Environmental and Atmospheric Monitoring Using Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)

The demands of environmental and atmospheric monitoring continue to grow in terms of the number of sites and types of facilities that are being monitored (fixed, mobile, shipboard, airborne) and the number of trace gases that must be tracked. These trace gases must be quantified for scientific applications, auditing, and/or as a result of government regulations. Fortunately, a single laser-based technology—off-axis integrated cavity output spectroscopy (OA-ICOS)—can address many of these applications with the requisite speed, sensitivity, accuracy, and reliability. Moreover, OA-ICOS analyzers also deliver the necessary combination of portability, remote operation, and affordability. These rugged analyzers can also report isotope ratios for several trace gases. This article briefly describes this versatile technology and examines some of the many applications that it currently supports.

Measurements of atmospheric gases

Increasing levels of greenhouse gases, specifically carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water (H2O) vapor, are widely recognized as one of the preeminent technological, social, and political challenges. Increased density and frequency of measurements are needed to fully understand the interrelationships of the various cycles involving these gases. In addition, the value of these concentration measurements can be greatly enhanced if the measurement technique also delivers stable isotope ratios (e.g., δ13C, δ17O, δ18O in CO2; δ15N and δ18O in N2O; δ13C in CH4) that provide telltale information about the origin (e.g., fossil fuel, biogenic, or other) of these gases.

There are also several other pollutants, usually manmade, that are increasingly being regulated and monitored. Nitrogen dioxide (NO2) is the main culprit responsible for the brown color in smog; it is produced by gasoline and diesel engines and other fossil-fuel burning systems. It efficiently produces low-altitude ozone, a health hazard.

Atmospheric ammonia (NH3) is mainly produced by intensified agricultural activity, and its principal damage comes from its role in aerosol formation. Atmospheric nitrous oxide (N2O) is also on the rise as a result primarily of agriculture and biomass burning. Moreover, it has a large greenhouse warming potential (310 times that of CO2 over 100 years).

Carbon monoxide (CO) is widely produced in fossil-fuel combustion and also by volcanic activity. It is highly toxic and also reacts in the atmosphere to produce CO2 and ozone. Hydrogen sulfide (H2S) is an even more toxic atmospheric component. The main anthropogenic source of this foul-smelling gas is from petroleum.

These are just some of the atmospheric gases that are currently being measured either for research purposes or to monitor compliance with increasingly stringent government regulations. And the list continues to grow.

OA-ICOS: an ultrasensitive and universal monitoring technology

Historically, these various gases have been measured in the lab, and occasionally in the field, using an incredibly wide array of methods. These methods ranged from gas chromatography to ultraviolet absorption to monitoring color changes in chemically treated paper strips, and even included measuring the chemiluminescence from controlled chemical reactions. All of these have drawbacks in terms of their sensitivity (absolute) accuracy, speed, linearity, size, dynamic range, reliability, and/or ruggedness for portability. Fortunately, OA-ICOS can now support all of these atmospheric monitoring applications with none of the previous limitations. It also delivers sensitivity down to parts per trillion, as is sometimes required for NO2, HF, or HCl monitoring.

What is OA-ICOS? All of these molecules have absorption spectra in the visible or near-IR containing numerous sharp lines corresponding to individual ro-vibronic transitions. However, at low concentrations, e.g., parts per million and parts per billion, the absorption is too weak to measure quantitatively over a short path with inexpensive diode lasers and conventional absorption spectroscopy.

Cavity ringdown spectroscopy

The first practical technique to overcome this limitation was cavity ringdown spectroscopy (CRDS). Here, a gas cell is bounded by highly reflective mirrors (99.99% typical) to create a resonant optical cavity. A laser beam is resonantly coupled into this cell, and bounces back and forth between these mirrors. After the beam is abruptly turned off, the time rate of decay of the laser light in the cavity (called the ringdown time) is measured and used to determine the gas concentration in the cell.

CRDS was a major advance because it was the first optical technique to match the sensitivity of conventional gas chromatography. However, CRDS has practical limitations that make it nonideal for many applications. Specifically, the resonant wavelength coupling requires subnanometer optomechanical precision and stability. Thus field service is impractical. In addition, resonant CRDS systems require extreme laser wavelength monitoring and control. These challenges can, at times, be overcome in the laboratory but with a trade-off of increased instrument cost, complexity, and robustness.

Figure 1In OA-ICOS, the laser enters the optical cavity (measurement cell) at an off-axis angle, resulting in a multibounce path between the mirrors. This eliminates limitations of earlier optical methods (extreme alignment stability and laser wavelength locking) based on resonance coupling of the laser, but maintains the extremely long effective pathlength (e.g., thousands of meters) for ultrahigh-detection sensitivity and precision.

OA-ICOS, originally developed in cooperation with scientists at Harvard University (Cambridge, MA) (Prof. James Anderson’s group), was created in order to overcome these limitations, while still maintaining extremely high sensitivity and precision. OAICOS uses off-axis, nonresonant alignment of the laser beam to the cavity (Figure 1). This beam alignment or trajectory is not unique and is easy to achieve, making OA-ICOS instruments orders of magnitude more robust and less sensitive to thermal changes and vibrations. As a result, OA-ICOS instruments are relatively simple to manufacture and service, inexpensive to build, and robust. The OA-ICOS measurement combines the advantages of CRDS with conventional high-resolution laser absorption spectroscopy by providing direct absorption measurements with optical pathlengths thousands of meters long.

Advantages of OA-ICOS: continuous scanning of laser wavelength and fast, real-time data reporting

Unlike CRDS, which enables at most only limited wavelength sampling (discrete jumps between wavelengths), OA-ICOS easily allows continuous scanning of the laser wavelength and thus enables recording of fully resolved absorption spectra. This is a critical advantage that permits OA-ICOS to provide accurate measurements of complex and/or contaminated samples without cross-sensitivity. Moreover, OA-ICOS can be operated at any wavelength from the UV through the IR. As a result, instruments can target the strongest absorption lines, allowing sensitivity below 1 ppt for some gases. Furthermore, because it is an intensity measurement, only OA-ICOS can deliver several decades of linear dynamic range.

OA-ICOS enables rugged, automated instruments with unmatched sensitivity and absolute accuracy, and which have been optimized for a wide range of gases (e.g., CO2, CH4, H2O, H2S, N2O, NO2, NH3, HF, HCl, OCS, and others). Moreover, the high sensitivity and excellent signal-to-noise of OA-ICOS means these instruments can be used for continuous measurement with real-time reporting of data at rates as fast as 10 Hz. Plus, these instruments can be extremely compact, enabling remote, portable, and airborne UAV (unmanned aerial vehicle) atmospheric monitoring applications.

OA-ICOS also allows stable isotope analysis, because the wavelengths of individual absorption lines are atomic mass dependent. For instance, in the case of water, a single laser scan can include absorption lines assigned to several isotopologues, including H218O, H217O, and HD16O. This permits simultaneous measurement of three stable isotope ratios—δ17O, δ18O, and δD—directly from samples, eliminating the time, cost, and complexity of isotope ratio mass spectrometry (IRMS).

Examples of OA-ICOS in atmospheric monitoring

Mobile urban nitrogen dioxide (NO2) mapping

Figure 2The high speed and rugged performance of a portable LGR Nitrogen Dioxide Analyzer enabled NO2 to be mapped at high spatial resolution around the entire SF Bay Area in less than 4 hr.

The characteristics of OA-ICOS analyzers make them well suited for mobile monitoring applications. As an example, a recent study involved a standard LGR Nitrogen Dioxide (NO2) Analyzer (Los Gatos Research [LGR], Mountain View, CA) being driven around the San Francisco Bay Area in a car. These data were overlaid on a Google Earth map graphic (Figure 2). The vehicle was driven at speeds up to 55 mph, so that the entire trip around the Bay Area was completed in less than 4 hr. Even at high speed, the 5-Hz data rate enabled very high spatial resolution and sensitivity. Not surprisingly, the data summary clearly shows the impact of high-traffic areas on elevated NO2 signals.

Airborne mapping of atmospheric ammonia (NH3)

One of the advantages of OA-ICOS over earlier optical methods like CRDS, which is effectively limited to lasers operating in the 1.3–1.6 μm range, is its ability to be used at any wavelength between 0.4 and 11 μm. For example, the LGR Trace Ammonia Analyzer (TAA), recently supplied to Pacific Northwest National Lab (PNNL, Richmond, WA), uses a quantum cascade laser operating near 9.65 μm to probe strong fundamental vibrational bands and yield sub-ppb sensitivity every second. PNNL researchers operated the TAA in an aircraft to obtain high-sensitivity measurements of ammonia (NH3) while flying at an altitude of 1000 feet over dairy farms. (The instrument is also capable of providing measurements at measurement rates up to 10 Hz.) The plot in Figure 3 shows a summary of the data that were recorded with 50-m spatial resolution even while flying at 100 meters per second.

Figure 3The LGR Trace Ammonia Analyzer recorded ammonia in air 1000 ft above dairy feedlots. Measurements, which were reported twice per second, provided 50-m spatial resolution while flying 100 m/sec. The map clearly shows significant ammonia levels (levels indicated by false color map and by height of “fence plot”) even at altitude. (Data courtesy of Pacific Northwest National Laboratory.)

Automated measurement of isotope ratios in rainfall

At the Hillslope and Watershed Hydrology Laboratory at Oregon State University (Corvallis), Prof. Jeffrey McDonnell and co-workers have been using OA-ICOS-based isotopic water analyzers (LGR model LWIA-24d) for long-term hydrological studies. (This is a dual-isotopic water analyzer that measures δ18O and δ2H ratios in real time, unlike the latest triple-isotope model that additionally measures δ17O.)

Stable isotope ratios provide a unique method of tracking the history and origins of individual rainfall events. The high-speed capabilities of OA-ICOS (unlike slow, expensive lab-bound IRMS) have enabled real-time monitoring of oxygen and hydrogen ratios, which have revealed significant variations during a single event (see Figure 4)

Figure 4Isotope hydrologists quantify the relationship between δ18O and δ2H to determine d-excess in liquid water or water vapor. This parameter would be expected to remain fairly constant during a rainfall event with a single origin. However, the high-frequency measurement capabilities of OA-ICOS revealed large isotopic variations during prolonged rainfall event (see inset) indicating multiple sources. (Data courtesy of Prof. Jeffrey McDonnell.)

Use of OA-ICOS analyzers in atmospheric monitoring and isotope hydrology

Numerous scientific applications, including atmospheric monitoring and isotope hydrology, require instrumentation that operates unattended and that reports measurements with extremely high precision with minimal maintenance. Fortunately, OA-ICOS now provides the requisite combination of sensitivity, selectivity, linearity, dynamic range, portability, and automated operation to support these disparate applications.

The authors are with Los Gatos Research, Inc., 67 East Evelyn Ave., Mountain View, CA 94041-1529, U.S.A.; tel.: 650-965-7772; fax: 650-965-7074; info@LGRinc.com.