Numerical results are not gospel. After crunching them again, even if they turn out to be valid, time is not lost. Investigating will often teach one something. So to learn a little more, I wondered how the EPA arrived at the figure of 8.92*10-3 metric tons CO2/gallon of gasoline.
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First, since I'm a middle-aged Canadian half-raised on imperial gallons and half on liters, I'll begin with a more trivial matter---a conversion:
8.92X10-3 metric tons CO2/gallon of gasoline * (1000 kg/metric ton)(U.S. gallon/ 3.7854 L) =
2.36 kg CO2 / L of gasoline burnt.
Gasoline is a liquid mixture consisting mostly of heptane (C7H16), isooctane (C8H18), cyclopentane (C5H10) and ethyl benzene (C8H10). A heptane molecule has 7 carbons with 12.0 units of mass apiece and 16 hydrogens at 1.01 units apiece. The fraction of carbon in heptane is 7*12/(7*12+16*1.01) = 0.839. Similarly, for the other compounds mentioned, the fractions are 0.840, 0.856 and 0.904, respectively. The proportion of each of the substances in gasoline varies from about 20 to 30%, but a weighted average translates into an approximation of 86% carbon (C) content.
If each carbon atom was oxidized into carbon dioxide( CO2 ), then every gram of carbon would yield 44.0/12.0 grams of carbon dioxide, the ratio being equal to the molar mass of CO2 divided by that of C.
In reality, although some of the carbon in gasoline becomes carbon monoxide(CO), most of it is eventually further oxidized to carbon dioxide. But a bit of the carbon gets trapped in a soup of particulate matter. According to this source, only 0.015 or 1.5 % of the carbon in gasoline remains unoxidized.
Since the composition of gasoline varies, so will its density, but we'll use the vehicle-value from the engineering toolbox of 0.737 kg/L.
Now let's combine all of the above and compare it to the EPA's figure:
1.00 L of gasoline *0.737 kg gasoline/L *0.86 C/gasoline * 44.0 CO2/12.0 C *(1-0.015) = 2.3 kg CO2, quite close to the EPA value of 2.36 kg/L. However, I'm wondering how the second decimal place and third significant figure is justified, given that the carbon percentage is known with less precision.
The unfiltered carbon dioxide emitted by gasoline combustion mixes with whatever is produced by other fossil fuel combustion, cement production, along with respiration, fermentation, volcano emissions and and organic decomposition. Much of this input is removed from the atmosphere by rain, ocean water, and photosynthesis.
The geological record indicates that the steady state gets has been disrupted continuously. At times the input rate of carbon dioxide into the atmosphere is greater, which contributes to an overall warming trend because the gas acts as an invisible blanket. (Water vapor is a stronger greenhouse gas, but whatever is added by man or the rest of nature is trivial because of the much larger original quantities involved.) When carbon dioxide levels decrease, cooling occurs.
The combustion of fossil fuels is the main reason that carbon dioxide in the atmosphere have increased from year to year. What's not mentioned too often is the role that the increase in population has played in boosting carbon dioxide emissions. From 1962 to 2011, the world population has increased by a factor of 2.23. If you consider the additional amount of cellular respiration per person and the extra animal domestication associated with a bigger population, those two alone account for 11% of the total amount of human emissions in 2011. In the same time span, consumption of fossil fuels has gone up faster (by a factor of about 3) than the population growth, up by a factor of 2.2 in half a century. (In the United States fossil fuel combustion has increased by a factor of 3.1, while population has gone up only by a factor of 1.7) But if the extra population born in the last half century was only burning as much fuel as we did in 1962, the increase in population would still account for 34% of all 2011 emissions. As it stands, in conjunction with the increased combustion rates, the 50 year-jump in population now accounts for 55% of all current anthropogenic contributions of CO2.
Notice in the table below, based on Mauna Loa measurements, that the average annual rate of CO2 increase has gone up from one decade to the next, as pointed out by CO2now.org. But I've added a new column to reveal that that the acceleration over the previous decade has been erratic.
The warming trend in the atmosphere has also been difficult to pin down---almost negligent in some areas, but pronounced in the Arctic in the last few decades. At some point, with a continuous increase in greenhouse gas emissions, there probably will be a significant increase in global temperatures, but there is no way to know when that will occur. The problem is that, on one hand, from studying isotope ratios in ice cores, we have non-detailed patterns over long periods of time, versus what we have in the present: meticulous data over a very short time span.
http://co2now.org/Current-CO2/CO2-Trend/
http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0889.1984.tb00245.x/pdf (free article)
First, since I'm a middle-aged Canadian half-raised on imperial gallons and half on liters, I'll begin with a more trivial matter---a conversion:
8.92X10-3 metric tons CO2/gallon of gasoline * (1000 kg/metric ton)(U.S. gallon/ 3.7854 L) =
2.36 kg CO2 / L of gasoline burnt.
Gasoline is a liquid mixture consisting mostly of heptane (C7H16), isooctane (C8H18), cyclopentane (C5H10) and ethyl benzene (C8H10). A heptane molecule has 7 carbons with 12.0 units of mass apiece and 16 hydrogens at 1.01 units apiece. The fraction of carbon in heptane is 7*12/(7*12+16*1.01) = 0.839. Similarly, for the other compounds mentioned, the fractions are 0.840, 0.856 and 0.904, respectively. The proportion of each of the substances in gasoline varies from about 20 to 30%, but a weighted average translates into an approximation of 86% carbon (C) content.
If each carbon atom was oxidized into carbon dioxide( CO2 ), then every gram of carbon would yield 44.0/12.0 grams of carbon dioxide, the ratio being equal to the molar mass of CO2 divided by that of C.
In reality, although some of the carbon in gasoline becomes carbon monoxide(CO), most of it is eventually further oxidized to carbon dioxide. But a bit of the carbon gets trapped in a soup of particulate matter. According to this source, only 0.015 or 1.5 % of the carbon in gasoline remains unoxidized.
Since the composition of gasoline varies, so will its density, but we'll use the vehicle-value from the engineering toolbox of 0.737 kg/L.
Now let's combine all of the above and compare it to the EPA's figure:
1.00 L of gasoline *0.737 kg gasoline/L *0.86 C/gasoline * 44.0 CO2/12.0 C *(1-0.015) = 2.3 kg CO2, quite close to the EPA value of 2.36 kg/L. However, I'm wondering how the second decimal place and third significant figure is justified, given that the carbon percentage is known with less precision.
The unfiltered carbon dioxide emitted by gasoline combustion mixes with whatever is produced by other fossil fuel combustion, cement production, along with respiration, fermentation, volcano emissions and and organic decomposition. Much of this input is removed from the atmosphere by rain, ocean water, and photosynthesis.
The geological record indicates that the steady state gets has been disrupted continuously. At times the input rate of carbon dioxide into the atmosphere is greater, which contributes to an overall warming trend because the gas acts as an invisible blanket. (Water vapor is a stronger greenhouse gas, but whatever is added by man or the rest of nature is trivial because of the much larger original quantities involved.) When carbon dioxide levels decrease, cooling occurs.
The combustion of fossil fuels is the main reason that carbon dioxide in the atmosphere have increased from year to year. What's not mentioned too often is the role that the increase in population has played in boosting carbon dioxide emissions. From 1962 to 2011, the world population has increased by a factor of 2.23. If you consider the additional amount of cellular respiration per person and the extra animal domestication associated with a bigger population, those two alone account for 11% of the total amount of human emissions in 2011. In the same time span, consumption of fossil fuels has gone up faster (by a factor of about 3) than the population growth, up by a factor of 2.2 in half a century. (In the United States fossil fuel combustion has increased by a factor of 3.1, while population has gone up only by a factor of 1.7) But if the extra population born in the last half century was only burning as much fuel as we did in 1962, the increase in population would still account for 34% of all 2011 emissions. As it stands, in conjunction with the increased combustion rates, the 50 year-jump in population now accounts for 55% of all current anthropogenic contributions of CO2.
Notice in the table below, based on Mauna Loa measurements, that the average annual rate of CO2 increase has gone up from one decade to the next, as pointed out by CO2now.org. But I've added a new column to reveal that that the acceleration over the previous decade has been erratic.
The warming trend in the atmosphere has also been difficult to pin down---almost negligent in some areas, but pronounced in the Arctic in the last few decades. At some point, with a continuous increase in greenhouse gas emissions, there probably will be a significant increase in global temperatures, but there is no way to know when that will occur. The problem is that, on one hand, from studying isotope ratios in ice cores, we have non-detailed patterns over long periods of time, versus what we have in the present: meticulous data over a very short time span.
| Decade | Total Increase | Annual Rate of increase | Acceleration over previous decade |
| 2002 – 2011 | 20.72 ppm | 2.07 ppm per year | 0.47 ppm/year2 |
| 1992 – 2001 | 16.00 ppm | 1.60 ppm per year | 0.09 ppm/year2 |
| 1982 – 1991 | 15.10 ppm | 1.51 ppm per year | 0.11 ppm/year2 |
| 1972 – 1981 | 13.95 ppm | 1.40 ppm per year | 0.51 ppm/year2 |
| 1962 – 1971 | 8.88 ppm | 0.89 ppm per year | Mauna Loa only started recording in 1958 |
SOURCES:
http://www.epa.gov/cleanenergy/energy-resources/refs.html
http://co2now.org/Current-CO2/CO2-Trend/
http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0889.1984.tb00245.x/pdf (free article)
