Heterogeneous reactions on environmental surfaces have been shown to influence the lifetime and abundance of a variety of trace gas species. In addition, these reactions may also alter the physical, chemical, and toxicological properties of the surfaces themselves. In our laboratory, we develop and use surface-sensitive spectroscopic techniques, including glancing-angle laser-induced fluorescence and glancing-angle Raman spectroscopy, to monitor kinetics at different interfaces of atmospheric relevance. We also measure the uptake of gas-phase species to environmental surfaces using Knudsen cell mass spectrometry and employ traditional chromatographic methods as needed.
We have measured the ozonation kinetics of polycyclic aromatic hydrocarbons (PAH) in artificial urban grime and found that degradation rates are much faster than those observed in the gas phase.1 Modeling studies have shown that heterogeneous ozonation on urban grime substrates is a major loss process for low volatility PAH.2
Recently, we have investigated the photoenhanced ozonation of PAH in laboratory proxies for urban grime. While the ozonation of some solid PAH films display a light enhancement3,4, the ozonation of pyrene contained in octanol solution does not4. These results indicate that substrate structure has a profound effect upon heterogeneous ozonation reactions.
Inorganic chemistry also occurs on urban grime: we have shown that illumination of nitric acid- containing films leads to loss of nitrate, likely as a result of the formation of HONO and/or NO25. This process could help to resolve the missing source of HONO in urban centers.
Currently we are using FTIR-ATR to study photochemical reactions on the proxy films. In this way changes in the film can be monitored to more accurately predict both inorganic and organic reactions that could be occurring. Real urban grime samples are also being grown in downtown Toronto for the purpose of performing more accurate uptake and reaction studies in the future.
(1) Kahan, T. F.; Kwamena, N. O. A.; Donaldson, D. J. Atmospheric Environment 2006, 40 (19),
(2) Kwamena, N.; Clarke, J.; Kahan, T.; Diamond, M.; Donaldson, D.J. Atmospheric Environment 2007, 41 (1), 37-50.
(3) Styler, S.; Brigante, M.; D'Anna, B.; George, C.; Donaldson, D.J. Physical Chemistry Chemical Physics 2009, 11(36), 7876-7884.
(4) Styler, S. A.; Loiseaux, M. E.; Donaldson, D. J. Atmospheric Chemistry and Physics 2011, 11(3), 1243-1253.
(5) Handley, S. R.; Clifford, D.; Donaldson, D. J., Environmental Science & Technology 2007, 41 (11), 3898-3903.
Snow and ice cover a large fraction of the earth’s surface. There is a growing body of evidence to suggest that chemistry occurring in or on snow and ice can significantly perturb the composition of the overlying atmosphere (the boundary layer).1 For example, bromide (Br-), which is a component of sea salt, can be ‘activated’ on frozen media and released to the atmosphere as gas-phase bromine (Br2). Once in the atmosphere, photolysis of Br2 leads to the production of reactive Br radicals, which initiate the catalytic destruction of boundary layer ozone (O3).2
However, a good understanding of air-ice chemical interactions is still lacking. This is largely due to challenges in characterizing the physical and chemical properties of ice and snow surfaces. At the ice surface there exists a disordered region which is often referred to as a quasi-liquid layer (QLL). Contaminants (or solutes) in ice are thought to be concentrated in the QLL a) via exclusion from the growing ice matrix during freezing and/or b) by deposition from the gas-phase to the ice surface. However, parameters such as surface pH, surface concentration, and QLL thickness are not well known. Since surface properties are not necessarily preserved when a sample is melted, many traditional analytical techniques cannot be used to investigate the air-ice interface.
Work in our group focuses on developing surface-sensitive spectroscopic tools with which to study the ice surface and air-ice chemical interactions. We have used glancing-angle Raman spectroscopy to study the reaction of gas-phase O3 with Brˉ and Iˉ at the ice surface (a mechanism for bromine activation), and find the kinetics to consistent with the exclusion of the halides to the ice surface.3 Chemical reactions on ice/snow (such as those involved in bromine activation) may exhibit important pH dependences. What is the pH of an ice/snow surface? How do trace gases (such as HNO3) interact with the QLL? We are currently using glancing-angle laser-induced fluorescence (LIF) spectroscopy to study the pH of ice surface.
We have also used glancing-angle LIF spectroscopy to study the photolysis of polycyclic-aromatic-hydrocarbons (PAHs, organic pollutants that undergo long-range transport to high-latitude snowpacks) at the ice surface.4 In this study (and others5,6) we find evidence for unique reactivity in the QLL. Such results have important consequences for how we model the ice surface: when does the ice surface behave like a thin liquid surface and when/how does it deviate? How can we connect surface properties to bulk properties? Future work aims to improve our understanding of air-ice chemical interactions from a microphysical perspective.
(1) F. Dominé and P. B. Shepson, Science, 297, 1506 - 1510 (2002)
(2) W. R. Simpson et al., Atmos. Chem. Phys. 7, 4375-4418 (2007)
(3) S. N. Wren, T.F. Kahan, K. B. Jumaa and D. J. Donaldson, J. Geophys. Res., 115, D16309, doi: 10.1029/2010JD013929 (2010)
(4) T.F. Kahan and D. J. Donaldson, J. Phys. Chem. A 111, 1277-1285 (2007)
(5) T.F. Kahan, N.-O. A. Kwamena, D.J. Donaldson, Atmos. Chem. Phys., 10, 10917-10922 (2010)
(6) T.F. Kahan, R. Zhao and D.J. Donaldson, Atmos. Chem. Phys. 10, 843-854 (2010).
The water surface is an important site for reactions in the environment for two main reasons: first, due to its abundance, and second, due to its very high surface free energy (resulting in high surface tension). One way to decrease this high surface energy is for solutes to partition to the air-aqueous interface. These ‘surface-active’ species can undergo chemical reactions on the water surface at faster rates than in the bulk phase. Understanding the factors that affect the rates of surface-mediated atmospheric reactions is crucial to construct accurate atmospheric models and also to understand the effect of changing anthropogenic emissions.
To study reactions at the air-aqueous interface, the Donaldson group has pioneered a surface-selective spectroscopic technique called glancing angle laser-induced fluorescence (LIF) and Raman.1 We have also coupled LIF to a profile analysis tensiometer (PAT), which allows for spectroscopic data as a function of organic coverage of a suspended drop. Another surface sensitive method used in the Donaldson group is second harmonic generation (SHG). This technique uses the slightly ordered nature of the air-aqueous interface to generate signal through a non-linear optical process allowed by the high field intensity of short-pulse lasers.
Current work in the Donaldson group is aimed at understanding the effect of organic coatings on environmentally-relevant reactions at the air-aqueous interface. For example, the influence of mid-length, functionalized hydrocarbon chains at the air-aqueous interface on the 2-D ozonation reactions of PAHs is quite interesting: the presence of a C8 alcohol (1-octanol) at the interface enhances the ozonation reaction relative to the uncoated surface, while the presence of a C8 acid (octanoic acid) suppresses the reaction.2,3 Also, compressed organic coatings at the air-aqueous interface have been shown to suppress the uptake of atmospherically relevant acids such as HNO3,4 N2O5,5-8 and acetic acid.9 Future work in the Donaldson group aims to study the effect of organic coatings on the uptake and hydrolysis of more atmospherically relevant species such as CO2, NO2, and SO2, and to elucidate the mechanism of the uptake inhibition – is there physical blocking? Or is there simply a decrease in available H2O needed for hydrolysis?
(1) Wren, S. N.; Donaldson, D. J. Phys. Chem. Chem. Phys. 2010, 12, 2648.
(2) Henderson, E. A.; Donaldson, D. J. J. Phys. Chem. A 2011, 116, 423.
(3) Mmereki, B. T.; Donaldson, D. J.; Gilman, J. B.; Eliason, T. L.; Vaida, V. Atmos. Environ. 2004, 38, 6091.
(4) Stemmler, K.; Vlasenko, A.; Guimbaud, C.; Ammann, M. Atmos. Chem. Phys. 2008, 8, 5127.
(5) McNeill, V. F.; Patterson, J.; Wolfe, G. M.; Thornton, J. A. Atmos. Chem. Phys. 2006, 6, 1635.
(6) Badger, C. L.; Griffiths, P. T.; George, I.; Abbatt, J. P. D.; Cox, R. A. J. Phys. Chem. A 2006, 110, 6986.
(7) Cosman, L. M.; Bertram, A. K. J. Phys. Chem. A 2008, 112, 4625.
(8) Griffiths, P. T.; Badger, C. L.; Cox, R. A.; Folkers, M.; Henk, H. H.; Mentel, T. F. J. Phys. Chem. A 2009, 113, 5082.
(9) Gilman, J. B.; Vaida, V. J. Phys. Chem. A 2006, 110, 7581.
The sea surface microlayer (SML), the uppermost hundreds of micrometers of the sea surface, contains enhanced amounts of dissolved organic matter and sea salt halides relative to bulk sea water. Processes at the sea water—air interface are of particular interest because species must pass through the SML to enter into either phase. In addition, chemistry that occurs at this interface may differ from reactions in corresponding bulk phases. Thus it is important to examine the physical, biological and chemical influences on the production and transport of species at salt water surfaces.
We have used glancing angle laser induced fluorescence (LIF) of aqueous surfaces to examine reactions of chlorophyll at salt water surfaces as an ozone sink and as a potential source of reactive halogens into the marine boundary layer.1-3
We have also used gas and solution phase absorption spectroscopy to study the ozonation of iodide at salt water—air interfaces with and without organic coatings to better understand the influence of organic matter on the production and release of molecular iodine via this reaction.4
We have recently set up an incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) to measure gases, such as NO2 or I2, produced from reactions in aqueous solutions or at salt water—air interfaces. This apparatus can also monitor products from photoreactions and has the potential to be used in tandem with laser induced spectroscopic techniques.
(1) Clifford, D., et al.,Environmental Science & Technology, 2008. 42(4): p. 1138-1143.
(2) Reeser, D.I., C. George, and D.J. Donaldson, Journal of Physical Chemistry A, 2009. 113(30): p. 8591-8595.
(3) Reeser, D.I., et al., Journal of Physical Chemistry C, 2009. 113(6): p. 2071-2077.
(4) Reeser, D.I. and D.J. Donaldson, Atmospheric Environment, 2011. 45: p. 6116-6120.
Although over a billion tons of photoactive mineral dust are released into the atmosphere each year from deserts and arid regions, we currently know little about the photochemistry that happens when gas-phase organic species partition to dust surfaces. Photochemical reactions on dust have the potential not only to alter the lifetime of organic species within the atmosphere but also to change the optical properties and cloud-forming potential of the dust particles themselves.
In our lab, we have developed a custom-built photochemical Knudsen cell to study the photochemistry of atmospherically abundant organic compounds at the surface of model and authentic atmospheric substrates, including metal oxides, Saharan sand, and Icelandic volcanic ash. We measure the production and loss of organic species using mass spectrometry and, in some cases, liquid chromatography.
In our first project using this apparatus, we investigated the photooxidation of isopropanol, a representative atmospheric alcohol, at the surface of TiO2, which we used as a laboratory proxy for the photoactive component of mineral dust aerosol. We found that illumination of isopropanol-saturated TiO2 films led to the production of gas-phase acetone, and that this production was enhanced in the presence of co-sorbed nitrate. This latter result is important because it implies that the reactivity of mineral dust aerosol may be enhanced by its interaction with urban pollution plumes, which is known to result in nitrate coatings.
In subsequent experiments, we have moved toward more atmospherically relevant conditions: we have modified our Knudsen chamber to allow for the introduction of gas-phase oxygen, and we have begun to conduct experiments on authentic samples, including desert dust and volcanic ash. In our most recent set of experiments, we showed that oxalic acid, the most atmospherically abundant dicarboxylic acid, undergoes efficient photooxidation at the surface of both dust and ash. These results imply that aerosol-catalyzed photochemical oxidation may be an important loss pathway for this species, especially in arid regions.