Atmospheric Chemistry

From the Surface to the Stratosphere

Grant Ritchie

• Editore: WSPC (EUROPE)
• Anno: 2017
• ISBN:
  9781786341785
• Argomento: Ciencias físicas
• Categoria: Química física y teórica

Understanding the composition and chemistry of the Earth's atmosphere is essential to global ecological and environmental policy making and research. Atmospheric changes as a result of both natural and anthropogenic activity have affected many of the Eart

Chapter 1

The Physical and Chemical Properties of the Earth’s Atmosphere

Introduction

The Earth’s atmosphere behaves as a dynamic fluid which can support a variety of motions on length scales spanning a few metres to the circumference of the entire planet. The atmosphere is also dynamic in the sense that its chemistry and physics are influenced not only by the photochemistry and physics induced by solar radiation, but also by interactions with the biosphere (all living organisms), the lithosphere (the crust and upper mantle), the hydrosphere (all water), and the cryosphere (a sub-division of the hydrosphere concerning only ice). Such interactions have important consequences, not least for the Earth’s climate.

99.9% of the present-day atmosphere comprises N₂, O₂, the noble gases (mainly Ar), and H₂O. All of these species apart from water have a constant abundance, with water present in variable amounts depending upon the atmospheric conditions, latitude and altitude. The remaining 0.1% of the atmosphere consists of trace

ases, including CO₂, CH₄, and others. Table 1.1 lists the abundances (expressed as mole fractions) and atmospheric lifetimes of the most important trace gases.¹ While the concentrations of trace species are many orders of magnitude smaller than those of N₂ and O₂, such species are present in chemically significant quantities, and have an important influence on many atmospheric processes. For example, the OH radical is the most important oxidant in the lower regions of the atmosphere, despite only being present at a concentration of ca. 10⁶ cm⁻³ (or mixing ratio of 10⁻¹³ at standard temperature and pressure, STP).² In addition to their chemical role, trace gases can also affect the physical properties of the atmosphere. The most important example is provided by CO₂, a species that has a large effect on Earth’s radiation budget despite its modest abundance of ∼400 ppmv. The chemical and physical properties of OH and CO₂ will be the subject of detailed discussion in Sections 4.2 and 2.3, respectively.

Table 1.1The abundances and lifetimes of the major constituents of the atmosphere.

Constituent	Mole fraction	Lifetime (yr)
N2	0.781	1.6 × 10^7
O2	0.209	9 × 10^3
Ar	9.3 × 10^−3	4.5 × 10^9
CO2	400 ppmv	5
H2O	(0–4)×10^−2	5 days
CH4	1.8 ppmv	10
H2	550 ppbv	4
N2O	320 ppbv	150
CO	40–200 ppbv	0.2
O3	20–80 ppbv	0.05
C2H6	1 ppbv	0.2
SO2	100 pptv	5 days
NO2	100 pptv	2 days

Trace gases have both natural (e.g. biogenic, geologic, and oceanographic) and anthropogenic (e.g. fossil fuel burning and agriculture) sources, with the result that they may not be distributed evenly throughout the atmosphere. The physical and chemical lifetimes of each trace species compared with the timescale of mass transport within the atmosphere are important in determining the degree of spatial inhomogeneity. Such considerations determine how far industrial emissions are likely to spread from their source. The factors that determine the lifetimes of species in the atmosphere are considered later in this chapter in Section 1.6. Before then, we will consider the physical structure of the atmosphere in terms of the variation in atmospheric pressure and temperature with altitude. We will investigate the role of moisture in the atmosphere, the factors governing atmospheric stability and transport, and then turn our attention to the kinetics pertinent to physical and chemical processing in the atmosphere. In Chapters 3 and 4, we will consider the chemistry of the two lowest regions of the atmosphere, the troposphere and the stratosphere, in more detail. While much of the chemistry of the atmosphere occurs in the gas phase, particulate matter in the form of aerosols are also an important atmospheric constituent with significant effects on both the chemical and physical properties of the atmosphere. Their effects will be considered in Chapter 5.

1.1 Structure of the Atmosphere

As shown in Figure 1.1, the atmospheric temperature does not simply decrease monotonically with increasing altitude above the surface, but instead displays a much more complex behaviour. The temperature profile is used to categorise various different regions within the atmosphere in terms of a series of layers. The detailed origins of the temperature profile will be considered in Section 1.3.

Figure 1.1(a) The temperature and pressure structure of the Earth’s atmosphere and (b) the definition of latitude.

1.1.1 The Troposphere

The bulk (90%) of the atmospheric mass lies in the troposphere, spanning the altitude range from 0 to 15 km. Within the troposphere, the temperature generally decreases with increasing altitude. In this region, the atmosphere is warmed by absorption of radiation from the Earth’s surface, an effect that diminishes with increasing altitude. The temperature profile within the troposphere results in convection currents that make the troposphere relatively turbulent, and its contents well mixed. For the same reason, the troposphere is the region of the atmosphere that is responsible for weather. As we shall see, the longer term climate, and climate change, are determined primarily by radiant processes near the surface, and are also directly influenced by biogenic and anthropogenic emissions into the troposphere.

The sublayer of the troposphere closest to the Earth’s surface, at altitudes below around 1 km, is known as the planetary boundary layer (PBL). The PBL is the region within which most pollutants are emitted into the atmosphere, from a variety of sources. Pollutants can become trapped within this layer by local weather conditions, with the res...