Sensitive and Versatile Tropospheric Hydrogen Chloride (HCl) Instrument

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Hydrogen chloride (HCl) gas is key to the understanding of tropospheric halogen cycles, since it is a relatively abundant gas-phase halogen species that serves as a source, an intermediate, or an end product in the chemistry of more reactive chlorine compounds. It is, however, greatly undersampled in the troposphere. Accurate in situ measurements of HCl are critically needed to better quantify its sources, sinks, transport, and ultimately to better understand the processes that govern tropospheric halogen activation. This project will develop an ultra-sensitive optical sensor for HCl gas. The sensor is based on cavity ring-down spectroscopy (CRDS), a method for high-sensitivity optical absorption. Recent advances in optical spectroscopy make possible the development of a versatile (i.e., small, lightweight, low power consumption), accurate and specific HCl sensor with high sensitivity and time response. The HCl sensor will employ a near-infrared diode laser for continuous-wave CRDS. Although the sensor is primarily targeted at HCl detection, it will allow simultaneous methane detection with high sensitivity and precision. The development of such an instrument will facilitate studies of the atmospheric role of HCl and may enable deployment on mobile platforms, such as ships and aircraft.Improved understanding of halogen activation in general, and HCl in particular, has broad implications for modeling of both regional air quality and climate. Within polluted coastal environments, HCl is a primary product in acid displacement reactions with sea salt and is a key source for atomic chlorine. Halogen activation cycles involving HCl may also influence the climate system through the oxidation of methane and other organic gases, conversion of marine sulfur emissions to sulfate aerosol, and, potentially, regulation of tropospheric ozone and other oxidant levels. In terms of future instrumentation, the sensor design and architecture proposed here will provide a blueprint for development of similar sensors for other species relevant to atmospheric chemistry (e.g., acetylene, ammonia). The project will also provide unique educational opportunities for graduate and undergraduate students in mechanical engineering at Colorado State University (CSU). Particular efforts will be made to recruit women students and students from underrepresented groups to participate in the project. The students will design and develop a state-of-the-art instrument for atmospheric sensing and will interact with researchers in atmospheric chemistry at CSU, from other universities, and from nearby federal environmental research laboratories. Results of the research will be incorporated into courses at CSU, and disseminated at academic conferences.