title: Observational insights into aerosol formation from isoprene.
abstract: Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F(MAE formation)). The strong temperature dependence of F(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m(-3)) and MAE-derived organosulfates (MAE-OS; ~1 ng m(-3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m(-3)) relative to MAE-OS (<0.0005 ng m(-3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10-100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.
authors.affiliation: Department of Environmental Science, Policy and Management, ∥Department of Chemistry, University of California , Berkeley, California 94720, United States.
authors.forename: David R
authors.initials: DR
authors.lastname: Worton
authors.forename: Jason D
authors.initials: JD
authors.lastname: Surratt
authors.forename: Brian W
authors.initials: BW
authors.lastname: Lafranchi
authors.forename: Arthur W H
authors.initials: AW
authors.lastname: Chan
authors.forename: Yunliang
authors.initials: Y
authors.lastname: Zhao
authors.forename: Robin J
authors.initials: RJ
authors.lastname: Weber
authors.forename: Jeong-Hoo
authors.initials: JH
authors.lastname: Park
authors.forename: Jessica B
authors.initials: JB
authors.lastname: Gilman
authors.forename: Joost
authors.initials: J
authors.lastname: de Gouw
authors.forename: Changhyoun
authors.initials: C
authors.lastname: Park
authors.forename: Gunnar
authors.initials: G
authors.lastname: Schade
authors.forename: Melinda
authors.initials: M
authors.lastname: Beaver
authors.forename: Jason M St
authors.initials: JM
authors.lastname: Clair
authors.forename: John
authors.initials: J
authors.lastname: Crounse
authors.forename: Paul
authors.initials: P
authors.lastname: Wennberg
authors.forename: Glenn M
authors.initials: GM
authors.lastname: Wolfe
authors.forename: Sara
authors.initials: S
authors.lastname: Harrold
authors.forename: Joel A
authors.initials: JA
authors.lastname: Thornton
authors.forename: Delphine K
authors.initials: DK
authors.lastname: Farmer
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Docherty
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Cubison
authors.forename: Jose-Luis
authors.initials: JL
authors.lastname: Jimenez
authors.forename: Amanda A
authors.initials: AA
authors.lastname: Frossard
authors.forename: Lynn M
authors.initials: LM
authors.lastname: Russell
authors.forename: Kasper
authors.initials: K
authors.lastname: Kristensen
authors.forename: Marianne
authors.initials: M
authors.lastname: Glasius
authors.forename: Jingqiu
authors.initials: J
authors.lastname: Mao
authors.forename: Xinrong
authors.initials: X
authors.lastname: Ren
authors.forename: William
authors.initials: W
authors.lastname: Brune
authors.forename: Eleanor C
authors.initials: EC
authors.lastname: Browne
authors.forename: Sally E
authors.initials: SE
authors.lastname: Pusede
authors.forename: Ronald C
authors.initials: RC
authors.lastname: Cohen
authors.forename: John H
authors.initials: JH
authors.lastname: Seinfeld
authors.forename: Allen H
authors.initials: AH
authors.lastname: Goldstein
mesh_terms: D000336:Aerosols / Q000032:analysis* / Q000737:chemistry*; D000812:Anhydrides / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry; D002070:Butadienes / Q000737:chemistry*; D004852:Epoxy Compounds / Q000737:chemistry; D045782:Hemiterpenes / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry; D008689:Methacrylates / Q000737:chemistry; D010084:Oxidation-Reduction; D010420:Pentanes / Q000737:chemistry*; D013431:Sulfates / Q000737:chemistry; D013696:Temperature; D013997:Time Factors
chemical_list: D000336:Aerosols; D000812:Anhydrides; D002070:Butadienes; D004852:Epoxy Compounds; D045782:Hemiterpenes; D008689:Methacrylates; D010420:Pentanes; D013431:Sulfates; C005059:isoprene; C008384:methacrylic acid; D017665:Hydroxyl Radical