Pm emissions from Aviation Current State of Research Coordinated Under the pm roadmap February 01, 2007



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PM Emissions from Aviation
Current State of Research Coordinated Under the PM Roadmap

February 01, 2007


The PM Roadmap is divided into five functional groups, namely, (1) policy, (2) measurement and methodology, (3) impact analysis, (4) technology development, and (5) database development. This paper summarizes the coordination of various research initiatives and the shared scientific knowledge amongst the five functional groups. Technical issues and data gaps in our current understanding of PM emissions from aircraft engines are acknowledged, and where possible, appropriate next steps are identified.

Introduction


Over the past decade, progress has been made towards understanding aviation PM emissions. However, there are still gaps and uncertainties in characterizing PM emissions and their potential environmental impacts. A National PM Roadmap for Aviation has been developed in the U.S. to guide, collaborate, and coordinate research on aviation PM emissions and their environmental and health impacts, with a focus on aircraft engines. The PM Roadmap brings representation from industry, academia, and federal agencies to identify the research gaps, define research goals, prioritize the key research needs, and define work plans to meet said goals. To date, the PM Roadmap has successfully raised awareness for research needs while securing limited multi-agency research funding to address them.

Research is needed for standardizing PM measurement techniques and equipment, understanding formation and composition of particles and their precursors formed in the engine, as well as their evolution in the exhaust plume. Key information is needed in these areas in order to understand related potential environmental and health impacts.

Several of these research thrusts are currently being advanced and coordinated under the PM Roadmap. Under this Roadmap, there are also ongoing efforts (e.g. research database) to disseminate findings of this research to wider communities for their practical use towards policy development and guiding future engine technology development.

The following section summarizes the current status of work and outlines upcoming activities under each group. The remaining sections of the paper describes the current funding sources, the next steps planned towards a better understanding of aviation PM emissions and their environmental impacts, and a conclusion drawn about our readiness to consider specific policy objectives and technology goals.


Current Status and Upcoming Work

Policy Group


Policy activities include liaison with the International Civil Aviation Organization (ICAO) Committee for Aviation Environmental Protection (CAEP) to ensure that regulatory considerations by CAEP are based on the best scientific knowledge available, technological feasibility, environmental benefit, and economic reasonableness. The current objective is to foster development of data and guidance to address the national requirements associated with aviation’s PM emissions and to coordinate the status of national developments with international entities. The policy work program includes the following activities.

  • Support for First Order Approximation, version 3.0 (FOA3) – CAEP’s Working Group 3 has reviewed and approved FOA3, which provides an interim methodology to quantify PM emissions from aircraft engines. The FOA methodology is employed by tools used to develop emissions inventories to demonstrate compliance with the PM national ambient air quality standards. The contributions of sulfate and organics to the volatile fraction are considered and the lubrication oil contribution may also be added in a future update. The interim FOA methodology will ultimately be replaced by a database of fully validated and verified PM emission indices, based on the work of the Database Development group (see below).

  • Liaison with the Society of Automotive Engineers (SAE), E-31 Committee – SAE E-31 is establishing aerospace recommended practices for measuring PM emissions from aircraft engines. National and international interests are coordinating with E-31 to ensure that its latest findings and recommendations are considered in policy developments (e.g. modifications to ICAO Annex 16, volume II). (See the following section for additional information on SAE E-31).

  • Liaison with the ICAO CAEP Working Group 3 on the future work program leading up to the eighth meeting of CAEP, which is anticipated to occur around February 2010. It is expected that the future work program will include a more aggressive consideration of the technical and scientific aspects concerning PM emissions from aircraft engines taking into account the work of SAE E-31. The status of the ICAO CAEP Working Group 3 understanding of the current state of science related to PM emissions from aircraft engines is summarized in the Appendix to this paper.

Measurement & Methodology Group


Techniques and equipment to accurately measure PM emissions (mass and size) from aircraft engines are rapidly advancing. Several significant projects have been conducted during the past few years to establish a baseline capability. Research in academia on aircraft combustors has evaluated PM composition, size, number, and mass and relationship to smoke number. Also, NASA has sponsored, along with various funding partners, and conducted several projects to develop and test new measurement and monitoring instruments and testing protocols, notably, the Aircraft Particle Emissions eXperiments, APEX, Delta-Atlanta/Hartsfield, JETS-APEX2 and APEX3, which have focused on developing and confirming PM measurement instruments and testing methods and collecting data from a variety of commercial aircraft engines.

PARTNER (Partnership for AiR Transportation Noise & Emissions Reduction), participated in project JETS-APEX2, in collaboration with Southwest Airline and Oakland airport, to collect PM and hazardous air pollutants (HAPs) data from dedicated aircraft that were parked in a test area. The project was conducted in August 2005. There were several objectives to this project, the majors ones included the use of collected data 1) to further understand emissions characteristics of older and newer engine technologies, taking into account the small sample size; 2) to quantify PM and HAPs emissions and to build up databanks; 3) to answer science questions that arise from previous measurement campaigns; 4) to look at field performance of the newly re-designed probe stand, probe and sampling lines, and state-of-the art measurement instruments. In addition to testing dedicated aircraft and engines, JETS-APEX2 also included downwind studies of aircraft take-offs and landings.


In November 2005, PARTNER conducted APEX3 campaign to continue the team effort to collect emissions data from wider varieties of engine sizes and engine manufacturers. Aircraft included in APEX3 were provided by FedEx, Continental and NASA. As for previous measurement campaigns where emissions results are influenced by atmospheric conditions at a particular geographical location, APEX3 data were taken during relatively colder weather conditions at Cleveland Airport. Similar to other campaigns, APEX3 will provide much needed information to characterize aircraft emissions.

An important consideration in understanding the environmental impacts of aviation PM emissions is distinguishing between non-volatile particles and volatile particles, and understanding the roles each plays in overall PM impacts. The non-volatile particles are small carbonaceous particles (primarily soot) formed by combustion processes within the engine. The volatile particles are formed by condensation of various gases after the exhaust leaves the engine. The condensing gases include unburned and partially oxidized fuel, sulfur species from sulfur compounds in the fuel, and engine lubricating oil. Some of the volatile material forms discrete particles while some condenses onto the non-volatile particles. Understanding these processes is crucial in developing the ability to predict the changes in PM mass loading, composition, and size in the ambient atmosphere and the subsequent potential impacts on human health and the environment.

The current state of technology for measuring non-volatile PM emissions from aircraft engines has been evaluated by SAE E-31 in AIR 5892 Non-volatile Particle Measurement Techniques, which was published in 2004. The information reported in this Aerospace Information Report is being advanced into drafts of three Aerospace Recommended Practice (ARP) reports, which are expected in the foreseeable future as Technical Appendices to a revised AIR 5892. However, there are some lingering concerns about non-volatile PM emissions and their interaction with volatile components under a wider range of conditions than those AIR 5892 considered.

Measurement capability with regard to the volatile component of the PM emissions is much less well established, as is our understanding of the generation and evolution of volatile PM emissions. The E-31 committee has circulated a position paper on volatile PM emissions from aircraft engines to solicit comments but an AIR for techniques to measure volatile PM emissions is not yet in process. Further research is planned by FAA through its PARTNER program, to investigate emissions during transient operations, the role of organics and lubrication oil in the volatile component, impact of probe location in the plume, effect of ambient weather conditions (particularly cold weather), and particle loss and creation (nucleation) in the sampling system. Other topics to be investigated include HC speciation as well as particle size distribution, number, composition, and mass. New instruments are slated to be tested as well. However, research into volatiles in 2007 is uncertain as a reflection of available research funding

The Department of Defense is using the current state-of-science, although not fully developed, in measuring PM emissions from aircraft engines to work with the Environmental Protection Agency (EPA) to gain approval for deploying the Joint Strike Fighter (JSF). Given current limitations, these procedures are being accepted only as an interim test method for this single application. The Strategic Environmental Research and Development Program (SERDP) funds this effort. The JSF Interim PM Test Method will be developed applying lessons learned during several commercial engine tests (APEX, JETS-APEX2 and APEX3) and testing representative military jet aircraft engines. The non-volatile and volatile components of PM emissions will be included in the program and particle size distribution, number of particles, and chemical species will be measured. Development and testing are expected by spring 2007. Other SERDP funded projects include a comprehensive emissions measurement program to develop emission indices for military aircraft and a program to develop a system for rapid real time measurement of PM emission indices from military aircraft engines.

National Aeronautic and Space Administration (NASA) plans to advance knowledge of aircraft engine particle emissions (non-volatile and volatile) formation and evolution under both Supersonic and Subsonic Fixed Wing Projects of the Fundamental Aero Program (FAP). Research efforts planned include fundamental understanding of particle sampling and measurement system, advancement of particle evolution models, and well designed and planned measurements to validate the models. Knowledge obtained from APEX series measurement campaigns led to a series of three studies on sample probes and sample lines in 2006. NASA released a NASA Research Announcement (NRA) for foundational research in support of the Aeronautics Research Mission Directorate (ARMD) in 2006. During 2006 NASA awarded two proposals under its FAP program on particulate emissions. One focuses on sample system studies, and the other on acquired but unprocessed APEX3 filter samples.


Impact Assessment Group


There is a well-developed understanding of the effects of particulate matter on human health and welfare. PM emissions are typically estimated to be responsible for 80% to 90% of the health impacts of poor local air quality in the U.S. It is not known if aviation PM emissions are substantively different from those of other combustion sources. However, for operation on or near the ground, after initial processing within the plume (which may indeed be unique due to the high temperatures relative to motor vehicle emissions for example), it is typically assumed that aviation PM (volatile particles, non-volatile particles, and precursor gases for additional particles) can be treated in a manner similar to emissions from other sources. Understanding distinction between volatile and non-volatile particles of emissions including the resultant combustion processes within the aircraft engine is crucial in developing the ability to predict the changes in PM mass loading, composition, and size in the ambient atmosphere and the subsequent impacts on human health and the environment. Further work is needed to determine if there are indeed unique features of aviation PM emissions. However, if after initial processing in the plume they are assumed to be similar to PM emissions from other sources, then one may apply the techniques that have been developed more generally for assessing PM impacts on local air quality and human health. These techniques typically involve simulation of the microphysical and chemical transformation of PM in the atmosphere, and the transport of the PM emissions geographically. An example of the methods used for such calculations is the Community Multi-scale Air Quality Model (CMAQ) developed jointly by The National Oceanic and Atmospheric Administration (NOAA) and EPA. Then estimates of ambient PM concentrations are used with concentration-response curves to evaluate changes in human mortality and morbidity. This requires combining PM concentration maps with census data and is typically done with geographical information system (GIS) applications or with specialized programs such as EPA’s BenMAP.

Under the Energy Policy Act of 2005 (EPACT) (§753, AVIATION FUEL CONSERVATION AND EMISSIONS, P.L. 109-58) a study is being performed to advise Congress on measures to conserve aviation fuel and reduce emissions. Elements of the study are to identify ways to promote fuel conservation measures to enhance fuel economy and reduce emissions, explore ways to reduce air traffic inefficiencies to improve fuel efficiency, and evaluate the impact of aircraft emissions on air quality in nonattainment areas. This study will include an initial assessment of PM emissions from commercial aircraft engines based upon a range of approximation methods, including FOA. It will also evaluate the impact of these emissions on human health and the environment.

The EPACT study is designed to understand the role of aircraft PM emissions within the context of emissions from other sources as well as to define their effects on the health of exposed individuals, effects on visibility, and global effects. In a similar vein, FAA is developing the Aviation Environmental Portfolio Management Tool (APMT). The APMT enables assessments of global, regional, national, and airport-specific environmental impacts of aviation and associated economic costs and societal benefits. It will model aviation system technology changes, operational impacts of aviation noise and emissions policies, manufacturer and operator costs of noise and emission reduction, environmental and health related costs associated with noise and emission exposure, and broader societal macroeconomic effects. Both the EPACT study and APMT model are using the techniques described above (modeling air quality impacts using CMAQ and then using BenMAP to evaluate health consequences). FAA is funding both the EPACT study and the development of AEDT, which together will provide an initial assessment of the impact of aviation PM emissions on human health and the environment. Preliminary results from these efforts are expected to be available in 2007.

Technology Development Group


Engine design and technology development for reducing PM emissions has not yet begun. Engine manufacturers are teaming with government agencies, research institutions, and universities working on the science of PM formation and on PM measurement methodology, which includes the identification of required measurement metrics. This research is also establishing what is needed to further evaluate health and environmental impacts. As the larger research community establishes the needs and assessment approaches, technology development can begin to respond.

Modeling to understand particle generation in a range of combustor designs and operating conditions, and particle evolution in the sample system (probes and lines) and plume is a component of the Supersonic Project; and experiments to validate these models is part of the Subsonic Fixed Wing Project. There may also be opportunities to expand interagency collaboration to include industry as a strategy for advancing understanding of particle formation more rapidly. Also, research in academia on aircraft combustors has evaluated PM composition, size, and mass and relationship to smoke number.

Alternative jet fuel may result in lower PM emissions than conventional jet fuel. Several alternatives are currently being studied including biodiesel jet fuel and synthetic kerosene. An advantage of alternative jet fuels is the reduced sulfur and aromatics content, which, particularly in the case of sulfur, would result in lower PM emissions. However, many of the alternative jet fuels studied to date have a much higher freezing point than conventional jet fuel, which is a problem in light of the cold temperature of an aircraft's operating environment. Other problems may arise where properties of alternative fuels vary from that of current jet fuel, which is performance based and has been developed over many years of use and evaluation. Any new fuel composition would require careful monitoring to ensure no performance degradation via a property not controlled by current jet fuel specifications but nevertheless inherent in petroleum based jet fuel. The NASA NRA also identifies research needs on alternative fuels with a goal to produce a new jet fuel that will achieve high combustion efficiency and minimize pollutant emissions.

While awaiting the results of planned research, investing in technology development to mitigate PM emissions may not be prudent. Until current and planned research has better quantified aviation PM emissions and defined their impacts on human health and the environment, technology development goals cannot be confirmed.


Database Development Group


Databases are needed to support both ongoing PM research activities and analysts interested in developing PM emissions inventories at airports. The research projects that have been instrumental in advancing PM measurement capabilities are expected to be excellent initial data (processed data are currently archived on a NASA secure web site) sources for a Research Database. The Research Database will integrate the up-to-date analyzed data and test conditions from all measurements that are responsible for underpinning the shared knowledge to date. At the same time, there is a need for the Policy group to foster guidance and liaise with organizations responsible for setting regulatory actions pertaining to environmental impacts of aviation. A second PM database, hereto known as the Emissions Index Policy Database comprised of confident research elements as well as approximation methodologies covering the today’s flying fleet, would provide a basis for informed decisions.

Funding is available in the coming year to initiate PM emissions database development. The initial research database will compile the experimental data from several research projects. Guidelines for adding data contributions and quality assurance procedures will be defined so the database can continue to expand as research sponsored by a variety of organizations becomes available in the future. The research data will also be evaluated and analyzed to produce emission indices for specific aircraft engines. The emission indices will be published in a database available to the public. Ongoing funding in future years will be necessary to maintain and grow these databases.



Next Steps in PM Research

Despite the progress to date, additional research is needed for:



  • standardizing PM measurement techniques and equipment,

  • understanding formation and composition of particles formed in the engine and evolved in the exhaust plume, and

  • understanding particle chemistry as a first step to understanding health and environmental impacts of the particles.

Several of these research efforts are currently being advanced to develop and evaluate sampling systems, characterize the particles, and understanding particle generation in the engine and its further evolution in the plume. Research to understand whether aviation particles’ impacts on human health and the environment are unique is only just beginning. Databases to make research findings available to scientists and analysts are in the early planning and development stages. These databases will be used in conjunction with modeling activities to assess the extent to which PM emissions from aircraft engines contribute to overall PM emissions and ambient air quality.

In addition, the Roadmap group has revisited an earlier decision to exclude Hazardous Air Pollutant (HAP) emissions from the Roadmap and concluded that since the measurement campaign set up, experimental techniques, and air quality and environmental impact modeling analyses for PM can be readily extended to consider HAPs, then it is reasonable to extend the Roadmap for characterization of HAP emissions from aircraft engines. This extension would seem to make sense in light of the fact that expertise in measuring and assessing HAPs already exists among campaign team members. Since HAPs reporting is frequently requested under the auspices of NEPA compliance, and the collection of HAPs data helps to shed light in its relationship with PM and other gaseous emissions, particularly HC, the PM Roadmap is therefore a good venue for understanding HAPs.



Funding Sources

In the U.S, the Airport Cooperative Research Program (ACRP), a cooperative, applied research program managed by the Transportation Research Board (TRB), a division of the U.S. National Academies of Science National Research Council, was recently established. ACRP is sponsoring a project for the coming year to identify information on PM emissions needed by airports. The project will also define gaps where the current knowledge base falls short of these needs. The project will produce problem statements that address the knowledge gaps, which can be used to define PM research projects for the coming years. It is anticipated that ACRP may fund some of the research identified by the project in the next 1-3 years. The ACRP research projects will be coordinated with the PM Roadmap work plan.

Additionally, the U.S. FAA also commits annual funds towards applied science through the Partnership for AiR Transportation Noise and Emissions Reduction (PARTNER) Center of Excellence. Currently, the FAA has existing research related to aircraft PM emissions under Projects 9 (Emission Measurements) and 16 (Air Quality Impacts) through the PARTNER program. The assessment of aviation health impacts is addressed under Project 11.

NASA FAP has established research plans and commits resources for foundational research in aircraft engine particle emissions with modeling efforts mainly under the Supersonic Project and experimental efforts mainly under the Subsonic Fixed Wing Project.


Conclusion


Significant progress has been made over the past several years in understanding and quantifying PM emissions from aircraft engines. Data from numerous measurement studies are becoming available to establish a scientific basis for understanding PM formation in engines and its further evolution in the exhaust plume. Additional studies planned for the coming year will advance our technical knowledge even further. Some preliminary studies of the potential health impact from PM emissions that can be attributed to aircraft engines have been completed or are underway. However, these studies are somewhat limited in their applicability to policy and technology considerations because of the lack of actual aircraft engine PM emissions data. As more data becomes available it is expected that additional studies of the impact of aircraft PM emissions on human health and the environment will be conducted, including any advancements in scientific understanding.

The lack of engine measurement data is still being impacted to some degree upon the absence of an agreed recommended practice from SAE E-31. However it is expected that SAE E-31 will progress their work on non-volatiles to a point that a recommended practice may be considered by the aviation community, including ICAO CAEP, in the near future.



The current state of science and technical knowledge with regard to PM emissions from aircraft engines is presently insufficiently mature to serve as a basis for setting specific policy objectives or guiding aircraft engine technology development for emissions. Research activities, model development, and full scale engine testing identified in the National PM Roadmap for Aviation will advance our understanding of PM emissions from aircraft engines in coming years. Formalized assessments with a more robust engine PM emissions database (based upon internationally agreed recommended practices), validated impact assessment methods, and periodic reassessments should be conducted to ensure action, if required, is taken to control or mitigate PM emissions from aircraft engines as soon as appropriate.



International Civil Aviation Organization

INFORMATION PAPER

CAEP/7-IP/xxx

../../06








C
Appendix
OMMITTEE ON AVIATION ENVIRONMENTAL PROTECTION (CAEP)


SEVENTH MEETING

Agenda Item

1:

Review of proposals relating to aircraft engine emissions, including the amendment of Annex 16, Volume II

Montreal, 5 to 16 February 2007

PARTICULATE MATTER CHARACTERISATION

(Presented by Working Group 3 Rapporteur)





SUMMARY

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/7 on 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 PM characterisation based on published papers (Section 2). Also included in this IP are current activities associated with SAE E-31 committee (Section 3), a brief summary of ongoing PM research programs (Section 4), and the latest advancements with the First Order Approximation (FOA) version 3 methodology for estimating aircraft PM emission inventories in the vicinity of airports (Section 5).




  1. INTRODUCTION

    1. 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.

    2. 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.

  2. CURRENT PM STATE-OF-THE-SCIENCE SUMMARY

    1. 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
      (2) to relate the climate effects to that due to other greenhouse gas emissions unrelated to aviation activities.

      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.



    2. 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.

      1. 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.

      2. 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.

      3. 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.


    3. 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.



      1. 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.

      2. 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.

      3. 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.

    4. 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.

      1. 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/).



      2. 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).

      3. 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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