Why smog makes us cry and how PANs can be used for good, rather than evil

Smog is an eye-irritant, but we didn't explain why. The reason smog makes us cry is PANs (peroyxaacetic nitric anhydrides) like CH₃C(O)ONO₂. PANs also irritate the respiratory system. PANs are produced when the oxidation of hydrocarbons results in aldehydes, which then react with diatomic oxygen and NO₂ in a series of reactions.

The lifetime of PANs is dependent on temperature. At 295 K it lasts only an hour, while at 250 K it can persist for several months¹⁰. Additionally, PAN decomposes to form NO_x, as seen in the reaction below: PAN → CH3C(O)OO + NO₂

Because it is so stable at low temperatures, it serves as a global chauffeur of sorts for NO_x. A remote island with no industry could be the dumping ground for NO_x from New Jersey or LA.

One way these PANs can be used for good instead of evil is to help us better understand the nitrogen cycle. Scientists can measure the uptake of PANs by leafs to estimate how much NO_x plants are actually taking in. They estimate that about 14% of an ecosystem's external nitrogen comes from NO_x³¹.

Raindrops Keep Falling on My Head

Environmental science might label this section "precipitation," but we're going to use the term "rain." When the text says rain, know it also means snow, sleet, and hail.

Rain is never just H₂O. Almost anything that is found in the atmosphere can also be found in rain. On the coasts in the United States, for example, rain contains more chloride and sodium because it picks it up from SSA²⁵. Rain almost everywhere in the world contains nitrates and sulfates, which are cations of strong acids. Rain also contains cations of strong bases like sodium, potassium, and magnesium.

The composition of rain depends on its geographic location and chemical processes in that particular region. The rain falling on a remote island is likely to be quite different from the rain in a place like Mumbai. Components of rain can come from soil, dust, the atmosphere, and bodies of water. Basically, if it's anywhere on earth, it can end up in the rain. In coastal areas, inputs by the sea dominate rainwater composition so rain contains lots of Na, Mg, and Cl. Near areas dominated by industry, rain contains more pollutants like SO₄²⁺ and NO₃^- and nitrates.


The composition of rain varies geographically. Rain in a coastal state like North Carolina, for example, will contain more sodium and chloride than rain in a landlocked state. (Source)

Brain Snack

Water droplets in cap clouds can sometimes cause iridescent clouds, like this one on National Geographic.

The ratio of ion concentration is like a chemical luggage tag. It lets scientists know the rain's place of origin. If the ratio of Mg to Na is 0.12 (the ratio of these ions found in seawater), then the ions probably came from the ocean. In the Southeast US, the ratio of Mg to Na can be anywhere from 0.29 to 0.76²⁵.

In the 1970s brought many things: leotards, ABBA, and acid rain. Rain is acidic most of the time. Acidic gases dissolve in cloud water droplets, altering the pH of rain. This process occurs fairly rapidly through acid dissociation reactions.

Here's a quick dissociation reaction (it uses vinegar as the acid, BTW).

CH₃COOH + H₂O ←→ CH₃COO^- + H₃O⁺

The water molecule steals an H from the vinegar. What is this, kindergarten? The basic formula for acid dissociation is:

HB ←→ H⁺ + B^-

Natural sources of acidity include carbon dioxide via decomposition, NO via lightning and SO₂ via forest fires and volcanic eruptions. Let's first examine how lightning can acidify precipitation. We know that 78% of the atmosphere by volume is N₂ and lightning converts it into NO:

Reaction 4.2: N₂ + O₂ → 2 NO

This NO then reacts with oxygen to form NO₂:

Reaction 4.3: 2NO + O₂ → 2NO₂

From the ozone smack down, we know that NO₂ is a key player in the production of tropospheric ozone. Now we can add the acidification of the rain to its lengthy list of misdeeds.

NO₂ then reacts with water to form nitric acid and NO:

Reaction 4.4: 3NO₂ + H₂O → 2 HNO₃ + NO

This reaction results in a decrease in the pH. In addition to natural sources like decomposition, there are also industrial sources of NO_x,(like burning of fossil fuels) that acidify the rain.

The overall effect of all these reactions is the acidification of the rain. NH₃ neutralizes close to 22% of this acidity (go ammonia!). Soil dust containing calcium carbonate can also neutralize these H+ ions.

Acidic rain can mobilize aluminum, along with other heavy metals we might find in soil. That means the aluminum can move around more easily.

Reaction 5.1: Al(OH)₃ + 3H⁺ (aq) → Al³⁺ + 3H₂O

This mobilization won't occur if the total number of base cations greatly outnumbers the total number of acid anions.

Why do we care if aluminum is free to move around? Mobilized aluminum is generally bad news for plants. It can damage root hairs and interfere with the uptake of phosphorous and other nutrients. Acid deposition can also leach important nutrients, like calcium and potassium, from the soil.


Blue hydrangeas: We sort of misled you when we said the mobilization of aluminum is always bad. The mobilization of aluminum in soils with a pH between 5 to 5.5 is what gives hydrangeas their blue color. Soil with a higher pH (and no mobile aluminum) results in pink hydrangeas¹³. One way of changing your pink hydrangeas to blue, then, is to add aluminum to the soil. These hydrangeas are located right next to the compost bin of a family of coffee guzzlers. The coffee grinds might be the reason for the soil's acidity, though coffee grinds can result in soil with a pH of anywhere from 4.6 all the way up to 8.4²⁴. (Source)   
  
Besides preventing aluminum from doing its damage dance, the cations can also buffer the acidity of the soil. They accomplish this by doing a chemical swap and trading places with one another. The H⁺ ions swap with a cation like calcium, releasing the calcium cation into the soil solution. The following reaction illustrates a typical cation exchange:

Reaction 5.2: Mg²⁺ + Clay-K₂ → 2K⁺ + Clay-Mg

Generally, the order in which these cations are exchanged is as follows:

Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ > NH₄⁺ > Na⁺

Side Note: Moles of Charge

One mole of charge (mol_c) of any ion is equal to one mole of charge of any other ion. One mole of magnesium ions has 2 moles of charge because magnesium has a +2 charge. One mole of potassium ions has 1 mole of charge since potassium has a +1 charge.

Moles of Charge = Oxidation #

When discussing moles of charge for soil ions (a discussion probably best saved for at least the second date), the units most often used are centimoles of charge. How often these cation wife swaps occur is known as the cation exchange capacity (CEC), expressed as centimoles of charge per kilogram of soil (cmol_c/kg soil). Quick tip: The more organic matter or clay in the soil, the greater the CEC.

Sample Problem

A soil has a CEC of 20 cmol_c/kg. 25% of the exchange sites contain H+. How much calcium carbonate, in cmol_c/kg, is required to neutralize ALL of the reserve acidity?

Answer: 25% of the exchange sites contain H+, which means there is 5 cmol_c/kg of H⁺.

To neutralize this, we would need an equal amount of calcium carbonate, so 5 cmol_c/kg of CaCO₃.


Rain with a pH below 5 can negatively affect plants, as is shown in this picture of a forest damaged by acid rain. (Source)   
Alkaline soils aren't affected as much by acid deposition as acidic soils are. Soils rich in base cations, like Na⁺, NH₄⁺, Ca^2+, Mg²⁺, and K⁺, for example, increase soil pH by releasing OH^- ions into the soil:

Reaction 5.4: Na₂CO₃ + H₂0 → 2Na⁺ + HCO₃^- + OH^-