Heterogenous oxidation reactions can occur upon mixing chemicals that are in two different physical states, such a liquid and a gas; for example, in the atmosphere the reaction of gaseous nitrate (NO3) free radicals reaching the wet surface of aerosol particles containing aromatic pollutants from wildfires.
Click here for original story, Sunlight-absorbing organic compounds are produced on the wet surfaces of atmospheric particles
Source: Phys.org