Linked from
The 12 pages that link to Tropospheric ozone, each with the reason it gives.
IsopreneRelated: Isoprene oxidation can promote ozone formation when nitrogen oxides are present.
MonoterpeneRelated: Reactions involving monoterpene emissions can alter ozone formation and loss.
Biomass burningRelated: Fire emissions supply precursors that can generate ozone downwind.
SmogRelated: It is a major harmful product of photochemical smog, not a directly emitted pollutant.