Article pubs.acs.org/est
Measurement of Naphthalene Uptake by Combustion Soot Particles David S. Liscinsky,*,† Zhenhong Yu,‡ Bruce True,† Jay Peck,‡ Archer C. Jennings,† Hsi-Wu Wong,‡ Jon Franklin,‡ Scott C. Herndon,‡ and Richard C. Miake-Lye‡ †
United Technologies Research Center (UTRC), East Hartford, Connecticut 06108, United States Aerodyne Research, Incorporated, Billerica, Massachusetts 01821, United States
‡
ABSTRACT: In this study, we designed and constructed an experimental laboratory apparatus to measure the uptake of volatile organic compounds (VOCs) by soot particles. Results for the uptake of naphthalene (C10H8) by soot particles typical of those found in the exhaust of an aircraft engine are reported in this paper. The naphthalene concentration in the gas phase and naphthalene attached to the particles were measured simultaneously by a heated flame ionization detector (HFID) and a time-of-flight aerosol mass spectrometer (ToF AMS), respectively. The uptake coefficient for naphthalene on soot of (1.11 ± 0.06) × 10−5 at 293 K was determined by fitting the HFID and AMS measurements of gaseous and particulate naphthalene to a kinetic model of uptake. When the gaseous concentration of naphthalene is kept below the saturation limit during these experiments, the uptake of naphthalene can be considered the dry mass accommodation coefficient.
1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) and soot particles from various combustion sources, such as aircraft engines, present significant environmental and health concerns.1−3 PAHs are known for their carcinogenic, mutagenic, and teratogenic properties and are also considered precursors of soot particles in combustion research.4,5 Soot particles have both a direct and indirect effect on the atmosphere, strongly absorbing sunlight, thereby warming the ambient air, which leads to further net warming, but also promoting cloud formation by providing condensation sites for water droplets (and thus potential net cooling or warming, depending upon cloud altitude). During atmospheric transport, soot particles can undergo chemical and physical changes via heterogeneous interactions with gas-phase species. Organic carbon aerosols can be a significant contributor to the radiative forcing of the Earth’s climate. Several laboratory and modeling studies have been conducted to investigate the oxidation reactions of PAHs on soot surfaces6,7 to understand the fate of organic aerosols and soot in the atmosphere. An investigation by Arens et al. found that HONO was formed from the interaction of NO2 and anthrarobin (a soot-like surface), which illustrates the potential importance of atmospheric gas−solid reactions.7 In addition, Chen et al. investigated uptake and photo-oxidation of naphthalene on humic-like thin water films.8,9 They found that the reaction rates with the surface-active organic substances were substantially higher compared to the bulk-phase reaction rate, again illustrating the importance of surface interactions. Because some of the combustion-generated PAHs are volatile, adsorption and reactions of such PAHs on soot are important © 2013 American Chemical Society
to the understanding of overall heterogeneous reaction mechanisms. The objective of this work is to characterize and quantify the uptake of a representative PAH, naphthalene (C10H8), by combustion soot particles. Two independent studies have identified appreciable naphthalene emissions from the engine exhausts of a commercial aircraft.10,11 The results of this research project measure the dry mass accommodation coefficient, which is a critical parameter in microphysical models, which attempt to quantify and accurately simulate volatile particle evolution in aircraft-emitted plumes. To further enhance the accuracy of microphysical modeling, ongoing projects are investigating the uptake of other volatile organic compounds (VOCs) found in aircraft exhaust using the same method. Uptake kinetics of a number of atmospherically important species found in the exhaust of aircraft engines, such as water, ozone, nitrogen dioxide, and sulfuric acid, has been experimentally investigated via a variety of spectroscopic and mass spectrometric techniques.12−18 However, only a few investigations have been performed on the uptake of organic vapors on combustion soot. For example, Saathoff and coworkers examined the coating of ozonolysis products of αpinene on soot at a high relative humidity (RH ∼ 90%) and found that the organic coating of soot particles leads to a compaction of the soot fractal agglomerates.19 Received: Revised: Accepted: Published: 4875
November 30, 2012 March 11, 2013 April 3, 2013 April 3, 2013 dx.doi.org/10.1021/es304912d | Environ. Sci. Technol. 2013, 47, 4875−4881
Environmental Science & Technology
Article
Figure 1. Schematic of the experimental apparatus.
a ultrafine condensation particle counter (CPC, model 3025A, TSI). Before mixing with the gaseous naphthalene, the soot stream was passed through a thermal soot denuder (Aerodyne Research, Inc.) operated at 150 °C to remove all remnants of organic carbon deposited during particle formation in the diffusion flame and, therefore, ensure very little organic coating on the soot particles. The specifications and performance of the thermal denuder have been described in detail elsewhere.22 The main difference in our design objectives versus those of previous designs is that our thermal denuder was designed with additional temperature control and reduced thermal mass to allow for temperature stepping. With the centerline temperature of the thermal denuder up to 250 °C, the removal efficiency for ammonium nitrate and ambient organic aerosol can reach as high as 98 and 85%, respectively. As demonstrated in Figure 2, the timeline of SMPS measurements shows the response of a typical soot stream to conditioning by the thermal denuder. The soot particle concentration remains nearly constant, while the particle volume decreased significantly when the temperature of the thermal soot denuder was at 150 °C compared to when the denuder temperature was at room
In this study, we examine the heterogeneous interaction of naphthalene on soot formed from a propane diffusion flame. Naphthalene is a white crystalline solid at room temperature and is the simplest PAH compound. It has a vapor pressure of 11.6 Pa at 298 K and a solubility of 30 mg/L in water. The United States Environmental Protection Agency (U.S. EPA) has declared naphthalene as one of the hazardous air pollutants (HAPs). Therefore, naphthalene is relevant as an atmospheric constituent.
2. EXPERIMENTAL SECTION This laboratory study investigated the uptake of naphthalene by combustion soot particles to provide physical insight to the heterogeneous interaction between PAHs and soot and also to yield critical parameters for a microphysical model to simulate the chemical and microphysical evolution of aircraft engine exhaust. Combustion soot particles were generated by a minicombustion aerosol standard (mini-CAST, Jing Ltd., model 5200) soot generator, which consists of a well-controlled propane diffusion flame.20 This soot generator has been applied as a reliable soot source in a number of investigations on combustion soot particles.21 The device is capable of generating stable soot flow over several hours, with a low organic carbon content (
