Linked from
The 13 pages that link to Secondary organic aerosol, each with the reason it gives.
Volatile organic compoundRelated: VOC oxidation can add fine particles to polluted air.
IsopreneRelated: Oxidation products of emitted isoprene can contribute to these particles.
MonoterpeneRelated: Oxidation of emitted monoterpenes contributes substantially to these particles.
Photochemical smogRelated: Organic vapors in smog can oxidize into fine particulate pollution.
Wildfire smokeRelated: Smoke gases can react in sunlight and form additional particles downwind.
PineneRelated: Oxidation products of emitted pinene contribute to these particles.