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.
Subject(s)
Aerosols/analysis , Aerosols/chemistry , Butadienes/chemistry , Hemiterpenes/chemistry , Pentanes/chemistry , Anhydrides/chemistry , Atmosphere/chemistry , Epoxy Compounds/chemistry , Hydroxyl Radical/chemistry , Methacrylates/chemistry , Oxidation-Reduction , Sulfates/chemistry , Temperature , Time FactorsABSTRACT
Photoelectron resonance capture ionization (PERCI) is demonstrated as a sensitive ionization technique involving minimal fragmentation of organic molecules. PERCI has been used successfully to softly and efficiently ionize both strongly UV absorbing and non-absorbing molecules. Tunable low energy (<1 eV) electrons are generated by focusing a pulsed UV laser on an aluminum photocathode in the presence of gas phase analyte. Negative ions are formed through a resonance electron capture process. Mass analysis is done using a reflectron time-of-flight mass spectrometer. PERCI is demonstrated for a number of gas phase compounds and simple mixtures, including sulfur hexafluoride, nitrobenzene, nitrophenol, 2-pentanone, hexanal, heptanal, and octanal. In all cases the molecular ion (or [M - H](-)) was observed to be the dominant peak. The 1sigma limit of detection was estimated to be on the order of 10(6) molecules in the ionization region.
ABSTRACT
Photoelectron resonance capture ionization (PERCI) is a soft and sensitive ionization method, based on the attachment of low-energy (<1 eV) photoelectrons to organic analyte molecules. PERCI has been developed in our laboratory for the real-time analysis of organic particles by mass spectrometry, and is employed here to monitor the heterogeneous reaction of ozone with oleic acid. Simplified identification of the reaction products is possible as a result of the soft nature of PERCI, giving predominantly the [M--H](-) ions. The major particle-phase products are identified as: 1-nonanal, nonanoic acid, 9-oxononanoic acid, and azelaic acid, consistent with proposed mechanisms. New insight into this well-studied heterogeneous reaction is gained as additional minor particle-phase products, consistent with the Criegee mechanism, are readily detected.