Photochemistry of Organic Aerosols
Chemical processes that convert volatile organic compounds (VOC) into secondary organic aerosols (SOA) are generally quite fast. For example, terpenes rarely last longer than an hour after they are released by vegetation on a hot sunny day. Once the condensable products of VOC oxidation partition into aerosols, the chemistry slows down considerably. However, it is incorrect to view SOA as a static collection of organic molecules which undergoes no chemistry after the initial particle condensation. A number of interesting “aging” processes leading to changes in the chemical composition and physical properties of aerosols occur on time scales ranging from minutes to days. Our group investigates mechanisms of chemical and photochemical aging in secondary organic aerosols using a combination of spectroscopic and mass spectrometric techniques.
Direct photochemical aging refers to processes initiated by absorption of solar radiation within the particle. This aging mechanism can be effective only if the following conditions are satisfied: (1) the organic aerosol material must have significant absorption in the tropospheric actinic window (that is, the wavelength must be > 300 nm); (2) the yields for condensed-phase photochemical reactions, such as photodissociation, must be large compared to that for fluorescence, vibrational relaxation, geminate recombination, and various other non-reactive processes. We have found that terpene SOA formed from the ozone-initiated oxidation of terpenes do indeed absorb light at atmospherically relevant wavelengths, leading to rich photochemistry within the particles such as direct photodissociation of carbonyl [57, 60] and peroxide  functional groups. We are currently working on the quantification of photolysis quantum yields of typical carbonyls and peroxides embedded into an organic matrix. These yields must be known accurately to model the lifetimes of organic aerosol constituents with respect to solar photolysis.
Another avenue of research we investigate is photochemistry of SOA dissolved in water, which is relevant for understanding cloud-processing of organic aerosols. Our recent cover article  investigated the effect of UV irradiation on the molecular composition of aqueous extracts of limonene+ozone low-NOx SOA. We showed that photolysis had a significant effect on the composition of the dissolved organics: oligomeric compounds were destroyed, carbonyl compounds were photolyzed, carboxylic acids were generated during photolysis, and large organic peroxides were recycled into smaller peroxides. In a related study on isoprene+OH SOA generated under high-NOx conditions  we discovered new photochemical reactions converting organic nitrates into unusual heterocyclic compounds containing nitrogen. These results suggest that biogenic SOA dissolved in cloud/fog droplets should undergo significant photolytic processing on a time scale of hours to days. We also conduct a fundamental studies of photodissociation of important atmospheric molecules in water and ice. For example, we found that photolysis of the simplest organic peroxide CH3OOH embedded in ice clusters results in rich photochemistry on a picosecond time scale . This was followed by a combined experimental and theoretical investigation of photodissociation quantum yields and absorption cross sections of CH3OOH in water and in ice .
As opposed to SOA, which appear in the atmosphere as a result of VOC chemistry, primary organic aerosols (POA) are released directly into the atmosphere by their sources. POA include sea-salt aerosol particles generated by wave-breaking, soot particles produced by internal combustion engines, smoke particles produced by biomass burning, dust particles produced by re-suspension, and smelly particles emitted in the atmosphere by the cooking industry. Such particles are often decorated by an outer layer of fairly hydrophobic organic material such as phospholipids, fatty acids, and aromatics. This layer is slowly oxidized by OH, ozone, and nitrogen oxides in a process known as "chemical aging". We are interested in understanding the role of direct photochemical processes in processing fresh and aged POA particles. We have previously studied photodegradation of partially oxidized unsaturated fatty acids  and self-assembled monolayers (SAM) [43,46] as a model of photochemical aging occurring in POA particles. We discovered that reactions taking place during the oxidation of unsaturated films and SAM transform them into a photochemically active state capable of photolysis in the lower atmosphere. We think that photochemistry occurring in the oxidized aerosol contributes significantly to their aging. Our current efforts in this area focus on investigation of surface-specific photochemistry in SAM containing carbonyl and peroxide functional groups.
Aerosol Photodegradation Research Tools
CRDS. This figure shows an IR-CRDS (Infrared Cavity Ring-Down Spectroscopy) apparatus designed in our laboratory to study photodegradation processes in oxidized organic films and in SOA samples. CRDS is an ultrasensitive technique that quantifies optical absorption of molecules placed between two highly-reflective mirrors from the rate with which photons escape this cavity. The sample is excited with a wavelength-tunable radiation source, and the evolved products are observed with the IR-CRDS approach. This instrument is especially suitable for the detection of small molecular weight photolysis products such as CO, CO2, H2CO, and HCOOH via their highly-specific rotational-vibrational infrared absorptions. Examples of using this instrument can be found in publications [46, 47, 50, 57].
CIMS. To monitor the gaseous products from the photodegradation of SOA we have built a Chemical Ionization Mass Spectrometer (CIMS) instrument . This apparatus uses a beta-source (63Ni) plus water vapor to generate H3O+, the ionizing agent. Because of the lower proton affinity of H3O+, protons are readily transferred from H3O+ to the organic molecules. After the proton transfer, the ions are sensitively detected with a quadrupole mass spectrometer (QMS). The advantage of this instrument compared to the IR-CRDS instrument is that it can detect more complicated volatile and semi-volatile, and monitor multiple products simultaneously.
Our newest photochemical tool makes it possible to study photolysis of ice and organic films at low temperatures. We used it for the first time to measure absorption cross-sections and photodissociation quantum yields of CH3OOH in ice . A small amount of sample and reference are placed in the form of liquid droplets on two identical slides, and covered with quartz cover slips. Slides are cooled by thermal contact with two Peltier coolers, which are in thermal contact with a water-cooled aluminum heat sink. To measure absorption spectra of the films, light from a deuterium light source is split and sent through the sample and reference and collected in two UV-Vis Ocean Optics spectrometers with optical fibers. Filtered photolysis radiation is delivered through a larger fiber from a Xenon arc lamp, and projected onto the sample slide under a small angle. An electronic shutter is used for blocking the photolyzing radiation without turning the lamp off. The compartment housing is continuously purged with dry, purified air to prevent water condensation on the microscope slides and cover slips.
The QCM (Quartz Crystal Microbalance) allows for very sensitive mass measurements for a thin film deposited on the surface of an oscillating AT-cut quartz crystal. According to a model developed by Dr. Günter Sauerbrey in 1959, such a film suppresses the oscillation frequency by an amount which is directly proportional to its mass. Our group uses QCM to study the efficiently of SOA photodegradation processes. Using this technique, it is possible to characterize processes leading to substantial reduction (or alternatively gain) in the SOA mass. Using QCM technique for the SOA characterization is not trivial for a number of reasons. The largest difficulty is making sufficiently thin and uniform films of SOA material required for the Sauerbrey equation to work. As this electronic microscopy image shows, some of the SOA films we make are nowhere near this limit (although they certainly look very cool). The first application of this instrument is described here.