Airborne minerals and related aerosol particles: effects on climate and the environment

Abstract

Aerosol particles are ubiquitous in the troposphere and exert an important influence on global climate and the environment. They affect climate through scattering, transmission, and absorption of radiation as well as by acting as nuclei for cloud formation. A significant fraction of the aerosol particle burden consists of minerals, and most of the remainder- whether natural or anthropogenic-consists of materials that can be studied by the same methods as are used for fine-grained minerals. Our emphasis is on the study and character of the individual particles. Sulfate particles are the main cooling agents among aerosols; we found that in the remote oceanic atmosphere a significant fraction is aggregated with soot, a material that can diminish the cooling effect of sulfate. Our results suggest oxidization of SO2 may have occurred on soot surfaces, implying that even in the remote marine troposphere soot provided nuclei for heterogeneous sulfate formation. Sea salt is the dominant aerosol species (by mass) above the oceans. In addition to being important light scatterers and contributors to cloud condensation nuclei, sea-salt particles also provide large surface areas for heterogeneous atmospheric reactions. Minerals comprise the dominant mass fraction of the atmospheric aerosol burden. As all geologists know, they are a highly heterogeneous mixture. However, among atmospheric scientists they are commonly treated as a fairly uniform group, and one whose interaction with radiation is widely assumed to be unpredictable. Given their abundances, large total surface areas, and reactivities, their role in influencing climate will require increased attention as climate models are refined.

Figure 1

TEM images of (NH₄)₂SO₄. (a) The selected-area electron-diffraction pattern (upper left) confirms the identification. The arrow points to a soot aggregate. (Azores, North Atlantic, ASTEX/MAGE); (b) Rings of small (NH₄)₂SO₄ crystals that formed as the sulfate particles dehydrated. The dimensions of the halos can be used to distinguish among particles that likely had different water contents while still airborne. (Southern Ocean, ACE-1.)

Figure 2

TEM images of an internal mixture of (NH₄)₂SO₄ and soot. (a) The halo is similar to those in Fig. 1. The arrow points to a soot aggregate. (Southern Ocean, ACE-1); (b) High-resolution image of the arrowed tip of the soot aggregate in a. A degree of ordering is evident in the onion-like graphitic layers, seen edge on. (c) A large branching soot aggregate; such aggregates are typical of combustion processes (95). (Southern Ocean, ACE-1.)

Figure 3

TEM image of ammonium sulfate (a) before and (b) after it was sublimated by the electron beam. We believe the dark films in b are residues of organic material that coated the aerosol particle before sampling. The arrow marks a small soot particle. (Southern Ocean, ACE-1.)

Figure 4

(a) AFM and (b) TEM images of identical sulfate particles. Note the decrease in size caused by dehydration within the TEM. The amount of lost water is larger than expected and suggests increased hygroscopicity through organic coatings. (AFM image by Huifang Xu) (Azores, North Atlantic, ASTEX/MAGE.)

Figure 5

TEM images of sea salt. (a and b) Subhedral halite (NaCl) and euhedral sulfate crystals. The particle in b belongs to the smallest sea-salt particles that occur in the ACE-1 samples. (Southern Ocean, Cape Grim, ACE-1); (c) Halite particles in various stages of conversion to sulfate and nitrate. Grain A is partly converted, whereas C has been completely converted to nitrate and grains B to sulfates. (Azores, North Atlantic, ASTEX/MAGE.)

Figure 6

Na-Cl-S plots showing changes in composition of aerosol particles with altitude (Upper) and time (Lower). Each of the upper triangles represents a sample from a single altitude (meters above sea level are indicated) and contains data from between 30 to 70 particles. The upper samples were collected during a series of flights on one day. The lower triangles contain composites of samples from several altitudes; the one on the left contains all particles indicated above, whereas the one on the right contains those collected 26 hr later. Each contains data from about 250 particles. Mg, K, and Ca are not included in the diagrams because their ratios to Na do not change (or are within our analytical error). (Southern Ocean, ACE-1.)

Figure 7

TEM images of mineral dust collected from the marine troposphere. (a) Internal mixture of presumably terrestrial silicate and anhydrite with sea salt (Azores, North Atlantic, ASTEX/MAGE); (b) smectite (clay) and quartz (Q). The small grain size of the clay is visible at the thin edge (the arrows mark hexagonal platelets). Selected-area electron-diffraction patterns of clay and quartz are at the upper left and lower right, respectively. (Canary Islands, North Atlantic, ACE-2); (c) TEM image of goethite, FeO(OH), collected 2,600 m above sea level. Fe-bearing minerals like this could be important nutrient sources in remote oceans. (Canary Islands, North Atlantic, ACE-2.)