The stratospheric ozone layer results from the photolysis of molecular oxygen by ultraviolet (UV) solar radiation in the high atmosphere. This large atmospheric layer is stable and, therefore, affects the general atmospheric circulation by decreasing significantly the vertical motions of air parcels. In addition, ozone protects the Earth from harmful UV radiation. Therefore, its destruction by anthropogenic activities may lead to public health impacts. This chapter presents first some fundamentals of atmospheric chemical kinetics (i.e., the speed at which chemical reactions occur in the atmosphere), which are needed to understand the processes leading to the presence of the ozone layer. These notions are also needed to understand the formation of gaseous and particulate pollutants, which is presented in the following chapters. Next, the processes that govern the ozone layer are described in terms of its natural formation and its destruction by man-made substances. Finally, the public policies introduced to address the protection of the stratospheric ozone layer are summarized.

Population exposure to air pollution occurs mostly near the Earth’s surface. Furthermore, most air pollution sources are located near the Earth’s surface (some exceptions include tall stacks, aircraft emissions, and volcanic eruptions). Therefore, the meteorological phenomena of the lower layers of the atmosphere are the most relevant to understand and analyze air pollution. The part of the atmosphere that is in contact with the Earth’s surface and is affected by it is called the atmospheric planetary boundary layer (PBL). This chapter describes the dynamic processes that take place within the PBL. Those include, in particular, turbulent atmospheric flows and heat transfer processes, which affect air pollution near the surface. Those processes are often referred to as “air pollution meteorology,” because they are the most relevant to air pollution. The major equations governing these processes are presented. A more detailed description of the PBL is available in books such as those by Stull (1988) and Arya (2001).

Air pollution is directly affected by various aspects of meteorology, such as winds, which transport pollutants (in some cases over long distances); turbulence, which disperses air pollutants; solar radiation, which initiates photochemical reactions leading to the formation of ozone, fine particles, and acid rain; high pressure systems, which are conducive to air pollution episodes because of their calm and sunny conditions; and precipitations, which scavenge air pollutants and transfer them to other media (e.g., acid rain). Therefore, it is essential to understand general meteorological features before addressing in detail the processes that are specific to air pollution. This chapter presents first some general considerations on the atmosphere (chemical composition, pressure, and temperature). Next, the main aspects of the general atmospheric circulation are described.

Atmospheric dispersion is a very important process in air pollution. The use of tall stacks to minimize the impacts of air pollutant emissions reflected the saying that “the solution to pollution is dilution.” Although this saying turned out to be wrong for several reasons, including the cumulative effect of a large number of individual sources and the formation of secondary pollutants at large regional scales, atmospheric dispersion is nevertheless one of the key processes that govern air pollution levels. This chapter presents the fundamental processes of atmospheric dispersion, their theoretical basis, as well as the advantages and shortcomings of different types of atmospheric dispersion models.

Air pollutants may be transferred via dry and wet deposition to other media such as soil, surface waters, vegetation, and buildings. These pollutants may then contaminate these surfaces and have adverse impacts on ecosystems, vegetation, and the built environment. In addition, some chemical species that do not have any adverse health effects via inhalation may become toxic via bioaccumulation in the food chain and subsequent ingestion. This chapter describes briefly the impacts of air pollutant deposition on ecosystems, agricultural crops, and buildings, as well as the indirect adverse effects on human health via the food chain.

The health effects of air pollution are difficult to characterize because of the large number of air pollutants present in the atmosphere and the relatively small contribution of their health effects compared to all other causes. In addition, air pollution does not affect all people in the same way. Some persons are more sensitive than others: for example, those suffering from asthma, chronic obstructive pulmonary disease (COPD) or cardiovascular problems, the elderly, and children. Also, some individuals are more vulnerable than others: those include, for example, workers and residents who tend to be in locations where air pollution exposure is greater than average. This chapter describes first how adverse health effects of air pollution can be identified and quantified using toxicological and epidemiological studies. Next, methods commonly used to conduct health risk assessments related to air pollution are presented. The use of such information to set up air quality regulations is presented in Chapter 15.

The atmosphere interacts with the Earth’s surface. Thus, air pollutants may be transferred toward surfaces and emitted (or reemitted) from surfaces toward the atmosphere. Atmospheric deposition processes are important because (1) they impact the atmospheric lifetime of air pollutants and (2) they may lead to the contamination of other environmental media. Processes of emission and reemission may contribute significantly to the atmospheric budget of some pollutants and it is, therefore, essential to take those into account. This chapter describes the mechanisms that lead to atmospheric deposition of pollutants, either via dry processes (dry deposition) or via precipitation scavenging (wet deposition). Emissions of particles by the wind (aeolian emissions), waves, and on-road traffic are also described.

Atmospheric particles and, in particular, fine particles are one of the major components of air pollution. They lead to significant adverse health effects, degrade atmospheric visibility, are involved in cloud formation and precipitation, and play a role in climate change. Particles have various sizes, ranging from ultrafine and fine to coarse, and different chemical compositions, since they may contain a large number of different inorganic and organic species. In addition, particles typically include a primary fraction, which has been emitted from various sources directly into the atmosphere, and a secondary fraction, which has been formed in the atmosphere via chemical reactions from precursor gases. The secondary fraction generally dominates the mass of fine particles. Therefore, the development of efficient emission control strategies to decrease the ambient concentrations of atmospheric particles is a challenging task, because it requires identifying the numerous sources of atmospheric particles, including those of the gaseous precursors of the secondary fraction, in order to properly characterize the processes that govern particulate matter (PM) formation and understand the complex relationships that link gaseous precursors and the secondary PM fraction.

Solar radiation is essential to life on Earth and is one of the major factors governing the atmospheric general circulation. Furthermore, solar radiation plays a major role in air pollution, since it leads to photochemical reactions when its radiative energy breaks apart some molecules. Then, these photochemical reactions initiate chemical and physico-chemical transformations that contribute to various forms of air pollution, including ozone, fine particles, and acid rain. In addition, the Earth emits radiation, which may be partially absorbed by anthropogenic greenhouse gases and some fraction of particulate matter, thereby leading to climate change. Finally, understanding how radiation is transferred through the atmosphere is useful to estimate the effect of air pollution on atmospheric visibility. This chapter describes first the radiative transfer processes in the atmosphere, i.e., solar radiation, its absorption by oxygen and ozone in the stratosphere, and its scattering by gases and particles.

Several gaseous chemical species may lead to adverse health effects and, therefore, several of those are regulated. Brief descriptions of those chemical species, including their major sources and atmospheric fate, are presented. Next, the focus of this chapter is on urban and regional pollution, since it corresponds to most of the population exposure to ambient air pollution. The gaseous pollutants that are currently the most relevant at the urban/regional scale in terms of adverse health effects are ozone and nitrogen dioxide. These pollutants are major components of photochemical smog, which results from chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOC) in the presence of sunlight. The fact that photochemical smog precursors such as NOx and some VOC (alkenes) are both producers and destructors of ozone makes the development of efficient strategies to reduce photochemical smog difficult.

Air pollution is due to emissions of pollutants in the atmosphere, which may be natural or of human origin. Thus, in order to understand air pollution, it is necessary to identify, characterize, and quantify those emissions. Furthermore, reducing air pollution requires either eliminating some of those emissions via a change in a product, process, or technology, or reducing those emissions using some control technologies. This chapter describes the main sources of air pollution and the technologies available to control those emissions. First, air pollutant sources are described. Next, the methods used to quantify the corresponding emissions and develop air pollutant emission inventories are presented. Finally, the main technologies used to control emissions of gaseous and particulate air pollutants are described.

This chapter describes the processes that lead to the formation of atmospheric pollutants in clouds and fogs via aqueous chemical transformations. Although the volume occupied by water droplets in the air is small, important chemical reactions occur in clouds. These reactions modify the atmospheric chemical composition and may lead to an increase of particulate mass when clouds and fogs evaporate or to acid rain when clouds precipitate. First, some general considerations on clouds and fogs are presented. Then, aqueous chemistry is addressed. Chemical equilibria and reactions have been described in other textbooks (e.g., Stumm and Morgan, 1995), and the focus here is on the processes pertaining to air pollution. This chapter treats in particular the transformations leading to the formation of sulfuric acid and nitric acid, two constituents of acid rain (if the cloud precipitates), as well as precursors of fine inorganic particles (if the cloud evaporates). The aqueous chemistry of organic compounds concerns mostly the formation of secondary organic aerosols (SOA) and is treated in Chapter 9. Finally, emission control strategies to reduce acid rain are discussed.

The Earth’s atmosphere is composed mostly of molecular nitrogen (N2, 78 % of dry air) and molecular oxygen (O2, 21 % of dry air). It holds also a fair amount of water vapor (H2O), which varies greatly in concentration (ranging from negligible in dry regions to a few % in humid regions) and leads to the formation of clouds and fogs in case of supersaturation. The Earth’s atmosphere also contains carbon dioxide (CO2), which has an average concentration of about 0.04 %. H2O and CO2 are gases that absorb infrared (IR) radiation, but let ultraviolet (UV) and visible solar radiation go through. Since they partially absorb IR radiation emitted by the Earth toward space, these species are called “greenhouse gases” (GHG).

Climate change is a major issue regarding the atmospheric environment. It differs from air pollution in spatial and temporal scales. Climate change is due mostly to greenhouse gases that have long atmospheric lifetimes and, therefore, are distributed relatively uniformly in the atmosphere. To the contrary, air pollution shows great spatio-temporal variability and offers a relatively short response time in terms of the relationship between emissions and atmospheric concentrations. Nevertheless, there are many links between climate change and air pollution. First, some air pollutants contribute to climate change. Second, climate change may affect air pollution. Finally, some sources emit both air pollutants and greenhouse gases, whereas other sources may emit mostly air pollutants or mostly greenhouse gases. Therefore, it is essential to identify all the emissions associated with a given source in order to avoid creating one problem when trying to solve another. This chapter first summarizes in general terms the main aspects of climate change and then describes the interactions between climate change and air pollution.

Improving air quality by decreasing air pollutant concentration levels requires promulgating regulations that protect public health, ecosystems, the agriculture, buildings, and atmospheric visibility. Then, public policies must be implemented to design, apply, and evaluate emission control strategies to meet the regulatory standards. In this chapter, the general approach for the development of regulations to protect public health via ambient air quality standards is described first. These regulations and the implementation of the associated public policies differ among countries. Those used in the United States and in France are presented here comparatively to illustrate slightly different approaches. Finally, approaches used at the national and international levels to regulate atmospheric deposition and global atmospheric issues (i.e., destruction of the stratospheric ozone layer and climate change) are presented.