The interaction between the electromagnetic radiation and the molecular species present in the earth atmosphere is of fundamental importance for studying the composition of the atmosphere itself and its evolution. Among all the constituents of the terrestrial atmosphere, water, even if it is a minor component, plays a significant role in the absorption of radiation and it is involved in many atmospheric phenomena by affecting atmospheric warming and cooling, weather and climate, electromagnetic waves propagation. Remote sensing applications require the spectroscopic parameters of the various atmospheric constituents to be known accurately. These parameters are collected in spectral databases such as HITRAN and, among them, the collisional broadening parameters have a crucial influence on reduction of remote sensing data. Actually, the largest sources of uncertainty in retrieved profiles are the pressure-broadening coefficient itself and its temperature dependence. HITRAN values are taken from measurements, when they are available, but in most cases come from calculations. Air broadening and shift, are deduced from a semiclassical treatment of the H2O collisions with N2 or O2, while self broadening parameter is obtained by an average of the J dependence of the existing experimental data. In order to check the accuracy of the existing values, measurements of pressure-broadened half-widths have been carried out in the THz frequency region. More precisely, the fol- lowing transitions have been investigated: J = 31;2 - 30;3 (1.097 THz), 11;1 - 00;0 (1.113 THz), 72;5 - 81;8 (1.147 THz), 31;2 - 22;1 (1.153 THz), 63;4 - 54;1 (1.158 THz), 32;1 - 31;2 (1.163 THz), 85;4 - 76;1 (1.168 THz), 74;4 - 65;1 (1.173 THz), 85;3 - 76;2 (1.191 THz), and 63;3 - 54;2 (1.542 THz). Since we focus on application to the Earth's atmosphere, nitrogen, oxygen and water have been considered as perturbing gas. Measurements were found in good agreement with semiclassical calculations. Our results show that old measurements are sometimes not very accurate and that the HITRAN self broadening parameters obtained by assuming a J dependence can involve large errors. Indeed, simple semiclassical calculations performed for more than 1500 rotational water lines with frequency lower than 1000 cm-1 cast doubts on many self broadening values reported in the HITRAN database, while are in good agreement with experiment. On the whole, we can conclude that when no reliable measurement is available semiclassical calculations should be resorted to also for the self broadening parameter.
Pressure-broadening of water lines in the THz frequency region: improvements and confirmations for spectroscopic database
CAZZOLI, GABRIELE;PUZZARINI, CRISTINA;
2008
Abstract
The interaction between the electromagnetic radiation and the molecular species present in the earth atmosphere is of fundamental importance for studying the composition of the atmosphere itself and its evolution. Among all the constituents of the terrestrial atmosphere, water, even if it is a minor component, plays a significant role in the absorption of radiation and it is involved in many atmospheric phenomena by affecting atmospheric warming and cooling, weather and climate, electromagnetic waves propagation. Remote sensing applications require the spectroscopic parameters of the various atmospheric constituents to be known accurately. These parameters are collected in spectral databases such as HITRAN and, among them, the collisional broadening parameters have a crucial influence on reduction of remote sensing data. Actually, the largest sources of uncertainty in retrieved profiles are the pressure-broadening coefficient itself and its temperature dependence. HITRAN values are taken from measurements, when they are available, but in most cases come from calculations. Air broadening and shift, are deduced from a semiclassical treatment of the H2O collisions with N2 or O2, while self broadening parameter is obtained by an average of the J dependence of the existing experimental data. In order to check the accuracy of the existing values, measurements of pressure-broadened half-widths have been carried out in the THz frequency region. More precisely, the fol- lowing transitions have been investigated: J = 31;2 - 30;3 (1.097 THz), 11;1 - 00;0 (1.113 THz), 72;5 - 81;8 (1.147 THz), 31;2 - 22;1 (1.153 THz), 63;4 - 54;1 (1.158 THz), 32;1 - 31;2 (1.163 THz), 85;4 - 76;1 (1.168 THz), 74;4 - 65;1 (1.173 THz), 85;3 - 76;2 (1.191 THz), and 63;3 - 54;2 (1.542 THz). Since we focus on application to the Earth's atmosphere, nitrogen, oxygen and water have been considered as perturbing gas. Measurements were found in good agreement with semiclassical calculations. Our results show that old measurements are sometimes not very accurate and that the HITRAN self broadening parameters obtained by assuming a J dependence can involve large errors. Indeed, simple semiclassical calculations performed for more than 1500 rotational water lines with frequency lower than 1000 cm-1 cast doubts on many self broadening values reported in the HITRAN database, while are in good agreement with experiment. On the whole, we can conclude that when no reliable measurement is available semiclassical calculations should be resorted to also for the self broadening parameter.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
