Investigating the Reactivity of Gas-Phase Species with Model Tropospheric Aerosols: Substrate, Bulk, and Interfacial Reactions Holly M.

Transcription

1 Investigating the Reactivity of Gas-Phase Species with Model Tropospheric Aerosols: Substrate, Bulk, and Interfacial Reactions Background Currently one of the most interesting pursuits in atmospheric chemistry is the development of an understanding of the reactivity of trace gases with tropospheric aerosols. The importance of these heterogeneous reactions first came to light when it was discovered that SO 2 is oxidized to sulfate in cloud droplets, leading to acid rain. This was an initially surprising find since oxidation of SO 2 by a trace gas such as O 3 was known to be negligible in the gas phase. However in aqueous solution this reaction can be rapid 1, illustrating the importance of gas-liquid reactions. A different heterogeneous system formation of HCl from HNO 3 reacting with solid NaCl (a model of sea-salt aerosol) was studied by Finlayson-Pitts, et al.. They found that in addition to reaction of HNO 3 with a bulk aqueous solution of Na + and Cl - a secondary mechanism utilizing surface-adsorbed water also appeared to be occurring 2. These results provide evidence that reactions of gaseous species within liquid aerosols and at aerosol interfaces like the NaCl-H 2 O interface can occur via alternate reactive pathways. Despite the potential atmospheric impact of these reactions including those that may contribute to global climate change very little is known about them and this leads to the failure of models that simulate atmospheric chemical processes. My research will fill this gap by determining the kinetics and mechanisms of the reactions of tropospheric gas-phase species with aerosols. It is furthermore anticipated that the results of this study will lead to improved atmospheric models. Tropospheric aerosol is complex, consisting mostly of sea salt, mineral dust, organic aerosols, and carbonaceous (soot) aerosols. This study will focus on three of these four types: sea salt, mineral dust, and carbonaceous aerosols (organic aerosols are structurally complicated and will be studied at a later time). Sea salt and mineral dust are important atmospheric constituents because they are the most widespread and concentrated natural aerosols 1,3. Chemically, sea salt is a potential source of

2 atmospheric free radical chlorine and/or bromine 4. Mineral dust may undergo electron transfer reactions with oxidants and also acid-base reactions to form a variety of gas-phase and condensed products. Photochemical reactions are also possible if species comprising the mineral dust have band gaps that fall within the solar spectrum. Understanding the chemical impact of mineral dust on the troposphere is particularly needed since it is expected to become more concentrated due to growing land use and erosion 5. Carbonaceous aerosol (CA) is a product of incomplete combustion processes and can therefore be locally concentrated in the troposphere as the result of fossil fuel and biomass combustion as well as in the lower stratosphere as a product of aircraft exhaust. Soot is of considerable environmental importance since it can act as a reducing agent in the otherwise oxidizing atmosphere 6. CA has also been predicted to have a cooling effect on the global climate 7. In addition to being chemically complex, tropospheric aerosol is also structurally complex. Depending on the atmospheric conditions inorganic aerosol can exist as ionic species in bulk liquid water (high humidity) or as a solid kernel surrounded by a film of water (low humidity). Based on this model, three reactive domains exist. Under high humidity conditions, one expects reaction primarily at the air-liquid film interface or possibly within the bulk liquid. Under low humidity conditions, reaction at the substrate-film interface should dominate (CA is similar except the condensed species would be organic). A full understanding of aerosol reactivity must account for these three domains and to do so one must be able to isolate them. This can be accomplished by tuning the film thickness. That is, reaction at the substrate-film interface can be observed if the film is a few nanometers thick while reaction in the bulk film can be observed if the film is several micrometers thick. Reaction at the air-film interface can then be determined by comparing the reactivity of the film supported by an inactive substrate with the reactivity of the bulk film. 2

3 The experimental strategy utilized in these studies is to follow the reaction in time using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) a technique that is sensitive to surface species on powdered samples for different film thicknesses. Film thickness will be measured using optical differential reflection (ODR), an inexpensive technique that allows the determination of the thickness of a film on a reflective substrate based on the difference in reflectance of parallel- and perpendicular-polarized light from the surface Both of these techniques are straightforward to implement and very suitable for undergraduates. Experimental Systems Investigated Table 1 lists the type of tropospheric aerosols to be investigated, how they will be modeled, and what reactant gases will be introduced to the system. Aerosol Model Substrate Aerosol Film Reactant Gas Sea Salt NaCl and NaBr Powder H 2 O NO 2, SO 2, HNO 3, N 2 O 5 Mineral Dust - and -Fe 2 O 3, CaCO 3, MgCO 3, SiO 2 Powders H 2 O NO 2, SO 2, HNO 3, NH 3 Carbonaceous n-hexane and Diesel Soot H 2 O, adipic acid NO 2, O 2 Table 1. Model aerosol systems. Methodology Reactions will take place inside a stainless steel vacuum chamber that is designed to fit inside the sample compartment of a FTIR spectrometer equipped with a diffuse reflectance accessory and MCT detector. The chamber will be evacuated by a turbomolecular pump to a base pressure of 10-7 Torr. Heating/cooling the sample in the chamber will enable a range of environmentally-relevant temperatures to be investigated. A typical experiment will consist of: 3

4 1. Insertion of the sample into the chamber for evacuation and bakeout, 2. Film deposition to an approximate thickness by backfilling the chamber with vapor (required pressure determined from adsorption isotherms), 3. ODR measurement of film thickness, 4. Introduction of reactant gas, and 5. DRIFTS measurement as a function of time. As new species are produced, IR absorption will increase at frequencies corresponding to new bonds and decrease at frequencies corresponding to broken bonds. Recording DRIFT spectra as a function of time will allow reaction rates and ultimately rate laws and mechanisms to be hypothesized (see below). Analysis Film thickness will be determined by nonlinear least squares fitting of the ODR data, with thickness and complex refractive index as variable parameters. Measurements will be made before and after reactant gas introduction to determine if reaction changes the thickness and/or refractive index. Using the refractive indexes of H 2 O ( ) and graphite (5.4(real); 8.4(imaginary) 14 ), the ODR signal can be estimated to be approximately 1% that of the initial light intensity for a 6.5 -thick water layer on graphite (a model for soot) at 70 incident angle and 650 nm incident wavelength, which is easily detected by standard Si photodiodes. Feasibility calculations have also been performed for water films on Fe 2 O 3 and NaCl with similar results. Different mechanisms occurring in the three reactive domains will be discerned by analyzing the identity and rates of product formation for the three regimes. Specifically: 4

5 Reaction at the substrate-film interface --investigated by comparing DRIFTS data from a bare substrate to that of a substrate with a thin liquid film. Reaction in the bulk liquid --investigated by comparing data in progressively thicker liquid films. Reaction at the liquid-air interface --investigated by comparing data from the thickest film to that from a film of similar thickness deposited on an unreactive substrate, such as a microscope slide. Role of Undergraduate and Graduate Student Researchers Undergraduate and graduate student researchers will contribute to all aspects of this project: sample preparation, data collection, data fitting, and analysis. Students will also assist in building the vacuum system and ODR set-up. Topics students will learn about include: gas-handling, vacuum, laser, and FTIR spectroscopic techniques atmospheric chemistry reaction kinetics and mechanisms at interfaces Current Status of Project The vacuum and optics systems were designed, assembled, and tested at Susquehanna University. The first system my (undergraduate) students and I examined was the reaction kinetics of NO 2 on vacuum-dried -Fe 2 O 3 (hematite) and -Fe 2 O 3 (maghemite) powders, which are components of mineral dust. As we did not deposit a film of water on the Fe 2 O 3 powders, this study targets the substrate-film interface regime. These experiments have been very successful and we have made a 5

6 number of discoveries not previously documented including reactive differences between - and - Fe 2 O 3, the permanent formation of NO 2- (which has led to a refining of the accepted reaction mechanism), and the permanent formation of NO +. The findings for -Fe 2 O 3 + NO 2 (the data for - Fe 2 O 3 are still being analyzed) are briefly discussed below. The remainder of this section may be skipped without loss of continuity. The nitrate products formed on the surface of -Fe 2 O 3 powder as a result of reaction with NO 2 are similar to those formed on the surface of -Fe 2 O 3, as evidenced by a previous study 15 (no investigations of NO 2 reacting with Fe 2 O 3 exist in the literature because it is assumed that the and phases react similarly). However, our studies indicate permanent nitrite and nitrosonium formation on -Fe 2 O 3 at low pressures (24 mtorr), unlike the -Fe 2 O 3 study. As an example of a typical data set for this study, Figure 1 shows the first twelve spectra collected during the reaction of 109 mtorr NO 2 with -Fe 2 O 3. Spectra similar to these were collected for 24, 153, and 210 mtorr NO 2 as well. To determine the rate law for this reaction, the area of the band extending from cm -1 ( Band 1 ) was measured and plotted as a function of reaction time for all pressures. The initial reaction rate at these pressures was then found via linear regression and the order of reaction found from the slope of a plot of log(reaction rate) vs. log(no 2 Pressure). We find that the order of reaction of NO 2 with -Fe 2 O 3 is first order with a value of 1.08 ± Assuming the amount of active sites on the Fe 2 O 3 powder is constant, pseudo-first order conditions exist and the rate law for this reaction may then be written as: d[no 3- ]/dt = k P(NO 2 ) 1 with k = 1.2 ± 0.3 x cm 3 mlcl -1 s -1. 6

7 A mechanism for this reaction has been proposed for -Fe 2 O 3 + NO 2 by Underwood et al. 15 In this mechanism, NO 2, once adsorbed, reacts with the surface to form nitrite. The surface-bound nitrite may then react with either another surface-bound nitrite group (Langmuir-Hinshelwood mechanism, LH) or gas-phase NO 2 (Eley-Rideal mechanism, ER) to form nitrate: NO 2 (g) NO 2 (surface) (1) NO 2 (surface) NO 2- (surface) (2) 2 NO 2- (surface) NO 3- (surface) + NO(g) (3a, LH) or NO 2- (surface) + NO 2 (g) NO 3- (surface) + NO(g) (3b, ER) No hypothesis was given regarding whether 3a or 3b would be more likely to occur, however one of these reactions is thought to be the rate-limiting step. As stated above we have found this reaction to be first order in NO 2. This order does not correspond to the LH (second order at low pressure, zeroth order at high) or ER (second order at low pressure, first order at high; we measure our reaction rate well before saturation therefore our conditions are closer to low pressure than to high) mechanisms. However it does correspond to unimolecular reaction of NO 2 with the surface to produce NO 2-, so we propose NO 2 (g) NO 2 (surface) to be the rate-limiting step. The reaction then appears to follow a LH mechanism after the initial NO NO 2 conversion, based on the fact that NO 2 remains on the surface in the presence of a continually refreshed supply of NO 2. The presence of nitrosonium (absorption at 2150 cm -1 ) indicates that the follow reaction 16 is also likely occurring: 2 NO 2 (surface) N 2 O 4 (surface) NO + (surface) + NO 3- (surface) 7

8 Due to the small intensity of the NO + - signal (approximately 5% of the dominant NO 3 signal in Band 1) it is clearly a minor product channel. We are currently investigating how this impacts the above mechanism. Future Directions In addition to altering concentrations of trace gases, aerosols also affect the surface temperature of the earth 1,4. The scattering of light by aerosols has been well known for years and is currently a vigorous field of climate change research, however it is unknown whether the index of refraction and hence the light-scattering ability of aerosols is altered by reaction. Since this property will be measured through ODR in this study, determining whether the index of refraction is changed upon reaction will be an interesting side-project to pursue. Future experiments will investigate the reactivity of O 3 and other radical species such as OH, HO 2, and Cl with the three types of model aerosol described above. The possibility of photochemistry on substrates with band gaps in the visible region (such as Fe 2 O 3 ) will also be examined. The effect of a surfactant layer on sea salt aerosols is yet another interesting question that will be investigated, as field studies have found marine aerosols that have an organic component 17. The reactivity of a polycyclic aromatic hydrocarbon layer on soot towards oxidation will also be investigated to more closely approximate reactivity of atmospheric aerosols. Funding As indicated in the overview, there are a number of private and government funding sources, both in chemical and environmental divisions, for this project. In fact, while at Susquehanna university I submitted a proposal to the ACS-PRF GB program to investigate the reaction of NO 2 with soot. Although the reviews were very favorable, the project was not funded, mostly because of my 8

9 temporary status. Since then I have used the reviewers comments to strengthen the proposal and submitted it for a Susquehanna University Research Grant (funded internally, reviewed externally). Reviewer comments were very positive they recommended the proposal be submitted to the PRF and I was awarded the grant in I intend to submit an updated version to the Research Corporation for a Cottrell College Science Award in the near future. References 1. Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley and Sons, Inc.: New York, Finlayson-Pitts, B. J.; Hemminger, J. C. Journal of Physical Chemistry A 2000, 104, Barnaba, F.; Gobbi, G. P. Journal of Geophysical Research 2001, 106, Finlayson-Pitts, B. J.; James N. Pitts, Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley and Sons: New York, Sheehy, D. P. Ambio 1992, 21, Ravishankara, A. R. Science 1997, 276, Liousse, C.; Penner, J. E.; Chuang, C.; Walton, J. J.; Eddleman, H.; Cachier, H. Journal of Geophysical Research 1996, 101, Dvorak, J.; Borguet, E.; Dai, H.-L. Surface Science 1996, 369, L Jin, X. F.; Mao, M. Y.; Ko, S.; Shen, Y. R. Physical Review B 1996, 54, Kwon, S.; Russell, J.; Zhao, X.; Vidic, R. D.; Johnson, J. K.; Borguet, E. Langmuir 2002, 18, McIntyre, J. D. E.; Aspnes, D. E. Surface Science 1971, 24, Wong, A.; Zhu, X. D. Applied Physics A 1996, 63, 1. 9

10 13. CRC Handbook of Chemistry and Physics; 71 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, Handbook of Optical Constants of Solids; Palik, D., Ed.; Academic Press: Boston, 1991; Vol Underwood, G. M.; Miller, T. M.; Grassian, V. H. Journal of Physical Chemistry 1999, 103, Marie, O., Malicki, N., Pommier, C., Massiani, P., Vos, A., Schoonheydt, R., Geerlings, P., Henriques, C., Thibault-Starzyk, F., Chemical Communications, 2005, 28, Mueller, P. K.; Mosley, R. W.; Pierce, L. B. Journal of Colloid and Interface Science 1972, 39,

11 Figure 1. DRIFT spectra of g-fe2o3 as a function of NO2 exposure time. P(NO2) = 109 mtorr. Spectra were recorded at 15 s intervals. 11