There are two general approaches to making atmospheric measurements. The first approach is in-situ and the second is remote sensing. The following sections give a description of these two approaches. For remote sensing, we explore in some detail the two types that exist, both passive and active. An example of how in-situ measurements are made is given in Section 3.0. Remote sensing techniques and examples are given in Section 4.0 (passive remote sensing) and Section 5.0 (active remote sensing).
In-situ measurements involve direct sampling of the atmosphere. A sample of the atmosphere (air) is brought into the instrument and is analyzed for its properties and/or its relative amount of various chemical components. This can involve the use of mass or optical spectroscopy, chemical assays, or observation of how intense light interacts with the molecules inside the sample. These techniques can be employed either from ground, aircraft, or balloon platforms, though not, of course, from space where there is no air!
We will explore the ozonesonde method of in-situ measurement in Section 3.0, including how it actually calculates the concentration or amount of ozone at different altitudes, thus giving a vertical distribution (referred to as a profile) of ozone. A profile of ozone (or any trace gas) is given in terms of partial pressure, which refers to that portion of total air pressure due solely to the existence of the molecule in question.
In remote sensing, some atmospheric parameter of interest (such as the amount of ozone) is derived indirectly by the changes in atmospheric radiation that result from the presence of the parameter. The remote sensing instrument detects changes in atmospheric thermal (infrared) and short wave (visible, ultraviolet, microwave) radiation produced by the parameter. The instrument, regardless its platform, is not measuring the parameter itself. For example, an ozone remote sensor does not measure ozone molecules. Instead, it measures how ozone molecules alter the radiation traveling through or emitted by the atmospheric regions being observed. In this way, remote sensing may but does not necessarily require that the observer be far removed from the location of the parameter. Satellites and ground based instruments are far removed from stratospheric ozone, whereas high flying balloons and aircraft are actually in the stratosphere.
2.2.1 Examples of Remote Sensing: Passive and Active -- Remote sensing can be either passive or active. It is conceptually easy to understand the difference between the two by considering the following examples. If you look outside the window and see that the sky is blue, you are making a passive remote sensing observation. On the other hand, if you walk into a dark room and shine a flashlight onto the wall and notice that the wall is blue, you are making an active remote sensing observation. In the passive case you are dependent on natural sources of light or other electromagnetic radiation. In the active sense the observer controls the energy source.
In the first example, you are doing nothing except to observe passively as the blue light reaches your eyes from its remote source. The Sun emits radiation over a wide range of wavelengths, including visible wavelengths. Some of the visible light is scattered by air molecules into the blue portion of the visible light region of the electromagnetic radiation spectrum. Merely by noting the blue light that reaches your eyes, you are making a passive remote sensing observation. In the second example, you are sending out a beam of electromagnetic radiation in the visible light region that interacts with the paint molecules on the wall and sends back the observation that the wall is blue. The paint appears blue because when light shines on it (assuming that the flashlight beam is "white" and hence contains all the primary or "rainbow" colors), it absorbs all the colors of the visible light spectrum except blue, which it reflects. The blue light is reflected back. We make the observation that the wall is blue.
The power of the remote sensing technique is that the observation platform does not have to sample the air directly. In the passive case, we are observing how the atmospheric parameters change the existing radiation that is in the atmosphere. In the active case, we are observing how the atmospheric parameters change the radiation that we introduce into the atmosphere (e.g. the flashlight beam). In neither case do we need to sample the parameter directly.
2.2.2 Passive Remote Sensing: Finding the Fingerprint -- Passive remote sensing, as noted above, is an indirect way of measuring an atmospheric molecule or particle that involves just observing the changes in electromagnetic radiation (including visible light) caused by the molecule or particle in question. We are able to observe these changes in radiation because each atmospheric constituent absorbs radiation at certain wavelengths in different regions of the electromagnetic spectrum. This radiation is emitted, reflected, and scattered by the atmosphere and the surface of Earth over a wide range of wavelengths from the infrared, visible, ultraviolet and microwave regions of the spectrum. The radiation is altered by the type, size, and amount of constituent molecules in the atmosphere it encounters. It is also affected by atmospheric temperature, particularly in the infrared and microwave regions. By knowing how and by what amount different molecules absorb radiation at different wavelengths, we can identify a "fingerprint" for each atmospheric constituent.
Passive remote sensing of the atmosphere relies on the fact that each atmospheric constituent has its own unique fingerprint which we refer to as its spectroscopic behavior. We can identify a molecule by observing it with an instrument that is sensitive to radiation at certain wavelengths. This allows us to identify its fingerprint.
2.2.3 The Lidar Technique of Active Remote Sensing -- Active remote sensing involves interacting with the radiation in the atmosphere that has already been changed by the presence of the parameter in question and measuring the response. In our example, the flashlight provided the initial electromagnetic radiation in the form of visible, white light that interacted with the paint on the wall, which then absorbed all the colors except blue. We were able to measure the response based on the color that the wall appeared to our eyes.
The lidar (short for light detection and ranging) technique employs a laser as a light source to probe the atmosphere. Laser light fired at the atmosphere is reflected back by the atmospheric molecules to a detector and the attenuation (reduction) of this light provides information on atmospheric particles and molecules. Changes in the returned wavelengths can provide information about atmospheric motion. The primary advantage of this technique is its ability to obtain high vertical resolution data at different altitudes. This is important for studies of how various trace gases (see Chapter 2) are transported by the wind. Lidar is also used to measure the cloud altitude. This information is very important for pilots and for meteorological observations. A more detailed discussion of the lidar technique is found in Section 3.5.