In this chapter we explored the topic of ozone and other atmospheric trace gas measurements using both in-situ and remote sensing techniques. In-situ measurements involve direct sampling of the atmosphere. A sample of air is analyzed inside an instrument to determine its properties and the relative amount of different chemical components. Remote sensing measurements involve measuring an atmospheric constituent gas (e.g. ozone, water vapor, methane, etc.) indirectly, by measuring the changes in atmospheric radiation that result from the presence of the gas. The remote sensing instrument detects changes in thermal (infrared) and short wave (visible, ultraviolet, microwave) radiation produced by the gas in question. By comparing the characteristics of the incoming beam with the outgoing beam we can deduce characteristics of the atmosphere.

Remote sensing can be either passive or active. In passive remote sensing, the instrument merely records the radiation reaching it. Radiation at some wavelengths will be absorbed by the constituent gas, while radiation at other wavelengths will not be absorbed. Knowing which gases absorb which wavelengths of radiation make remote sensing possible. By knowing how and by what amount different molecules absorb radiation at different wavelengths, we can identify a "fingerprint" for each atmospheric constituent. Based on the radiation measured by the instrument, retrieval algorithms are used to infer physical measurements (such as number density, partial pressure, column amount) of the different gases. In active remote sensing, radiation is actually sent out by a laser beam. The light beam is then altered by interacting with atmospheric gases. The instrument then measures the altered beam and deduces characteristics of the atmosphere.

Both passive and active remote sensing can be carried out from one of four platforms, including satellite (space based), balloon, aircraft, and the ground. There are four different passive remote sensing techniques based on the viewing geometry of the instrument. They include backscatter ultraviolet (BUV), occultation, limb emission (or limb viewing), and limb scattering. We discussed each of these four techniques in the context of satellite platforms, though the viewing geometry concepts behind them can also be used in other platforms. We also discussed the advantages and disadvantages associated with each technique, as well as many of the satellite instruments past and present that use these techniques.

The BUV technique looks directly down at the atmosphere (nadir viewing) to measure the amount of shortwave solar radiation scattered back up to the satellite. Through this, it is able to measure total column amounts and profiles of ozone and some trace gases. The TOMS instrument uses a BUV technique to measure total column ozone. The Nimbus-7 TOMS and its successors, Meteor 3, ADEOS, and EP-TOMS, have provided global measurements of total column ozone since 1978. SBUV, SBUV/2, and GOME are BUV nadir viewing instruments that measure total and profile ozone. The BUV technique provides good horizontal resolution, but poor vertical detail.

The occultation technique uses solar, lunar, and even stellar radiation as measured directly though the limb of the atmosphere during satellite sunrise and sunset events. By measuring the amount of absorption of radiation through the atmosphere at different wavelengths (e.g. UV, visible, infrared), occultation instruments can infer vertical profiles of a number of trace constituents, including ozone. The advantage of the occultation technique is improved vertical resolution, while the disadvantage is the limited spatial coverage, since measurements historically could only be made at sunrise and sunset events. New occultation instruments can also use the moon and stars, increasing the spatial coverage. The SAGE and POAM series, as well as the HALOE instrument were among the satellite missions using this technique.

The limb emission or limb viewing technique measures the longwave radiation (infrared or microwave) thermally emitted in the atmosphere along the line of sight of the instrument. The instrument looks into the limb of the Earth's atmosphere down to an altitude called the tangent altitude. It is from this altitude that the most longwave radiation comes. The viewing geometry is similar to the occultation technique, but limb emission instruments measure thermal and microwave radiation, from which they infer vertical profiles of trace gases. LIMS, CLAES, HiRDLS, TES, and MLS are some of the acronyms for the satellite instruments that we reviewed using this viewing technique.

The limb scattering technique has a viewing geometry similar to that of both limb emission and occultation. This provides for good vertical resolution. But it also measures scattered solar radiation in a manner similar to the BUV measurement, using the light source in the limb of Earth's atmosphere. This allows for continuous overage through the daylight portion of the upper troposphere and stratosphere. The SCIAMACHY instrument aboard the future ENVISAT-1 employs this technique.

Passive remote sensing from the ground includes the Dobson and Brewer Spectrophotometers.

An example of an active remote sensing instrument is the lidar (light detection and ranging). The differential absorption lidar (DIAL) technique uses lidar at two different wavelengths, one absorbed by the trace gas of interest, the other not absorbed. From this, it is possible to infer measurements of trace gases, including stratospheric ozone with high vertical resolution.

Finally, we discussed current and future missions on all four different platforms using both in-situ and remote sensing instruments.

Space based platforms include the current UARS and the future EOS-Chem, SAGE III, NPOESS, ENVISAT-1, EUMETSAT, and ADEOS II satellites, as well as future Space Shuttle missions. Balloon missions are used as complementary, correlative data to satellite missions. They also cover an extensive altitude range and provide an excellent testing venue for measurements. For aircraft measurements, ten separate missions were listed. The two main aircraft platforms used are the ER-2 and the DC-8. The ER-2 carries primarily in-situ instruments, while the DC-8 carries both in-situ and remote sensing instruments. Finally, ground based experiments include the AGAGE and the NDSC global networks. Using the global monitoring network established for both, AGAGE monitors surface manmade and natural trace gases, while NDSC monitors the chemical composition of the stratosphere.