Molecular Composition of Organic Aerosols
Atmospheric aerosols refer to the liquid or solid “clumps” of molecules floating through the ambient air. Typical sizes of aerosol particles range from several nanometers to several micrometers. They take a long time to settle to the ground because of the negligible gravitational pull relative to the Brownian motion forces exerted by random collisions with the gas-phase molecules and the forces originating from convective air currents. Aerosols play an important role in controlling the climate, driving atmospheric chemistry, and contributing to air pollution problems worldwide. For example, every single cloud or fog droplet requires a seed particle in order to grow. Atmospheric aerosols have received a lot of scientific attention in recent years because of the profound effects they have on the climate. Through multiple field observations, scientists have established that ambient particles contain a significant fraction of organic compounds. Particles that are dominated by the organic material are loosely defined as “organic aerosols” (OA), and further divided into primary and secondary by the mechanism of their generation. 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.
Imagine entering a garage in a house that has been occupied by a large, disorganized family for years. You will likely find dozens of dusty boxes filled with thousands of items that the family members used in the past but are now only vaguely aware of. Each particle in an organic aerosol is similar to such a garage in that it contains a bewildering number of products of atmospheric oxidation of various organic molecules, which were discarded by the air because they were not volatile enough to remain in the vapor state. Identifying each molecule in an organic aerosol is far more challenging then sorting through the mess of an old garage. However, it is worth doing because detailed knowledge of molecular composition of organic particles holds clues to their important properties. For example, if we knew what kinds of molecules particles contain, we could predict what types of effects they would have on our health when we inhale them. 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 (HR-MS) 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 study of the composition of isoprene/ozone aerosol ; 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 [51,56,61,67,69,84], isoprene+ozone SOA , biomass-burning organic aerosol , SOA prepared by photooxidation of isoprene [70, 71,75,79,80], naphthalene , and a number of other biogenic and anthropogenic VOC . We have also analyzed ambient particulate matter samples collected in urban  and rural  areas. For example, in collaboration with the group of Allen Goldstein from UCB, we analyzed aerosol samples from the 2010 CalNex field study in Bakersfield and Pasadena using positive mode nanospray-desorption electrospray ionization mass spectrometry . That study represented the first systematic comparison between high-resolution mass spectra of field and chamber generated aerosols, and demonstrated that diesel was a major source of SOA in Bakersfield.
We have also worked on the development of new methods designed to make HR MS analysis more sensitive, quantitative, selective, and user-friendly. This method development work has been primarily driven by Drs. A. Laskin and J. Laskin from PNNL. For example, we were the first to apply desorption electrospray ionization (DESI) , to SOA analysis, which made it possible to examine aerosol filter samples without prior extraction. This was followed by the development of nanospray desorption electrospray ionization (nano-DESI) by Drs. A. Laskin and J. Laskin, and applications of nano-DESI to the analysis of both laboratory , and field  SOA samples. We were the first to apply reactive nano-DESI to SOA analysis , an approach that makes it possible to selectively and quantitatively observe carbonyl compounds in SOA. We suggested an original approach for an approximate quantification of concentrations of SOA extracts in HR ESI-MS without using standards , described a new method for doing HR ESI-MS analysis of organic aerosols using a particle-into-liquid-sampler (PILS) , demonstrated that HR ESI-MS is a useful method for quantitative measurements of the average O/C ratios in SOA , and studied solvent interference in the electrospray ionization (ESI) analysis of SOA .
Our Research Tools
Aerosol Chamber. We use a 5 m3 Teflon aerosol smog chamber , 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 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  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. ). 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 (this image was created by Tran Nguyen as part of her PhD dissertation; click on the image to see a higher-resolution version)