals formed at 293 K by aerosol particles composed of SO42-, NO3-, NH4+, and H+ are determined by aerosol flow tube infrared spectroscopy. An innovative experimental protocol is employed to restore water content to the aerosol particles and thus remove the ambiguity of their physical state after exposure to low relative humidity. The six crystals formed include (NH4)2SO4, (NH4)3H(SO4)2, NH4HSO4, NH4NO3, 2NH4NO3(NH4)2SO4, and 3NH4NO3(NH4)2SO4. The dependence of which crystals form on aqueous chemical composition is reported. The infrared signatures of these crystals are determined. The infrared spectra of 2NH4NO3(NH4)2SO4 and 3NH4NO3(NH4)2SO4 and their formation in aerosol particles are reported for the first time. The formation of NH4HSO4 and NH4NO3 in initially homogeneous aerosol particles is also reported for the first time: the crystallization occurs only after another crystal has already formed, indicative that heterogeneous nucleation is necessary for their formation. For some chemical compositions, in a fraction of the aerosol particles, metastable crystals that form at low relative humidity reconstruct to thermodynamically stable crystals at higher relative humidity. An externally mixed aerosol results. Contact ion pairs are apparent in the infrared spectra of aerosol particles that do not crystallize even to 1% relative humidity. Taken together, our findings suggest a more diverse array and more frequent occurrence of crystalline SO42-/NO3-/NH4+/H+ aerosol particles in the troposphere than currently considered in the literature.
http://www.deas.harvard.edu/environmental-chemistry/pubs.php