**1. Introduction**

Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary field of research and draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and volcanology and other disciplines. It also deals with chemical compounds in the atmosphere, their distribution, origin, chemical transformation into other compounds and finally their removal from the atmospheric domain. These substances may occur as gasses, liquids or solid. The composition of the atmosphere is dominated by the gasses nitrogen and oxygen in proportions that have been found to be invariable in time and space at altitudes up to 100 km. All other compounds are minor ones, with many of them occurring only in traces.

The composition and chemistry of the atmosphere is of importance for several reasons, but primarily because of the interactions between the atmosphere and living organisms. The composition of the Earth's atmosphere (Figure 1) has been changed by human activity and some of these changes are harmful to human health, crops and ecosystems. Examples of problems which have been addressed by atmospheric chemistry include acid rain, photochemical smog and global warming. Atmospheric chemistry seeks to understand the causes of these problems, and by obtaining a theoretical understanding of them, allow possible solutions to be tested and the effects of changes in government policy evaluated.

Observations, lab measurements and modeling are the three important methodologies in atmospheric chemistry. Progress in atmospheric chemistry is often driven by the interactions between these components and they form an integrated whole. For example observations may tell us that more of a chemical compound exists than previously thought possible. This will stimulate new modelling and laboratory studies which will increase our scientific understanding to a point where the observations can be explained. Measurements

© 2012 Rozaini, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Rozaini, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

made in the laboratory are essential to our understanding of the sources and sinks of pollutants and naturally occurring compounds. Lab studies tell us which gases react with each other and how fast they react. Measurements of interest include reactions in the gas phase, on surfaces and in water. Also of high importance is photochemistry which quantifies how quickly molecules are split apart by sunlight and what the products are plus thermodynamic data such as Henry's law coefficients.

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 325

will only have a very crude representation of mixing in the atmosphere. In contrast, 3D models represent many of the physical processes of the atmosphere but due to constraints on computer resources will have far fewer chemical reactions and compounds. Models can be used to interpret observations, test understanding of chemical reactions and predict future concentrations of chemical compounds in the atmosphere. One important current trend is for atmospheric chemistry modules to become one part of earth system models in which the links between climate, atmospheric composition and the biosphere can be

An aerosol is a system (in the sense of a system as used in thermodynamics or chemistry) comprising liquid and/or solid particles in a carrier gas. It is generally defined as a suspension of liquid or solid particles in a gas, with particle diameters in the range of 10-9-10-4 m (lower limit: molecules and molecular clusters: upper limit: rapid sedimentation). The most evident examples of aerosols in the atmosphere are clouds, which consist primarily of condensed water. The suspension of the particles in the gas must be significantly stable and homogenous. Hence the assumptions of stability and homogeneity, and consequently the possibilities to use statistical descriptors, are limited to understand and to predict the system, the particle properties, i.e. their size, shapes, chemical compositions, their surfaces, their optical properties, their volumes and masses must be known (Preining, 1993). Aerosol particles scatter and absorb solar and terrestrial's radiation, they are involved in the formation of clouds and precipitation as cloud condensation and ice nuclei, and they affect the abundance and distribution of atmospheric traces gases by

The atmospheric aerosol in the boundary layer and the lower troposphere is different for

Chemically or photochemically produced from precursor gases, continental or oceanic biosphere or from anthropogenic releases including sulphates, nitrates, hydrocarbons, soot and so on. The continental aerosols are strongly influenced by man's activities and include urban and rural aerosols. Dust storms produce another type of continental aerosol. Aerosols with a lifetime of up to several years exist in the stratosphere, the sources of which are volcanic injections, and particles or gases entering the stratosphere via diffusion from the troposphere as well as interplanetary dust entering from space. The most important source

heterogeneous chemical reactions and other multiphase processes.

a. continental aerosol - a main component of which is mineral dust;

b. maritime aerosol - a main component of which is sea salt; c. background aerosol - aged accumulation mode aerosol.

studied.

**2.1. Aerosol** 

**2.2. Aerosol types** 

different regions, the main types are:

**2. Background knowledge** 

**Figure 1.** Schematic of chemical and transport processes related to atmospheric composition.

Modelling for instance is important to synthesize and test theoretical understanding of atmospheric chemistry. Computer models (such as chemical transport models) are used. Numerical models solve the differential equations governing the concentrations of chemicals in the atmosphere. They can be very simple or very complicated. One common trade off in numerical models is between the number of chemical compounds and chemical reactions modelled versus the representation of transport and mixing in the atmosphere. For example, a box model might include hundreds or even thousands of chemical reactions but will only have a very crude representation of mixing in the atmosphere. In contrast, 3D models represent many of the physical processes of the atmosphere but due to constraints on computer resources will have far fewer chemical reactions and compounds. Models can be used to interpret observations, test understanding of chemical reactions and predict future concentrations of chemical compounds in the atmosphere. One important current trend is for atmospheric chemistry modules to become one part of earth system models in which the links between climate, atmospheric composition and the biosphere can be studied.
