Chemical analysis of atmospheric aerosols

Abstract

An aerosol is an assembly of liquid or solid particles which are suspended in a gaseous medium long enough to enable observation and measurement. The atmospheric aerosol thus describes the system of particles suspended in our atmosphere. Generally, the sizes of aerosol particles are in the range 0.001–100 μm. Sampled aerosol particles are also called particulate matter (PM). According to the cutoff used during sampling, a suffix is often used, thus, PM10 denotes all aerosol particles with an aerodynamic particle diameter d≤10 μm. Most legal standards are related to PM10. Atmospheric aerosols are of interest mainly because of their effects on health and climate. Concerning health, many epidemiological studies have shown a link between increased mortality/morbidity and increased PM10 or PM2.5 (see [1] and references therein). Concerning climate, aerosol particles scatter and absorb light (known as the direct effect on climate), and modify cloud properties (with a variety of effects known as indirect effects) [2]. These effects are influenced by the chemical and physical properties of the aerosol particles, which makes these properties important to be measured. In addition, the chemical composition reveals information about the sources of the atmospheric aerosols. Thus, for optimal mitigation processes the chemical composition needs to be known. Figure 1 presents the average chemical composition of PM10 from Zurich (data from [3]). Elemental carbon (EC), trace elements and mineral dust are so-called primary aerosol constituents, i.e., they are directly emitted into the atmosphere in particulate form. On the other hand, nitrate and sulfate are only formed in the atmosphere through the oxidation of the gaseous species NOx (i.e., sum of NO and NO2) and SO2 to nitric and sulfuric acid and subsequent neutralization of ammonia. These components are thus called secondary. Organic matter (OM) is composed of organic carbon (OC), which includes a variety of heteroatoms such as oxygen, hydrogen, or nitrogen. A conversion factor is therefore used to calculate OM from the measured OC concentration. OM can be of primary or secondary origin. The sum of OM and EC is also called carbonaceous aerosol. The unknown fraction consists of a considerable amount of water [3]. While the sources of the inorganic constituents are quite well known, very little is still known about the chemical composition of OM. This paper therefore focuses on the chemical analysis of OM, and includes a discussion of primary versus secondary and anthropogenic versus biogenic carbon.