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.
The present research focuses on development of an ultra-sensitive optical sensor for hydrogen chloride (HCl) mixing ratios appropriate for tropospheric measurements. Halogen species are well-known for their important roles in atmospheric chemistry, influencing, for example, the abundance of ozone in the stratosphere. Within the troposphere, halogens have been implicated in numerous processes that affect air quality and climate, yet their tropospheric sources and the mechanisms for release of reactive halogen species to the gas phase remain poorly understood. Hydrochloric acid, HCl, 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 under-sampled 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. We have developed a versatile (i.e., small, lightweight, low power consumption), accurate and specific HCl sensor with high sensitivity and time response. The sensor employs a near-infrared diode laser with the continuous-wave (cw-) CRDS technique. A distributed-feedback diode laser at 1742 nm coupled to a high finesse optical cavity to make sensitive and quantifiable concentration measurements of HCl based on optical absorption. The instrument has a (1-sigma) limit of detection of <20 pptv in 1 min and has high specificity to HCl. The measurement response time to changes in input HCl concentration is <15 s. Validation studies with a previously calibrated permeation tube setup show an accuracy of better than 10 %. In 2011, as part of the NACHTT campaign, the CRDS sensor was preliminarily tested in the field with two other HCl instruments (mist chamber and chemical ionization mass spectrometry), all of which were in broad agreement. The mist chamber and CRDS sensors both showed a 400 pptv plume within 50 pptv agreement The sensor also allows simultaneous sensitive measurements of water and methane, and minimal hardware modification would allow detection of other near-infrared absorbers.
