Volatile particles measured by vapor-particle separator
Introduction
Combustion particles from aircraft engines are composed of condensable and non-volatile fractions (Cheng, Corporan, DeWitt & Landgraf, 2009), and all turbine engine particles are ultrafine (Cheng et al. (2008), Cheng, Corporan, DeWitt, & Landgraf (2009)). Small size of particles enhances their ability to translocate if they are inhaled (Oberdörster & Utell, 2002). Numerous epidemiological studies have clearly shown a relationship between particulate matter (PM) concentrations and mortality rates. PM2.5 or particles <2.5 μm in aerodynamic diameter is a criteria pollutant by the U.S. Environmental Protection Agency (EPA). The U.S. EPA and international environmental agencies continue to implement more stringent air quality standards. These standards and regulations will no doubt affect the transportation sector, including aviation industry that relies heavily on the use of fossil fuels. In the case of aviation, the regulation may slow the growth of commercial aviation worldwide and limit operations in U.S. military bases, especially those located within National Ambient Air Quality non-attainment areas.
More than 50% of the atmospheric PM mass is classified as secondary in many regions such as the Southeast U.S. Secondary PM is a result of the complex gas-to-particle transformation and mass transfer between the two phases. Molecules of higher molecular weight, including sulfur-based and organic compounds, are responsible for the formation of secondary PM (referred to as volatile PM – VPM) in the atmosphere. As the exhaust leaves the engine, it rapidly mixes with the ambient air and cools down, high molecular weight vapor molecules in the exhaust plume would condense onto the non-volatile fraction (or soot) (Ristimaki, Vaaraslahti, Lappi & Keskinen, 2007). Given a sufficient concentration, the vapors could nucleate into new particles through the condensation-nucleation processes (Ronkko et al., 2007) that would change the particle size distribution from a single mode distribution to possibly a bi-modal one of non-equal size. Representative sampling and accurate measurement of the volatile aircraft engine PM has been challenging as these are greatly influenced by the ambient conditions and composition of the engine exhaust. At the present, only soot or soot-like materials or non-volatile PM (primary PM) can be measured consistently in the particulate phase by modern aerosol instrument and sampling technology as demonstrated in the Particle Measurement Programme (PMP) (Giechaskiel, Dilara, Sandbach, & Anderson (2008), Mamakos, 2012, Zheng et al. (2012)).
There exists no standard method for sampling and measurement of VPM. Currently, researchers remove aircraft volatile PM by using devices such as thermodenuder (Burtscher et al., 2001), catalytic stripping (Swanson & Kittelson (2010), Mamakos, 2012), and Volatility Tandem Differential Mobility Analyzer (VTDMA) (Johnson, Ristovski, & Morawska (2004), Villani, Picard, Marchand, & Laj (2007)). None of these enable one to observe engine particle dynamics, molecular transfer, or evaporation process under varying temperature conditions. For example, the thermodenuder and the catalytic stripper were intended for rapid removal of volatile components from soot particles, while VTDMA was to investigate volatilization and hygroscopicity of single ambient aerosol particles. Although highly detailed, the VTDMA is too slow to be suitable for aircraft VPM measurement. In addition, the concentrations of water vapor, unburned hydrocarbons, and particles in the engine exhaust are so high that the VTDMA would be overwhelmed.
For 30 years, thermodenuder of different variations has been a popular device designed to desorb volatile species for studies of “solid” ambient particles (Johnson, Ristovski, & Morawska (2004), Newman (1978), Cobourn, Husar, & Husar (1978), Slanina, Keuken, & Schoonebeek (1987), Sturges & Harrison (1988), Wehner, Philippin, & Wiedensohler (2002), Fierz, Vernooij, & Burtscher (2007), Park, Kim, Choi, & Hwang (2008), Huffman, Ziemann, Jayne, Worsnop, & Jimenez (2008), Wu, Poulain, Wehner, Wiedensohler, & Herrmann (2009)). Thermal stripping devices have also been used on diesel engine soot and aircraft emissions (Burtscher et al. (2001), Maricq et al., 1999, Vaaraslhti, Virtanen, Ristimaki, & Keskinen (2004), Virtanen, Ristimaki, Vaaraslhti, & Keskinen (2004), Petzold et al. (2005), Mamakos, Ntziachristos, & Samaras (2006)). Although volatile components are removed from the PM, the adsorbent used in these designs retain the volatile components. Once the adsorption capacity is exceeded, the volatiles could and have been found to re-condense on existing particles or form new ones (e.g., Swanson & Kittelson, 2010).
Section snippets
Description of vapor particle separator (VPS)
To improve measurement and characterization of aircraft VPM, a new technology called Vapor-Particle Separator (VPS) has been designed and tested at the Oak Ridge National Laboratory (ORNL) (Cheng & Allman, 2011). The VPS technology was tested against the PMP VPR (Giechaskiel, Dilara, Sandbach, & Anderson (2008), Mamakos, 2012, Zheng et al. (2012)) using tetracontane particles and exhaust particles from turbine engines (T63 and F117 engines at the Wright-Patterson Air Force Base). The PMP VPR
Materials and method
Evaluation of the VPS was performed in laboratory and field conditions. The evaluation in the laboratory was designed to (1) understand particle transmission efficiency at non-heated condition (Cheng (2010), Cheng & Allman (2011)), (2) assess the feasibility for separation of gas phase species vaporized from the particles, and (3) evaluate the removal efficiency of particles as a function of temperature. Early studies (Cheng & Allman (2011), Cheng (2010)) had addressed the first two to some
Thermal behavior of C40 aerosol particles
Thermal degradation of tetracontane was previously studied (Németh, Blazsó, Baranyai & Vidóczy, 2008) in many applications including waste treatment and polymer manufacturing. Reactions at high temperature (e.g., 500 °C) were found to involve simultaneous or subsequent H-abstraction, β-scission and backbiting reactions (intramolecular H-shifts) leading to the formation of radicals, smaller alkane, alkene, diene molecules, and terminally unsaturated polymer residues (Németh et al., 2008). Some or
Conclusions
The Vapor-Particle Separator (VPS) is a new technology for analyzing aerosol volatility. The VPS was designed to separate volatile and non-volatile particles to enhance the characterization of both fractions by using a novel metallic membrane and cross-flow separation. The VPS was field-tested on raw PM samples from military aircraft engines. The utility as a particle remover was compared with that of a PMP VPR device in this study. It was learned from both field (actual engine) and laboratory
Disclaimers
Mention of the commercial instruments, model numbers, trade names, chemicals and chemical manufacturers do not represent the endorsement of the authors nor the organizations the authors are associated with.
Acknowledgements
The design and development of VPS was originally supported by SERDP under Project #WP-1627 in the Weapons Systems and Platforms Thrust Area. This demonstration research was financially supported by the ESTCP administered by the AFRL under ESTCP Project # WP-201317 and DOE Contract number 2340-V672-13. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract number DE-AC05-00OR22725. Dr. John M. E. Storey (ORNL) was instrumental in the
References (33)
- et al.
Separation of volatile and non-volatile aerosol fractions by thermodesorption: Instrumental development and applications
Journal of Aerosol Science
(2001) - et al.
Continuous in situ monitoring of ambient particulate sulfur using flame photometry and thermal analysis
Atmospheric Environment
(1978) - et al.
An improved low-flow thermodenuder
Journal of Aerosol Science
(2007) - et al.
Method for measuring the hygroscopic behavior of lower volatility fractions in an internally mixed aerosol
Journal of Aerosol Science
(2004) Techniques for determining the chemical composition of aerosol sulfur compounds
Atmospheric Environment
(1978)- et al.
Thermal degradation of polyethylene modeled on tetracontane
Journal of Analytical and Applied Pyrolysis
(2008) - et al.
Micron-pore-size metallic filter tube membranes for filtration of particulates and water purification
Journal of Microbial Methods
(2008) - et al.
Evaluation of thermal denuder and catalytic stripper methods for solid particle measurements
Journal of Aerosol Science
(2010) - et al.
Design and calibration of a thermodenuder with an improved heating unit to measure the size-dependent volatile fraction of aerosol particles
Journal of Aerosol Science
(2002) - et al.
Characterization of the volatile fraction of laboratory-generated aerosol particles by thermodenuder-aerosol mass spectrometer coupling experiments
Journal of Aerosol Science
(2009)