The history, by region, of anthropogenic SO2 emissions into the air over the past century and a half is shown in Figure 3-17. Europe—including the eastern
PROBLEM 3-10
PROBLEM 3-11
countries—was the largest source of SO2 emissions through the second half of the twentieth century, but their contribution—along with that from North America, which had been the second-largest emitter—have greatly dimin- ished. East Asia became the leading contributor of the emissions in the 1990s, and continues to be so today.
The global total of SO2 emissions (not shown; equal to the sum of all the curves in Figure 3-17 plus smaller contributions from other regions) reached its maximum in the 1970s and had been steadily declining from then until the turn of the millennium, but now is again increasing. This recent increase has occurred due to the rise in emissions from coal burning, which has always been the dominant source. Petroleum production and combustion, including emissions from fueling international shipping, is still globally the second largest contributor of SO2 emissions, with metal smelting the third largest.
The distribution of anthropogenic SO2 emissions by sector for the United States and Canada is shown in the pie charts in Figure 3-16; emis- sions for the United States are dominated by electricity generation (coal burning) and those for Canada by smelting (classed as “Industrial” on the graphs). Because of federal regulations, the amount of sulfur dioxide emitted into the air in North America has fallen substantially from its peak level in 1973.
The 1991 Air Quality Accord between the United States and Canada required both countries to reduce substantially their sulfur dioxide emissions beyond those of previous laws and agreements. Such emissions in the United States are restricted in accordance with the Clean Air Act and its amendments.
1850 1900 1950 2000
50,000
Emissions (Gg SO2)
Year 40,000
30,000
20,000
10,000
0
Former Soviet Union East Asia (including China) North America
Middle East Europe FIGURE 3-17 Global
anthropogenic SO2
emissions by region. [Source:
S. J. Smith et al., Atmospheric Chemistry and Physics 10 (2010): 16111–16151.]
Improving Air Quality: Sulfur-Based Emissions 115
The average concentration of sulfur dioxide in air over the United States fell from 13.2 ppb to 7.5 ppb from 1975 to 1991, and further to 3.3 ppm by 2008.
Whereas Phase I (1995 deadline) of the Clean Air Act imposed controls on only the largest coal-fired plants, those of Phase II, which began in 2000, imposed more stringent requirements, and applies to almost all plants. There is an SO2 tonnage limit for each power plant that emits this gas, based upon the power it produces, and a cap on overall national emissions as well. By 2009, the SO2 emissions in the United States had dropped by 67% compared to 1980 levels.
The reductions in sulfur dioxide emissions by power plants in the U.S.
Midwest has been achieved at lower-than-expected cost, due in part to the availability of cheap, low-sulfur coal (1% S, versus more than 3% S in the high-sulfur coal used previously) and inexpensive scrubbers, and in part to the implementation of a system of tradable emission permits. This permit system, in operation since the early 1980s, allows industries to buy emission allowances if they need to exceed their allowed levels, or to sell excess allow- ances on the open market (through the Chicago Board of Trade) if they do not need their whole allowance. A similar program has been initiated for nitrogen oxide emissions.
In Europe, the EU issued a directive in 1988 specifying reductions from large power plants of 50–70% of their SO2 emissions from 1980 levels by 2003. This cutback was achieved mainly by switching from coal to natural gas in power stations, and by the use of low-sulfur coal and the scrubbing of emissions in facilities where coal was still burned. According to the 1999 Gothenburg Protocol, Europe’s sulfur dioxide emissions are to be cut beyond 1990 levels by another 63% by 2020.
Much of the increase in global SO2 emissions in the early 2000s was due largely to those from China (included in East Asia in Figure 3-17), which increased by 53% from 2000 to 2006.This was due in large part to the expan- sion of coal burning, which supplies about two-thirds of China’s energy.
Emissions from this source began to decline around 2006 when SO2 began to be removed by flue-gas desulfurization—more than half its power plants now have scrubbers—and by closure of many small coal-fired power plants. The decline was reflected in atmospheric SO2 concentrations above eastern China, which began to decrease after 2007. Nevertheless, China is now the world’s leading source of the gas.
Japan initiated tight controls on SO2 emissions in the 1970s, and by 1980 their power plants had almost eliminated such emissions by the wide- spread installation of scrubbers. The rate of emissions from Japan and from South Korea both now continue in gradual decline.
By the turn of the millennium, the high rate of SO2 emissions from the former Soviet Union had declined by half (Figure 3-17), presumably due more to economic problems than to intentional controls, but have begun to increase again.
Ironically, volcanic releases of the gas from Japan outrank its anthropogenic sources.