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.
There is widespread concern over the enhanced global warming that might result from the buildup of greenhouse gases in the atmosphere. The effects of aerosols (suspensions of solid or liquid particles in air) on Earth's radiation balance is less widely realized, and recognition of the role of airborne minerals has occurred only relatively recently.
Climate is fundamentally influenced by Earth’s energy budget, which depends on radiation received from the sun and energy radiated back to space. Incoming radiation is primarily in the visible range, whereas exiting radiation is largely in the IR. Greenhouse gases (H2O, CO2, CH4, N2O, etc.) absorb IR radiation and radiate it back to Earth's surface. Anthropogenic emissions of greenhouse gases cause increases in surface temperature (the greenhouse effect) and can have profound effects on climate and thus on societal welfare (1, 2).
Aerosol particles also have a major influence on global climate and climate change; they can locally either intensify or moderate the effects of the greenhouse gases through the scattering or absorption of both incoming solar radiation and thermal radiation emitted from Earth’s surface. Aerosols also act as cloud condensation nuclei (CCN) and thereby modify the radiative properties of clouds. The profound effects of atmospheric aerosols are surprising in view of their exceedingly low concentrations: the volumetric ratio of aerosol particles to atmospheric gases is between roughly 10 raised to the -10 and 10 raised to the negative 14 (3). The focus of this paper is on those particles, their compositions and structures and their effects on climate and, to a lesser extent, on the environment.‡
A growing awareness of the impact of particulate aerosols on climate, and the incompletely recognized but serious effects of anthropogenic aerosols, is summarized in several recent reviews (4–6). One reason for the relatively slow recognition of the role of particulate aerosols is that their study has fallen to disparate groups of scientists. Radiative transfer and other physical properties tend to be handled by one group (largely meteorologists and physicists), whereas chemical effects such as acid rain are emphasized by different scientists (mainly chemists). Perhaps the least attention to date has been on the geochemistry and mineralogy of aerosol particles and the effects of speciation.
Efforts to control greenhouse gases have been formalized by international treaty, e.g., the 1997 Kyoto Protocol on Climate Change. A comparable international effort to understand and control anthropogenic aerosol emissions has not (yet) occurred, at least in part because the extent to which they affect climate is not satisfactorily known. The incremental effects of anthropogenic increases in greenhouse gases are long lived (decades to centuries), whereas those of aerosols are shorter (weeks) (7). However, the sizes, compositions, and atmospheric lifetimes of particulate aerosols can vary spatially and temporally, and their strongest effects tend to be near their sources. If aerosols indeed offset climate responses to greenhouse gases, then the climate effects of greenhouse gases are even more substantial than has been recognized.
What role do mineralogists and geochemists have in addressing these and related issues of fundamental importance for human society and welfare? The main difference between most aerosol particles and the materials that are routinely studied by mineralogists is that most terrestrial minerals are not as fine grained. There are major problems with studying fine-grained materials, and therefore atmospheric chemists have traditionally emphasized bulk analyses to determine aerosol types. However, it is the individual chemical species that affect the radiative balance and climate as well as visibility and health. Paraphrasing a recent statement (8) and allowing for slight exaggeration, interpreting environmental and health effects of aerosols from bulk rather than individual-particle analyses is like interpreting mortality reports in a war zone from bulk airborne lead concentrations rather than from bullets.
Our group has focused on the painstaking but necessary analysis of individual particles. High-spatial-resolution methods, using electron beams as the primary probes of both chemistry and structure, have been developed to study increasingly fine-grained minerals. We examine the inorganic and, in special cases, the organic fraction of aerosol particles with electron microprobe analyzers and scanning electron microscopes (SEMs) and transmission electron microscopes (TEMs). In this paper we provide a background to the above issues and indicate ways in which mineralogical experience and experimental techniques can provide uniquely useful information. We first review the broad problems and briefly describe the analytical techniques, then discuss some of our recent transmission electron microscopy results regarding sulfate, soot, sea salt, and mineral aerosols.
