Title: Effects of sensor location and the atmospheric stability on the accuracy of an inverse-dispersion technique for lagoon gas emission measurements Authors
|Flesh, Thomas -|
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 30, 2012
Publication Date: N/A
Technical Abstract: Measuring gas emission rates from wastewater lagoons and storage ponds using currently available micrometeorological techniques can be an arduous task because typical lagoon environments contain a variety of obstructions (e.g., berm, trees, buildings) to wind flow. These non-homogeneous terrain conditions violate the basic assumptions of these techniques and create uncertainty in the accuracy of the techniques. In these difficult environments, the placement of the sensors within the landscape may significantly affect the accuracy of the methods. In addition, the value of atmospheric stability parameter such as Obukhov length (L) also affects the accuracy of the micrometeorological methods. This study evaluates the impact of concentration and wind sensor locations and the different values of L obtained from two different methods on the accuracy of the backward Lagrangian stochastic inverse-dispersion technique (bLS) in determining gas emission rates from treatment lagoon environment. Open-path tunable diode absorption spectrometers (TDL) were used to obtain path-integrated concentrations (PICs). Wind statistics were measured using 3-dimensional (3D) anemometers. These concentration and wind data were collected at different locations within the lagoon landscape. The values of L were estimated either directly using the wind statistics from a 3-D sonic anemometer or from the gradient Richardson number from wind speed and temperature gradient data obtained from two wind and temperature sensors on a 10-meter (m) tower. A 45m x 45m perforated pipe network floating on 65m x 65m irrigation pond was used as a synthetic distributed emission source with known release rates of methane gas. Measurements were collected with both smooth and rough berm surfaces. In this study, the PICs obtained on downwind berm yielded the best bLS accuracy when used with wind sensors located on the upwind, side berm, or downwind berm (78 - 101% accuracy). The PICs obtained from the middle of the pond consistently produced lower accuracy with all 3D anemometer locations (< 64% accuracy). These results indicate that the combination of concentration and wind sensor locations can be optimized to yield the highest accuracy of the bLS technique. Considering the practical difficulties in setting up equipment to measure emission from lagoons and the accuracies associated with various sensor locations, we concluded that the best configuration for a bLS measurement is to set both the wind and concentration sensors on the downwind berm. The effects of different values of L on the accuracy are ongoing and will be presented at the meeting.