Review pubs.acs.org/CR
Ambient and Modified Atmospheric Ion Chemistry: From Top to Bottom Nicholas S. Shuman, Donald E. Hunton, and Albert A. Viggiano* Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, Albuquerque, New Mexico 87117, United States References
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1. INTRODUCTION 1.1. Historical Overview and Goals
The knowledge that charged particles exist in the atmosphere dates to the 1700s with Ben Franklin’s discovery that the electrical properties of the atmosphere were the same as those that occurred in the laboratory. The most famous of his experiments is imbedded in the history of the U.S. and science: in 1752, he used a kite to raise a conducting cord into the sky during a storm.1 The cord led to a crude capacitor, which collected charge from the atmosphere, thereby showing that there was an electrical fluid produced. Franklin made this remarkable discovery over a hundred years before the existence of the electron was proposed. The discovery that the Earth has charged particles in space is a direct consequence of Marconi’s first transmission of radio waves from England to Canada in 1901. The waves had traversed the curved Earth over the horizon, a seemingly impossible task. In response to an attempt to explain this phenomenon by invoking the æther, Lodge responded in Nature, “I can assure Prof. Joly that his explanation will not do. The observed effect, which if confirmed is very interesting, seems to me to be due to the conductivity, and consequent partial opacity, of air, under the influence of ultraviolet solar radiation.”2 Just after this statement, Kennelly and Heaviside independently attributed the transmission to reflection due to electrons in the atmosphere, and the relevant layer of the ionosphere (the E-region) is often referred to by one or both of their names. In 1923, Appleton confirmed the existence of the ionosphere by studying the frequency modulation resulting from radio wave propagation at sunset.3 He observed interference between a ground wave and a secondary wave caused by reflection off the ionosphere. That insightful study earned him the Nobel Prize in 1947. While the interaction of radio waves with the Earth’s ionosphere is still an intense area of study, the goal of this Review is not to focus on that aspect directly, but to discuss the chemistry of charged particles in the atmosphere. Current knowledge has come from a wide variety of both laboratory and in situ measurements, and those methods will be addressed briefly. We will discuss production processes, chemical reactions including charge transfer, and loss processes such as mutual neutralization and dissociative recombination. The
CONTENTS 1. Introduction 1.1. Historical Overview and Goals 1.2. Background Basics of the Atmosphere 2. The Exosphere 3. Thermosphere 3.1. Composition and Positive Ion Chemistry 3.2. Ion−Molecule Reactions 3.2.1. O2+ Reactions 3.2.2. O+ Reactions 3.2.3. N+ Reactions 3.2.4. N2+ Reactions 3.3. Airglow and Dissociative Recombination 4. Mesosphere 4.1. Positive Ion Composition and Chemistry 4.2. Negative Ion Composition and Chemistry 4.3. Mutual Neutralization 4.4. Meteoric Smoke 5. Stratosphere and Troposphere 5.1. Acid−Base Chemistry 5.2. Ion-Induced (Mediated) Nucleation 6. Active Chemical Release Experiments 6.1. Background and Historical Perspective 6.2. Atmospheric Tracer Experiments 6.3. Plasma Enhancement Releases − Samarium Chemi-ionization 6.4. Plasma Depletion Experiments − Ionospheric Holes 6.5. Plasma Depletion Experiments − Electron Holes 7. Summary and Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments
© XXXX American Chemical Society
A A B D E E F F G I I J K K M N N O O Q R R R S T U W X X X X Y
Special Issue: 2015 Chemistry in Climate Received: July 1, 2014
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DOI: 10.1021/cr5003479 Chem. Rev. XXXX, XXX, XXX−XXX
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Figure 1. Summary of the composition and conditions of the atmosphere. Regions of the atmosphere are indicated toward the leftmost side of the figure, and are overlapped by the D-, E-, and F-regions of the ionosphere indicated toward the right side of the figure. Ion, electron, and neutral concentrations vary significantly with time of day, season, and latitude, and fluctuate in time; values shown are typical during daytime at midlatitudes. Typical altitude ranges of satellite, rocket, and balloon-borne measurement devices are shown. Altitudes of well-known orbiting objects are shown for context. MSP stands for meteoritic smoke particle; see section 4.4. Data compiled from a variety of sources.5−7,14−21
1.2. Background Basics of the Atmosphere
chemistry of the charged constituents ranges from extremely simple to very complex reaction sequences. Several earlier reviews have covered portions of this same subject matter. Ferguson provided an early review of laboratory measurements relevant to the ionosphere.4 Torr and Torr provided a detailed overview of chemistry in the E- and Fregions as it stood in 1978 and later of metastable composition and chemistry.5,6 Viggiano and Arnold wrote an in-depth review of ionospheric chemistry in 1995.7 In more recent years, Richards has incorporated some updated reaction measurements.8 Pavlov has extensively compiled the literature of laboratory measurements for ionospheric processes as of 2012.9,10 Several researchers intimately involved in deciphering the chemistry of the ionosphere have written personal retrospectives on their careers, which cover the topics through a historical perspective.11−13 This Review includes discussion of contributions from the recent literature and offers a different structure than earlier works: the higher the altitude, the simpler the chemistry of the atmosphere, and therefore we will start the discussion at the top of the atmosphere and proceed downward. As the altitude is reduced, the number of reactions occurring increases significantly, usually building on the chemistry that occurs at higher altitude. Finally, we will finish with efforts to influence the ion chemistry by chemical releases into the atmosphere. Before we go into the details of the chemistry, we start with a brief overview of those aspects of the structure of the atmosphere most significant for an understanding of charged particles.
To understand the ion chemistry in the atmosphere, numerous parameters including temperature, density of neutral reactant species, solar UV flux, and cosmic ray flux must be accounted for. The purpose of this section is to give a brief overview of the most relevant parameters. It is well beyond the scope of this Review to provide enough detail to model the atmosphere or to discuss changes due to season, latitude, solar cycle, auroras, etc. The atmosphere is divided into regions by a variety of conventions. The most common, and that by which much of this Review is ordered, is by temperature. Figure 1d shows a typical temperature profile of the atmosphere. At the highest regions of the atmosphere, called the exosphere and then the thermosphere, neutral species, ions, and electrons are not well described by a single temperature, but temperatures generally decrease with decreasing altitude. Below about 100 km, species become thermalized to one another, and the temperature reaches a minimum at the separation between the thermosphere and the mesosphere, called the mesopause, then slowly increases again with decreasing altitude. There is a temperature maximum at the separation between the mesosphere and the stratosphere (the stratopause), and then decreasing temperature, in large part due to ozone absorbing solar radiation, down to lower altitudes. Finally, the bottom region where we live is the troposphere, where the temperature increases with decreasing altitude from a minimum at ∼10 km. Alternatively, the atmosphere may be divided by the nature of mixing of constituent gases. From the ground to approximately the mesopause, the atmosphere is uniformly B
DOI: 10.1021/cr5003479 Chem. Rev. XXXX, XXX, XXX−XXX
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times-of-flight of reflected waves are recorded. The time-offlight is a function of the altitude at which the wave reflects off of the ionosphere, with reflection occurring at a critical electron density related to the frequency of the radio wave by23
mixed, and the relative concentrations of the long-lived species (O2, N2, Ar, CO2...) are constant at the familiar ratios of air at ground level; thus the region is called the homosphere. Above that altitude is the heterosphere where the air is very thin and molecular diffusion becomes important. Species separate by weight, photodissociation depletes the major species N2 and O2, and the mixing ratios of all species become altitude dependent. Charged particles are also affected by electric and magnetic fields leading to different temperatures for electrons, neutrals, and ions. Already at 100 km, the electron temperature is elevated from the molecular species. At ∼300 km, the ion and neutral temperatures also deviate from each other. As shown below, this has distinct effects on ionospheric chemistry and shows the need for both thermal and nonthermal measurements. The atmosphere can also be divided into layers by maxima in the charged particle density. Figure 1a and b shows the concentrations of positively and negatively charged species versus altitude; note that at all altitudes the net charge of the atmosphere is zero. There are several regions where the density peaks, and for historical reasons these are called the F-, E-, and D-regions with decreasing altitude. The E-region can be more of a bump than a true maximum, and sometimes there are secondary bulges where the E- and F-regions are subdivided, all depending on the particulars of the geophysical conditions. As will be discussed, these density peaks are of high importance to radio communication, determining what frequencies will be refracted or reflected while traversing the atmosphere (Figure 2). Altitudes below the D-region are not labeled in this scheme
ν = 0.009 Ne
(1)
where ν is in MHz and Ne is the critical electron density in cm−3. The question of when a radio wave of a given frequency will or will not reflect or refract off of the ionosphere has motivated much of the research into the ion composition and related chemistry, which largely determines the electron density. The degree of ionization in the different regions is largely, but not entirely, due to solar radiation. Figure 3 shows typical
Figure 3. Typical rate of ion production for the indicated species as a function of altitude. The dashed curve is the sum of all ions produced by cosmic ray impact; because cosmic rays ionize all species with a similar efficiency, the ions are produced in proportion to the concentrations of the parent neutrals.
photoionization production rates of various ions as a function of altitude. The solar flux decreases with decreasing altitude as light is absorbed as it passes through the atmosphere, while the concentrations of neutral species increase dramatically with decreasing altitude. The rate of photoionization (or conversely the attenuation of solar radiation) is the product of these two factors. Total photoionization is small at the highest altitudes of the ionosphere, and increases down to ∼150 km, where it begins to peak due to attenuation of the solar radiation. The ionization in the E- and F-regions is dominated by absorption of extreme UV and X-rays by the dominant species N2, O, and O2, which have large ionization energies of 15.58 (80 nm), 13.62 (91 nm), and 12.07 (103 nm) eV, respectively. These are by far the most ionized regions with densities orders of magnitude above other regions. Radiation with wavelengths below 100 nm (12.4 eV) is absorbed more or less completely by ∼100 km (the mesopause), and below this altitude the charged particle density decreases substantially. There is a peak in the solar spectrum at Lyman-α (121.6 nm, 10.2 eV), which is too low in energy to ionize O2 or N2 and therefore penetrates to lower altitudes. NO has an ionization energy of only 9.26 eV (134 nm), and ionization of NO by Lyman-α is one of two major ionization sources in the D-region, the other being
Figure 2. Variation in typical electron density between daytime and nighttime. Paths illustrate the behavior of radio waves of the indicated frequencies under these conditions.
because all negative charge is in the form of ions, which unlike electrons interact only weakly with radio waves. Significant effort has been made to map out the electron density profile of the ionosphere at varying times-of-day, times-of-year, latitude, solar activity, etc. One important technique involves an ionospheric sounder, or ionosonde, the principles of which date back to the 1920s and are well detailed elsewhere.22 Briefly, radio wave pulses of varying frequency, generally from 0.5 to 20 MHz, are transmitted vertically, and the round trip C
DOI: 10.1021/cr5003479 Chem. Rev. XXXX, XXX, XXX−XXX
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nighttime conditions, and the concentration plateaus at ∼104 cm−3. The He+ concentration follows a pattern similar to that of the H+ concentration. Typically, the He+ concentration is about 1 order of magnitude smaller; however, this is latitudinally and seasonally dependent, and He+ may even be the dominant ion under certain conditions at very low latitudes during periods of high solar activity.20 Helium has an ionization energy of 24.59 eV, much larger than that for other atmospheric (or indeed any) species. He+ cannot be formed via reaction with other molecules. Instead its only atmospheric sources are photoionization of He by high energy solar photons and to a smaller extent electron impact with energetic electrons. He may also be ionized by cosmic rays, but the contribution is so small that it may be ignored. Charge transfer between He+ and other atomic species such as H or O proceeds with a very small rate constant due to the large energy differential and lack of degrees of freedom in which to distribute that energy. These reactions could proceed much more readily via a three-body process; however, at the low neutral concentrations of the exosphere, this may be disregarded entirely. Other ionic species present in the exosphere, primarily O+, are also produced predominantly by dissociative photoionization or electron impact with parent neutrals (e.g., O2) in lower regions of the atmosphere. The ions diffuse upward to the exosphere. A very small amount of photodissociation occurs in the exosphere itself, but is limited by the small concentrations of neutral species, such that the bulk of ion formation happens below 200 km where neutral concentrations are many orders of magnitude larger. Formation of negative ions by radiative electron attachment to atomic species H or O proceeds with very small rate constants, and, in any case, any negative ions formed at altitudes above ∼100 km, and certainly above 500 km, will quickly decay via photodetachment. As a result, there are no negative ions in the exosphere, and all negative charge density exists as free electrons in concentrations equal to the sum of all positive ions, to obey charge neutrality. The electron density in the atmosphere peaks below the exosphere at around 106 cm−3 and falls off quickly as altitude increases, dropping about 1 order of magnitude by 500 km and 2 orders of magnitude by 1000 km. A plausible loss process for cations and electrons in the exosphere would seem to be recombination; however, unlike molecular ions, which may rapidly undergo dissociative recombination with electrons (i.e., AB+ + e− → A + B), the monatomic cations of the exosphere may recombine with electrons only through a radiative stabilization mechanism. Radiative recombination occurs with small rate constants, on the order of 10−12 cm3 s−1. At the concentrations of cations and electrons in the exosphere, this loss process is then negligible. In fact, the physics occurring in the exosphere is more important than the chemistry in controlling ion loss. A brief overview is provided here, while numerous discussions and models have covered the topic in detail.25,27 Some ions are lost to space through diffusion, with ion diffusion following the Earth’s geomagnetic field lines. The solar wind (the stream of high energy particles emitted from the outer atmosphere of the sun) compresses the magnetic field on the sun-facing side and creates a tail on the opposite side. Exospheric ions in the polar regions can be affected by this effect, termed the polar wind, and diffuse along the tail and into space. However, the primary loss process of ions from the Earth’s atmosphere to space is
cosmic rays (high energy particles from outside the solar system), which ionize all molecules with about equal efficiency. The bulge in charged particle concentration near the tropopause is an indication that the cosmic rays have been significantly absorbed. Finally, near the ground, radioactive decay, mostly of radon, is a source of ionization and creates a small increase in ion density, not shown in Figure 1 due to the thinness of this region. Other local sources can include lightning, combustion, and high voltage power lines. The sum of all of these effects produces an ion density that is a few thousand per cm3 within a factor of 2 or 3 from 100 km to the ground. In reality, there are many influences that create large differences from these values such as solar storms that cause auroras, day/night, and latitude, which has an effect in large part due to magnetic fields. To understand the ion chemistry of the atmosphere, concentrations of neutrals must also be known. The chemistry of the lower atmosphere is dominated by minor species (∼parts per million by volume (ppmV) or less), and we save that discussion for later. Figure 1c shows typical concentrations of the main neutral atmosphere components. At the bottom of this region (i.e., ground level), O2 and N2 dominate. As altitude increases, first O2 and then N2 dissociate due to extreme UV (EUV, 10−124 eV) and X-rays. The dissociation also leads to NO formation. For much of the ionosphere, the neutral reactant partners for charged species are these five species: N2, O2, NO, N, and O. H and He become important at very high altitudes due to gravitational separation of species.
2. THE EXOSPHERE The exosphere, the uppermost region of the atmosphere beginning at an altitude of about 500 km, consists of the lightest neutral species (H, He) and positive ions (H+, He+, to a lesser extent O+, and to a much lesser extent N+ and possibly Ne+)17,24 as well as electrons. Concentrations of particles in this region are sufficiently low that collisions are rare; most models treat the region as collisionless.25 Particles are essentially on ballistic paths, and their behavior is dominated by gravitational pull resulting in higher concentrations of lighter species at higher altitudes. Little chemistry actually occurs in the exosphere due to the lack of collisions. This section will discuss the sources and sinks of the species present, which involve chemistry that occurs at lower altitudes. The dominant ionic species in the exosphere is H+. H+ may be produced by photoionization of H or H2, but its concentration in the exosphere is primarily controlled by the equilibrium:21 O+ + H ↔ H+ + O
(2)
which is a function of the relative concentrations of H and O. O+ is produced by ionization of O2 at lower altitudes and diffuses upward. Unlike most atom−atom charge exchange reactions, this reaction involving O and H proceeds rapidly due to the nearly identical ionization energies of the two species; that is, there is an accidental resonance. The reaction is slightly exothermic to yield H+, and has been measured to proceed with a forward rate constant of 6.0 × 10−10 cm3 s−1 at 300 K.26 At increasing altitude, the lighter neutral species increasingly dominate due to gravitational effects, shifting the equilibrium toward H+. The O+ concentration falls rapidly from its peak in the F-region, and the H+ concentration rises in complement. H+ becomes the dominant ion at altitudes varying from ∼500− 1000 km, depending heavily on latitude, season, and daytime or D
DOI: 10.1021/cr5003479 Chem. Rev. XXXX, XXX, XXX−XXX
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what is termed Jeans escape. At altitudes above ∼100 km, the turbulence that leads to mixing of gases at lower altitudes becomes insignificant, and molecular diffusion dominates the vertical distribution of species, separating them as a function of mass. The concentration of a particular species decreases exponentially with altitude, such that at some altitude the mean free path between collisions is equal to the scale height (the exponential factor by which the concentration decreases). This critical altitude defines the start of the exosphere, with the physical significance that this region will be essentially collisionless, meaning that any particle with sufficient velocity to escape Earth’s gravitational pull will be unimpeded and escape to space. The critical velocity to escape Earth is 10.8 km s−1, meaning that a hydrogen atom requires 0.6 eV to escape, and other particles need much more energy. The thermal population of any species above this critical point is fairly small (exospheric hydrogen is at ∼1000 K (0.1 eV), meaning
