Sulfur/Aerosol/Cloud Chemistry Problems:

1. In the 1980s a large copper smelter was located in Southeastern Arizona which emitted large quantities of SO₂ into relatively clean air. Say that the background aerosol consisted of moderate levels of coarse dust particles and low levels of an accumulation mode of aerosols originating from natural and pollution sources.

a) Describe one pathway by which the sulfur from the SO₂ can impact the background aerosols.

The SO₂ (almost always) needs to first be oxidized to be able to form aerosol particles. This can occur in the gas or the aqueous phase.

i) In the gas phase, SO₂ reacts with OH as follows:

SO₂ + OH + O₂ → SO₃ + HO₂

SO₃ + H₂O → H₂SO₄(g)→ aerosol suflate

ii) In the other pathway, SO₂ dissolves into cloud droplets forming H₂SO₃, HSO₃^-, and SO₃^2- which can react with ozone or hydrogen peroxide to form sulfate. Once the cloud droplet evaporates, this sulfate is converted to the aerosol phase.

b) Which aerosol mode(s) would you expect the sulfur to end up in based on your path in a? Explain how you know this, and what, if any, factors affect what mode(s) it ends up in. (12 pts)

i) Because the sulfuric acid vapor can form new particles or condense onto existing particles, the sulfate will end up in ultrafine particles, in accumulation mode particles, and in coarse mode particles. Most of it should end up on accumulation mode particles, because these particles typically have the most surface area (even if at low levels vs. coarse mode aerosol). Factors affecting what modes it ends up with will include the surface area of existing aerosol particles.

ii) Because only accumulation and coarse mode sized aerosols can nucleate cloud droplets, the sulfate would contribute to the fine or coarse mode aerosols – but not the ultrafine. What size it ends up in will depend on which particles nucleate more droplets (typically the accumulation mode aerosol).

2. Nitric acid is a strong acid formed from an atmospheric reaction and can affect precipitation acidity. Give a reaction in which its acidity is neutralized. What is a source of the compound which neutralizes its acidity? (8 pts)

1) NH₃(g) + HNO₃(g) → NH₄NO₃ (s) – ammonia is emitted with fertilizer application and from livestock.

2) CaCO₃(s) + 2HNO₃(g) → Ca(NO₃)₂ (s) + CO₂(g) + H₂O (g)