Understanding the chemistry of atmospheric sulfur dioxide is crucial as it can lead to the formation of sulfuric acid, ultimately forming photochemical smog. Sources of SO2 include volcanic eruptions, phytoplankton emissions, and human activities like energy generation and smelting. Clouds play a key role in oxidizing SO2 into sulfates or sulfuric acid. Cloud and fog droplets act as chemical reactors, transforming gas-phase compounds and contributing to atmospheric reactions. Acidification of atmospheric vapor from SO2 has implications for air quality and ecosystems affected by acid rain.

Understanding the chemistry on environmental SO2 is important. It generally derives from the combustion of sulfur-rich material, whereupon it combines with oxygen. SO2 in its gas phase is a lung irritant, but it is its journey into atmospheric vapor - and its associated transition into sulfuric acids - that can cause major impacts. 

Its sources are varied, including volcanic eruptions  and the emissions of dimethyl-sulfide compounds from phytoplankton, which generate nitrate or OH radicals to become SO2. But the biggest source of environmental SO2, by some distance, is through human emissions through combustion in energy generation and smelting. 

So, the next question to ask is how does SO2 become oxidized into sulfates or sulfuric acid?

Well, one way is using the most common chemical reactor on the planet - clouds. 50% of our planet’s surface is covered by clouds at any given point, with up to 15% volume of the troposphere being cloud. Clouds serve to use liquid water droplets (mainly located in the lower atmosphere) as the location for chemical reactions that take gas phase compounds and transform them into something different. 

A famous historical example is from London in the winter of 1952. Clouds at ground level - otherwise known as fog - become the focus of reactions with the vast quantities of SO2 released through the domestic and industrial combustion of coal. The SO2 reacted in droplets of H2O fog and became oxidized to sulphuric acid. The result - a condensation of smoke and fog, or smog - was grim. 

The inhabitants of London were, in effect, breathing in sulphuric acid in oxidized form. The implications of this from a public health perspective were stark, with somewhere between 4 and 12 thousand people dying of respiratory conditions associated with the environmental air quality. 

It is important to be reminded, however, this phenomenon is not something from history. In the 1980s fog sampled in the LA basin registered on a pH scale of less than 2 - this is a markedly strong level of acidity. 

So, let’s consider what is happening chemically at the droplet scale.

The original composition of each droplet of water in cloud or fog is given by the nature of the condensation nuclei. This could be an aerosol particle, inorganic salts, organic material, metals…it might not even be water soluble - a solid in suspension. It could even be a biological particle, a bacteria, that reanimates in water and begins to interact with the compounds surrounding it. 

The liquid phase of the droplet allows all sorts of gas phase compounds to dissolve in the water, and so changing the nature of the droplet and the compounds. If the droplet is supersaturated relative to the gas phase, it can push compounds back into the gas phase.

In this way, with billions of tiny droplets acting as chemical reactors in any given cloud or fog bank, atmospheric reactions at global scale take place, not just the oxidation of sulfur.

With this in mind, it is worth considering that clouds are not the pure, distilled, fluffy white things of our imagination - they are chemical cocktails of impressive complexity and can often harbor a considerable loading of environmental pollutants. 

The nature of the acids produced from SO2 can vary widely and is actually a complex function associated with, amongst other things, the pH of the droplet itself, which can be determined by the condensation nuclei. 

For now, suffice to say that acidifying atmospheric vapor has major implications for the air we breath, and the ecosystems that this vapor ultimately falls on as acid rain.