Relevant chemical process during thunderstorm

During a thunderstorm, lightning provides enough energy for nitrogen and oxygen in air to react: $\mathrm{N_2 + O_2 \rightarrow 2NO}$

Then nitric oxide gets oxidized further: $\mathrm{2NO + O_2 \rightarrow 2NO_2}$

Formation of acids in rain water

The nitrogen dioxide formed dissolves in rain water and produces acids such as nitric acid: $\mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}$

Since $\mathrm{HNO_3}$ is a strong acid, its presence increases the concentration of $\mathrm{H^+}$ ions in rain water.

Effect on pH

pH is defined as: $\mathrm{pH = -\log[H^+]}$

If $[\mathrm{H^+}]$ increases, pH decreases.

Therefore, rain water collected during a thunderstorm becomes slightly more acidic than ordinary rain water.

Evaluation of options

• A: slightly higher pH $\rightarrow$ incorrect, because acidity increases.
• B: uninfluenced $\rightarrow$ incorrect, thunderstorm causes acid formation.
• C: depends on dust in air $\rightarrow$ not the main reason here.
• D: slightly lower pH than rain water without thunderstorm $\rightarrow$ correct.

Final conclusion

The pH of rain water during a thunderstorm is slightly lower due to formation of nitric acid from atmospheric nitrogen oxides produced by lightning.