4.1 Acid Deposition: A Regional Challenge

Acid deposition, commonly known as acid rain, is a serious environmental problem on a regional scale, affecting large areas of North America, Europe, and Asia. The issue stems from the emission of two primary precursors: sulfur dioxide (SO₂) and nitrogen oxides (NOx). As polluted air masses are transported over hundreds of kilometers, these precursors are oxidized in the atmosphere to form strong inorganic acids—primarily sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Understanding this long-range transport and chemical transformation is the goal of sophisticated long-range transport (LRT) models.

4.2 Nitric Acid (HNO₃) Formation Pathways

Nitric acid is formed in the atmosphere through several key gas-phase processes:

1. Daytime Pathway: The dominant process during the day is the reaction of the hydroxyl radical (OH) with nitrogen dioxide (NO₂). This is the same reaction (Reaction 21) that acts as a chain-terminating step in ozone chemistry.
2. Nighttime Pathways: At night, two other pathways become important. The first involves the reaction of dinitrogen pentoxide (N₂O₅) with water vapor.

These pathways are highly efficient. Under typical summertime conditions, the 24-hour average conversion rate of NO₂ to HNO₃ is estimated to be between 15% and 20% per hour.

4.3 Sulfuric Acid (H₂SO₄) Formation: Gas-Phase vs. Aqueous-Phase

The transformation of SO₂ to sulfuric acid occurs through both gas-phase and aqueous-phase pathways, but their relative importance is very different.

Gas-Phase Oxidation

The primary gas-phase pathway for SO₂ oxidation is initiated by the hydroxyl radical:

HO + SO₂ + M → HOSO₂ + M (Reaction 33) HOSO₂ + O₂ → HO₂ + SO₃ (Reaction 34) SO₃ + H₂O → H₂SO₄ (Reaction 35)

For modeling purposes, this sequence is often simplified to:

OH + SO₂ (+ O₂, H₂O) → H₂SO₄ + HO₂ (Reaction 36)

While important, this gas-phase process is significantly slower than the conversion of NO₂. Maximum daytime SO₂ oxidation rates are typically in the range of 3-5% per hour, far less than the rate for NO₂.

Aqueous-Phase Oxidation

A very important pathway for SO₂ oxidation occurs in the aqueous phase within cloud droplets. First, gaseous SO₂ dissolves in water, where it primarily exists as the bisulfite ion (HSO₃⁻). The concentration of HSO₃⁻ is inversely proportional to the acidity ([H⁺]) of the droplet.

Several oxidants present in cloud water can then convert S(IV) (e.g., HSO₃⁻) to S(VI) (sulfate, SO₄²⁻):

• Metal-Catalyzed Oxidation: Transition metals like manganese (Mn²⁺) and iron (Fe³⁺), often present from industrial emissions, can catalyze the oxidation.
• Ozone Oxidation: Ozone dissolved in the droplet is an effective oxidant, but its reaction rate decreases significantly as the solution becomes more acidic (i.e., as pH drops).
• Hydrogen Peroxide (H₂O₂) Oxidation: This is often the most important pathway in polluted environments. H₂O₂ is highly soluble in water, and unlike ozone, its reaction rate increases as the pH decreases (down to a pH of about 2.0).

Observe Figure 8 closely. Notice how at a pH of 5.5, ozone is a competent oxidant. But as acidity increases—a common occurrence in polluted clouds—the line for ozone plummets. In contrast, the H₂O₂ pathway remains highly effective, even becoming more effective down to pH 2. This is why H₂O₂ is the workhorse for SO₂ oxidation in acidic cloud water.

[Insert Figure 8: Comparison of aqueous-phase SO2 oxidation paths]

4.4 Natural and Organic Sources of Acidity

While anthropogenic emissions are the main driver of acid deposition, natural sources also contribute. The most significant natural sulfur compound is Dimethyl sulfide (DMS), which is emitted in large quantities from the oceans. In the atmosphere, DMS reacts with the OH radical via two potential pathways:

• Addition Pathway: OH adds to the sulfur atom, ultimately forming methane sulfonic acid (MSA).
• Abstraction Pathway: OH abstracts a hydrogen atom, leading to a reaction sequence that can produce SO₂.

This demonstrates how biogenic processes can be a source of the atmospheric SO₂ that contributes to sulfuric acid formation.

In addition to inorganic acids, organic acids make a substantial contribution to atmospheric acidity. The most important gas-phase carboxylic acids are formic acid (HCOOH) and acetic acid (CH₃COOH). They are produced from biomass burning, vehicle exhaust, and biogenic emissions, and are also formed through atmospheric reactions (e.g., ozone-alkene reactions). Their primary removal mechanism is through wet and dry deposition.

The chemistry of acid formation thus connects local and regional pollution sources to widespread environmental impacts, bridging the gap from tropospheric processes to the global-scale chemistry of the stratosphere.