Active remote instruments such as lidars and radars use electromagnetic radia-tion to observe aerosol particles and clouds. Here, a short introducradia-tion to what is radiation and how the physical and chemical properties of these atmospheric com-ponents together with the radiation facilitate their detection and classification is pre-sented (Sect. 2.1). The most relevant aerosol types are prepre-sented in Section 2.2 and cloud properties in Section 2.3. Lastly, the importance of aerosols and clouds and their link to climate is found in Section 2.4.
Electromagnetic radiation (EMR) is a form of energy that propagates through space as waves, traveling in packets of energy called photons. EMR consists of a spectrum with variable wavelengths (denoted with λ) and travels through the air with the speed of light (2.997×108 m/s). The wavelength is the distance between two successive wave crests or troughs and determines the energy of a photon (Fig. 1). The shorter the wavelength, the higher the frequency of the electromagnetic wave, and the greater the energy of the waveform. From shorter to longer wavelengths: gamma rays, x-rays, ultraviolet, visible, infrared, microwaves and radio waves constitute the elec-tromagnetic spectrum (Fig. 1). The visible light is a tiny region of the EMR spectrum (0.4–0.7 μm), yet it defines our perception of the world as human eyes are sensitive at these wavelengths. Life on this planet have been evolved to have their best sensi-tivity to the visible light. EMR is emitted by any object having temperature greater than absolute zero (-273.15 °C). This practically means that the Sun, the Earth, and the atmosphere having extremely different temperatures from each other radiate at different electromagnetic spectra. The Sun, for example, emits shortwave radiation;
the maximum intensity of the emitted energy is around 0.5 µm at the top of the at-mosphere and the energy distribution is skewed to the shorter wavelengths, meaning that about half of the energy is in the visible wavelengths below 0.7 µm. The Earth emits longwave radiation at a peak wavelength of about 10 µm and its intensity is orders of magnitude lower than that of the Sun.
Energy transfer in the atmosphere is accomplished through EMR. Not all the shortwave radiation from the Sun is transmitted all the way to the Earth. Some of the wavelengths reach the Earth’s surface while others are partly or fully filtered out by the atmosphere (Fig. 1). For example, the ozone layer located in the stratosphere ab-sorbs most of the solar ultraviolet radiation, while in the troposphere, aerosol parti-cles and clouds interact with radiation both by absorbing and reflecting it. Then, the
---Earth absorbs the remaining shortwave radiation and emits it back to the atmosphere. Overall, the atmosphere regulates this transfer of energy. The amount of energy reaching the Earth and the amount of energy escaping from the Earth, i.e.
the radiative balance, ultimately controls the climate of the Earth and it is critical in ecosystems functionality. An imbalanced radiation budget forces the components of the climate system to adjust and eventually pose warmer/cooler surface temperatures over time reaching a new energy balance (equilibrium).
Figure 1. Range of electromagnetic spectrum and interaction with the Earth’s atmosphere. As shown in the uppermost part, the smaller the wavelength the more frequent the wave form is.
The ability of certain wavelengths to penetrate the Earth’s atmosphere from space is shown with vertical lines. Credit: STScI/JHU/NASA.
As mentioned, EMR interacts with ozone molecules, aerosol particles and many other atmospheric components. The form of interaction between radiation and mat-ter depends on the size, shape, and chemical composition of the component, as well as the wavelength of the incident radiation. There are three main processes affecting
---the propagation of radiation through ---the atmosphere: absorption, emission, and scat-tering. Eventually, these processes reduce the amount of radiation propagating through the atmosphere. In real conditions, rarely a single wavelength strikes a mol-ecule or an aerosol particle. Instead, radiation of various incident wavelengths head towards a target. When this occurs, molecules and particles may selectively absorb or scatter radiation at certain wavelengths. Lidars use such properties to measure, for example, the water vapor in the atmosphere (see Sect. 3). The selection of the emitted wavelength together with the size of the targets are the two decisive param-eters for the observation of the various atmospheric components.
When a photon is absorbed by a molecule, it ceases to exist, and its energy is transferred to the molecule. This energy can be transferred to vibrational, rotational, electronic, or translational forms which combined are called the internal energy of the molecule. The above energies are quantized and in absorption the energy transfer occurs only when the energy of the incident photon exactly matches the energy dif-ference between two energy states in the molecule. In this case, the incident radiation becomes part of the internal energy of the molecule positioning the molecule to a higher energy level (excited state). As a result of absorption, atmospheric compo-nents increase their internal energy which further increases their temperature. In emission, molecules that are excited decay to lower energy levels by emitting radia-tion (e.g. fluorescence). Therefore, emission increases the outgoing radiaradia-tion at cer-tain wavelengths and absorption reduces it. When the incident radiation is less than the energy difference between two levels in the molecule for it to be absorbed, scat-tering can occur. In scatscat-tering, the electrons within the molecule are perturbed at the same frequency as the incident wave.As a result, the electrons within the molecule are momentary separated, inducing a dipole moment. The scattered light is the result of emitted EMR induced by this dipole. There are two different scattering processes depending whether the molecule returns to its original state or not upon scattering the radiation: elastic and inelastic. Elastic scattering is when the scattered radiation has the same wavelength as the incident radiation and (almost) no energy loss has occurred. Therefore, the molecule returns to the initial energy state upon emission of EMR. Correspondingly, in inelastic and particularly in Raman scattering, the wave-length is shifted between the incident and scattered radiation as the induced dipole is adjusted by molecular motions like vibrational or rotational (Raman & Krishnan, 1928). In this case, the initial and final energy states are different. There is a variety of rotational/vibrational excitation states which leads to several bands of Raman ra-diation (Wandinger, 2005). The scattered Raman rara-diation is characteristic of the mol-ecule which allows temperature determination of the molmol-ecule.
Scattering is highly dependent on the size parameter, x = πnr/λ, which is a func‐
tion of the particle radius r, the incident wavelength (λ), and the refractive index,n, (defined by the chemical composition of the molecule, hereafter scatterer). There are
---three regions depending on the size parameter which defines the scattering proper-ties (Fig. 2).
Firstly, when the size of the particle is much smaller than the wavelength of the incident radiation, the scattering is considered as Rayleigh or molecular scattering in which the optical properties of the scattered particles can be predicted by the so-called Rayleigh theory (Strutt, 1871). As almost 99% of the Earth’s atmosphere con‐
sists of molecules of nitrogen and oxygen (very small compared to both solar and terrestrial radiations), this type of elastic scattering is the most dominant in the upper atmosphere. Moreover, Rayleigh scattering exhibits a strong wavelength depend-ence. The wavelength dependence of the scattered intensity is proportional to λ-4, meaning that shorter wavelengths are more efficiently scattered than longer wave-lengths. For example, the sky appears blue because molecules in the air scatter blue light from the Sun almost 10 times more than red light.
Secondly, when the particle is comparable in size with the wavelength of the in-cident radiation, the scattering is better described by Mie theory (Mie, 1908). Alt-hough, Mie theory covers the Rayleigh region, it is optimally used for particles whose sizes are comparable to the wavelength of the radiation, or larger. The scattering in-tensity in this case varies strongly with the wavelength and can therefore be used to identify atmospheric particles. Nevertheless, dissimilar to the Rayleigh scattering in which the radiation is scattered similarly, in Mie scattering the scattered radiation is angular dependent. The scattering angle is the angle between the incident and scat-tering directions. In Mie scatscat-tering, the scatscat-tering intensity distribution is weighted in the forward direction (0o). This implies that more light is scattered forward than backwards (180o). Mie calculations assume that the particles are spherically shaped, but that is not the case for all atmospheric particles. The assumption of perfect spheres to retrieve optical properties for irregularly shaped particles increase the er-rors and impair forecast accuracies, producing potentially misleading results (Kylling et al., 2014). Therefore, Mie theory is often a rough approximation in the case of large and non-spherical particles such as dust and ice crystals where different ap-proaches are more appropriate (Redmond et al., 2010). Rayleigh theory also considers the particles to be spherically shaped but due to their small size compared to that of the wavelength of the incident radiation the errors can be neglected.
Thirdly, when the size of the particle is much bigger than the wavelength of the incident radiation, the scattering is non-selective, and the light propagation is better described by geometric (or ray) optics. Geometric optics is not widely used in atmos-pheric research, but some applications have been reported (e.g. Hulley & Pavlis, 2007).
---Figure 2. Wavelength dependence of incident radiation and particle radius for various atmos-pheric components assuming satmos-pherically shaped particles.
Credit: Dr. Luca Lelli (http://www.iup.uni- 21remen.de/~luca/?download=01_LL_VO.pdf).