Development and Use of an Experimental near Infrared Open Path Diode Laser Prototype for Continuous Measurement of CO₂ and CH₄ Fluxes from Boreal Hydro Reservoirs and Lakes

Abstract

Diode laser second derivate modulation spectroscopy combined with an open atmospheric path is a well suited technique for trace gas monitoring above wide areas. This paper presents the development of a portable long optical path near infrared spectrometer based on telecommunication laser diodes in order to provide a powerful tool for real time simultaneous measurements of greenhouse gas (GHG) concentrations above lakes and hydro reservoirs. CO₂ and CH₄ are respectively monitored at 1572 and 1653 nm along optical paths of several hundreds of meters above the area of interest. Simultaneous measurements of the target gases at two different heights above the water surface allows to detect concentration gradients on a continuous basis. A simplified turbulent diffusion model involving both the measured concentration gradients and local wind data has been used to estimate average GHG fluxes emitted by lakes and hydro reservoirs in different regions. Recent optimizations of the developed prototype allow quick on site set-up and operation of the laser device, even in remote areas without local facilities, as well as the continuous measurement of low GHG concentration gradients during long time periods with a minimum of local surveillance. Further improvements of this laser system would allow simultaneous detection of other trace gases such as N₂O emitted by agricultural soils and other gases present in various environments.