Ground Based Measurements of Atmospheric Gases by Spectroscopic Methods

Spectroscopic methods afford a number of advantages for making atmospheric measurements. It is a passive technique which is capable of making real time, in situ measurements. Since each molecules has a characteristic spectrum it is possible to make simultaneous measurement of a number of species. In addition, unequivocal identification of the target species is possible if the instrument has sufficient spectral resolution. Detection limits as low as 1 part in 10 13 have been reported as well as dynamic ranges as high as 10 8 . Temporal resolution of fractions of a second can be achieved for “eddy correlation” flux measurements. – Fourier Transform Infrared Spectroscopy (FTIR) is, perhaps, the most universal method and is particularly useful in identifying new species in heavily polluted air. However, it lacks the sensitivity required for measurements of trace gases in clean air. Differential Optical Absorption Spectroscopy (DOAS) operates in the visible and the near UV which limits the number of measurable species. But it is capable of measuring highly reactive gases such as OH, NO 3 , and HONO. Tunable Diode Laser Absorption Spectroscopy (TDLAS) has very high spectral resolution, sensitivity and time response. Its main disadvantage is the need for a number of laser diodes to cover a wide spectral range. Laser Induced Fluorescence (LIF) is also capable of both high sensitivity and specificity. However, rather complex excitation sources have to be devised for each gas being investigated. Photoacoustic Absorption Spectroscopy (PAS) is, in principle very simple and capable of simultaneous measurements. In practice, however, it suffers from lack of resolution and serious interferences from CO 2 and H 2 O.