Elsevier

Atmospheric Environment

Volume 38, Issue 36, November 2004, Pages 6253-6261
Atmospheric Environment

Heterogeneous chemistry of individual mineral dust particles from different dust source regions: the importance of particle mineralogy

https://doi.org/10.1016/j.atmosenv.2004.07.010Get rights and content

Abstract

The heterogeneous chemistry of individual dust particles from four different dust source regions is investigated on a particle-by-particle basis using state-of-the-art scanning electron microscopy techniques including computer-controlled Scanning Electron Microscopy/Computer-Controlled energy dispersive X-ray (CCSEM/EDX) analysis. Morphology and compositional changes of individual particles as they react with nitric acid are observed. Clear differences in the reactivity of mineral dusts from these four different dust regions with nitric acid could be observed. Mineral dust from source regions containing high levels of calcium, such as those found in parts of China and Saudi Arabia, are found to react to the greatest extent. Calcium containing minerals, such as calcite (CaCO3) and dolomite (CaMg(CO3)2), react to form nitrate salts whereas other calcium containing minerals such as gypsum (CaSO4radical dot2H2O) do not react. The importance of particle chemical composition and mineralogy in the heterogeneous chemistry of mineral dust aerosol is definitively borne out in this study of individual dust particles.

Introduction

Mineral dust aerosol makes up a large fraction of the tropospheric aerosol mass and therefore impacts the Earth's climate and the atmospheric environment in several ways. First, mineral dust aerosol influences climate through direct and indirect climate forcing by scattering and absorbing incoming solar radiation and by nucleating clouds (Tegen, 2003; Buseck and Pósfai, 1999). Second, mineral dust aerosol influences biogeochemical cycles as iron-containing minerals provide an important nutrient for ocean life (Zhu et al., 1993). Third, mineral dust in the respirable size range is deleterious to human health (Guthrie, Jr. and Mossman, 1993). Finally, mineral dust aerosol influences the chemistry of the Earth's atmosphere by reducing photolysis rates of gas-phase species due to the fact that dust can decrease the incident solar flux and through heterogeneous chemistry (Bian and Zender, 2003; Tang et al., 2004).

Heterogeneous chemistry of mineral dust is not only intimately linked to the impact that dust has on the gas-phase composition of the atmosphere it can also influence how mineral dust aerosol impacts climate, biogeochemical cycles and health. For example, as mineral dust reacts in the atmosphere the physicochemical properties of the particles change including the optical properties of the particles and their effectiveness to serve as cloud condensation nuclei (Martin, 2000). This is important as there is increasing evidence that mineral dust aerosol impacts cloud formation, cloud properties and precipitation (Levin et al., 1996; Rosenfeld et al., 2001; Yin et al., 2002; Rudich et al., 2002; Sassen et al., 2003; Toon, 2003; Mahowald and Kiehl, 2003; Rudich et al., 2003). In addition, the bioavailability of iron in iron-containing dust particles may differ after the particles react with trace gases, especially acidic gases, in the atmosphere (Meskhidze et al., 2003). Finally, as particles age in the atmosphere they can become coated with carcinogens from the uptake of compounds such as polycyclic aromatic hydrocarbons (Garçon et al., 2000). Once coated, these particles are potentially even more of a health concern. Thus, an increased understanding of the chemistry of mineral dust is warranted.

Although mineral dust aerosol is often discussed as a single entity aerosol, similar to sea salt, it should be immediately obvious that this is a poor representation of the rich mineralogy and varying chemical composition of the dust. Not only is the mineralogy of the dust rich it is also source specific (Claquin et al., 1999). Thus, when atmospheric chemistry or global climate models represent mineral dust aerosol as a size distribution with a single kinetic parameter for reaction with a particular trace gas (Dentener et al., 1996; Bian and Zender, 2003; Liao et al., 2003; Martin et al., 2003; Bauer et al., 2004) or a single refractive index to model climate forcing (Dickerson et al., 1997), the results of these calculations can be called into question.

Here we present a case study of the heterogeneous chemistry of mineral dust with nitric acid, an important trace atmospheric gas. The composition and corresponding chemistry of individual particles from four different dust sources including China Loess from Asia, Saharan Sand from Africa and coastal and inland dust from Saudi Arabia have been investigated. The map shown in Fig. 1 identifies the source regions of these different dust samples. Several of these regions have been identified as being in the “dust belt”. The “dust belt” consists of the largest and most persistent sources of dust in the world (Prospero et al., 2002) Digital images of the powdered samples are also shown in Fig. 1. The chemistry of individual dust particles was analyzed with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. As discussed in detail below, the data show that the reactivity of nitric acid with mineral dusts is dust source dependent because of the varying mineralogy of the different dust sources. We show that calcium-containing carbonate particles, including calcite and dolomite, react to the greatest extent with nitric acid to form nitrate salts. These particles show unique morphological changes upon reaction and a change in hygroscopicity of the particle after reaction. Reactions of these particles are not surface limited and continued consumption of the available carbonate is observed (Krueger et al., 2003a., Krueger et al., 2003b). As a result, calcium-containing carbonate particles are converted into aqueous droplets of nitrate salts following the reaction. Other calcium-containing particles such as gypsum (CaSO4·2H2O) are not observed to be reactive. Quartz and aluminum silicate clays, a large component of the mineral dust, appear less reactive, at least with respect to bulk reactivity of the particle. Importantly, the results of this study indicate substantial differences in individual particle reactivity and therefore clearly show the need to include dust mineralogy in atmospheric chemistry and climate models.

Section snippets

Experimental methods

To obtain particles of atmospherically relevant sizes, the samples of sand from different source regions were first sieved to <40 μm particle sizes and then dispersed onto TEM grids for analysis and chemical processing. TEM grids, coated with a thin (∼50 nm) carbon film, were purchased from Ted Pella, Inc. (Carbon Type-B on Au 200 grid) and used as substrates in this work. Once the particles were dispersed onto the grids, the larger particles and particles that poorly adhered to the grid were

Average elemental composition of different dust source regions determined from single particle analysis

Mineral dust samples from four different desert regions of the world were examined in this study. Fig. 1 displays the source regions of these samples along with digital images of the corresponding bulk samples. Even visual comparison of the dust color in the four images indicates a different mineralogy makeup for each of the samples. Since mineral dust entrained in the atmosphere is typically in the micron size range, the dust samples used in our study were purposely prepared to remove the

Conclusions and atmospheric implications

In this study, using individual particle analysis that allows for imaging and chemical analysis, we have shown that dust particle reactivity depends on the mineralogy of individual particles. The carbonate component of the dust is particularly reactive. In terms of the percentage of reactive particles, the two most reactive dust sources in this study are China Loess and coastal Saudi Arabia sand. This is because a large number of the calcium particles are associated with carbonate minerals. In

Acknowledgements

The authors acknowledge support provided by the Biological and Environmental Research Program (BER), U.S. Department of Energy, Grant no. DE-FG01-98ER62580, and the National Science Foundation, through Grant no. CHE-9988434. This research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Pacific

References (41)

  • Caquineau, S., Gaudichet, A., Gomes, L., Legrand, M., 2002. Mineralogy of Saharan dust transported over northwestern...
  • T. Claquin et al.

    Modeling the mineralogy of atmospheric dust sources

    Journal of Geophysical Research

    (1999)
  • F.J. Dentener et al.

    Role of mineral aerosol as a reactive surface in the global troposphere

    Journal of Geophysical Research

    (1996)
  • R.R. Dickerson et al.

    The impact of aerosols on solar ultraviolet radiation and photochemical smog

    Science

    (1997)
  • El Sayed, M.A., Basaham, A.S., Gheith, A.M., 2002. International Journal of Environmental Studies 59,...
  • M.A.H. Eltayeb et al.

    Elemental composition of mineral aerosol generated from Sudan Sahara sand

    Journal of Atmospheric Chemistry

    (2001)
  • A.H. Falkovich et al.

    Chemical and mineralogical analysis of individual mineral dust particles

    Journal of Geophysical Research

    (2001)
  • P. Formenti et al.

    Inorganic and carbonaceous aerosols during the Southern African Regional Science Initiative (SAFARI 2000) experimentchemical characteristics, physical properties, and emission data for smoke from African biomass burning

    Journal of Geophysical Research

    (2003)
  • Guthrie Jr., G.D., Mossman, B.T., 1993. Merging the geological and biological sciences; an integrated approach to the...
  • Krueger, B.J., Grassian, V.H., Laskin, A., Cowin, J.P., 2003a. The transformation of solid atmospheric particles into...
  • Cited by (262)

    View all citing articles on Scopus
    1

    Tel.: +1-509-376-8741; fax: +1-509-376-6066.

    View full text