**1. Introduction**

In the industrialized society air pollution has become a major concern for the environment and public health. Pollutants such as particulate matter and harmful chemicals are causing disease or death to humans and other living organisms and strongly affect ecosystems (e.g. acidifica‐ tion, eutrophication). Although significant progress has been made in recent years in tackling the emissions of some air pollutants (e.g. sulphur dioxide), a large fraction of the population is still exposed to excessive concentrations of certain air pollutants, in particular volatile organic compounds, particulate matter, and ammonia. Therefore, environmental standards are continuously being raised and more effective technologies for depollution of gas streams are needed.

Air non-thermal plasmas (NTPs) are strongly oxidizing environments and thus useful means for the activation of advanced oxidation processes for air and water remediation. NTPs are used for the generation of ozone as an important oxidant for water or air cleaning and the removal of dust from flue gases in electrostatic precipitators [1, 2, 3]. Within the last years deodorization by means of NTP or NTP-supported methods has reached the level of com‐ mercialisation.

The term plasma denotes an ionised gas containing free electrons, ions and neutral species (atoms and molecules) characterized by collective behaviour. Often referred to as the "4th state of matter", plasmas have unique physical and chemical properties distinct from solids, liquids and gases. They are electrically conductive, respond to electromagnetic fields, contain chemically reactive species as well as excited species and emit electromagnetic radiation in

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various wavelength regions. Plasmas are generated artificially by supplying energy to gases, liquids or solids.

In principle, gaseous plasma depollution can be done by an increase of the gas enthalpy in socalled "translational" plasmas, which are non-equilibrium plasmas (electrons and heavy particles have different mean kinetic energies) but with gas temperatures reaching several thousand K. Plasma torches, arcs, arc jets or gliding arcs are examples for such plasma-based incineration sources [4]. However in the following we will focus on non-thermal plasmas which stays at moderate gas temperatures, also referred to as "cold" non-thermal plasmas. Another approach, which is also excluded in this chapter, is the electron beam injection, which is under development for very large combustion facilities (coal fired power plants) [5].

The most common method of producing "cold" non-thermal plasmas for technological applications is based on the application of an electric field to a gas. If the applied field exceeds a certain threshold value (breakdown field strength) a gas discharge and thus plasma are generated. The specific feature of so-called non-thermal plasmas is that most of the coupled energy is primarily released to the free electrons which exceed in temperature that of the heavy plasma components (ions, neutrals) by orders of magnitude. Thus, strong non-equilibrium conditions are achieved in which the gas temperature remains nearly at or slightly above room temperature ("cold") while "hot" electrons initiate chemical processes resulting eventually in the oxidation of pollutants.

This chapter is meant to provide some insight into the application of NTP for air remediation. After a brief introduction to summarize the principles of NTP generation, the main aspects of discharge physics and plasma chemistry involved in air treatment are described and discussed. Special focus is on the removal of volatile organic compounds (VOCs). The two major types of plasma sources for such applications, namely dielectric barrier discharges and corona discharges, are described in the next section. Various aspects of plasma generation and the ensuing chemical processes will be discussed in two separate sections based largely on work carried out by the Authors in Padova and in Greifswald. Finally, some conclusions and perspective outlook on the field are given.
