MIT12_009S11_lec6_9

MIT12_009S11_lec6_9 - 3 Short-term evolution of atmospheric...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 3 Short-term evolution of atmospheric CO 2 We have spoken of volcanism as the long-term source of CO 2 . Among the other sources, the respiration ux is about 3 orders of magnitude greater than volcanism, and fossil-fuel combustion is about one and one-half orders of magnitude greater. Whatever the source, we consider the following questions: How can we identify the source? How long does CO 2 stay in the atmosphere? We shall answer these question in a precise way that pertains to short, roughly decadal, time scales. We first consider the growth of atmospheric CO 2 since the mid-20th century. 3.1 The Keeling curve Atmospheric CO 2 concentrations have been measured monthly at Mauna Loa Observatory, Hawaii, since 1958. The measurements, begun by C. David Keeling, report CO 2 concentration as a dry mole fraction: the number of molecules of carbon dioxide divided by the number of molecules of dry air multiplied by one million (ppm). The resulting plot is known as the Keeling curve : 38 1960 1970 1980 1990 2000 2010 300 320 340 360 380 400 year pCO2 (ppm) Data from P. Tans, NOAA/ESRL ( www.esrl.noaa.gov/gmd/ccgg/trends/ ). Studies of CO 2 in ice cores show that this increase is the latest chapter in process that started about 200 years ago: 500 1000 1500 2000 260 280 300 320 340 360 380 year pCO2 (ppm) Data from the Law Dome ice core, Antarctica [11]. An obvious question arises: Why is CO 2 increasing? To provide a precise answer, we first digress to a discussion of carbon isotopes. 39 3.2 Carbon isotopes 3.2.1 Natural abundance Reference : Emerson and Hedges [12]. Every atom of carbon has Z = 6 protons. Z is the atomic number . When there is no net charge, each atom of C has 6 electrons. However the mass number A varies: A = 12, 13, or 14 . The variation in mass number derives from the variations in the neutron number N = Z A. Each isotope of carbon corresponds to a specific neutron number N , and therefore mass number A . These are named according to their mass number: carbon-12, carbon-13, carbon-14 , and are generally written symbolically as 12 C , 13 C , 14 C . 14 C is radioactive, and 12 C and 13 C are stable. About 98 . 9% of Earths carbon is 12 C and nearly all the rest is 13 C; thus 13 C 10 2 , C where the denominator represents the sum of all carbon. 14 C is much less abundant: 14 C 10 12 . C 14 C is naturally produced in the upper atmosphere by cosmic rays, which can shatter a nucleus (N or O), releasing neutrons, some of which are absorbed by 14 N such that 14 14 7 N + n 6 C + p + + e . Thus 14 N is converted to 14 C, releasing a proton and an electron. 40 3.2.2 Radioactive decay 14 C, being unstable, converts back to 14 N....
View Full Document

Page1 / 22

MIT12_009S11_lec6_9 - 3 Short-term evolution of atmospheric...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online