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The pH of Rain

If you're crazy for reactions, you probably should read this section on the reactions that acidify the rain. We'll start with sulfur dioxide and carbon dioxide. The dissolution of both sulfur dioxide and carbon dioxide are similar to that of NO2:

Reaction 4.5: SO2 + H2O → H2SO3

Reaction 4.6: CO2 + H2O → H2CO3

The dissociation of sulfurous acid is shown below:

Reaction 4.7: H2SO3 → H+ + HSO3-

We know that NH3 works to neutralize about 22% of the acidity in rain, but we never explained ourselves. Hold on to your baseball caps, because here comes the answer. It accomplishes this Herculean task by scavenging H+ ions:

Reaction 4.8: NH3 (aq) + H+ → NH4+

Calcium carbonate also works to neutralize H+ ions:

Reaction 4.9: CaCO3 (s) → Ca2+ + CO32-

Reaction 5.0: CO32- + 2H+ → CO2 (g) + H2O

In agricultural areas like the middle of the United States, the pH of the rain is usually higher due to soil dust and the emission of NH3 from fertilizers and livestock.

Crazy Cations

After the Industrial Revolution, the concentrations of other anions like HNO3 and H2SO4 have increased. These acids transfer the cations into the soil solution at a faster rate than carbonic acid does. At first, this increases the growth of plants because they are taking in more nutrients (giant beanstalks everywhere). At the same time, the sulfate and nitrate ions also keep the cations in solution, which means they are more likely to drain out of the soil. The overall effect is to leach nutrients from the soil.

Temperate soils tend to have greater CECs and so these soils buffer acidity more easily than tropical soils do. Tropical soils depend on aluminum to buffer the soil. These reactions, shown below, are reversible. Even though they lead to a more acidic soil, they buffer the soil against additional H+ ions by the dissolution of aluminum hydroxide. The reaction is complex and involves many steps, but the overall reaction is shown below:

Reaction 5.3: Al(OH)3 + 3H+ → Al3+ + H20

It's Getting Hot in Here

The temperature of the earth is mostly determined by solar radiation. Gases and aerosols in the atmosphere are the main determinants for how much solar radiation is reflected and absorbed. Gases and aerosols are the main factors determining the temperature of earth. Kind of reads like a proof in geometry, right?

Statement

1. Earth's temperature is determined by solar radiation
2. Gases and aerosols reflect / absorb solar radiation
3. Gases / aerosols determine earth's temperature

Reason

1. Given
2. It says so in the aerosol section
3. Re-read 1 and 2 and connect the dots

Let's say 100 units of solar radiation reach earth. Just for fun, imagine these units as Skittles and you are the earth. Yum. It's raining Skittles and you're frantically trying to catch them in your mouth.

Six Skittles bounce off your face, but don't worry, they're the gross green apple ones. 24 Skittles never even make it to you because they're deflected by your cat Cloudy. That means you're down 30 Skittles, which still gives you a way better average than any Major League Baseball player.


The diagram shows the candy-free version of the earth's energy budget. Every diagram will have slightly different numbers, but this is one case where "close enough" is "good enough."   
  
Skittles in should equal Skittles out, meaning Energy In = Energy Out. If it doesn't, that means there is radiative forcing (or someone else is eating your Skittles). A simple explanation of radiative forcing is: Energy In – Energy Out. A positive value indicates warming, while negative values indicate cooling. Typically, radiative forcing is expressed as W/m2. Deforestation, greenhouse gas emissions, and aerosols all can contribute to radiative forcing.

Greenhouse gases greatly contribute to radiative forcing. The main greenhouse gases are, in order of appearance: water, carbon dioxide, methane, NOx, ozone, CFCs, and aerosols.

Removal of ghg's

Most greenhouse gases (ghg), with the exception of carbon dioxide and water, are removed from the atmosphere by chemical reactions. Greenhouse gases with more than one hydrogen atom are primarily removed by reactions with OH radicals7.

The amount of OH in the atmosphere depends on the concentrations of NOx, CH4, CO, VOCs, O3, and H20 and the amount of UV radiation greater than 300 nm. All of these gases vary with location, time of day, and season.

N2O, PFCs, SF6, and CFCs are all greenhouse gases that don't react with OH in the troposphere, but instead are removed by solar radiation (< 240nm) in the stratosphere7.

Basics of the Greenhouse Effect

The greenhouse effect of any gas can be determined using its residence time and how it reacts with other molecules in the atmosphere. Any chemically reactive gas will contribute to the greenhouse effect because it will affect atmospheric chemistry.

We can quantify the contribution each gas makes to the greenhouse effect by using the global warming potential (GWP), which is the ability of a gas to trap heat and warm the atmosphere compared to carbon dioxide over a specific time period. The 100-year GWP of methane is 23, while the 100-year GWP of carbon dioxide is 1. This means that methane is 23 times stronger than carbon dioxide.

Residence time also affects GWP. For gases with short residence times, the GWP for 100 years is smaller. For example, CFC-11 and SF6 both affect radiative forcing to approximately the same degree, but because SF6 has a lifetime of 3200 years compared to CFC-11's 45 years, the GWP of SF6 is almost five times greater than that of CFC-11.

Ghg's absorb radiation between 5 to 50 μm, which is the wavelength of most radiation reflected from the earth's surface. This is what makes them greenhouse gases; they trap the radiation that normally would have been sent out into space. Without the greenhouse effect, the earth would have a frigid mean temperature of -18 ºC. With the greenhouse effect, the earth is a much more livable 15 ºC. The greenhouse effect is one factor that makes life on earth possible. Although ice worms probably wouldn't care one way or the other.

Water Vapor

Water vapor is the strongest greenhouse gas. It has a concentration of up to 5% in the atmosphere. This ghg doesn't get a lot of attention because human inputs to the concentration of water vapor in the atmosphere are minimal. This is one cycle any superhero would have a hard time changing.

Water is involved in both positive and negative feedback processes that affect the earth's temperature. Increasing temperatures lead to an increase in evaporation, which leads to increasing temperatures because water vapor is a greenhouse gas. This is a positive feedback loop because it pushes the system to one extreme. In this system, the extreme is higher and higher temperatures.



  
  
Water is also involved in a negative feedback loop that can lead to lower temperatures. As clouds increase, more solar radiation is reflected, leading to decreased temperatures. Carbon Dioxide


The graph shows the atmospheric concentration (ppm) of carbon dioxide during the past 417,000 years. The rapid increase of atmospheric carbon dioxide since the Industrial Revolution is one reason it is cast at the villain. Another reason for this unfortunate casting is that most of these emissions are anthropogenic. (Source)   
  
We know that the greenhouse effect is why earth is a livable temperature, so why the attack on a greenhouse gas like carbon dioxide? If the greenhouse effect were a comic book, carbon dioxide would be the villain. Carbon dioxide is responsible for 60% of the greenhouse effect. Since 1900, the earth's average surface temperature has increase by 1.3ºF, and most people link this to carbon dioxide since human activities are pumping so much of it into the atmosphere. Estimates put anthropogenic carbon dioxide at 8.0 Gtons per year.

Between carbon dioxide and water vapor, there is only a small window allowing radiation to escape back into space. Methane, our next villain on trial, absorbs some of the radiation that would otherwise escape through this window.

Methane

Methane is an up and coming greenhouse gas, contributing approximately 0.48 W/m2 to radiative forcing. In recent years the concentration of methane in the atmosphere has increased by approximately 1% per year. This is greater than the rate at which atmospheric carbon dioxide has increased. Since the Industrial Revolution, the concentration of methane in the atmosphere has more than doubled7.

Imagine a car windshield after it has been hit by a pebble. That spider-web of cracks would let air through. Now imagine taping up those cracks. Less air would go through, right? That windshield is water vapor and carbon dioxide; methane is the tape. Methane absorbs radiation that carbon dioxide and water vapor miss.
The wavelengths of radiation absorbed by carbon dioxide overlap that of water vapor. Methane, however, absorbs wavelengths not absorbed by either carbon dioxide or water. It blocks the radiation that carbon dioxide and water vapor missed, making it an important greenhouse gas.   
  

The image shows an unexpected source of methane in the Arctic: the ocean surface. (Source)   
   
NOx and N2O


Nitrous oxide is a direct ghg, absorbing IR between 7.4 to 8.7 μm and contributing 0.16 W/m2 to radiative forcing. Because nitrous oxide hangs out in the troposphere for 120 years, it contributes to the greenhouse effect just as much as ozone does. The biggest source of nitrous oxide is not the cars from Fast and Furious, but rather the soil7.

The NOx are indirect greenhouse gases. They catalyze tropospheric ozone formation, and tropospheric ozone is a potent greenhouse gas. The main source of NOx is fossil fuel combustion. Tropospheric emissions of these species have a significant impact on global greenhouse gases. Because NO converts HO2 to OH, NO increases the amount of OH in the troposphere:

Reaction 6.2 : HO2 + NO → NO2 + OH

NO concentrations also impact the amount of CO, CH4, and HFCs because all of these are removed through reactions with OH:

Reaction 6.3: CO + OH → CO2 + H

Ozone

Ozone is a surprisingly efficient ghg, absorbing infrared radiation between 9 and 10 μm. As we learned in the ozone smack down, increasing NOx emissions have lead to an increase in tropospheric (bad) ozone. More tropospheric ozone means warmer temperatures (break out the bathing suits and flip-flops), but this is somewhat counteracted by the decrease in stratospheric ozone (darn, put the yellow polka-dot bikini back in the closet). Overall, ozone results in warming.

0.35 W / m2 in the stratosphere + -0.05 W/m2 in the troposphere = 0.30 W/m2 overall.19


This image shows a Dobson ozone spectrophotometer, which measures ozone in a column of the atmosphere. (Source)   
  
CFCs


In addition to having the dubious distinction of destroying the ozone layer, CFCs also act as greenhouse gases. They, along with other halogenated gases like HFCs, contribute an estimated 0.34 W/m2 to radiative forcing. They contribute a paltry 2% to global ghg emissions, but with a GWP of anywhere from 140 to 23,000 and a lifetime of up to 17,000 years, these synthetic chemicals are strong contenders for greenhouse gas of the year.


CFC molecules. In addition to being ozone destroyers, these synthetic molecules are also potent greenhouse gases, having a GWP as high as 23,000 and an atmospheric lifetime up to 17,000 years. (Source)   
  
Aerosols


Most are classified as indirect greenhouse gases and thus do not have a number for GWP. Behind every great greenhouse gas is a great indirect greenhouse gas. These indirect greenhouse gases contribute to the greenhouse effect by controlling the abundance of the direct greenhouse gases like carbon dioxide. NOx and CO are both examples of indirect greenhouse gases.

Molecular hydrogen is another indirect greenhouse gas because it can decrease the amount of OH7. We know that OH is a chemical scrubbing brush, removing methane and HFCs.

Less OH = More Methane and HFCs
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