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Molecular Composition of Organic Aerosols
Primary Organic Aerosols (POA) are emitted in the atmosphere directly by various sources such as traffic, waves breaking, wind-blown soil, biomass burning, cooking, and so on. The molecular make-up of POA usually reflects the environment they came from. Secondary Organic Aerosols (SOA) are produced in the atmosphere as a result of complex chains of reactions that start with the oxidation of Volatile Organic Compounds (VOC) by ozone (O3), hydroxyl radical (OH) and nitrate radical (NO3) and end with the condensation and coagulation of low-volatility oxidation products into aerosols. The presence of nitrogen oxides (NOx), an important component of urban pollution affects the formation of the aerosols. One especially important group of VOC that efficiently form SOA is terpenes, a class of hydrocarbons emitted predominantly by tree foliage. Terpenes include isoprene (C5H8), monoterpenes (C10H16) and larger terpenes.
SOA made from terpenes are distinguished by an astonishingly high degree of chemical complexity. Even SOA generated from a single terpene under controlled laboratory conditions may contain thousands of different organic compounds. The real-world SOA are even more complicated than that. Several years ago no one thought that figuring out the molecular composition of terpene SOA would even be possible but recently this has all changed due to exciting developments in mass spectrometry. In collaboration with Dr. Alexander Laskin and Dr. Julia Laskin from the DOE Pacific Northwest National Laboratory (PNNL), we have been developing and applying methods of high resolution mass spectrometry to characterize the molecular structure of SOA. This approach provides an unprecedented amount of detail about the composition of organic aerosols by supplying molecular formulas for thousands of aerosol species in a single measurement. Detailed analyses of the distribution of chemical formulas reveal useful mechanistic information about the chemistry leading to the initial formation and subsequent aging of biogenic and anthropogenic organic aerosols. The following figure is a highlight of our recent study of the composition of isoprene/ozone aerosol [64]; it displays a map of the O:C versus H:C ratios for about 1000 organic molecules that we were able to observe in this type of aerosol.
The collaboration between the UC Irvine and PNNL research groups has resulted in the identification of multifunctional monomeric and oligomeric aerosol compounds in limonene+ozone SOA [51], isoprene+ozone SOA [64], biomass-burning organic aerosol [67], and SOA prepared by photooxidation of isoprene [70, 71], observation of time evolution in SOA composition [61], and a detailed study of solvent interference in the electrospray ionization (ESI) analysis of SOA [56]. In addition, we discovered chemical pathways leading to colored compounds in “brown” SOA [62, 66, 72, 77] discussed in more detail here. Our work has recently been reviewed in a perspective [68] on molecular chemistry of organic aerosols through the application of high resolution mass spectrometry. We are currently working on applications of this powerful method to aerosols generated by fuel combustion and biomass burning.
Our Research Tools
 Aerosol Chamber. We recently constructed a 5 m3 Teflon aerosol smog chamber [64], which is housed inside a 20 m3 protective enclosure. The chamber is surrounded by 40 UV-B lamps for photochemical generation of OH from precursors like H2O2, HONO, and RONO. The OH we generate then reacts with the VOC of interest, making SOA. SOA is collected through denuders onto filters or impacted onto other substrates for analysis. The reaction can be conducted at different VOC and NOx concentrations. The chamber is connected to a suite of state-of-the-art instruments that help control and monitor the reaction conditions: zero-air generator, NOy monitor, O3 monitor, RH/T probe, scanning mobility particle sizer (SMPS), chemical ionization mass spectrometer (CIMS), time-of-flight aerosol mass spectrometer (ToF-AMS), proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS, see below). This chamber is our main “reactor” for making model SOA.
Aerosol Flow Reactor. In addition to static aerosol chambers, like the one described above, we can use a flow tube reactor to make aerosol samples. Terpenes or other unsaturated chemicals of interest are slowly injected with a syringe pump into a flow of air and ozone and carried into a meter long glass flow tube. The gases exit the after a few minutes of reaction. The remaining ozone is stripped from the mixture with a denuder, while particles are collected with a filter or an impactor. Compared to the static aerosol chambers, the main advantages of this system include the ability to keep the aerosol concentration constant over a significant period of time (hours) and the ability to collect large amounts of SOA material (mg quantities). We routinely use this chamber for producing model terpene/O3 SOA.
 PTR-ToF-MS is a sophisticated mass spectrometer that is optimized for quantitative measurements of gas-phase alcohols, carbonyls, esters, amines, olefins, aromatic compounds, etc. with sensitivities approaching parts-per-trillion by volume. The PTR-ToF-MS instrument has sufficiently high resolving power (m/∆m = 6,000) to unambiguously identify small VOCs by their accurate molecular masses and sufficiently fast response time to monitor the reaction kinetics with 1 s time resolution. The ionization mechanism involves a charge-transfer from H3O+ onto VOCs. The VOC carbon skeleton remains intact upon the proton-transfer in most cases, which significantly simplifies the data analysis. This PTR-ToF-MS instrument is shared with other research groups in the AirUCI, an institute at UCI focusing on atmospheric science research.
High-resolution ESI-MS. For our high resolution mass spectrometry research on SOA, we rely on a Linear Ion Trap Quadrupole Orbitrap™ Mass Spectrometer with a high resolving power (m/∆m = 100,000) and an Electrospray Ionization (ESI) ion source. This state-of-the-art instrument is one of the capabilities of the Environmental Molecular Science Laboratory (EMSL), a DOE scientific user facility located at the Pacific Northwest National Laboratory (PNNL). In addition to the traditional ESI-MS approach, a Desorption-ESI (DESI) capability has recently been implemented [66] on this instrument making it possible to sensitively ionize aerosol-phase compounds without first extracting them into a solvent. This was followed by an introduction of an even more power ionization method, nano-DESI by the PNNL team (an example of application of nano-DESI is described in Ref. [70]). In collaboration with Dr. Alexander Laskin and Dr. Julia Laskin, we have also been developing a method specifically designed for a detailed analysis of “brown” aerosol. This method relies on separation of the SOA compounds by their polarity using liquid chromatography (LC), followed by simultaneous measurement of the UV/Vis spectrum of the eluting fractions with a photodiode array absorption detector (PDA) and the high resolution mass spectrometer. We have successfully applied this approach to the analysis of aged “brown” biogenic SOA, and we plan to use it for molecular characterization of the light-absorbing species in the anthropogenic SOA.
The analysis of the mass spectra produced by this instrument takes advantage of specialized computer programs developed by our group and at PNNL. The validity of the formula assignments is checked against information we know about isotopic patterns, chemical families, and known chemical mechanisms. The following figure schematically represents the steps we usually take in the data analysis.
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