Carbon Dioxide Analysis Heats Up
Over the last few decades, governments and businesses worldwide have been galvanized to address the threat of global warming. In December, 170 countries will meet in an attempt to pass the Copenhagen Protocol, the successor to 2005’s Kyoto Protocol. Set to expire in 2012, the Kyoto Protocol calls for a 5.2% yearly reduction in emissions from a country’s 1990 level. Recent efforts on a regional level include newly proposed emissions monitoring regulations in the US, China and the EU.
Legislative and regulatory actions relating to resource utilization and energy production are based on climate change and regional air quality data reported by global greenhouse gas (GHG) analysis centers. These centers and their measurement technologies are focusing on obtaining atmospheric GHG measurements in the field.
The quantification of GHG constituents is necessary to determine the rate of global warming. Carbon dioxide is the largest man-made contributor to the greenhouse effect. Today, global and national monitoring networks operate the most extensive field GHG-measurement programs. The largest network of CO2 measurement sites is the Global Atmosphere Watch (GAW) network for GHGs run by the World Meteorological Organization (WMO). The WMO is a part of the Global Climate Observing System, formed under the United Nations Framework Convention on Climate Change. GAW’s World Data Centers consist of 24 global, 640 regional and 73 contributing stations. Other GHG monitoring networks include the National Oceanic and Atmospheric Administration’s (NOAA) Earth Science Research Laboratory, the largest US-based monitoring network, which operates eight baseline observatories and 40 North American CO2 monitoring sites. Fluxnet, an umbrella network of regional flux-measurement networks, operates over 400 tower sites for measuring GHG flux, which is the rate at which GHG constituents are exchanged within the atmosphere. CarboEurope is an EU-based monitoring network, with 61 research centers in 17 European countries. The EU’s Integrated Carbon Observation System will add 40 atmospheric concentration-measurement sites and will be operational in 2013.
Atmospheric CO2 is measured in the field and the lab, but monitoring centers would like to increase the amount of CO2 monitoring in the field. The most common method of field-based CO2 monitoring is Nondispersive Infrared Spectroscopy (NDIR). According to Joe Bisby, product manager at LumaSense NDIR is more cost effective for CO2 measurements compared to other techniques, including Fourier Transform Infrared Spectroscopy (FTIR), Integrated Cavity Output Spectroscopy (ICOS) and Cavity Ringdown Spectroscopy (CRDS). According to Mr. Bisby, NDIR measures quantities of known substances, such as CO2, as opposed to both identifying and measuring unspecified substances. “That also allows those of us in the NDIR business to fine tune our instruments for that particular gas and its one optimum frequency, so that we can be very precise,” he explained.
Ilari Patrakka, product manager at Gasmet, an FTIR provider, told IBO that FTIR’s ruggedness, ability to measure multiple GHG constituents and simple calibration procedure make it a good fit for GHG monitoring in the field. Relatively new to this application are CRDS instruments, which are rugged and possess extremely high sensitivity, especially for their small size. CRDS systems can also measure CO2 isotopes, allowing researchers to determine if CO2 is of biogenic or petrogenic origin. Los Gatos Research provides ICOS systems and Picarro sells wavelength-scanned CRDS systems.
According to Dr. Arlyn Andrews, a physicist with the NOAA’s Carbon Cycle Greenhouse Gases group, the equipment chosen for global monitoring sites must be accurate and robust because it is generally left unattended and only visited once a week. However, the instruments that global monitoring networks are using for CO2 monitoring in the field are changing. “Our current instrument suite uses Li-Cor 7000 CO2/H2O analyzers for CO2. The NDIR approach has been the state of the art for a long time, but cavity-enhanced techniques, such as ICOS and CRDS, are beginning to play a larger role now, thanks to their demonstrated robustness and long-term stability,” said Dr. Andrews. According to Michael Woelk, president of Picarro, the use of CRDS in global monitoring networks is increasing. “[The WMO has] decided that they want to use a Picarro instrument to travel the globe and calibrate their instrumentation. We’re not at every one of these stations, but we’ll see how that plays out in the future.” Mr. Patrakka also noted a steady increase in FTIR sales for CO2 monitoring, and told IBO that increases in research funding and new carbon-trading schemes have raised interest in such systems.
NOAA sites are located on mountaintops, islands and other remote locations, where well-mixed air without high GHG variability can be sampled; but this too is changing. “Under those conditions, even collecting a flask sample just once per week provides a very useful record. Now, however, we are increasingly interested in sampling near important sources and sinks. For CO2, that means sampling near cities and in regions where biological fluxes of CO2 are large; for example, in croplands and forested regions,” said Dr. Andrews. The main factors driving interest in sources and sinks are studies showing that the current reporting of data is insufficient as well as the increasing availabilities of models with high resolution to better simulate point measurements, according to Dr. Andrews.
As Mr. Woelk explained, global monitoring networks and regulatory bodies need to increase the number of point-source measurement sites. “The WMO understands what is happening at a very high level, measuring what is called the ‘global well-mixed atmosphere.’ The challenge now is to be able to dissect that into continental and regional area fluxes, so you can actually measure what is coming off of Europe independently of what is coming off of, let’s say, Russia. The fact of the matter is that this can be done, but it cannot be done today because there aren’t enough measurements being taken around the world.”
Dr. Andrews explained that additional NOAA sites are in the works. “Plans that are ‘on the books’ call for expanding the NOAA network to 15 sites in the US, but we hope that urgency in the call for GHG regulation will lead to revised plans that call for NOAA to maintain something like 30–50 core sites,” said Dr. Andrews. “When GHG regulation is fully implemented, we anticipate that federal, state, and local partners may eventually establish and maintain something like 300–500 sites nationwide.”
Measurements of CO2 in the field are also taken using instruments on aircraft. The GAW’s Integrated Global Atmospheric Chemistry Observations program manages such programs. According to Dr. Andrews, much of the progress in aircraft measurement is taking place in the EU and Asia, while the US is slowly catching up. Although some of the same techniques used in the field can also be used in aircraft, the use of NDIR on aircraft can be problematic due to the technology’s high sensitivity to changing environmental conditions. CRDS instruments may also be used for this application. “Many of the newer CRDS and ICOS sensors can do multiple species such as CH4, N2O, CO, which in the past was achieved by flying multiple instruments, including GCs or tunable diode laser absorption spectroscopy systems—all of which were extremely customized, expensive and difficult to integrate onto airborne platforms,” she said. Future plans for CO2 monitoring include extending the use of satellites for global atmosphere measurements.