Thermodynamics

Chapter 2 | 89a certain amount of acid, but the amounts produced by the power plantsusing inexpensive high-sulfur coal has exceeded this capability, and as aresult many lakes and rivers in industrial areas such as New York, Pennsylvania,and Michigan have become too acidic for fish to grow. Forests inthose areas also experience a slow death due to absorbing the acids throughtheir leaves, needles, and roots. Even marble structures deteriorate due toacid rain. The magnitude of the problem was not recognized until the early1970s, and serious measures have been taken since then to reduce the sulfurdioxide emissions drastically by installing scrubbers in plants and by desulfurizingcoal before combustion.The Greenhouse Effect:Global Warming and Climate ChangeYou have probably noticed that when you leave your car under direct sunlighton a sunny day, the interior of the car gets much warmer than the airoutside, and you may have wondered why the car acts like a heat trap. Thisis because glass at thicknesses encountered in practice transmits over 90percent of radiation in the visible range and is practically opaque (nontransparent)to radiation in the longer wavelength infrared regions. Therefore,glass allows the solar radiation to enter freely but blocks the infrared radiationemitted by the interior surfaces. This causes a rise in the interior temperatureas a result of the thermal energy buildup in the car. This heatingeffect is known as the greenhouse effect, since it is utilized primarily ingreenhouses.The greenhouse effect is also experienced on a larger scale on earth. Thesurface of the earth, which warms up during the day as a result of theabsorption of solar energy, cools down at night by radiating part of itsenergy into deep space as infrared radiation. Carbon dioxide (CO 2 ), watervapor, and trace amounts of some other gases such as methane and nitrogenoxides act like a blanket and keep the earth warm at night by blocking theheat radiated from the earth (Fig. 2–66). Therefore, they are called “greenhousegases,” with CO 2 being the primary component. Water vapor is usuallytaken out of this list since it comes down as rain or snow as part of thewater cycle and human activities in producing water (such as the burning offossil fuels) do not make much difference on its concentration in the atmosphere(which is mostly due to evaporation from rivers, lakes, oceans, etc.).CO 2 is different, however, in that people’s activities do make a difference inCO 2 concentration in the atmosphere.The greenhouse effect makes life on earth possible by keeping the earthwarm (about 30°C warmer). However, excessive amounts of these gases disturbthe delicate balance by trapping too much energy, which causes theaverage temperature of the earth to rise and the climate at some localities tochange. These undesirable consequences of the greenhouse effect arereferred to as global warming or global climate change.The global climate change is due to the excessive use of fossil fuels suchas coal, petroleum products, and natural gas in electric power generation,transportation, buildings, and manufacturing, and it has been a concern inrecent decades. In 1995, a total of 6.5 billion tons of carbon was released tothe atmosphere as CO 2 . The current concentration of CO 2 in the atmosphereGreenhousegasesSome infraredradiation emittedby earth is absorbedby greenhousegases andemitted backFIGURE 2–66The greenhouse effect on earth.SUNSolar radiationpasses throughand is mostlyabsorbedby earth’ssurface