Zeeman modulation contrasted with pressure modulation in its use in measuring atmospheric NO from the ground

The Zeeman Modulator Radiometer (ZMR), recently built at Oxford to measure the column of NO from the ground by observing the sun, is contrasted with the calculated performance of a pressure modulator radiometer (PMR) observing in the same way. The ZMR is limited in the number of gases that can potentially be measured, but it is more suitable for absorption measurements from the ground because radiometric errors and errors in the correction for other atmospheric absorbers are less important. Both instruments are relatively insensitive to tropospheric NO, although the ZMR is more so. In principle, a PMR containing a variable mean pressure of NO can measure the vertical profile of NO from the ground. The vertical resolution would be 10km between 10 and 25km, and it would measure the total column above 25km. As such it would be especially useful in Antarctica since 10-25 km is the altitude region of the disappearance of ozone there in the spring. Because of its greater energy grasp, the PMR can also make useful measurements observing the moon. Again this would be especially useful in Antarctica, where the stratosphere is sunlit long before the surface.