Pm emissions from Aviation Current State of Research Coordinated Under the pm roadmap February 01, 2007
OMMITTEE ON AVIATION ENVIRONMENTAL PROTECTION (CAEP)
Montreal, 5 to 16 February 2007
PARTICULATE MATTER CHARACTERISATION
(Presented by Working Group 3 Rapporteur)
CAEP Steering Group (SG) at its meeting in Montreal (October 2005) identified specific deliverables required from every Working Group and Task Force, listed in the Appendix to CAEP7-SG20051-SD/4. One of SG’s requirements of WG3’s Alterative Emissions Methodology Task Group (AEMTG) is a report to CAEP/7on the characterisation of particulate matter (PM) emissions from aircraft engines.
This information paper (IP) fulfills the CAEP work items E5(c), E5(d), and E6 by providing a state-of-the-science executive summary on aircraft particulate matter (PM) characterisation based on published papers (Section 2). Section 3 describes current activities associated with SAE E-31 Committee. Section 4 provides a brief summary of ongoing PM research programs. Section 5 presents the latest advancements on the First Order Approximation (FOA) version 3 methodology for estimating aircraft PM emission inventories in the vicinity of airports.
CURRENT PM STATE-OF-THE-SCIENCE SUMMARY
Summary: Significant work in the past decade has focused on characterizing the PM emissions1 from aircraft engines. Building on work done to understand aircraft emissions at altitude in the 1990s, measurements and modelling work has progressed to better understand the way that non-volatile particles (soot), charged molecular clusters that form during combustion (chemiions), and condensable gases (sulphuric acid, formed from a small part of the sulphur in the fuel, and organics, from incomplete combustion) contribute to particle emissions. In addition to the direct emission of carbonaceous soot, the emitted condensable gases interact with each other and with the soot to form the mixture of compositions and particle sizes that are deposited in the atmosphere. Section 2.2 summarizes the measurement works. Section 2.3 discusses the various modelling efforts that aim to predict the change in concentrations in the ambient atmosphere. Section 2.4 reviews the environmental impacts associated with the changes in concentrations.
Current information suggests that different tactics should be employed in assessing specific environmental impacts from PM emissions. In the area of global climate change, one can state with confidence that the direct forcing from the PM emitted is small. There is large uncertainty associated with the direct forcing from contrails and cirrus clouds, and the quantitative role of PM emissions at cruise in affecting the formation of contrails and cirrus clouds has yet to be validated. In this area, one must take the “end-to-end” approach in tracking the formation of the contrail, and quantify the associated change in radiative forcing. The challenge in this case is two fold:
(1) to predict the occurrence of contrails, contrail cirrus, and changes in aerosols associated with operation of an aircraft fleet, and
For issues on local air quality and visibility, the infrastructure is in place to address the impacts from all sources. The challenge here is to isolate the changes in ambient PM concentrations attributable to aviation sources. Once this identification is made, one can start asking additional questions whether there are specific health impacts associated with changes in PM concentrations and/or PM composition peculiar to aviation.
Measurements: Early work in the mid-1990s measured PM emissions from both engines in ground-based facilities as well as employing chase planes for in-situ sampling of aircraft plumes 100 meter to 20 km behind airplanes operating at cruise conditions. The interest was mainly on understanding the initial particle emissions and how the resulting aerosol may affect climate. These studies soon focused on the important role of gaseous sulphur species in the exhaust in determining aerosol properties and how combustion generated ions mediated the formation of new particles from sulphuric acid nucleation. Subsequent developments have taken two different directions. One area of interest is on how airplanes affect the formation of contrail and cirrus at altitude. The other is use test rig on the ground to sample exhaust gases at the engine exit plane and a few meters downstream to better understand how PM emissions evolve in the new field.
Airborne Measurements: The feasibility of making airborne measurements of engine exhaust was demonstrated during the Airborne Arctic Stratospheric Expedition (1992) where clear signatures on several instruments indicated when aircraft exhaust plumes were sampled accidentally on numerous occasions. During the ASHOE/MESA (1994) campaign, the ER-2 had several opportunities to sample its own plumes as well as a fortuitous sampling of an aged Concorde exhaust plume in the lower stratosphere.
Subsequently, several campaigns were designed with the purpose of sampling exhaust plumes. In the USA, these include the SASS Near-Field Interactions Flight Experiment (SNIF) (1996-1997) where aerosol and aerosol precursor instruments deployed on the NASA T-39 aircraft were used to characterize aircraft particle emissions at cruise altitudes for commercial aircrafts and an Air Force F-16 at a trailing distances ranging from 100 m to 20 km. In the case of the F-16, the aircraft was deployed at different times burning high, medium and low sulphur content fuels. The European effort was conducted under the lead of DLR Institute of Atmospheric Physics within the framework of the SULFUR experiments between 1994 and 1999 (for an overview, see Schumann et al., 2002) and the POLINAT study in the North Atlantic flight corridor (Hagen et al., 1996). As part of the SULFUR experiments, measurements were conducted with the instrumented DLR research aircraft in the plume of source passenger aircraft, sometimes only 50 m behind the source aircraft. Collected data provided detailed information on particle emission indices for black carbon and volatile particles, aerosol properties like particle size and composition in aircraft plumes and aerosol processing in the plume and contrail and their dependence on fuel sulphur content, engine type, and overall propulsion efficiency. In the SASS Ozone and NOx Experiment (SONEX) (1997) the DC-8 was again used to sample aged plumes from hundreds of different commercial aircraft in the North Atlantic corridor, in collaboration with the EU research project, POLINAT. A special study was performed to evaluate aircraft particle source strength relative to the strength of other aerosol sources.
The Subsonic Assessment Cloud and Contrail Effects Special Study (SUCCESS) (1996) used instruments on the NASA DC-8 to characterize aircraft emissions plus contrail and cirrus microphysical properties. The CRYSTAL-Florida Area Cirrus Experiment (FACE) in 2002 represents the most recent airborne effort to investigate cloud processes and microphysical/radiative properties. This study was unique in that the WB-57 aircraft was deployed to self-sample its own contrail in the upper troposphere. The German national project Particles and Cirrus (PAZI) was initiated in 2000 and will continue until end of 2007. For a summary of progress to date, see Kärcher et al., 2004.
Exhaust Gas Sampling Techniques: Exhaust gas sampling and handling is difficult, since ensuring that the particles do not change during the sampling process is paramount to measuring them accurately. At the engine exit plane, no condensable species are in the particle phase, so volatile particle measurement is particularly challenging. Measurements have been made downstream of the engine, after the exhaust has mixed with ambient air, with a commensurate cooling and slowing of the exhaust flow. But the exhaust is also more dilute and has been subjected to varying ambient conditions at the same time. In 1999, an experiment workshop was held to compare performance of different aerosol instruments and inlet probes (SASS Aerosol Instrument Inter-comparison Workshop). In 2006, NASA planned and executed three sample system studies. These studies included different sample probes (gas and particle, cooled and un-cooled), sample lines with different diameters, and inter-comparison of multiple measurement systems.
Ground-based Measurements: Several ground-based experiments were carried out using different test venues. In 1995 and 1997, emissions were acquired from a single engine, in both cases an F100, mounted on a propulsion cell to examine particulate emissions (SASS Engine Emission Characterization Experiment). In 1996, the chemical composition of aircraft exhaust particles was studied by Petzold and Schröder (1998). Sampling was performed using a single engine from the DLR ATTAS research aircraft which was operated on an airfield under ICAO LTO conditions. For the first time the split of combustion particles into organic and black carbon was reported for ICAO LTO operation conditions. Studies on a combustor and a full engine using the same combustor design were carried out in collaboration between NASA and QinetiQ in 2000 and 2001 (Whitefield et al., 2002) to explore the post-combustor PM evolution in an engine. In 2002, a study to characterize PM emissions from the NASA aircraft B757 with RB211 engines (Experiment to Characterize Aircraft Volatile Aerosol and Trace Species Emissions, EXCAVATE) was completed. In 2004, multiple research teams were invited to collaborate on a parametric study of aircraft particle emissions from the NASA aircraft DC8 with CFM56 engine (Aircraft Particle Emissions eXperiment, APEX). Particle emissions were acquired from three different downstream locations and from three different fuels. In mid-2005, similar research teams went to Oakland Airport to acquire PM emissions from an array of B737s with CFM56 engines at several downstream locations (JETS-APEX2). In late-2005, similar research teams went to Cleveland Hopkins Airport to acquire particle emissions from a wide range of aircraft/engine combinations which include large commercial aircraft (B737/CFM56, B757/RB211), cargo (A300/PW4168), regional jet (ERJ145/AE3007), and general aviation type (NASA Lear25/CJ610) at several downstream locations (APEX3).
The European project PartEmis (Petzold et al., 2005) on the measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines was a comprehensive test rig study combining experimental and modelling approaches. Particle emissions from a gas turbine were studied for various operational conditions and fuel sulphur contents. The results showed a clear effect of fuel sulphur content and sample dilution on the formation of volatile particles. The emission of combustion particles depends mainly on the operating conditions of the combustor, while the effect of different fuel sulphur content on particle formation is weak. The turbine section of an engine showed no significant influence on the properties of the combustion particles but influenced the formation rate of volatile particles.
A different type of measurement also had been tested. Measurement systems were located at downwind site alongside an active runway. Emissions from aircraft taxiing near-by and taking-off were acquired. This type of operational sampling was first carried out by making measurements outside of the JFK Airport. It was followed by a similar, but more detailed study that was done inside Atlanta Hartsfield airport in 2004. Most recently, runway tests were also carried out as additional part of the JETS-APEX2 field mission.
Modelling: The evolution of the deposited particle mixtures in the ambient atmosphere depends upon microphysical processes that include nucleation of new volatile particles, condensational growth of both volatile particles and soot, and coagulation of particles of various types with one another. As of this writing, particle emissions have been measured from a wide variety of aircraft gas turbine engines. Particle numbers, sizes, masses, and composition have been measured, and studies have been carried out to better understand the microphysical evolution of these properties after emission from the engines. While important questions regarding the effects of engine technology, fuel properties, and ambient conditions still remain, there is now a well founded understanding at least of the major contributions to the particles that an aircraft engine emits into the atmosphere. This understanding will continue to be advanced by the on-going programs that are currently underway.
Not withstanding the important questions regarding the effects of engine technology, fuel properties, and ambient conditions, much progress has been made in both measuring the particles and understanding their microphysical evolution. The major components of the particles have been identified, and the primary microphysical processes are understood in concept. The combustion-generated soot particles that leave the engine are joined by newly nucleated volatile particles, and are soon coated with condensed species as well. The volatile composition of both the volatile particles and the coatings on the soot particles includes contributions of both sulphuric acid and organic species. The major microphysical processes of chemiion-assisted particle nucleation, condensational growth, and coagulation determine the eventual particle composition and size distributions. Models that simulate these processes have been developed, and the general features of the particle microphysical evolution are captured by these models. However, the complexity of the multiple component mixture and how it evolves has yet to be fully captured in the models and further work in this area continues. In particular, the inclusion of the plethora of hydrocarbon species potentially involved in the particle evolution is a daunting task that will require significant new work. Detailed model calculations that attempt to predict how the multi-component aerosol mixtures evolve up to the scale of a large-scale model have not been carried out, but are urgently required to understand the potential of soot particles to initiate cirrus formation.
PM emissions evolve (chemical transformations of gaseous components, formation of secondary particles, changes in sizes and composition of primary and secondary particles) in the near field in the plume before the plume is diluted and cooled to ambient conditions. Local air quality models and global models typically do not have small enough spatial and temporal resolution to simulate these changes. Thus, one needs to provide to the large scale models the appropriate effective emissions after the near field evolution. See further description of the work in the PAZI program in section 4.6.
The sulphuric acid arises from the oxidation of the sulphur in the fuel, most of which is emitted as SO2, but some of which is further oxidized to sulphuric acid (a few percent or less of the fuel sulphur). The organics that contribute to volatile particulate mass arise from low vapour pressure species represented in the small levels of products of incomplete combustion. These may include both partially oxidized fuel fragments as well as product species from pyrolysis reactions. There is also evidence that small amounts of vaporized lubrication oil may be contributing to the condensed mass composition. In a collaborative study between German and American groups, the key role of chemiions in nucleation of volatile aerosol particles shortly after emission was studies experimentally within the SULFUR missions (Schröder et al., 1998; Kärcher et al., 1998; Yu et al., 1998). An example of a parameterization scheme for volatile particle emissions is described in Kärcher et al., 2000. Such schemes typically predict the number and size of volatile particles as a function of plume age in cruise conditions, and identifies the conversion efficiency of emitted SO2 into condensable sulphur, the initial number of chemiions, and the emission index of condensable organics.
The German national project Particles and Cirrus (PAZI) was initiated in 2000 and will continue until end of 2007. For a summary of progress to date, see Kärcher et al., 2004. The main goal of PAZI is to better understand the formation of the ice phase in cirrus clouds from natural and anthropogenic aerosols and to improve microphysical and optical parameterizations of cirrus clouds in global models. This will permit to determine the impact of aviation soot-induced cirrus and contrail cirrus relative to cirrus formed on particles from other anthropogenic and natural sources, to compare the climate impact from aviation with the climate impact caused by other atmospheric change, and to develop means to reduce the aviation impact through changes in engine technology or air traffic management.
Environmental Impacts: With the recognition that engine emissions from aircraft can increase the ambient concentrations of PM in the atmosphere, the aviation community has been trying to quantify the corresponding environmental impacts. These include visual air quality and health impacts on local and regional scales from airport operations, and global issues on global air chemistry, contrail and global climate from cruise. Ideally, one would like to start with PM emissions from the engine, follow its evolution in the plume and the ambient atmosphere, and quantify the impacts. Unfortunately, the sources for information on PM emissions and their environmental impacts are fragmented. The reason is that the information is compiled by several different communities of practitioners for their own needs.
Focus on Air Quality Modelling and Health Impact: A nice summary on the state of PM air quality modelling in the USA can be found in the NARSTO Assessment Report on PM Science (NARSTO, 2004). The assessment plan for the report was conceived in 2000. As part of the process, two review boards (Board on Environmental Studies and Toxicology and Board on Atmospheric Sciences and Climate) from the National Research Council (NRC) provided comments on the draft in 2002. The report was published in book form in 2004. In the USA, the work on PM health effects is most closely aligned with the U.S. EPA’s PM Research Program (http://www.epa.gov/pmresearch/). The overall directions of the EPA program are guided by a number of activities, notably the review activities by the NRC, and the NARSTO Assessment report on PM Science. U.S. EPA organizes its activities under 10 topics as outlined by the NRC report (NRC, 1998) published in 1998. The NRC provides continuing updates on progress in this area, the latest published in 2004 (NRC, 2004). U.S. EPA tracks its own progress internally according to the 10 areas described in the NRC report. The EPA’s latest report was published in 2004 (EPA, 2004).
The European view on the relevance of particle emissions from aviation-related sources for airport air quality issues is reflected in the European Network of Excellence ECATS. As is expressed in the work programme of ECATS, there are many deficiencies in the understanding of the composition of aircraft emissions and their contribution to local airport air quality. The understanding of some emission indices, such as speciated hydrocarbons, fine and ultra fine particles, polycyclic aromatic hydrocarbons (PAHs) and odours, is very limited. An improved dataset of aircraft PM emission indices together with a methodology such as the FOA (see Section 5) and detailed aircraft traffic modelling at airports will enable the development of realistic spatially-resolved emission inventories of airports which are the basis of AAQ modelling. A detailed understanding of the aircraft’s engine exhaust emissions at the engine-atmosphere interface is important if we are to model accurately the transport of exhaust components in the near field of the engine. A major uncertainty in modelling aircraft exhaust emissions is a lack of knowledge of the physical dimensions and initial dispersion characteristics of the exhaust plume. A further major deficiency in the understanding of the impact of aircraft emissions on local air quality is the composition of the plume. Background concentrations of primary and secondary pollutants will affect the speciation of nitrogen oxides (NOy) emissions from aircraft and this has not been adequately evaluated in previous airport air quality studies (ECATS website: http://www.pa.op.dlr.de/ecats/).
Focus on Visual Air Quality: Regional haze impair visual air quality by obscuring scenes. This has been identified by the US EPA as a serious issue for many National Parks and Wilderness areas. That said, the effect of aircraft engine PM emissions on visibility degradation is unknown at this time. Chapter 9 of the NARSTO report contains a concise summary of the science behind the visibility issue. At first glance, the fact that light extinction can be computed given the number, size, and composition of the particles may suggest that one can objectively predict the change in visibility given emissions. Unfortunately there are several complications as discussed in the report. The same emissions can result in different PM concentrations because of local conditions such as temperature, humidity, availability of sunlight, and trace species in the atmosphere. Furthermore the change in visibility is not linear in the changes in PM concentration. Small amounts of PM can degrade visibility in clear condition, while visibility in polluted areas is relatively insensitive to the same change in concentration. Finally, there remains the question of human perception (perceived visual air quality).
Focus on Global Climate Change and others: The most comprehensive source of information for aviation PM emissions and their effects on contrails and clouds can be found in chapter 3 in IPCC (1999). More recent information was presented at the International Conference on Transport, Atmosphere and Climate in Oxford in 2006 (http://www.pa.op.dlr.de/tac/). The IPCC report, however, is the “gold standard” for a consensus report. It also has a single focus of looking at the impacts from aviation. The findings on the relative impacts from various aircraft emissions on global climate in the IPCC (1999) report were based on instantaneous radiative forcing. They can be summarized as follows:
The direct instantaneous climate forcing from changes in ambient concentration of PM due to aircraft is small compared to the forcing from CO2, ozone, and H2O from the same fleet.
The direct instantaneous radiative forcing of contrails could be large (comparable to the effect of CO2 from the fleet) with a large uncertainty (factor of 4). While much progress has been made in modelling the processes that control contrail formation, our ability to predict how contrail formation may change with changes in operation remains very limited.
Extensive cirrus clouds have been observed to develop after the formation of persistent contrails. However, the mechanisms associated with cirrus cloud formation and the role aerosols play as ice nuclei are not well understood. As a result, it is difficult to quantify the indirect instantaneous radiative forcing associated with change in cirrus cover associated with PM emissions. Forcing comparable to the CO2 effect is possible.
Key reactions that occur on the surfaces of aerosol particles have important effects on the photochemical balance that affects ozone concentrations. The impacts depend on the surface areas available as reaction sites. Changes in PM will shift this balance and change ozone in the atmosphere. For airplanes flying in the stratosphere, the emitted SO2 and PM could lead to increases in the surface area of the sulphate aerosols in the stratosphere. This would lead to decrease in ozone concentration in the stratosphere. Results from previous studies were summarized in Section 4.3 of the IPCC report. The effect on the tropospheric aerosol is expected to be smaller. Since NOx emission from aircraft is expected to have a larger impact on tropospheric ozone than PM emissions, less effort has been spent in studying this issue.
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