Advances in Environmental Measurement Systems: Remote Sensing of Urban Methane Emissions and Tree Sap Flow Quantification

Abstract

Two projects were designed and implemented in an effort to gather high quality data focused on two metrics important to environmental science: methane emissions from cities and the flow rate of sap in trees. Methane is a powerful greenhouse gas that is emitted to the atmosphere in many ways. In cities, these sources include landfills and fugitive emissions from natural gas infrastructure, and the quantification of these emissions is an important area of research. Sap flow, which is a direct measure of the water use of a tree, is also vital to the understanding of rates of carbon exchange between the atmosphere and the biosphere. A recently developed technology, the EM27/SUN spectrometer, was deployed in the city of Indianapolis for a five day measurement campaign. Five of the sensors were spread across the greater urban area so that atmospheric methane concentrations could be measured as air moved across the city. An inverse modelling framework was designed which combined these observations with a trace gas transport model and information on the location and predicted magnitude of methane sources. This framework was able to estimate the magnitude of emissions from small diffuse sources, such as individual pipeline leaks, spread across the urban domain. Diffuse emissions from the city were found to be 47.0 +- 14.8 Gg/yr, which was more twice the suggested by some bottom-up inventories, but consistent with other observation and modeling efforts. The insights gained during this project are important to future urban measurement and modeling efforts, especially those using remote sensing sensors like the EM27/SUN or current and future satellites. The second project was the development of new sensor to measure the flow rate of sap in trees. There are two issues with conventional sap flow sensors which limit their widespread use. The first is that the probes are costly and difficult to manufacture. The second major issue is that installing the probes correctly, so that they are aligned properly with each other, is quite difficult which introduces large uncertainties in the data. The new design, the ribbonized sap flow (RSF) sensor solves these issues by placing all of the components of the sensor onto a single circuit board that can be easily slid into a slot made in the tree, and can be manufactured at much lower cost. This sensor, and a custom control and data collection system, were designed, built, calibrated, and tested in field conditions in multiple locations. The results from the calibration suggest that the RSF system has the sensitivity and accuracy needed to make meaningful sap flow measurements, and the reduced cost over conventional systems make in an appealing option for future sap flow deployments.