Calibration for sea salt spray and tilt angle for Li-cor 7500 open path infrared CO2/H2O gas analyzer for on-board flux measurement over Sea

The influence of sea salt accumulation over the gas analyzer mirror of Li-cor 7500 is a serious problem while observing the turbulent fluxes CO₂ and H₂O over sea. The sea spray causes deposition of salts and forms salt crust in due course, thus affecting the validity of the fluxes observed. A simulation experiment was undertaken at the Department of environmental engineering, National Chung Hsing University, Taichung, Taiwan to study the error due to sea salt, sea salt removal technique and the effect of analyzer tilt angle over the flux values. The experimental set up consists of two set of Li-7500 CO₂ /H₂O infra red gas analyzer. The two sensors are positioned at a distance of 0.95 meter apart. One analyzer was kept as control and the other one was subjected to simulation studies. Exposure to sea salt spray to the gas analyzer was done artificially through laboratory simulation. Then the sea salt depositions were washed away using automatic timer controlled water spray system developed for onboard use at sea. The results reveals that during the presence of sea salt over the analyzer mirror, the analyzer over estimated both CO₂ /H₂O. Out of different spay duration time tested; it was evident that spraying distilled water for 60 seconds was sufficient enough to remove the salt deposition. However, the flux values recorded during the spray were highly erroneous. Moreover it was noticed that the analyzer takes three more minutes after stopping the water spray over the mirror. Hence it should be noted that, the data recorded in the data while processing the data, the data recorded during the water spray and the subsequent three minutes period should be discarded from the flux analysis and the gap shall be filled through suitable interpolation technique. As a measure to reduce the impact of sea spray over the gas analyzer, and to facilitate the sea spray to easily drop out from the analyzer mirror a study consists of two different tilt angles was investigated. The tilt angle consist of 0º as control and two other tilt angle as 10º and 20º were adopted to test verify how the influence of sea spry stay over the analyzer mirror. Since the tilt angle cause the sea spray to roll out of the mirror due to gravity, the behavior was investigated. From the observed values at 10º and 20º sensor tilt angle, it can be seen that 20º sensor tilt angle has less biased value than that of 10º tilt angle with respect to water vapor. When the tilt angle increases, the water vapor flux tends to become closer to the value observed over the control treatment without any sea spray. This is because higher the tilt angle, lesser shall be the opportunity for the sea sprays to stay over the analyzer mirror surface. In contrast to water vapor flux, CO₂ flux has an opposite trend with respect to sensor tilt angle. It is evident that unlike water vapor flux, the fluxes of CO₂ become higher than the control as tilt angle increases. Hence, it is concluded that a further investigation is necessary to understand the influence of sensor tilt angle with respect the fluxes of water vapor and CO₂.