When a photon interacts with a molecule, such as happens when the laser radiation from a lidar scatters from atmospheric molecules, it can gain or lose energy as a result of transitions within the molecule. This is known as Raman scattering, which is an inelastic quantum-mechanical scattering process. It results in a change in wavelength of the radiation, and this change is a characteristic of the scattering molecule. This makes it possible to select radiation from different molecules. This scattering process is much weaker than the simple elastic process in which there is no change of wavelength. Raman scattering is used in many applications other than lidar, such as the analysis of chemicals in laboratory work.
Water vapour measurement
The water vapour mixing ratio (the proportion of water molecules by either number or mass) is measured from the ratio of the Raman scattering signal from water vapour to that from nitrogen. The proportion of nitrogen in the atmosphere is close to constant as a function of height over the range of interest in these measurements. Both of the transitions used for the measurement are from the first vibrational level of the molecule to the ground state. For our transmitted wavelength of 354.7nm, the water vapour and nitrogen lines are at 407.8 and 386.7nm respectively.
At a temperature T, the fraction of molecules which are in an excited state of energy E is proportional to the factor exp (-E/kT). This factor is small for the first excited vibrational state (as used in water vapour measurements), but is large for the first few rotational states, which are much more closely spaced in energy. The distribution of molecules between these energy states is a function of temperature, so by measuring Raman signals corresponding to two different sets of rotational states it is possible to determine the temperature of the scattering molecules. In our system we use filters centred at 353.0 and 353.9nm. These select scattering from groups of lines from both nitrogen and oxygen molecules.
The radio refractivity, N, defined as (refractive index-1)x10^6, at a point in the atmosphere can be expressed as where P is the atmospheric pressure (mbar), T is the temperature (K) and pwv is the partial pressure of water vapour (mbar). N is dimensionless but is commonly referred to in N-units.
The gradient of the radio refractivity with altitude gives important information on the paths over which radio waves are propagated. For this purpose it is useful to convert to the quantity of modified radio refractivity, M, also a dimensionless quantity, defined as (N+157z), where z is altitude in km . For the majority of time dM/dz > 0, but in cases where it is less than 0 a condition known as ducting occurs and it is possible for radiowaves to propagate to sites far beyond the horizon. Ducting conditions exist for 5-10% of the time in Northern Europe.
Implicit in all these Raman scattering measurements is the need to precisely select wavelengths and also to be able to reject the much larger elastic scattering signal from them. In the case of temperature measurements the wavelength shift is only 1-2nm, which requires the use of high precision filters. As is common with Raman lidar systems, our system uses custom made interference filters for wavelength selection, but it is also possible to use more complex systems such as Faby Perot Etalons.