To understand the chemistry of our atmosphere, we need to be able to measure which chemical species are present in what concentrations and how those concentrations vary in space and time. For some purposes, very broad measurments can be made from satellites, but when looking closely at chemical reations, in situ instruments offer more precise and often more accurate measurements. In order to understand the atmosphere, we need to deliver our instruments to the upper troposphere and lower stratosphere.
Jim Anderson's first instruments flew into the stratosphere on small rockets. After that, we built instruments for large research balloons. These large balloons have the advantage of being able to get much higher than most aircraft, but the logistics and expense required for launching them means it may only be possible to get one or two flights off in a year, and then with only a single ascent and a single descent.
In 1987, we built our first instrument for aircraft. After the unexpected discovery of the Antarctic ozone hole, NASA was looking for an instrument to measure Chlorine Monoxide (ClO) to augment the suite of instruments already flying aboard their ER-2 aircraft. The ER-2 is a longer-winged version of the U-2 spy plane which can reach altitudes above 65,000 feet. NASA repainted theirs white, changed the name to "Earth Resources" and put its cameras to use mapping land usage, monitoring forest fires and storm damage, and making scientific measurements in the stratosphere.
Switching from balloons to aircraft resulted in a significant increase in observations. Given fair weather, the ER-2 could fly every other day for up to eight hours. In August and September of 1987, we participated in the Airborne Antarctic Ozone Expedition, based in Punta Arenas, Chile. From there, the ER-2 flew south to Antarctica's Palmer Peninsula, penetrating the polar vortex and documenting the unprecidented levels of ClO which were the lynchpin in the catalytic destruction of ozone.
We continued to design and fly instruments aboard the ER-2 through 2000. In 2001, we started working with NASA's WB-57. The WB-57 is a modified bomber. It is not capable of getting quite as high as the ER-2, but it has fewer constraints on the weather conditions it can tolerate, and is capable of sustained flight at lower altitudes, which is desirable for tropospheric measurements.
While these aircraft have been crucial in helping map out chemistry and dynamics, the limitations on their range, altitude and capacity means we have only sampled a small fraction of the earth's atmosphere. As the evidence of climate change mounts, we need a truly global means of monitoring the earth's vital signs. There are a wealth of weather satellites in orbit , but since their purpose is weather and not climate, they are located over the populated parts of the globe and their instruments are designed with only short-term relative accuracy in mind. To obtain a global measure of temperature, we need satellites in polar orbit capable of imaging the entire atmosphere and designed from the start with a solid calibration based on international standards such as those documented by the National Institue of Standards and Technology.