The primary goal of GOMOS is the accurate detection of stratospheric ozone, allowing long-term monitoring of global trends. Whilst providing ozone profiles from UV-visible occultation spectra in an altitude range of ~15 – 80 km and with a vertical resolution of better than 1.7 km, the instrument yields small-scale turbulence measurements and high-resolution temperature profiles using two fast broadband photometers in the visible spectrum.
Primary Scientific Objective: Ozone Trend Observations
The primary scientific objective of GOMOS is the global monitoring of the stratospheric and mesospheric vertical ozone distribution with high accuracy, high vertical resolution and full global coverage. The expected high accuracy of the ozone profile measurements should allow ozone trends to be studied over the lifetime of Envisat in the stratosphere and mesosphere.
GOMOS ozone trend observations will address:
The primary scientific mission objective points to the use of an instrument applying the occultation technique, as this is self-calibrating, protecting the measurements from long-term instrument drifts. Stars as light sources have been chosen in order to provide sufficient global coverage of the measurements, which could not be achieved with, for example, a solar occultation instrument.
The occultation technique has one significant advantage, which is that an absolute estimate of the molecule density is obtained from the ratio of two measurements taken with the same instrument within a few seconds. This makes the method inherently self-calibrating, since even if the spectral sensitivity of the instrument is changing with time, the ratio will be measured correctly. This protection against long-term drift is ideal for the study of atmospheric trends.
GOMOS measures in the UV and visible wavelength regions where ozone shows strong absorption. The measured wavelength range is 250 nm to 675 nm, covering the ozone Huggins and Chappuis band. The UV wavelengths range is particularly well suited for mesospheric and upper stratospheric measurements. The visible band is well suited for the probing the stratosphere, including the lower stratosphere, where the UV signal is no longer strong enough anymore because of the UV absorption by ozone. Roughly, the UV yields the best results for the mesosphere and stratosphere above 35 km and the visible band for the stratosphere below 35 km.
A highly desirable follow-on programme with GOMOS or COALA-type instruments could provide long-term trend observation.
Secondary Scientific Objectives
NO2 and NO3 play important roles in the nitrogen chemistry relevant to ozone.
Observations of NO2 and NO3 will help to address:
Future NOx observations relevant to the ozone recovery. As NOx dominates ozone loss on a global scale, NOx observations are important to interpret ozone changes as halogens decrease.
Stratospheric water vapour is fundamental to the budget of many trace gases in the stratosphere. It is therefore important to determine its three-dimensional distribution and long-term trends. Water vapour is measured in a near-infrared channel at 926-952 nm.
A further secondary objective is to study stratospheric dynamics. This requires measuring the atmospheric temperature and density profiles. Temperature and air density will be derived from a near-infrared channel (756-773 nm). Complementary to this, a high-resolution temperature profile can be retrieved from atmospheric scintillation, which will be measured using two fast photometers sensitive in the blue and red, respectively.
The measurement of temperature is also needed for supporting the primary mission objective, ozone monitoring. The ozone cross-sections in the Huggins band (310-350 nm) are strongly dependent on the atmospheric temperature. Therefore, a long-term variation of UV absorption might be due to a temperature variation and could be wrongly attributed to an ozone variation. Temperature measurements are possible with the A-band of O2 at 760 nm. O2 is a perfectly mixed gas and the air can be assumed to be in hydrostatic equilibrium. Therefore, its scale height is directly connected to the atmospheric temperature. The O2 measurements also allow to relate all measurements of ozone density (and other species) to the air density to yield the mixing ratio [O3]/[air], which is a quantity most readily used in models. Furthermore, temperature and air density are essential parameters for atmospheric dynamics, including mixing of gases.
The mission objectives have also been described by Bertaux (1999).
Long-term, campaign and permanent objectives
The most important of these objectives is the monitoring of stratospheric ozone. The second group consists of campaign type objectives. These have more limited scope than those of the long-term objectives. The activation of a campaign objective is based on an agreement among the partners of the GOMOS mission planning.
The third group consists of a few permanent objectives that must always be considered when a new mission plan is being prepared. In particular, whenever possible, certain stars (e.g. Sirius) should always be occulted and predefined locations (e.g. selected validation sites) should be prioritised. Therefore, these objectives have an overruling priority.
In more detail, the three groups include:
Campaign type objectives
Inter-comparison with MIPAS and SCIAMACHY:
The permanent objectives have overruling priority over the other mission objectives. But it must be noted that the permanent and other mission objectives can be fulfilled - or nearly so - at the same time, because of the large number of occultations per orbit. More generally, almost all mission objectives are to some extent fulfilled in any mission plan. The degree of fulfilment, of course, varies. The best results are obtained for the mission objective, which will be the target of the optimisation.