Environ. Sci. Technol. 2003, 37, 1371-1378
Even-Odd Alternation of Evaporation Rates and Vapor Pressures of C3-C9 Dicarboxylic Acid Aerosols MERETE BILDE,* BIRGITTA SVENNINGSSON, JACOB MØNSTER, AND THOMAS ROSENØRN Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Aliphatic straight-chain dicarboxylic acids have been identified as common water-soluble organic components of atmospheric aerosols. To model the partitioning of such compounds between gas and particle phase in the atmosphere, information about their vapor pressures is essential. In this work, vapor pressures of C3-C9 dicarboxylic acids are derived from measured evaporation rates of submicron aerosols over the temperature range of 290314 K using the tandem differential mobility analyzer technique. Vapor pressures obtained from the experimental data were as follows: log(p°malonic, Pa) ) - 4822 K/T + 12.9, log(p°succinic, Pa) ) - 7196.8 K/T + 19.8, p°glutaric (296 K) ) 6.7 × 10-4 Pa, log(p°adipic, Pa) ) -8065.0 K/T + 22.2, log(p°pimelic, Pa) ) -7692.8 K/T + 21.8, log(p°suberic, Pa) ) -9629.4 K/T + 26.5, and log(p°azelaic, Pa) ) -7968.7 K/T + 21.7. Vapor pressures of C3-C9 dicarboxylic acids are shown to alternate strongly with the parity of the number of carbon atoms. Higher vapor pressures of the odd acids fit the less stable crystal structure, the propensity of polymorphism in the odd acids, and the evolution of melting temperatures. Results are compared with available literature data.
Introduction Aliphatic straight-chain dicarboxylic acids have been identified as common water-soluble organic components of urban (1-5) as well as rural (5), marine (6, 7), and polar (8, 9) submicron aerosols. Their general chemical formula is HOOC(CH2)n-2COOH where n is the number of carbon atoms. In this work, we focus on acids with n between 3 and 9. Common names for these acids are malonic (n ) 3), succinic (n ) 4), glutaric (n ) 5), adipic (n ) 6), pimelic (n ) 7), suberic (n ) 8), and azelaic (n ) 9) acids. Atmospheric dicarboxylic acids originate from photooxidation of biogenic and anthropogenic compounds as well as direct emissions. Since the late 1970s, field measurements and smog chamber experiments have been performed to identify gaseous precursors and to understand the mechanisms for the formation of dicarboxylic acids and other water-soluble organic compounds in the atmosphere (1013). Even so, the formation of dicarboxylic acids and their partitioning to the particle phase is not well understood. * Corresponding author e-mail:
[email protected]; phone: +45 35 32 03 29; fax: + 45 35 35 06 09. 10.1021/es0201810 CCC: $25.00 Published on Web 02/27/2003
2003 American Chemical Society
Cycloalkenes such as cyclopentene and cyclohexene found in gasoline and automobile exhaust have been suggested as potential anthropogenic precursors of atmospheric dicarboxylic acids with n in the range of 3-6 (1, 10, 14). Biogenic precursors include aliphatic diolefins and unsaturated carboxylic acids. As an example, oleic and linoleic acids from terrestrial plants have been suggested as biogenic precursors of suberic and azelaic acids (4, 15, 16). Kawamura et al. (8) have speculated that unsaturated fatty acids of marine origin are photooxidized in the marine atmosphere to form a homologous series of dicarboxylic acids including azelaic, succinic, malonic, and oxalic acids. Primary sources of C3C9 dicarboxylic acids include wood burning (17), automobile exhaust (18, 19), and meat cooking (20). Concentrations of dicarboxylic acids in atmospheric aerosol samples vary by orders of magnitude from one region to another. For example, Kawamura et al. (9) found total concentrations of dicarboxylic acids of 25 ng/m3 in the remote aerosol of Alert, AK. This is 20 times lower than the 480 ng/ m3 observed in Tokyo aerosol by the same authors (9, 14). There is evidence that gaseous precursors of dicarboxylic acids can travel long distances and that photochemical oxidation to dicarboxylic acids may take place at locations far away from their sources (21, 22, 6). As an example dicarboxylic acids of anthropogenic origin have been observed in the Arctic atmosphere at polar sunrise (9). Partitioning of dicarboxylic acids between gas phase and particle phase in the atmosphere is determined by many factors including solubility in water, adsorption onto available particle surfaces, and dissolution into a potential organic aerosol phase. Pankow (23, 24) has derived the governing equations for the last two effects (adsorption and dissolution into an organic aerosol phase) and shown that the vapor pressure is a key parameter in describing partitioning of low volatility organic compounds between gas phase and particle phase. For purposes of atmospheric science, experimental values of vapor pressures at ambient temperatures are therefore needed. While methods of estimating the vapor pressure of low volatility organic compounds exist in the literature (25, 26), low vapor pressures are difficult to measure, and very few experimental data are available. As recently pointed out by Asher et al. (26), the lack of experimental data is very unfortunate since compounds with low vapor pressures partition to the particulate phase to the greatest extent. Vapor pressure of organic aerosol components have previously been measured using light scattering (27), effusion methods (28-30), and the tandem differential mobility analyzer (TDMA) technique (31, 33, 34, 37, 39) where vapor pressures are derived from measurements of evaporation rates. Recently, Chattopadhyay et al. (32) presented a new temperature-programed thermal desorption method where organic particles were collected and cooled to -50 °C in a vacuum chamber before allowed to evaporate. Gas phase molecules were detected using quadropole mass spectrometry. An advantage of this technique is that a complete Clausisus-Clapaeyron plot can be obtained in a few hours. TDMA experiments are time-consuming as compared to the thermal desorption method, but precision is apparently also better as pointed out by Chattopadhyay et al. (32). In this work, we demonstrate the capability of the TDMA technique to capture variations in vapor pressures of submicron aerosol particles with solid-state structure. To the best of our knowledge, experimental vapor pressure data for aliphatic dicarboxylic acids at ambient temperatures are only available for glutaric, adipic, pimelic, and suberic VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Physical Properties of C3-C9 Dicarboxylic Acids Mi (10-3 Tm Gi (103 Vc (cm3 kg mol-1) (K)a kg m-3)b mol-1)c malonic acid succinic acid glutaric acid adipic acid pimelic acid suberic acid azelaic acid
104.06 118.09 132.12 146.14 160.17 174.20 188.22
408 461 369 426 376 415 380
1.616 1.566 1.424 1.362 1.281 1.272 1.251
255 310 365 420 475 530 585
σii (Å)d
Eii/kB (K)d
5.33 5.69 6.01 6.30 6.56 6.81 7.03
784.80 885.41 708.77 818.21 722.21 797.09 728.93
a Melting points were obtained from Fluka and Aldrich Chemical Companies. b Densities from ref 49. c Critical volumes are estimated using the group contribution method by Lyderson (54). d σii ) 0.841Vc1/3 (ref 41, p 22), ii/kB ) 1.92Tm (ref 41, p 22). The Lennart-Jones parameters σi,air (m) and i,air/kB are estimated as follows: σi,air ) 1/2(σii + σair), i,air ) xiiair, where σair ) 3.617 Å and air/kB ) 97 K.
acids. The vapor pressures of glutaric acid and adipic acid were determined using the TDMA technique by Tao and McMurry (33), who found the following relationship between vapor pressure (Pa) and temperature (K): log(p°) ) -(792n + 1387)/T + 0.81n + 10.9 where n (number of carbon atoms) is 5 or 6 over the temperature range of 285-313 K. The vapor pressure of glutaric acid was later measured by Bilde and Pandis (34) (also using a TDMA technique) to be in good agreement with the value reported by Tao and McMurry (33). Chattopadhyay et al. (32) measured the vapor pressures of C6-C8 dicarboxylic acids using the new temperatureprogramed thermal desorption method described above. The combined data set of Tao and McMurry (33) and Chattopadhyay et al. (32) suggests that the vapor pressure decreases monotonically with the number of carbon atoms in the dicarboxylic acid chain. Using the TDMA technique, we show herein that vapor pressures in fact alternate strongly with the parity of the number of carbon atoms. We provide a microphysical explanation and discuss our results in the context of atmospheric science. In the following, we first describe the recently constructed TDMA system at the University of Copenhagen, Denmark, and present a series of experiments performed to quality check the system. On the basis of measured evaporation rates at different temperatures in the range of 290-314 K, we infer vapor pressures and heats of vaporization. We compare vapor pressures of glutaric, adipic, pimelic, and suberic acids obtained in this work with literature data and provide new experimental results for malonic, succinic, and azelaic acids at ambient temperatures. In the following, dicarboxylic acids will often be referred to as “diacids”, and “odd” and “even” will refer to the parity of the number of carbon atoms in the dicarboxylic acid chain.
Experimental Section Chemicals. All diacids studied are soluble in water, and they are white crystalline solids or powders at room temperature with melting points in the range of 96-188 °C (35), see Table 1. Dicarboxylic acids were obtained from commercial sources: malonic acid (>99%), succinic acid (>99.5%), suberic acid (>98%), pimelic acid (g99%), and azelaic acid (g99%) were obtained from Fluka. Glutaric acid (99%) and adipic acid (99%) were from Aldrich Chemical Co. All chemicals were used as received. Double-deionized water purified using a Milli-Q PLUS Ultrapure water system was used for making aqueous solutions of the dicarboxylic acids. Typical concentrations of the aqueous solutions used were in the range of 0.1-0.2 g/L. Experiments were performed in a carrier gas of air purified and dried using a TSI 3074 filtered air supply. TDMA System. Evaporation rates of organic particles were measured using the TDMA technique (36, 37). The basic idea 1372
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FIGURE 1. Schematic drawing (not to scale) of the experimental setup. The length of the laminar flow reactor is 3.5 m; the i.d. is 2.6 cm. Particles can be sampled via four ports along the length of the reactor. Monodisperse particles are generated with an atomizer. Dew point temperature is measured after drying and passage through a Kr-85 neutralizer. If necessary, dry particle-free dilution air is added to lower the humidity. A monodisperse size fraction is selected with the first DMA. Particles enter the laminar flow reactor through a stainless steel tube reaching into the reactor. Sheath air can be added through surrounding holes in the end of the reactor. Initial particle size was measured with DMA-2 and the CPC either directly after DMA-1 or at port 4. After passage through the laminar flow reactor, particle final diameters were measured using DMA-2 and the CPC again. Excess air or dilution air can pass through a particle filter immediately after the reactor. is to let monodisperse particles (selected with a DMA) evaporate for a given time in a laminar flow reactor and measure their final size using a second DMA. A schematic drawing (not to scale) of our system is shown in Figure 1. Aerosols were generated by atomizing aqueous solutions of organic compounds in a TSI 3076 constant output atomizer operated in recirculation mode. After passage through two or three silica gel diffusion dryers (each of length 54 cm with an inner wire screen diameter of 1.3 cm), the particles were charged by exposing them to a Kr-85 bipolar ion source (TSI, 3077 Aerosol Neutralizer). Dried purified air was used to dilute the aerosol stream. At this point the relative humidity, as measured using a General Eastern Hygro M4 dew point monitor was below 6% in most experiments and below 10% in all experiments. A TSI 3081 electrostatic classifier (DMA1) was set to select an almost monodisperse part of the aerosol size distribution. To get the size distribution as narrow as possible, the ratio of aerosol to sheath air in the DMA column was typically 1:10 or smaller. The DMA operates with recirculating sheath air, and a filter with activated charcoal was inserted in the recirculation loop (before the particle filters) to avoid buildup of organic vapors. Test experiments were performed without the charcoal filter, and in some cases the gas phase became saturated with organic vapor. These data were discarded. The central part of the system is a 3.5 m long stainless steel laminar flow reactor with an i.d. of 0.026 m. The reactor has four ports along its length for sampling or measurement of temperature and relative humidity. Particles are introduced into the reactor trough a moveable stainless steel cylinder of 7 mm i.d. Sheath air can be introduced from the end of the reactor through holes surrounding the aerosol inlet. A stable flow of sheath air was ensured by a GFC mass flow controller (Aalborg Instruments and Controls, Inc.). Volumetric flow rates were measured using the built-in TSI flow meters and an external Gilibrator flow cell. All flow meters and flow controllers were calibrated relative to the Gilibrator flow cell. Sheath flow rates through the laminar flow reactor were in the range of 0-4 L/min, and aerosol flow rates were typically in the range of 0.2-0.5 L/min. An opening for excess air at the end of the reactor guaranteed atmospheric pressure in the system.
FIGURE 2. Size distributions of succinic acid before and after evaporation in the laminar flow reactor for 38 s at 307 K. The temperature of the reactor was controlled by flowing water from a circulating bath (Heto CBN) through a jacket surrounding the reactor. Reactor sheath air was preheated or cooled to the same temperature as the reactor by passing it through tubing immersed in the water bath. For temperatures other than room temperature, the first 0.5 m of the reactor was used as a temperature heating/cooling and equilibration zone. Temperature and relative humidity in the reactor were measured using a Rotronic I-1000 hygromer transmitter (absolute accuracy (0.3 K, reproducibility
