Photochemical Smog

Photochemical smog is a brownish, oxidising haze that forms over sunny cities with heavy

vehicular traffic -- Los Angeles and, more relevantly for us, many large Indian cities on a clear,

bright day are classic settings for it. Unlike classical smog, it needs no fog and no coal-burning:

its two essential raw ingredients are oxides of nitrogen ($\text{NO}_x$) and unburnt hydrocarbons,

both supplied in abundance by vehicle exhaust, and its essential extra ingredient is strong sunlight, which drives the whole sequence of reactions.

The formation sequence, in outline (see the accompanying figure for the full cycle), runs as

follows:

1. Vehicle exhaust releases $\text{NO}$, which is oxidised in air to $\text{NO}_2$.
2. Sunlight splits $\text{NO}_2$ back into $\text{NO}$ and a free oxygen atom: $\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$.
3. That free oxygen atom reacts immediately with $\text{O}_2$ in air to form ozone: $\text{O} + \text{O}_2 \rightarrow \text{O}_3$.
4. Meanwhile, unburnt hydrocarbons react with oxygen and with $\text{NO}_2$ through a chain of free-radical steps to produce peroxyacetyl nitrate (PAN).
5. Ozone and PAN accumulate through the day as sunlight keeps driving the cycle, both being strong oxidising agents and irritants -- which is why photochemical smog, chemically, is described as oxidising, the opposite character to classical smog.

The nitrogen dioxide in this cycle plays what is often called a catalytic role: it is consumed

in step 2 but regenerated as $\text{NO}$ is re-oxidised back to $\text{NO}_2$ in step 1, so the

$\text{NO}$-$\text{NO}_2$ pair keeps cycling through the reaction rather than being used up,

sustaining ozone production for as long as sunlight and hydrocarbons are available.

Effects. On humans, ozone and PAN irritate the eyes (a stinging, watering sensation) and the

respiratory tract, and can aggravate asthma and other lung conditions; prolonged exposure is linked

to reduced lung function. On plants, ozone damages leaf tissue, causing visible bronzing or …