Different roles of water in secondary organic aerosol formation from toluene and isoprene

Abstract. Roles of water in the formation of secondary organic aerosol (SOA) from the irradiations of toluene-NO₂ and isoprene-NO₂ were investigated in a smog chamber. Experimental results show that the yield of SOA from toluene almost doubled as relative humidity increased from 5 to 85 %, whereas the yield of SOA from isoprene under humid conditions decreased by 2.6 times as compared to that under dry conditions. The distinct difference of RH effects on SOA formation from toluene and isoprene is well explained with our experiments and model simulations. The increased SOA from humid toluene-NO₂ irradiations is mainly contributed by O–H-containing products such as polyalcohols formed from aqueous reactions. The major chemical components of SOA in isoprene-NO₂ irradiations are oligomers formed from the gas phase. SOA formation from isoprene-NO₂ irradiations is controlled by stable Criegee intermediates (SCIs) that are greatly influenced by water. As a result, high RH can obstruct the oligomerization reaction of SCIs to form SOA.