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| The Aura spacecraft (NASA image from aura.gsfc.nasa.gov). |
Imagine standing outside in the early afternoon and looking up through a cloudless deep blue sky. Even though you can't see it, the Aura spacecraft flies overhead. Onboard Aura, the Ozone Monitoring Instrument (OMI) measures ultraviolet (UV) radiation from the sun reflected from Earth and its atmosphere. Back on the ground, scientists use these data to calculate how much UV radiation reaches the surface. This is an important question because of the effects of UV radiation, both good and bad, on you and your environment. With the global coverage possible from a satellite, measurements from OMI are used to produce global maps of UV radiation at Earth's surface that would be impossible to obtain just from surface-based measurements.
What's wrong with this picture? The "cloudless deep blue sky" assumption is definitely a problem. Under these ideal conditions, scientists can do an excellent job of predicting surface UV based on sunlight reflected from the Earth/atmosphere system. However, these conditions hardly ever occur! Real atmospheres contain clouds, water vapor, and aerosols. When scientists try to interpret OMI measurements made in the real world, there are many difficulties. What does Earth's surface actually look like as seen from space? What are the effects of aerosols, water vapor, and clouds on the reflection of UV radiation back to space and its transmission through the atmosphere to the surface?
Now let's imagine how you might help OMI scientists. Instead of just standing on the ground looking up, you could measure the actual UV radiation coming through the atmosphere as Aura flies overhead. Then scientists can compare this value against the value predicted by OMI. Such measurements can be used to evaluate and, over time, perhaps improve the performance of OMI algorithms. Mission accomplished? Not so fast...
When OMI looks down at the Earth/atmosphere system, it sees not just a single "point," but an area at least 13x24 km. This is called the instrument "footprint" - a measure of its spatial resolution. The UV values calculated by OMI are, essentially, averages over this footprint. However, an area of this size, and the atmosphere over this area, are not homogeneous. All the important atmospheric properties - clouds, aerosols, and water vapor - can change significantly within this footprint. OMI's algorithms try to take this variability into account. For example, scientists have built mathematical models of the effects of clouds on surface UV radiation. Depending on their size, shape, and composition, clouds can sometimes even increase the amount of UV radiation reaching the surface, compared to a cloud-free sky. Especially when the sky is partly cloudy, the relationship between the actual average UV radiation over an entire OMI footprint and a simultaneous ground measurement at a single point within that footprint is very complicated and still very uncertain.
This fact brings us to a critical conclusion: In order to evaluate OMI UV products, several simultaneous measurements must be made within a single OMI footprint during overflights of the OMI instrument. It is only in this way that spatial variability in atmospheric properties can be related to the calculations that OMI makes.
In addition, it is not sufficient just to make a single measurement simultaneously at several points within a footprint. UV radiation varies with time as well as spatially. One of the OMI products is the total daily UV radiation dose; this so-called "erythemal dose" is related to the UV index published in newspapers and elsewhere. So, not only must a single OMI measurement over a footprint be accurately converted to a spatial average over that footprint, it must also be related to an entire day's worth of UV radiation.
Because UV radiation varies in both space and time, and because much of this variation is due to the movement of clouds, some scientists believe that it may be possible to relate spatial variability in radiation, based on simultaneous measurements at different places, to temporal variability, based on continuous measurements made at the same place. This is a hypothesis that can be tested only by collecting a time series of measurements at several places within an OMI footprint. The practical effect of this requirement is that instruments for recording UV radiation must be attached to data loggers that record measurements starting well before and ending well after an Aura overflight - perhaps even during an entire day.
These requirements for properly validating OMI measurements pose daunting challenges. High-quality instruments for measuring UV radiation are very expensive to purchase, operate, and maintain. In fact, it is generally agreed that it is prohibitively expensive to collect proper validation measurements with such instruments. This is where you can make a contribution! Scientists who have studied this problem have concluded that a reasonable approach to validating OMI measurements involves using a combination of one high-quality instrument, to provide a reliable measurement and calibration standard, and several "simple radiometers" (to quote one author writing on this topic) distributed around an OMI footprint.
There is one additional restriction on these simple radiometers. Scientists would like to have ground measurements taken at a wavelength that is unaffected by varying amounts of ozone in the atmosphere, thus removing one variable from their calculations. Ozone absorbs significant amounts of solar radiation across most of the UV part of the solar spectrum. (This is why the well known atmospheric "ozone hole" affects the amount of UV radiation reaching Earth's surface.) At higher UV wavelengths, in the UV-A part of the spectrum, ozone absorption is negligible.
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| The GLOBE/GSFC UV-A radiometer/sun photometer. |
There are several relatively inexpensive commercial instruments for measuring UV radiation. However, they do not measure just that small part of the UV spectrum that is unaffected by ozone. In cooperation with NASA's Goddard Space Flight Center, GLOBE scientist David Brooks and his colleague Forrest Mims have developed an inexpensive radiometer that measures UV radiation in a narrow band of wavelengths centered around 372 nm, in the desired part of the UV-A spectrum. The cost of such an instrument is about 100 times less than a high-quality reference instrument! If several of these instruments are deployed around an area equal in size to an OMI footprint, the requirements for evaluating OMI UV measurements can be met in a very cost-effective way.
This is an ideal project for GLOBE scientist/teacher/student partnerships:
I look forward to your participation in this exciting project!