THE PHYSICS AND CHEMISTRY OF THE ATMOSPHERE
Extracts from: Physique et chimie de latmosphère edited by Robert Delmas, Gérard Mégie and Vincent-Henri Peuch and published by Belin
Atmospheric dynamics and transport
Matter-radiation interactions and radiative transfer
Atmospheric aerosols and heterogeneous chemistry
Anthropogenic and natural emissions and depositions
The chemistry of the stratosphere
The chemistry of the troposphere
The polluted boundary layer
Paleoenvironments and ice core data
The role of atmospheric chemistry in global changes
Measurement principles and instruments
Regulation and management of the atmosphere
The use of the expression 'atmospheric chemistry' to encompass the entire range of processes that control the composition of the atmosphere is an oversimplification, considering that chemical reactions are only some of the many complex mechanisms that determine its composition.
To understand the atmospheres chemical composition and the way it evolves, a knowledge of atmospheric dynamics and the impact of solar radiation on the atmosphere is required. The following basic notions of atmospheric physics and chemistry are necessary for a full understanding of our atmospheric system: atmospheric circulation, radiation and both chemical and photochemical reactions in homogeneous and heterogeneous phases. To these should be added an understanding of atmospheric aerosols, the mechanisms of emission and transferral across interfaces and the quantification of the principal sources of the minor components of the atmosphere. In atmospheric chemistry we in fact distinguish between the chemistry of the troposphere, with its oxidising capacity and that of the stratosphere, where the main issue is the ozone layer.
The field of atmospheric chemistry is also implicated in several contemporary issues, in particular the way human activities affect the evolution of the chemical composition of the atmosphere. Pollution influences the atmosphere in several ways. We need only think of its impact on the atmosphere itself and thence the climate (ozone layer, oxidising capacity, climate change), or again of its impact on the biosphere. Any impact on the atmosphere and the climate in turn effects urban and industrial pollution at the local and regional levels, paleo-environments (or what the past can teach us about the future) and the way atmospheric chemistry is related to climate change and the other large-scale effects of atmospheric pollution. If we could control the state of the atmosphere we could influence ecosystems and our own health.
The study of atmospheric chemistry is highly complex, requiring as it does laboratory work (chemical kinetics and spectroscopy), field work (involving observation of both dynamics and chemistry on different scales), observation networks, measurements taken on a very large scale from aircraft or satellites and , lastly, numerical modelling for analysing all the data and producing forecasts. Our tools for investigation include instruments and methods for measuring atmospheric pollutants, computer modelling and research strategies that combine measurements and models on different scales of time and space.
Our galaxy is probably 10 billion years old and our solar system only around half that (4.6 billion). After the formation of the Earth and its atmosphere, as temperatures gradually dropped to less than 100 °C at the surface, water (H2O) began to condense and form the oceans. Liquid water started to appear on the surface of the Earth about 3.8 billion years ago and the oceans reached their present volume after a relatively short period. Once liquid water was present, several gases began to dissolve into it, so that a large proportion of hydro-soluble gases disappeared from the atmosphere. The primitive atmosphere was then composed principally of nitrogen dioxide (N2), which has a very low solubility in water (0.029 g/L1). This atmosphere must also have contained a number of compounds in limited quantities, such as carbon dioxide (CO2), methane (CH4, formed by a reaction between degassed hydrogen and CO2) and ammonia (NH3), but contained no oxygen molecules (O2). This only began to accumulate in the atmosphere about 600 million years ago, when its rate of production began to exceed its rate of consumption by the reduced substances. If we consider the quantity of oxygen produced since the appearance of primitive photosynthesis, 96% has remained stored in the Earth's crust (58% in the form of ferrous oxides and 38% in the form of sulphates). Free oxygen, in its molecular form in the atmosphere and in the oceans, therefore only represents 4% of this global production.
The appearance of dioxygen in the atmosphere was the decisive event which allowed the biosphere to evolve by initiating new processes of aerobic biochemistry. The photolysis of O2 in the stratosphere (above an altitude of 15 km) led to the formation of the ozone layer which, through its capacity for absorbing ultraviolet rays, considerably reduced the amount of ultraviolet radiation reaching the surface of the Earth, thus enabling the development of multi-cellular organisms. The first multi-cellular organisms found in the sediments of the oceans date from 680 million years ago, but plants only appeared at
400 million, less than one 10th of the age of the Earth.
The chemical composition of today's atmosphere is mainly regulated by the biosphere and in particular the proportion of carbon dioxide. Our Earth is the only one of the planets of the solar system whose average temperatures allow for the presence of liquid water and the development of life forms. Were it not for the atmosphere, the Earth's radiation budget (a function of the distance from the Sun and the planets albedo) would result in a temperature of 255°K (or 18°C). The water vapour and carbon dioxide in the atmosphere produce a natural greenhouse effect which raises this temperature by 33°K, bringing average surface temperatures up to 288°K (+15°C) which allows water to exist in all three phases (see chapters 2 and 10). It is clear that biological systems developed due to a process of evolution in which the greenhouse effect and the presence of the ozone layer determine the fundamental equilibrium of the planet and allow for the existence of life as we know it. So we can see that any disruption to this equilibrium by human activities, such as an alteration to the ozone layer or a reinforcing of the greenhouse effect, are major issues for the global community which have justified the development of this young science, no more than 20 years old, known as atmospheric chemistry.
Atmospheric dynamics is the study of the movements of the Earth's atmosphere to determine the physical laws which govern them. These laws make up a set of constraints which control atmospheric movements capable of transporting physical and chemical properties from one point in the atmosphere to another (convection is one example). Transport is what displaces the sources and sinks of the chemical species in the atmosphere: without transport the chemical composition of the atmosphere at a local level would tend towards photochemical equilibrium, where the production and destruction of species would be balanced. It is also true that the time constants which characterise chemical reactions are partly determined by the distribution of pressure and temperature which are themselves subject to the laws of atmospheric dynamics. Dynamics thus play a major role in the distribution of chemical species in the atmosphere. Inversely, the distribution of the different components of the atmosphere together with their radiative characteristics have an impact on atmospheric dynamics.
Atmospheric dynamics and transport are characteristic of two layers of the atmosphere, the troposphere and the stratosphere.
Atmospheric movements are governed by three principal forces: gravity, the pressure gradient force and the Coriolis force. The laws governing fluid flows in the atmosphere follow the principles of conservation of mass , the amount of movement and energy, which are valid for isolated systems. The presence of water in all its forms (vapour, liquid and solid) plays an important role in atmospheric dynamics. Depending on the space-time scale of the atmospheric phenomena studied, the equations expressing atmospheric fluid flows can be simplified and modified. On a large scale, the vertical distribution of the atmosphere's mass is determined by an equilibrium between the force of gravity and the vertical component of the pressure gradient force (this is known as the hydrostatic equilibrium); whereas horizontally, the Coriolis force and the pressure gradient force tend to cancel each other out (which is known as the geostrophic equilibrium). From these geostrophic and hydrostatic balances which are valid for large-scale phenomena we can deduce the thermal wind relationship which is a combination of the vertical wind gradient and the horizontal temperature gradient. Circulation in the troposphere plays an important role in these processes. Fluctuations around the mean state over time cause perturbations and blockages in the mid-latitudes such as the monsoons of the tropical regions. Convection phenomena can cause unstable buoyancy. In the first kilometre above the surface, flow is modified by friction due to boundary layer phenomena. The stratosphere contains practically no water and has neither convection nor boundary layer. In addition, chemical processes are particularly favoured at this level by stratospheric general circulation, tropical and extra-tropical dynamics and exchanges between the stratosphere and troposphere.
Electromagnetic radiation in the ultraviolet, visible and infrared wavebands plays a fundamental role in many atmospheric processes. It provides the initial source of energy for different physical mechanisms and so, quite logically, most climate change scenarios involve radiated energy. In addition, photochemical reactions depend on the availability of electromagnetic radiation at the appropriate frequencies. Inversely, chemical mechanisms influence the distribution of many atmospheric components, while the way they interact with radiation through absorption, emission and diffusion strongly influences the climate system. Moreover, interaction between matter and electromagnetic radiation in the broad sense is the basis for remote sensing and certain in situ techniques for measuring gaseous chemical species and aerosols.
Most of the species emitted in the atmosphere are eliminated as a result of chemical transformation. The atmosphere is an oxidising medium and these transformations therefore lead, for the most part, to gradual oxidation of the elements concerned (from carbon into CO2, from hydrogen into H2O, from nitrogen into HNO3, from sulphur into H2SO4 and so on). The oxidising species whose budget is most affected is molecular oxygen, O2. As far as reaction mechanisms are concerned, this atmospheric oxidation involves a series of complex stages, particularly catalysis and many species. It would be very difficult to give a complete description of all the stages. Many different processes are involved: radical oxidation in the gaseous phase initiated by solar radiation, oxidation in the aqueous phase inside cloud droplets, heterogeneous chemistry at the surface of aerosols (particles in suspension in the atmosphere), etc. In addition these transformations involve extremely variable timescales (and therefore variations in space) ranging from a fraction of a second for the most reactive species to several years for the least reactive.
Some concepts of physical chemistry will help, firstly, to understand how the chemical transformations take place and secondly, to identify the principal factors that control their speed. These concepts concern:
Aerosols are some of the principal components of the atmosphere. They play an important role in the Earth's radiation budget and can also cause a reduction in atmospheric visibility. In addition, very high levels of fine particles in urban and peri-urban atmospheres have a major impact on the health of the population, probably causing hundreds of thousands of hospitalisations and tens of thousands of deaths in Europe every year. These high levels also cause deterioration to historic monuments as their acidity attacks limestone.
Considering the extent of their effect on health and the environment, it is clearly essential to implement policies adapted to local and regional conditions in order to monitor and reduce aerosol emissions. This can only be done with greater knowledge of the properties of atmospheric aerosols, their sources of emission and the transformations that their particles undergo during transport through the atmosphere. To understand these phenomena we need more information on the physical and chemical properties of atmospheric aerosols and the mechanisms involved when a population of particles evolves in the atmosphere, from formation to deposition.
The periodic table of elements lists around a hundred chemical elements, more than a score of which are essential for life. Six elements, C, H, O, N, S and P are the major constituents of living tissue and make up 95 % (mass) of the biosphere. Even if living organisms influence the circulation of certain heavy elements, the biogeochemistry of life remains principally the chemistry of the six principal light elements. The manner in which these six circulate in the environment (and particularly through the atmosphere) characterise their biogeochemical cycles. The most important cycles concern water (H2O), carbon and nitrogen, to which we can add the sulphur cycle. Phosphorus is of less importance, as this element has no known gaseous phase in the atmosphere. These biogeochemical cycles determine the concentrations of trace elements in the Earths natural atmosphere. How much of them is present in the atmosphere depends on the processes governing emission into and elimination from the atmospheric reservoir (sources and sinks respectively). These processes are of many different types, essentially physical in the case of water, biological and chemical in the other cases (carbon, nitrogen and sulphur).
At present, human activity is significantly disrupting the biogeochemical cycles of all the elements. Any study of the impact of these activities must use the physics of the exchanges of matter between the ground and the atmosphere, which in turn determines the mechanisms of emission and deposition. The minor components of the atmosphere primarily have natural sources and those created or modified by human activity (anthropogenic sources). Global budgets for the different sources of carbon, nitrogen, sulphur and aerosol compounds are calculated using various methodologies from which emissions can be mapped, an essential step in any attempt to model atmospheric chemistry.
The stratosphere extends from the tropopause (whose altitude can vary between 7 km and 16 km from the Poles to the Equator), up to the stratopause, at an altitude of about 50 km. This region of the atmosphere is stable and vertically stratified, because of the positive temperature gradient in relation to altitude. The reason for this phenomenon is the warming caused by what is commonly known as the stratospheric ozone layer, which contains almost 90% of the total quantity of ozone in the Earth's atmosphere. Although the concentration is relatively weak, being only a few molecules of ozone per million molecules of air, stratospheric ozone is of fundamental importance because this compound is essential for maintaining life on the Earth's surface. By absorbing solar radiation between 240 and 320 nm, ozone filters out the UV-B ultraviolet waves that would otherwise destroy the molecular structures of living organisms. Chemical mechanisms control the natural balance of ozone in the stratosphere and these are easily disrupted by human activities.
The troposphere differs from the stratosphere by its proximity to ground emission sources, so that to understand the way in which its chemical composition evolves we must consider the sources of emission, the frequent redistribution of species caused by meteorological phenomena unique to the troposphere and the significant reduction in UV radiation for wavelengths shorter than 300 nm. The chemical composition of the lower levels of the atmosphere can be seen to vary widely, as do the meteorological parameters (temperature, wind and humidity). Analysing the spatial and temporal variability in the distribution of the minor species reveals the reactivity of the tropospheric chemical system. The length of time that surface gases (such as oxides of nitrogen, or halogen and carbon compounds) reside in the atmosphere, whether emitted naturally or through human activity, depends primarily on the chemical reactivity of the troposphere. These compounds, which are affected by the oxidising power of the atmosphere, also play an important role in stratospheric chemistry and affect the planets climate by contributing to its radiation budget. Any description of the tropospheric chemical system must also take into account the presence of water in its condensed form and consequently those chemical reactions occurring in the aqueous phase. Another factor is that solid particles of micrometre and sub-micrometre size modify the reactivity of the atmosphere and are responsible for the elimination of certain reactive gases from the atmospheric medium.
The boundary layer is the slice of atmosphere in which we live. It is between one and three kilometres thick and it is here that most pollution resides. It is directly or almost directly influenced by the emission of pollutants that we inject into it. Unlike the impact on our climate of greenhouse gas emissions, which take tens or hundreds of years to become apparent, the impact of pollutants on air quality in the boundary layer is much more rapid: from a few hours to a few weeks. These times depend on the physical and chemical processes that act on the pollutants: dispersion, chemical transformations and deposition on the ground or vegetation.
The air we breathe is not always as good as we would like. The gases or particles it contains sometimes have mechanical or chemical properties that render them toxic. This type of pollution, with its direct effect on human health, is a result of the economic development of the most industrialised countries. With the exception of fire pollution', which has been present since prehistoric times, atmospheric pollution has only existed in Europe since the 19th century. It was not until the middle of the 20th century that our society became aware of this danger. At that time, industry was discharging such quantities of pollutants into the atmosphere that very serious episodes were occurring, such as the death of a thousand people in London in 1952. Since then, the international community has sought to monitor air quality and to control the emission of pollutants. For certain types of pollution, success has been undeniable. For others, we still have a long way to go before we can breathe air as good as we would like.
Many vestiges of the past testify to the condition of our environment at different moments in history, such as marine sediments and corals, lake beds, tree rings and continental pollen records. In this list, ice core data have a place apart, providing as they do the widest range of information about our climate and the chemical composition of the Earth's atmosphere in the past. An important reason for studying air bubbles trapped in ice is the fact that scientists only began documenting the chemical composition of our atmosphere quite recently, when it had already been considerably perturbed by human activity. Amongst greenhouse gases, carbon dioxide has only been measured since 1957, nitrogen protoxide since 1978 and methane since 1983. Proportions of anthropogenic aerosols in the atmosphere such as sulphates linked to sulphur dioxide emissions have also been studied only recently (in the last twenty or thirty years). We need to know more about the state of our atmosphere before it was modified by human activity, if we wish to understand (and forecast) the way this fragile equilibrium reacts to human activity. Ice cores also help us understand the complex role played by the atmosphere in the great climate changes of the past, which will help us to anticipate correctly the extent of climate change we can expect over the coming decades.
The environmental parameters stored in the ice are especially valuable because of the timescales they cover and the extent of the information they contain. Deep drilling into the polar icecaps of the Antarctic and Greenland has revealed the history of our atmosphere over several thousand years and has provided crucial information on the link between the chemical composition of the atmosphere (concentrations of greenhouse gases and aerosols, aerosol speciation) and past climates. Other ice samples, covering the last few centuries and taken from the Antarctic, Greenland and the Alps, have made it possible to reconstruct the history of atmospheric pollution by greenhouse gases and aerosols. For certain chemical species whose emissions, transformations and depositions are still not fully understood, the information gleaned from ice cores allows us to test certain hypotheses.
The impact of human activity on the chemical composition of the atmosphere can now be clearly demonstrated through observation. Disruption is occurring more rapidly now than at any other time in the history of the climate. However, we are still a long way from understanding the cycles of all the pollutants and the complex exchanges occurring between the atmosphere, vegetation and the oceans. What exactly are these chemical constituents which, despite their extremely low concentrations in the atmosphere, manage to upset our planets energy budget on a global scale? Carbon dioxide, CO2, is often blamed, understand ably, for the whole issue of climate change. But we should not forget the many other gases, some of them reactive, as well as particles in suspension in the atmosphere which also play an essential role in these perturbations. Amongst these gases, tropospheric and stratospheric ozone seem to play a key role by participating directly in the disruption of the energy budget and by affecting the oxidising power of the atmosphere. Understanding these gases and particles and their effect on radiation, helps us bridge the gap in our knowledge of two major problems affecting the environment: local pollution and global climate change. The challenge before us today is clearly to understand how the massive emission of pollutants by industrial and agricultural activities or the smoke from forest fires will modify the composition of the atmosphere over the coming decades.
The Earth's atmosphere contains a large number of compounds from many different sources whose concentrations vary widely over space and time. The so-called primary compounds (SO2, NOx, CO, COV, metals, particles etc.) are emitted by natural or anthropogenic sources. They are transported, dispersed and undergo chemical transformation or phase changes in the atmosphere, producing secondary compounds under certain meteorological conditions. They are then deposited in either dry or humid form. All these processes and the pollutants that result upset the balance of the Earths atmosphere and have an impact on human health, the ecosystem and even materials.
In the 1950s, smog (a contraction of smoke and fog) was very apparent in heavily polluted areas, which helped to raise awareness and warn of the dangers of air pollution. The link between atmospheric pollution and public health was demonstrated by several epidemiological studies. From this moment on, air protection policies and national and European legislation began to be developed and harmonised.
Measuring the impact of atmospheric pollution on public health requires suitable indicators, which means developing operational tools for monitoring and forecasting. Certain toxic substances found in the atmosphere in very low concentrations and not yet covered by any legislation require new measurement techniques.
It is therefore urgent that we acquire precise, specific, high-resolution measurements for a wide range of concentrations. We need to adopt quality-control policies involving certain essential phases such as the calibration and evaluation of instruments and the organisation of comparison campaigns. For a better understanding of distributions over time and space we also need measurements using appropriate tools, not just at ground level but also at different altitudes. Various measurement principles and techniques are available for quantifying atmospheric pollutants (either in situ or by remote sensing from space).
As in the sphere of meteorology, numerical modelling is today one of the most important tools for research and applications in the field of atmospheric physical-chemistry. What is a numerical model? We might suggest the following definition: a set of physical and chemical facts or hypotheses, expressed in the form of mathematical equations or empirical relationships and resolved in a manner which is usually approximate with the help of appropriate computational resources. Although numerical models for the atmosphere have been theoretically possible for almost a century, it was difficult to use them until recent advances in computing power. In the field of atmospheric physical chemistry, three-dimensional models have only become available over the last 15 years or so. The above definition gives a glimpse of one of the most important goals of modelling which is to verify the validity of hypotheses and of state-of-the-art knowledge, by comparing computer simulations with observed reality. Models can be seen as extremely important tools for integrating and extending our knowledge by enabling us to identify our chief uncertainties: in which atmospheric regions do the models show the most significant defects? Is there any way of linking these defects to the simulation of a particular process? And so on. There are several reasons for continuing to develop numerical models that are ever more complete, detailed and therefore realistic. Moving from equations to the discretised digital codes that the computer is based on three important processes: advection, the temporal resolution of the systems of chemical equations and the representation of transport and mixing caused by convective processes. One application for the models is the resolution of questions about the atmospheric environment, in particular for forecasting the "chemical weather" by assimilating data and developing coupled chemistry-atmosphere models.
by Philippe Ricaud (LA), Christain Elichegaray (ADEME), Céline Mari (LA), Jean-Louis Brenguier (CNRM), Philippe Ciais (LSCE), Jean-Pierre Pommereau (SA), Jean- Baptiste Renard (LPCE)
The observation and study of atmospheric chemistry remains a huge challenge for scientists. A challenge because the public is increasingly sensitive (and with good reason) about today's air quality and tomorrow's climate. Huge because atmospheric chemistry concerns the entire Earth, it changes rapidly and the chemical and meteorological phenomena occur on scales ranging from the molecular for chemical reactions, to thousands of kilometres, for the intercontinental transport of pollution. No current method can satisfy this need for continual and simultaneous observation of all the relevant parameters on a global scale. Exploring the chemistry of the atmosphere means coordinating information from several observation platforms: this is the experimental strategy.
New experimental strategies were developed in the 20th century which aimed to integrate selected measurements collected by observation networks or from specially equipped sites (using research or commercial aircraft, ground stations at both sea level and high up, marine stations, etc) as well as airborne or satellite remote sensing observations and numerical modelling. These experimental strategies vary according to whether the objectives concern public health (monitoring air quality) or pure science (studying a poorly understood process). The two approaches are far from exclusive, however and can often be complementary.
Any experimental strategy starts with selecting the most suitable place and time for observing the phenomenon to be studied, in other words when the signature of the phenomenon in the atmosphere is most obvious. For example, to study peaks of pollution we would choose large urban areas in the summer, because if we tried to detect an increase in concentrations of atmospheric carbon at oceanic sites far from any direct anthropogenic emissions, we would need to gather continual measurements over several years. It may therefore be said that an experimental strategy relies also on scientific knowledge handed down from past experience and discoveries. Once the time and place have been chosen, the strategy can be defined: the scientists choose their instruments and observation systems in terms of their complementarity and the scientific objective, rather as a coach chooses his team for a particular sporting challenge!
This, then, is the scientific approach which, by skilfully combining the right measurements and ensuring their coherence, helps increase our knowledge and improve our forecasting of chemical weather and the climate. Experimental strategies will obviously have to evolve rapidly in response to changes in national and international regulations, improvements in measurement techniques and numerical models and above all the new issues which we will certainly be facing in the future.
New concepts and regulatory instruments are being developed to try and control the atmosphere and various actions are being implemented at a national or international level. Scientific knowledge and experience of how the atmosphere works as a machine is needed to help decision-makers and guide actions to preserve air quality.