Synchronized N2O/CH4/H2O/NH3 plume mobile measurement system based on low-power open-path laser analyzers

In recent years, vehicle-based, multiple-gas mobile sensing platforms have been developed and extensively utilized for greenhouse gases (GHGs) and air pollutant emission studies. Closed-path analyzers are currently the primary equipment used for plume observations. However, the closed-path approach, poses sampling challenges for the species such as water vapor (H₂O) and ammonia (NH₃) that readily adsorb and desorb from the instrument inlets, tubings, and optical cells. Due to the different adsorption characteristics of each gas, the plume signals generated during the sampling process may become desynchronized. In addition, many mobile systems are deployed on fuel-powered vehicles, which emit exhaust that can contaminate the detected plume signals. These issues can increase the complexities in subsequent data processing tasks.

This work reports the field deployment of a multiple trace gas plume sensing platform, equipped with open-path N₂O, CH₄, H₂O and NH₃ quantum-cascade laser analyzers (model HT8500, HT8600P, HT8700, respectively) with a 10 Hz sampling time resolution. The plume monitoring system with a total power consumption of no more than 150W allows it to be easily driven by an electric vehicle. Utilizing the open-path N₂O/CH₄/H₂O/NH₃ gas analyzers eliminates the need for a pressure-controlled enclosed gas cell, the associated tubing systems, and power- hungry pump. The ambient air flows unrestricted through the optical path, enabling analyzers to achieve high temporal resolution, high response rates, and reduced sampling artifacts and power consumption compared to their closed-path gas analyzer counterparts. This open-path configuration not only eliminates the influence of exhaust emission signals from vehicles using fossil fuel engines, but also achieves perfect plume synchronization, which is crucial for the real-time identification of diffuse sources using correlations between different molecules in measured plumes.

The mobile platform has been field deployed in different field experiments including livestock farms, ammonia plants, cold storage facilities, wastewater treatment plants, and urban traffic roads in China. Our study has identified a substantial increase in ammonia concentrations adjacent to rivers, with an average increment of ~37 ppb relative to a few ppb background concentration. We observed that the peak methane concentration near a wastewater treatment plant reached 7539 ppb. Furthermore, the ratio of methane plume signal intensity to ammonia plume signal intensity in the vicinity of industrial areas is ~10, as opposed to non-industrial areas where this ratio is significantly reduced. The synchronized plume significantly enhances the efficiency of extracting effective plume data from the raw signals acquired from different gas analyzers.