0 The Analytical Process THE “MOST IMPORTANT” ENVIRONMENTAL DATA SET OF THE TWENTIETH CENTURY 400 350 Keeling's data: Increase in CO2 from burning fossil fuel 300 250 200 150 (parts per million by volume) 2 100 CO 50 0 800700 600 500 400 300 200 100 0 Thousands of years before 1950 Atmospheric CO2 has been measured since 1958 at Mauna Loa Historic atmospheric CO2 data are derived from analyzing air bubbles trapped Observatory, 3 400 meters above sea level on a volcano in Hawaii. [Forrest in ice drilled from Antarctica. Keeling’s measurements of atmospheric CO2 give M. Mims III, www.forrestmims.org/maunaloaobservatory.html, photo taken in 2006.] the vertical line at the right side of the graph. [Ice core data from D. Lüthi et al., Nature, 2008, 453, 379. Mauna Loa data from http://scrippsco2.ucsd.edu/data/in_situ_co2/ monthly_mlo.csv.] In 1958, Charles David Keeling began a series of precise measurements of atmospheric carbon dioxide that have been called “the single most important environmental data set taken in the 20th century.”* A half century of observations now shows that human beings have increased the amount of CO2 in the atmosphere by more than 40% over the average value that existed for the last 800 000 years. On a geologic time scale, we are unlocking all of the carbon sequestered in coal and oil in one brief moment, an outpouring that is jarring the Earth away from its previous condition. The vertical line at the upper right of the graph shows what we have done. This line will continue on its vertical trajectory until we have consumed all of the fossil fuel on Earth. The consequences will be discovered by future generations, beginning with yours. *C. F. Kennel, Scripps Institution of Oceanography. n the last century, humans abruptly changed the composition of Earth’s atmosphere. We Ibegin our study of quantitative chemical analysis with a biographical account of how Charles David Keeling came to measure atmospheric CO2. Then we proceed to discuss the general nature of the analytical process. 0-1 Charles David Keeling and the Measurement of Atmospheric CO2 Notes and references appear after the last Charles David Keeling (1928–2005, Figure 0-1) grew up near Chicago during the Great chapter of the book. Depression.1 His investment banker father excited an interest in astronomy in 5-year-old Keeling. His mother gave him a lifelong love of music. Though “not predominantly interested in science,” Keeling took all the science available in high school, including a wartime course in aeronautics that exposed him to aerodynamics and meteorology. In 1945, he enrolled in a 0-1 Charles David Keeling and the Measurement of Atmospheric CO2 1 FIGURE 0-1 Charles David Keeling and his wife, Louise, circa 1970. [Courtesy Ralph Keeling, Scripps Institution of Oceanography, University of California, San Diego.] summer session at the University of Illinois prior to his anticipated draft into the army. When World War II ended that summer, Keeling continued at Illinois, where he “drifted into chemistry.” Upon graduation in 1948, Professor Malcolm Dole of Northwestern University, who had known Keeling as a precocious child, offered him a graduate fellowship in chemistry. On Keeling’s second day in the lab, Dole taught him how to make careful measurements with an analytical balance. Keeling went on to conduct research in polymer chemistry, though he had no special attraction to polymers or to chemistry. A requirement for graduate study was a minor outside of chemistry. Keeling noticed the book Glacial Geology and the Pleistocene Epoch on a friend’s bookshelf. It was so interest- ing that he bought a copy and read it between experiments in the lab. He imagined himself “climbing mountains while measuring the physical properties of glaciers.” In graduate school, Keeling completed most of the undergraduate curriculum in geology and twice interrupted his research to hike and climb mountains. In 1953, Ph.D. polymer chemists were in demand for the new plastics industry. Keeling had job offers from manufacturers in the eastern United States, but he “had trouble seeing the To vacuum future this way.” He had acquired a working knowledge of geology and loved the outdoors. Professor Dole considered it “foolhardy” to pass up high-paying jobs for a low-paying post- Thermometer doctoral position. Nonetheless, Keeling wrote letters seeking a postdoctoral position as a measures chemist “exclusively to geology departments west of the North American continental divide.” temperature He became the first postdoctoral fellow in the new Department of Geochemistry in Harrison Brown’s laboratory at Caltech in Pasadena, California. Stopcock One day, “Brown illustrated the power of applying chemical principles to geology. He CO gas 2 suggested that the amount of carbonate in surface water . might be estimated by assuming the water to be in chemical equilibrium with both limestone [CaCO ] and atmospheric carbon Pressure 3 Calibrated measured in dioxide.” Keeling decided to test this idea. He “could fashion chemical apparatus to function volume millimeters in the real environment” and “the work could take place outdoors.” of mercury Keeling built a vacuum system to isolate CO2 from air or acidified water. The CO2 in Glass dried air was trapped as a solid in the vacuum system by using liquid nitrogen, “which had pointer Mercury recently become available commercially.” Keeling built a manometer to measure gaseous CO marks 2 calibrated by confining the gas in a known volume at a known pressure and temperature (Figure 0-2 and volume Box 3-2). The measurement was precise (reproducible) to 0.1%, which was as good or better than other procedures for measuring CO . FIGURE 0-2 A manometer made from a 2 glass U-tube. The difference in height between Keeling prepared for a field experiment at Big Sur. The area is rich in calcite (CaCO3), the mercury on the left and the right gives the which would, presumably, be in contact with groundwater. Keeling “began to worry . pressure of the gas in millimeters of mercury. about assuming a specified concentration for CO2 in air.” This concentration had to be Box 3-2 provides more detail. known for his experiments. Published values varied widely, so he decided to make his own 2 CHAPTER 0 The Analytical Process measurements. He had a dozen 5-liter flasks built with stopcocks that would hold a vacuum. He weighed each flask empty and filled with water. From the mass of water it held, he could calculate the volume of each flask. To rehearse for field experiments, Keeling measured air samples in Pasadena. Concentrations of CO2 varied significantly, apparently affected by urban emissions. Not being certain that CO2 in pristine air next to the Pacific Ocean at Big Sur would be constant, he collected air samples every few hours over a full day and night. He also collected water samples and brought everything back to the lab to measure CO2. At the suggestion of Professor Sam Epstein, Keeling provided samples of CO2 for Epstein’s group to measure carbon and oxygen isotopes with their newly built isotope ratio mass spectrometer. “I did not anticipate that the procedures established in this first experiment would be the basis for much of the research that I would pursue over the next forty-odd years,” recounted Keeling. Contrary to hypothesis, Keeling found that river water and groundwater contained more dissolved CO2 than expected if the dissolved CO2 were in equilibrium with the CO2 in the air. Keeling’s attention was drawn to the diurnal pattern that he observed in atmospheric CO2. Diurnal means the pattern varies between Air in the afternoon had an almost constant CO2 content of 310 parts per million (ppm) by night and day. volume of dry air. The concentration of CO2 at night was higher and variable. Also, the higher 13 12 the CO2 content, the lower the C/ C ratio. It was thought that photosynthesis by plants would draw down atmospheric CO2 near the ground during the day and respiration would restore CO2 to the air at night. However, samples collected in daytime from many locations had nearly the same 310 ppm CO2. Keeling found an explanation in a book entitled The Climate Near the Ground. All of his samples were collected in fair weather, when solar heating induces afternoon turbulence that mixes air near the ground with air higher in the atmosphere. At night, air cools and forms a stable layer near the ground that becomes rich in CO2 from respiration of plants. Keeling had discovered that CO2 is near 310 ppm in the free atmosphere over large regions of the Northern Hemisphere. By 1956, his findings were firm enough to be told to others, including Dr. Oliver Wulf of the U.S. Weather Bureau, who was working at Caltech. Wulf passed Keeling’s results to Harry Wexler, Head of Meteorological Research at the Weather Bureau. Wexler invited Keeling to Washington, DC, where he explained that the International Geophysical Year commencing in July 1957 was intended to collect worldwide geophysical data for a period of 18 months. The Bureau had just built an observatory near the top of Mauna Loa volcano in Hawaii, and Wexler was anxious to put it to use. The Bureau wanted to measure atmospheric CO2 at remote locations around the world. Keeling explained that measurements in the scientific literature might be unreliable. He proposed to measure CO2 with an infrared spectrometer that would be precisely calibrated with gas measured by a manometer. The manometer is the most reliable way to measure CO2, but each measurement requires half a day of work. The spectrometer could measure several samples per hour but must be calibrated with reliable standards.