Aerosol Photochemistry Group Research

Aging Processes in Secondary Organic Aerosols

Chemical processes that convert volatile organic compounds (VOC) into secondary organic aerosols (SOA) are reasonably fast. For example, terpenes rarely last longer than a few hours 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 photochemical and thermal (dark) aging in secondary organic aerosols using a combination of spectroscopic and mass spectrometric techniques. To learn more about this research topic please check our recent papers on photodegradation of organic aerosols, photochemistry of select atmospherically-relevant compounds, photosensitized processes in aerosols, ROS in aerosols. and dark aging processes in aerosols.

Aging of Secondary Organic Aerosols by Condensed-Phase Photochemistry

Direct photochemical aging of SOA refers to processes initiated by absorption of solar radiation within the aerosol particles. 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; (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 SOA formed by ozonolysis and photooxidation of VOCs do indeed absorb light at atmospherically relevant wavelengths [107], leading to rich photochemistry within the particles such as direct photodissociation of peroxide [50] and carbonyl [57, 60] functional groups.

We have demonstrated the importance of condensed phase photochemistry in aging of organic aerosols. For example, we found that condensed-phase photochemical processes in alpha-pinene SOA result in a decrease in mass and diameter of particles occurring on atmospherically-relevant time scales [98]. Photodegradation of SOA produces a variety of small, oxygenated VOCs, which evaporate from the particles leading to the observed mass loss. This photodegradation process could produce upwards of a few Tg/year of formic acid in the atmosphere – comparable to its primary sources [111]. However, uptake of VOCs during irradiation is also possible in a so-called photosensitized process. We recently showed that photosensitized uptake of limonene occurs in competition with the photodegradation, but that the photodegradation occurs on a much faster time scale, and is therefore more important aerosol aging [122].

Another avenue of our research is photochemistry of SOA in aqueous environments, which is relevant for understanding cloud-processing of organic aerosols. In our first paper on this topic, we reported the effect of UV irradiation on the molecular composition of aqueous extracts of limonene ozonolysis 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 high-NOx isoprene SOA [80] we discovered new photochemical reactions converting organic nitrates into unusual heterocyclic compounds containing nitrogen. In our follow up studies, we showed that aqueous photodegradation is common to a broad range of SOA [93, 100, 118]. We found that the SOA material became more volatile on average after the photolysis [118]. These results suggest that SOA dissolved in cloud/fog droplets should undergo significant photolytic processing on a time scale of hours to days, and may need to be considered by atmospheric models.

Our group also performs experiments on photochemistry of SOA material in a variety of matrices, where we control the viscosity of SOA (by adjusting temperature and humidity), and/or the embedded molecules. To investigate the role of the particle viscosity on photochemistry, we investigated photolysis of various molecules embedded in alpha-pinene SOA at high (liquid) and low (glassy) temperatures and showed that photochemistry is suppressed in the glassy state [91]. In a follow up study, we studied this effect in three types of SOA and found that the photochemistry is suppressed at lower temperature or lower relative humidity in all three model systems [110]. We investigated the photolysis of 4-nitrocatechol and 2,4-dinitrophenol in semisolid isomalt as a new type of surrogate for glassy SOA and compared it to photolysis in liquid water, isopropanol, and octanol showing remarkable sensitivity of photochemistry of these compounds to the surrounding matrix [168]. We also studied photooxidation of toluene, the brown color of which is mostly due to nitrophenols, and found that SOA lifetime with respect to photobleaching and lifetimes of individual chromophores in SOA with respect to photodegradation depend strongly on the sample matrix in which SOA compounds are exposed to sunlight [174].

Photochemistry of Selected Atmospheric Compounds

In an effort to better understand the effect of the matrix on photochemical properties, we conduct fundamental studies of photodissociation of important atmospheric molecules in gaseous phase and condensed phases. 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 [63]. 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 [78]. Another study involved the absorption spectra and aqueous photochemistry of alkyl nitrates. We determined that the aqueous photolysis of the alkyl nitrates was insignificant compared to the gas-phase photodegradation and aqueous OH reactions [104]. We have investigated the mechanisms of photochemistry of cyclohexanone, an atmospherically-relevant carbonyl compound [115].

Thermal (Dark) Aging of Organic Aerosols

In addition to the photochemical aging experiments, we also investigate chemical changes in SOA resulting from thermal reactions. For example, explored the effects of water on the chemical composition of SOA during long-term aging processes [167]. We also did experiments on hydrolysis of oxaloacetic acid (OAA) catalyzed by ammonium ion [165]. We also worked with the Shiraiwa group members to better understand spontaneous formation of reactive oxygen species (ROS) during dissolution of SOA in water [160].