Effect of relative humidity on SOA formation from isoprene/NO photooxidation: role of particle-phase esterification under dry conditions

Abstract

The effect of relative humidity (RH) on secondary organic aerosol(SOA) formation from the photooxidation of isoprene under initiallyhigh-nitric oxide (NO) conditions was investigated in a dual outdoorsmog chamber. Based upon particle volume concentration measurementsand the detailed chemical characterization of isoprene SOA usinggas chromatography/mass spectrometry (GC/MS) and ultra performanceliquid chromatography/electrospray ionization high-resolution quadrupoletime-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS), enhancedSOA formation was observed under lower RH conditions (15?40%) comparedto higher RH conditions (40?90%). 2-methylglyceric acid (2-MG) andits corresponding oligoesters, which have been previously shown toform from further oxidation of methacryloylperoxynitrate (MPAN),were enhanced in the particle-phase under lower RH conditions. Inaddition, an abundant unknown SOA tracer likely derived from thefurther oxidation of MPAN was detected and enhanced under lower RHconditions. In contrast, the 2-methyltetrols, which are known toform from the reactive uptake of isoprene epoxydiols (IEPOX) underlow-NO conditions in the presence of acidified aerosol, did not substantiallyvary under different RH conditions; however, isoprene-derived organosulfateswere found to be enhanced under high-RH conditions, indicating thelikely importance of the aqueous aerosol phase in their formation.Based upon the detailed chemical characterization results, particle-phaseorganic esterification is proposed to explain the observed enhancementsof isoprene SOA mass under lower RH conditions. This is one of onlya few chamber studies that have examined the effect of RH on isopreneSOA formation. In comparison to our recent results obtained fromaromatic SOA formation, the effect of RH on isoprene SOA formationis reversed. The results of this study highlight the importance ofelucidating the key reactive intermediates that lead to SOA formation,especially since RH likely affects their ability in forming SOA.Furthermore, ignoring the effects of RH may significantly affectthe accuracy of both regional and global SOA models.