Green Aviation

By Allan Seabridge

Green Aviation is the first authoritative overview of both engineering and operational measures to mitigate the environmental impact of aviation.

It addresses the current status of measures to reduce the environmental impact of air travel.  The chapters cover such items as:

• Engineering and technology-related subjects (aerodynamics, engines, fuels, structures, etc.),
• Operations (air traffic management and infrastructure)
• Policy and regulatory aspects regarding atmospheric and noise pollution.

With contributions from leading experts, this volume is intended to be a valuable addition, and useful resource, for aerospace manufacturers and suppliers, governmental and industrial aerospace research establishments, airline and aviation industries, university engineering and science departments, and industry analysts, consultants, and researchers.

• Language: English
• Publisher: Wiley
• Release date: Sep 21, 2016
• ISBN: 9781118866436

Book preview

Contributors

Ramesh K. Agarwal

Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO, USA

Kevin L. Anderson

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

David Angland

Faculty of Engineering, University of Southampton, Southampton, UK

Alfred J. Bedard Jr.

Cooperative Institute for Research in Environmental Sciences, Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA

Mike Bennett

Centre for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Gaudy M. Bezos-O'Connor

NASA LaRC, Hampton, VA, USA

Cees Bil

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Bilal M.M. Bomani

NASA Glenn Research Center, Cleveland, OH, USA

Alice Bows-Larkin

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

D. Douglas Boyd

NASA Langley Research Center, Hampton, VA, USA

Peter Braesicke

Chemistry Department, NCAS Climate, University of Cambridge, Cambridge, UK

Simon I. Briceno

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering,Georgia Institute of Technology, Atlanta, GA, USA

Rachel Burbidge

EUROCONTROL, Brussels, Belgium

Imon Chakraborty

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Jeffrey D. Crouch

Boeing Commercial Airplanes, Seattle, WA, USA

Oliver Dessens

Centre for Atmospheric Science, Cambridge University, Cambridge, UK

Graham Dorrington

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Lynnette M. Dray

UCL Energy Institute, University College London, London, UK

Wenbo Du

University of Illinois at Urbana-Champaign, Urbana, IL, USA

Antony D. Evans

UCL Energy Institute, University College London, London, UK

Peter Frederic

Tecolote Research, Inc., Santa Barbara, CA, USA

Joshua E. Freeh

NASA Glenn Research Center, Cleveland, OH, USA

Astrid Gühnemann

Institute for Transport Studies, University of Leeds, Leeds, UK

Elena Garcia

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Alessandro Gardi

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

François A. Garnier

Physics and Instrumentation Department, ONERA, Châtillon, France

Klaus M. Gierens

Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany

James Gill

Faculty of Engineering, University of Southampton, Southampton, UK

Susan A. Gorton

NASA Langley Research Center, Hampton, VA, USA

Jonathan Hart

Rolls-Royce plc, Derby, UK

Robert C. Hendricks

NASA Glenn Research Center, Cleveland, OH, USA

Paul D. Hooper

Center for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Ken I. Hume

Center for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Dawn C. Jegley

NASA Langley Research Center, Hampton, VA, USA

Wayne Johnson

NASA Ames Research Center, Mountain View, CA, USA

Marcus O. Köhler

School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK

Trevor Kistan

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Lee W. Kohlman

NASA Glenn Research Center, Cleveland, OH, USA

Craig Lawson

Centre for Aeronautics, School of Aerospace Manufacturing and Transport, Cranfield University, Cranfield, UK

David S. Lee

Dalton Research Institute, Manchester Metropolitan University, Manchester, UK

John C. Lin

NASA Langley Research Center, Hampton, VA, USA

Mujeeb R. Malik

NASA Langley Research Center, Hampton, VA, USA

Sarah L. Mander

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

Matthew Marino

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Joaquim R.R.A Martins

Aerospace Engineering Department, University of Michigan, Ann Arbor, MI, USA

Janet A. Maughan

Center for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Dimitri N. Mavris

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Michael McCune

Division of UTC, Pratt & Whitney, East Hartford, CT, USA

Philippe J. Mirabel

Surfaces et Procédés pour la Catalyse, Université de Strasbourg, Laboratoire des Matériaux, Strasbourg, France

Adrian Mouritz

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Craig Nickol

NASA LaRC, Hampton, VA, USA

Bethan Owen

Dalton Research Institute, Manchester Metropolitan University, Manchester, UK

Daniel E. Paxson

Communications and Intelligent Systems Division, NASA Glenn Research Center, Cleveland, OH, USA

Alexia P. Payan

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

H. Douglas Perkins

Propulsion Division, NASA Glenn Research Center, Cleveland, OH, USA

Christopher A. Perullo

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Clément Pornet

Bauhaus Luftfahrt, Munich, Germany

Subramanian Ramasamy

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Dave Raper

Centre for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Tom G. Reynolds

Air Traffic Control Systems Group, MIT Lincoln Laboratory, Lexington, MA, USA

Carl R. Russell

NASA Ames Research Center, Mountain View, CA, USA

Roberto Sabatini

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

William S. Saric

Department of Aerospace Engineering, Texas A&M University, College Station, TX, USA

Thomas W. Schlatter

Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA

Jeff S. Schutte

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Ravinka Seresinhe

Centre for Aeronautics, School of Aerospace Manufacturing and Transport, Cranfield University, Cranfield, UK

William Sheridan

Division of UTC, Pratt & Whitney, East Hartford, CT, USA

Keith P. Shine

Department of Meteorology, University of Reading, Reading, UK

Wei Shyy

Hong Kong University of Science and Technology, Hong Kong, P. R. China

Christopher A. Snyder

NASA Glenn Research Center, Cleveland, OH, USA

Aleksandar Subic

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Callum S. Thomas

Center for Aviation, Transport and the Environment, Manchester Metropolitan University, Manchester, UK

Egbert Torenbeek

Department of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands

Michael B. Traut

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

David R. Trawick

Aerospace Systems Design Laboratory, The Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA

Xavier P. Vancassel

Physics and Instrumentation Department, ONERA, Châtillon, France

Alexander Velicki

The Boeing Company, Huntington Beach, CA, USA

Zia Wadud

Institute for Transport Studies and Centre for Integrated Energy Research, University of Leeds, Leeds, UK

Chun Wang

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia

Edward A. Whalen

Boeing Research & Technology, Hazelwood, MO, USA

John Whurr

Rolls-Royce plc, Derby, UK

Michael Winter

Division of UTC, Pratt & Whitney, East Hartford, CT, USA

F. Ruth Wood

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

Nansi Xue

Zee.Aero, Mountain View, CA, USA

Gloria K. Yamauchi

NASA Ames Research Center, Mountain View, CA, USA

Larry A. Young

NASA Ames Research Center, Mountain View, CA, USA

Dale E. Van Zante

Acoustics Branch, Propulsion Division, NASA Glenn Research Center, Cleveland, OH, USA

Xin Zhang

Department of Mechanical and Aerospace Engineering, School of Engineering, The Hong Kong University of Science and Technology, Hong Kong SAR, China

Foreword

When our respective predecessors introduced the Encyclopedia of Aerospace Engineering, they called it an ambitious effort to meet the challenge of distilling the collective body of aerospace engineering knowledge into a single cohesive reference framework.

The ambition and challenge inherent in this effort derives in part from the relatively short time span the aerospace discipline has existed as well as the dynamic technology-driven discipline it is. Since the publication of the first volume in 2010, new technologies have developed and matured, new technical challenges have emerged and have been confronted, and the resulting experience and knowledge must be incorporated into the professional body of knowledge. A prime example of this is the emergence and maturation of green aviation technologies.

The growing awareness of our responsibilities to preserve our planet and its environment for future generations is evident from increased media attention, the growth of noise and emissions management as drivers for our design engineers and mission planners, and the passionate views of our young people. In addition to being good corporate citizens by sincerely endeavoring to go greener, in a time of global economic uncertainly, it makes good business sense to fly aircraft and rotorcraft as efficiently as possible. In short, we as an industry can do good and do well at the same time.

As a field that is both quickly expanding and becoming ever more complex, we welcome this addition to the Encyclopedia of Aerospace Engineering as both timely and comprehensive. Mirroring the character of the parent publication, this handbook is multidisciplinary and multinational; however, this supplementary publication meets the particular need to capture the risks and mitigation strategies associated with the topical and important environmental aspects of aviation.

From aerodynamics and propulsion to systems and operations and from noise mitigation to atmosphere and climate science, this volume is an essential primer for new and experienced aerospace professionals alike. It will also most certainly be of interest to the larger scientific community, whose members are trying to better understand the past, current, and future effects of aviation on the environment.

The year 2016 marks both the 150th anniversary of the Royal Aeronautical Society (RAeS) and the 85th anniversary of the American Institute of Aeronautics and Astronautics (AIAA). With a combined membership of more than 50 000 aerospace professionals, our two organizations celebrate these milestones, and our members' never-ending quest for knowledge and solutions is not just to the problems and challenges of today but also of the next impossible thing.

It can be easy to imagine that the level of innovation of today cannot match the pioneering work predating heavier-than-air flight and of the subsequent early years of aviation: yet today we service an industry that underpins the global economy; leads the world in safety; invests enormous sums in product development; and solves a vast array of interconnected technological challenges. These factors make the contributions of today's innovators as important, inspirational, and intriguing as they were 150 years ago. We particularly applaud those whose efforts to understand and manage the environmental impact of aviation are essential for the continued health and well-being of both our industry and our global society.

Aerospace makes the world safer, more connected, more accessible, and more prosperous. It is our hope that the addition of this volume to the Encyclopedia continues this trend and is as professionally valuable and influential to its readers – and the industry – as were the first nine volumes.

We are proud, jointly, to commend to you this new contribution to the aerospace engineering body of knowledge.

Mr. James Maser

President, American Institute of Aeronautics and Astronautics and Vice President, Operations Program Management, Pratt & Whitney, East Hartford, CT, USA

and

Dr. Chris Atkin

President, Royal Aeronautical Society and Professor of Aeronautical Engineering,City University London, UK

Preface

The Wiley Encyclopedia of Aerospace Engineering in printed and electronic formats provided knowledge to all disciplines of the aerospace industry in a series of learned chapters. It has continued to do that successfully since its publication. This knowledge was principally applicable to aerospace systems currently in operation or in development. Built upon this extensive and comprehensive list of chapters, the Editors decided that further coverage of selected topical areas of emerging importance can further serve the community's interests. The present volume intends to address opportunities, challenges, and technical issues related to aerospace emissions, environment, and green aviation.

The discussion and debate on this issue has grown steadily over the years, and it is clear that there is no universal agreement. Does the aerospace industry in general pollute the environment to a detrimental extent and if so by what mechanisms? More importantly, can the effect be measured, the impact predicted with certainty, and what can be done about it? The debate has been conducted in the press, in textbooks, in the political arena, and in journals. In so doing, a set of polarized views has emerged, and it is difficult to separate scientific and popular viewpoints, objective and vested opinions, and serious and provocative statements. Who is right and who is wrong, in fact is there a right and wrong?

This volume has been developed with the goal of providing informed assistance to achieving an understanding of the issues in hand. It is a collection of chapters written by scientists, engineers, designers, and academics in order to provide a wide ranging, but also deep, understanding of many of the key topics. It is to address the interaction between aerospace and the environment and to provide information and advice to key players in the field. This includes those who directly contribute to the development of new aircraft and infrastructure – engineers, designers, operators, maintainers, and regulators as well as those who have the capability to influence that development such as policy makers, decision makers, planners, politicians, and journalists.

The range of topics is wide, covering innovative aircraft and engine designs, aerodynamics and propulsion, aircraft operations and air traffic management, alternative fuels, lightweight high-strength materials, onboard auxiliary power units, noise reduction technologies, nonconventional aircraft (electric, solar, hydrogen), and so on, but are directly relevant to the issue, and have been written by the authors with pertinent and solid experience. As far as possible, the chapters are factual and objective – this is not a forum for personal opinion or speculation. It is intended to stimulate discussion and to encourage debate.

We expect that with the publication of this volume as part of a comprehensive Encyclopedia of Aerospace Engineering, more exchanges and collaborations will take place to advance new concepts and practices, and in due time, the convenience and pleasure of flight will be offered in an increasingly more sustainable manner.

Ramesh Agarwal

The William Palm Professor of Engineering Washington University, St Louis, MO, USA

Fayette Collier

NASA Langley Research Center, Hampton, VA, USA

Andreas Schäfer

UCL Energy Institute, London, UK

and

Allan Seabridge

Independent Aerospace Consultant, Lytham St. Annes, UK

Part 1

Overview

Chapter 1

Aviation and Climate Change – The Continuing Challenge

Alice Bows-Larkin, Sarah L. Mander, Michael B.Traut, Kevin L. Anderson, and F. Ruth Wood

Tyndall Centre for Climate Change Research, School of Mechanical Aerospace and Civil Engineering, University of Manchester, Manchester, UK

1 Introduction to Aviation & Climate Change Policy

2 Trends in the Aviation Sector’s CO2 Emissions

3 Drivers of Demand for Air Travel

4 Technical Options for Cutting CO2 in Aviation

5 Future of Aviation and Climate Change

6 Conclusion

References

Further Reading

1 Introduction to Aviation & Climate Change Policy

International aviation's contribution to global CO2 emissions has come under scrutiny since the early 2000s. Prior to that, mitigation focused on the CO2 released within national borders, given the exclusion of international aviation from the Kyoto Protocol's national targets. Although a considerable body of research has since interrogated aviation's CO2 contribution, discussing cuts in the CO2 produced by flights remains controversial and unpopular for many reasons voiced by industrial stakeholders and the general public (Budd and Ryley, 2013). So, while there are arguments for treating aviation on a level playing field with other sectors and implementing stringent mitigation policies aimed at tackling CO2 (Bows, 2010; Budd and Ryley, 2013; Peeters, Williams, and, Haan, 2009), this is not a universal view.

Aviation's economic importance is regularly cited as a key reason to avoid stringent CO2 mitigation (Wood, Bows, and Anderson, 2012). Another argument can be attached to its role in connecting nations at different stages of development. The sector's growth rate, coupled with few options for reducing carbon emissions per passenger-km (gC/RPK), drives up aviation's CO2 emissions. Increasing mobility and high rates of economic growth in industrializing nations influence demand. These industrializing nations do not in general foresee CO2 targets for their other sectors before 2020, and therefore few direct drivers toward cutting emissions. Globalization supports arguments for treating international aviation and shipping differently to sectors that do not operate within international airspace or waters, with policies that can allow for high growth rates in some countries. While this may have some traction, it only holds within a climate change context if globally averaged growth rates do not jeopardize the international commitment to remain within the 2 °C global temperature target.

There has been widespread political consensus enshrined in various accords, agreements, and declarations that 2 °C represents the threshold between acceptable and dangerous climate change. Controlling emissions of greenhouse gases across sectors is critical if the carbon budgets underpinning this commitment are not to be exceeded. Yet the sizable and growing emissions from international aviation (and shipping) were exempt from national targets enshrined in the Kyoto Protocol. Domestic aviation emissions were included, but as the United States, with its dominant share of the CO2 from all internal flights when the Protocol was adopted (63% in 1997, IEA, 2014), did not ratify it, the already weak constraints on aviation emissions were watered down still further.

In a bid to include international aviation's CO2 within global climate commitments, the Kyoto Protocol tasked the UN's specialist agency, the International Civil Aviation Organization (ICAO), with responsibility for mitigating CO2 from aviation. However, slow progress during the 1990s led the EU Commission, having voiced its frustration, to independently develop proposals for including aviation within its Emissions Trading Scheme (ETS) and impose a carbon price on the industry (Bows, 2010). So, by the Kyoto Protocol's final official year, the EU had included aviation within its ETS despite concerns coming from the industry regarding elevated costs and doubt surrounding the resulting impact on CO2 emissions.

Yet even before the policy began operating, the EU suspended the inclusion of non-EU nations' flights in response to progress by ICAO toward establishing a global trading scheme, and in light of strong opposition to the scheme from some countries including the United States (Bows-Larkin, 2014). This suspension remains in place until 2016, when the ICAO mechanism is scheduled to be agreed. Other policy mechanisms promoted through ICAO include a voluntary global annual 2% fleet fuel efficiency improvement up to 2050, with a 50% reduction in net emissions from 2005 levels, and an aim for carbon neutral growth from 2020 (ICAO, 2013a).

By October 2013, development of ICAO's global trading scheme was underway, with derived revenue hypothecated to alleviate the impact of aircraft engine emissions, and developing low-carbon alternative fuels. However, with no mechanism agreed before 2016, and then further time needed for implementation, emissions are expected to rise unabated at least until then. Meanwhile, there have been developments within the United States. In 2014, the US Supreme Court upheld the United States Environmental Protection Agency's (EPA) power to regulate CO2 under the Clean Air Act. Now, the EPA is considering if aviation has an impact on human health, releasing an information sheet with potential plans to impose a CO2 standard on aircraft.

With cuts of at least 80% from 2010 levels by 2050 necessary across all sectors for a reasonable chance of avoiding 2 °C (Bows-Larkin, 2014), the current mitigation strategy for international aviation assumes other sectors will proportionally cut CO2 by more than aviation. Bows (2010) assessed aviation's climate impact, comparing scenarios for future aviation CO2 with carbon budgets associated with 2 °C. The paper presented the mitigation challenges for aviation and highlighted the importance of understanding the broader climate change context when assessing aviation's climate impact. This chapter updates Bows (2010) by comparing updated aviation scenarios with more recently published 2 °C carbon budgets. It discusses insights in the context of emerging developments, to reassess if that paper's conclusion that without a large reduction in growth rate or significant penetration of alternative fuel by 2050, aviation's projected CO2 emissions will be incompatible with the 2 °C target remains sound.

2 Trends in the Aviation Sector's CO2 Emissions

The civil aviation industry's CO2 emissions have grown almost consistently year-on-year since its emergence. The regional profile of aviation-related CO2 emissions shifts as economies develop. Nevertheless, growth remains high even in industrialized economies for two principal reasons: its role in connecting nations especially at different stages of industrialization; the shift from occasional use to frequent flying, facilitated by falling air fares (Randles and Mander, 2009). Its high growth rate poses great challenges for climate change mitigation. Unlike almost all other sectors, technologies available for deployment in the time window consistent with avoiding 2 °C are few and far between. Thus, long lifetimes of aircraft coupled with the longevity of design specifications risk leaving the sector locked into conventional technologies for many decades. Even as fuel efficiency improves, Figure 1 illustrates that high demand growth leads to rising CO2 emissions.

A graphical representation for trends in aviation indicators, where carbon dioxide emissions (MtCO2 year–1) is plotted on the y-axis on a scale of 0–800 and years on the x-axis on a scale of 1975–2010. Passenger demand (billion km) is plotted on the right hand side and parallel to the y-axis on a scale of 2000–6000. Total aviation, international aviation, domestic aviation, RPK, and ASK are represented by different curves in the graph.

Figure 1 Trends in aviation indicators. Revenue Passenger-km (RPK), Available Seat-km (ASK), and the CO2 emissions for aviation for international and domestic flights. (Source: CO2 data from the International Energy Agency (IEA, 2014); 1971–2006 RPK and ASK data, Owen (2008); 2007–2013 data calculated from passenger growth rpk and passenger load factor achieved (IATA, 2014).)

Regions currently experiencing rapid growth in aviation CO2 differ depending on whether domestic or international travel is interrogated. Obviously, nations with large land masses have much greater propensity for domestic air travel than smaller nations, where international flights dominate. Nevertheless, the construct of domestic and international aviation is important in a climate policy context, as international flights are not subject to national mitigation strategies.

2.1 Trends in Domestic Aviation CO2 Emissions

CO2 emissions from aviation have historically been dominated by those from domestic flights in the United States – and they remain a major share (50% of the CO2 from all domestic flights globally in 2011; IEA, 2014; Figure 2). However the picture is in flux, with CO2 from domestic flights in the United States recently declining. China has just one-fifth of the domestic aviation CO2 of the United States, but is growing rapidly at rates close to 10% per year (IEA, 2014). Other parts of Asia are experiencing high growth rates too: 6% per year since 1990, but it is the vast geographical area of some regions that will inevitably lead to high levels of CO2 from domestic flights.

A graphical representation for regional split of CO2 emissions from domestic flights, where carbon dioxide emissions (MtCO2 year–1) is plotted on the y-axis on a scale of 0–200 and years on the x-axis on a scale of 1975–2010. Africa, middle East, Asia (excluding China), China (including Hong Kong), Non-OECD Europe and Eurasia, Non-OECD Americas, OECD Americas, OECD Asia Oceania, and OECD Europe are represented by different curves in the graph.

Figure 2 Regional split of CO2 emissions from domestic flights. (Data: IEA statistics (IEA, 2014).)

2.2 Trends in International Aviation CO2 Emissions

The international aviation CO2 emissions are captured in Figure 3. OECD Europe dominates in recent years, with OECD Americas and then Asia (excluding China) following. In terms of growth, China has experienced the highest annual rates at 8% on average since 1990 with the rest of Asia at 5%. OECD Europe continues to grow at 3% annually and even in North America where aviation is considered to be a mature industry, average growth has been 2% per year since 1990 – including both the events of September 11, 2001 and the recent global economic downturn.

A graphical representation for regional split of CO2 emissions from international flights, where carbon dioxide emissions (MtCO2 year–1) is plotted on the y-axis on a scale of 0–200 and years on the x-axis on a scale of 1975–2010. Africa, middle East, Asia (excluding China), China (including Hong Kong), Non-OECD Europe and Eurasia, Non-OECD Americas, OECD Americas, OECD Asia Oceania, and OECD Europe are represented by different curves in the graph.

Figure 3 Regional split of CO2 emissions from international flights. (Data: IEA statistics (IEA, 2014).)

2.3 Future Aviation CO2 Emissions

In recent years, aviation's CO2 profile shifted emphasis from domestic to international, driven primarily by a rise in international travel by EU citizens, as well as fall in demand for US domestic flying, particularly following the events of 11 September 2001 and recent recession. However, the emergence of China as a rapidly growing source of domestic aviation CO2 suggests the balance could shift back again. Another driver of CO2 will stem from rapid growth in international flying across all nations, influenced by connections with nations experiencing high levels of economic development. This new landscape raises questions. What might be the impact on innovation and climate policy of rapidly growing domestic aviation in China? Will the growing source of international flight CO2 continue to fall outside of policy regimes aimed at other fossil-fuel-consuming sectors? And finally, what drives passenger demand for flying in the first place?

3 Drivers of Demand for Air Travel

To explore some of the drivers of demand for flying, it is worth considering what air travel is primarily used for. In summary, aviation accounts for approximately 10% of all transport (vehicle) km traveled and moves 35% by value of goods traded internationally (ATAG, 2014). Fifty-three percent of international travelers arrive at their nondomicile destination by air compared with 47% by other modes – 40% by road, 2% by rail, and 5% by water (UNWTO, 2014). Fifty-two percent of flyers do so for leisure, 27% to visit friends and family, religious, or health reasons, while 14% fly for business (UNWTO, 2014). Understanding the drivers of demand for air travel is a key piece of the jigsaw.

3.1 Growing Demand Around the World

Recently, passenger numbers have been on an upward trend following a decline around 2008/2009 (Figure 1), with monthly passenger kilometres for September 2014 in the region of 510 billion (IATA, 2014). Emphasizing the link between the state of economies and demand for aviation, as of September 2014, the International Air Transport Association (IATA) judged the outlook for aviation to be positive though inconsistent across the globe; faltering economic improvement in the EU contrasted with recovery in the United States and on Asia Pacific routes (IATA, 2014). While year-on-year growth in passenger kilometers for September 2014 compared with September 2013 averaged 5.3% globally, there is significant regional variation. The Middle East is the only region where the rate of growth continues to increase due to its strong regional economy, with a fall in other regions related to economic slowdown and factors such as strikes and market volatility. Overall growth in demand is highest in emerging economies (IATA, 2014). Forty percent of the international market is centered on Europe, followed by Asia Pacific with 25%. Forty-one percent of the domestic market is the United States with China in second place at 23% (IATA, 2014) and industry forecasts suggest China will become the largest domestic market within 10 years. In terms of flights per capita, in 2014 North Americans and Europeans are the most likely to fly, averaging, 1.6 and 1 flight per person per year, respectively. In 2033, Airbus predicts flights per capita in China will reach 0.95 up from 0.25 in 2014, with India reaching 0.26 up from 0.06.

3.2 The Role of Socio Economic Factors in Shaping Demand

Demand for aviation is shaped by factors that affect people's ability and desire to fly, and by supply-side factors within the aviation industry such as capacity or infrastructure. To unpack demand for aviation, a variety of modeling approaches can be used including econometric modeling (for instance, Department for Transport, 2013). Aviation demand is often related to economic activity and air fares (see, for example, Department for Transport, 2013), while other studies (see Further Reading section) include variables such as exchange rates, purchasing power overseas, and perceived level of household wealth (O'Connell et al., 2013).

Studies show that most people do not fly because they want to fly per se, but because flying enables them to do things they wish to do. Thus, to gain a more nuanced understanding of drivers for demand for aviation, much can be gained from complementing economic modeling studies with sociological and psychological research. Migration and changing household demographics are an important driver for aviation given that increasing globalization means friends and family can be dispersed; this is reflected in figures that suggest around a quarter of flights are to visit friends and family (Hibbert et al., 2013). Urry (2012) argues that society views high-mobility lifestyles positively, whereby someone's standing is reflected by the places they have visited and mobility patterns; frequent flyer programs and the marketing strategies of airlines also link status with mobile lifestyles.

Supply-side changes, namely, the emergence of low-cost airlines specializing in domestic and short-haul routes has helped create new markets. For those who can afford it, the low cost of flights in the United Kingdom has contributed to raising the standard of occasions such as a hen party or trip with friends (Randles and Mander, 2009). Changing practices of celebrating, holidaying, and visiting friends and family abroad represents an upward ratchet on the number of flights taken per year. Growth is facilitated further by interacting aspects such as easy Internet booking and online check-in, speeding up the purchasing and delivery of service (Randles and Mander, 2010).

Finally, aviation is an area of consumption where there is a gap between attitudes toward the environment and behavior. Climate awareness does not lead to people not flying. Instead, a lack of alternatives and the habitual nature of flying lead people away from sustainable choices, in contrast to some other areas of decision-making (Hares, Dickinson, and Wilkes, 2010). Frew and Winter (2009) highlight how concerns about the time, family commitments, and a desire to see the world can outweigh consideration of the environmental cost of travel (Frew and Winter, 2009). The extent to which flying practices in many wealthy parts of the world transfers to emerging markets, particularly those now served by their own low cost airlines such as AirAsia, remains to be seen.

3.3 Future Demand for Air Travel

Future demand for aviation is likely to be driven by an increasing middle class population, primarily in emerging economies, whose desire to travel is enabled by supply-side developments such as expanding infrastructure and deregulation of markets. Although national models of deregulation and liberalization differ, a competitive aviation industry is seen as crucial to reduce costs and improve service, both domestically and as carriers look to compete internationally (O'Connell et al., 2013). Airport expansion has supported the enlargement of domestic aviation in emerging economies as shown by the development of airports in smaller cities to support regional commuter travel in China for example. In emerging markets, the development of regional airports can drive growth in international air travel as Hub and spoke configurations can be used to attract international travelers (O'Connell et al., 2013). With the support that national governments are giving to the aviation industry within emerging economies, the building blocks that enable industry growth predictions to come to fruition are being put in place (O'Connell et al., 2013).

4 Technical Options for Cutting CO2 in Aviation

Redrawing attention to climate change, how can rising demand be met if absolute CO2 emissions from the sector are to be reduced? The obvious place to seek solutions resides in innovative technologies and operational practices available to deliver change. However, previous work highlights the very incremental nature of much of what is available (Bows-Larkin, 2014).

4.1 Improving Aircraft Efficiency

Most technological improvements to reduce fuel burn address structural weight, aerodynamics, or engine efficiency. Decreasing the structural weight of the aircraft and reducing drag to increase the lift-to-drag ratio, both cut propulsive power and thereby reduce fuel consumption. Increasing aircraft engine fuel efficiency also lowers the CO2 intensity of flight. Incremental change across all three leads to efficiency improvements between different aircraft generations. With fuel making up a significant share of the running costs for airlines, unsurprisingly, fuel efficiency has been a major research and development goal for many decades. Advances in engine design, optimized wing and body shapes, and material science contributed to new generations of aircraft that consume less fuel per Available Seat Kilometer (ASK) than their predecessors. Coupled with increased utilization factors, this has led to significant fuel efficiency improvements (Figure 4). While there is still potential for future generations of aircraft to be more fuel efficient than today's, the best-fit regression line in Figure 4 illustrates how aviation faces diminishing returns as technology matures. Aircraft are highly optimized and must meet rigorous safety standards as well as respond to other constraints on noise and local pollution, meaning that fundamental design changes are difficult and costly to achieve. So, while there is certainly promise over the longer term for more advanced, more fuel-efficient, and lower-carbon intensity aircraft, the current challenge is that such changes are highly unlikely within a time frame compatible with climate change targets. Alternative fuels have therefore become attractive, but they come with unresolved issues, such as their full life-cycle CO2 impacts and wider sustainability concerns.

A graphical representation where energy intensity E, (MJ ASK–1) is plotted on the y-axis on a scale of 0–4.0 and years of market introduction on the x-axis on a scale of 1940–2060. Solid and dashed curves are representing best-fit regression and exponential, respectively. Filled-circle, rhombus, clear-square, and filled square are representing Penner et all (1999), A380, piston-powered airlines, and B787, respectively.

Figure 4 Trends in energy intensity in the aviation sector since the emergence of the civil aviation indus-try projected out to 2050. (Reproduced with permission from Peeters, Williams, and Haan (2009), p. 294. © Taylor and Francis.)

4.2 Beyond Efficiency

To reduce CO2 emissions beyond annual incremental change, alternative fuels will have to come into play. The only ones considered realistic in the near-term are biofuels or other synthetic fuels. The fuel itself would not be fundamentally different from kerosene, but rather than of fossil origin, produced from biomass or from another low-CO2 feedstock. There is a burgeoning range of initiatives in this direction but emission savings delivered depend on feedstocks and production methods. Some of the CO2 from biofuels may be negated by the CO2 taken up by the plants grown to produce them. However, the CO2 benefits of producing jet fuel from, for example, Jatropha, depend on the current vegetation or land use it displaces and for many areas this may be negative. Furthermore, large-scale production raises wider sustainability issues as feedstock production displaces food production.

In general, life-cycle assessments yield a wide range of potential emission savings or otherwise, depending on the specific production process (Hileman and Stratton, 2014). On-going research seeks to resolve issues around biofuels with different properties from standard jet fuel and create production routes for second-and third-generation biofuels, such as jet fuel from algae. Replacing plants with alternative mechanisms of using solar (or another source of) energy to combine water and CO2 into hydrocarbons could open a production route for a wholly synthetic jet fuel. Going beyond hydrocarbons, hydrogen and battery-electric powered propulsion have been considered, but are far from offering a realistic alternative within an appropriate timescale (Hileman and Stratton, 2014).

5 Future of Aviation and Climate Change

With rising demand and a limit to the technical options available for cutting CO2 emissions, it is essential to place aviation in the context of broader climate change policy objectives.

5.1 Global Climate Policies and Scenarios

In 2014, the Intergovernmental Panel on Climate Change (IPCC) released its latest synthesis of the global climate change challenge. One of the new areas brought to the fore is the importance of cumulative greenhouse gas emissions in dictating the future global mean temperature increase. Global cumulative carbon budgets constrain emission pathways for all sectors, varying depending on the desired climate outcome. In short, from 2015 onward there remains around 1100–1400GtCO2 for a 50 : 50 (reasonable) chance of avoiding the 2 °C target threshold between acceptable or dangerous climate change (IPCC, 2014). Naturally, there are various paths that can be followed to remain within this constrained budget, in addition to a range of contributions by sectors that release CO2. However, when the numbers are scrutinized it becomes clear how very challenging this global budget is for any sector. For a reasonable chance of avoiding 2 °C, all sectors will need significant and absolute cuts in CO2 in the coming decades. More realistically, energy systems in wealthier countries, including the transportation sector, need to eliminate CO2 emissions by very soon after 2050 (Anderson and Bows, 2011).

A more conservative take is presented by the Representative Concentration Pathways (RCP) (Figure 5). The most constrained scenario (RCP2.6) has the best chance within this suite of avoiding 2 °C. Considering this pathway in detail however, it becomes clear that global CO2 emissions have in reality risen faster than this 2 °C scenario. Furthermore, in the vast majority of energy scenarios assumed to fit with this emissions pathway, negative emissions play a significant part in maintaining low temperature rises. In other words, much confidence is placed in the large scale and rapid deployment of biomass energy sources coupled with carbon capture and storage and/or carbon sinks through land-use change. Without this, the budgets become much more challenging, with all sectors needing to radically adjust expected levels of future emissions to avoid 2 °C.

A graphical representation for representative concentration pathways (RCPs) and historical emissions of fossil fuel and industrial CO2, where carbon dioxide emissions (MtCO2 year–1) is plotted on the y-axis on a scale of 0–100 and years on the x-axis on a scale of 1980–2100.

Figure 5 Representative Concentration Pathways (RCPs) and historical emissions of fossil fuel and industrial CO2. (Source: Historical data taken from the Global Carbon Project (Le Quéré et al., 2014) and RCPs taken from Meinshausen et al. (2010) and Moss et al. (2010)). The number after each RCP stands for the amount of future radiative forcing.

5.2 New Scenarios and Forecasts of CO2 From Aviation

To formulate a response to the challenge posed by climate change, it is necessary to look out into the future. There are many outlooks, forecasts, and scenarios in the literature, some of which are making predictions, while others are formulated to explore potential future change. What-if type scenarios have gained prominence in climate and energy policy, given the nature of end point climate objectives, such as the 2 °C temperature target. They are also used in various forms within sectors, and aviation is no exception. It is therefore instructive to revisit existing scenarios and consider them in the context of recent trends in CO2 emissions from aviation. Figure 1 presents data on emissions from domestic and international aviation, which followed a very similar growth trajectory until the mid-1990s. Since then, growth in domestic and international CO2 combined has for the most part been due to increases in emissions from international aviation, but as discussed, there is a large potential for domestic aviation within China, and other emerging economies with large land masses, to change this.

Taking a future perspective, Figure 6 presents examples from two generations of aviation emission scenarios alongside the historical CO2 trajectory from Figure 1. Ten scenarios from the literature (Newton and Falk, 1997; Penner et al., 1999; Vedantham and Oppenheimer, 1994) including a new suite from ICAO (2013b) are scaled to 1992. Out of the older ten scenarios, the three highest and one lowest growth scenarios were considered at the time to be implausible. Indeed over the time period since 1992, many of the older scenarios projected emissions significantly higher than has materialized. This is in part due to the events of September 11, 2001 and recent global economic downturn. It is unsurprising that scenarios influenced by industry perspectives tend to be optimistic about expected demand or lack incentives to develop low-growth scenarios.

A graphical representation for future aviation emission scenarios, where carbon dioxide emissions (MtCO2) is plotted on the y-axis on a scale of 0–4500 and years on the x-axis on a scale of 2000–2050.

Figure 6 Future aviation emission scenarios. The ICAO scenarios (ICAO, 2013b) are underpinned by one rising demand scenario. (Source: Data for the other scenarios is from Newton and Falk (1997), Penner et al. (1999), and Vedantham and Oppenheimer (1994)). The Environmental Defence Fund (EDF) scenarios (designated E**) have high growth assumptions (designated E*h) and base growth assumptions (designated E*b). The Forecasting and Economic Analysis Sub group (FESG) scenarios are designated F**, with variations in growth differentiated by a, c, or e.

The set of scenarios considered in ICAO's 2013 Environmental Report (ICAO, 2013b) in Figure 6 all use the same mid-range demand scenario with different assumptions about efficiency gains from technology and operations, resulting in a range of future CO2 levels. There has not been sufficient analysis yet to include the potential CO2 savings from alternative fuels out to 2050, although some assessment is made up to 2020 in ICAO's 2013 report, where it is estimated that approximately 3% of fuel consumed could be from sustainable alternative sources. ICAO scenarios' average annual passenger demand growth of 4.9% is similar to the rate expected over the time period 2014–2033 by Boeing in its Market Outlook 2014. Of course, this and other assumptions can be called into question. Nevertheless, comparing the outlook with results from climate science allows for conclusions to be drawn.

The ICAO scenarios in Figure 6 all show a growing trajectory for aviation's CO2 emissions. This is in stark contrast to ICAO's target of carbon-neutral growth from 2020 onward (ICAO, 2013b) and subsequent cutting of emissions in half by 2050. This demonstrates that a step change either in technological or in operational advancements is required, or the CO2 reductions will need to be met by other sectors through emission trading. While reasonable to assume some sectors will cut emissions more rapidly than others and at different times over the coming decades, the aviation sector will too need to cut its CO2 significantly during that timeframe, given the constraining 2 °C CO2 budget. Furthermore, as emissions trading has so far failed to deliver emission reductions in line with 2 °C, and such a scheme is unlikely to be operational for aviation before 2020, pinning hope on trading to deliver on mitigation objectives is arguably misplaced (Bows-Larkin, 2014).

5.3 Contrasting the Outlook for Aviation with Avoiding 2 °c

The most recent future aviation scenarios taken from ICAO are contrasted with the global CO2 scenarios compatible with avoiding 2 °C (Figure 7). In addition to the RCP2.6 scenario, which is already out of kilter with the current global CO2 trajectory, (global CO2 emissions are closer to a much higher climate impact scenario since 2000, RCP8.5 and Figure 5) three global scenarios from Anderson and Bows (2011) are overlaid to emphasize the scale of CO2 reduction necessary across all sectors. These additional global pathways from Anderson and Bows (2011) are commensurate with a reasonable chance of avoiding 2 °C, but take into account a more explicit recognition of momentum within energy systems (for more information on their derivation, see Anderson and Bows, 2011). All trajectories are indexed to 1992 = 1 to highlight how projected aviation emissions, under all scenarios, are at odds with the cross-sector 2 °C pathway. Comparing Figures 5 and 6, it is also notable that the absolute level of aviation emissions in 2050 is ∼30–40% of total global CO2 under the RCP2.6. However, these ICAO scenarios do not have a storyline consistent with RCP2.6 where strong mitigation is assumed across all sectors.

A graphical representation where carbon dioxide emissions (indexed) is plotted on the y-axis on a scale of 0–7 and years on the x-axis on a scale of 2000–2050.

Figure 7 Four global CO2 scenarios for all sectors (RCP2.6, AB C + 1 C + 4 and C + 5) and a suite of ICAO aviation CO2 scenarios from ICAO, 2013b (red). (Note: All scenarios are indexed to 1992 = 1.)

Looking back to Bows (2010), where this gap was previously illustrated, it is clear that its conclusion …without a large reduction in growth rate or significant penetration of alternative fuel by 2050, aviation projected CO2 emissions will be incompatible with the 2 °C target remains, albeit within a now more constrained 2 °C budget. There are few indications that any new technical or operational advances will start to make CO2 cuts in real terms and across the world's aircraft fleet that would negate the rise in CO2 due to growth in activity. Many parts of the world continue to see very high rates of growth, which, without a significant acceleration toward overcoming technical and sustainability barriers around the use of biofuels for aircraft propulsion, will only serve to maintain the high and growing levels of CO2 from air transport. While other sectors will too struggle to decarbonize in line with 2 °C, it will be necessary for all sectors to play their part. Unpopular as it is, and as long as emissions trading remains inconsistent with the 2 °C goal, there is a clear role in aviation for demand management.

6 Conclusion

The headline conclusion is clear and unequivocal; the aviation industry's current projections of the sector's growth are incompatible with the international community's commitment to avoiding the 2 °C characterization of dangerous climate change. Even a highly optimistic uptake of the most promising technologies for reducing the CO2 intensity of flying cannot deliver the rapid and deep rates of mitigation illustrated in Figure 7 to comply with the IPCC's carbon budgets for a reasonable to likely chance of staying below 2 °C. This stark conclusion holds even with the heroic assumption that other sectors may be able to shoulder some additional mitigation effort to compensate for the aviation sector mitigating less than its counterparts. Ultimately, however construed, the maths forthcoming from the IPCC's 2 °C carbon budget, mandates that the demand for aviation will need to be constrained if the global community is not to renege on its 2 °C commitments.

In contrast to such demand management, market analysis highlights how many nations have rapidly growing aviation sectors, and that if China's domestic aviation broadly follows that of the United States, it will emerge as a strong driver of future CO2 emissions. As it stands, there is a clear and significant risk that current expansion plans will extend the flying practices of today's frequent fliers both within wealthier nations, and to and within emerging economies. Such a prospect plays against the international community's commitments to mitigate emissions in line with 2 °C.

Juxtaposing existing scenarios and forecasts of aviation-related CO2 emissions with global CO2 scenarios for 2 °C illustrates a huge and widening gap between the two. At present, there are few signs that the sector, like most sectors, takes the 2 °C target and carbon budget framing of climate change seriously. While the industry has set out proposals for carbon neutral growth from 2020 and to reduce the sector's emissions by 50% by 2050, there is little evidence to demonstrate that this is at all feasible, even with emissions trading. Furthermore, the 50% reduction falls short of cuts commensurate with a reasonable chance of avoiding 2 °C, leaving the industry relying on other sectors curbing their emissions even more. It will be an enormous challenge for all sectors to reduce their own emissions in line with a reasonable to likely chance of avoiding 2 °C, so any assumption that sufficient sectors will be in a position to make much greater cuts than aviation misunderstands the scale of the mitigation challenge. Consequently, if the aviation sector is to reduce emissions in line with the 2 °C commitment, it must acknowledge the veracity of the climate challenge, and put in place the internal mechanisms to manage its own demand in accord with the necessary levels of mitigation.

With the publication of the IPCC's fifth report and explicit inclusion of carbon budgets associated with the 2 °C threshold, a clear framework within which to consider emissions from aviation now exists. Against this backdrop, the United Nations Environment Programme's Gap report (UNEP, 2014) draws attention to the high-level and widespread failure of the global community to constrain emissions in line with 2 °C. This failure is exemplified by ICAO predicting a significant and ongoing rise in emissions, while at the same time continuing to emphasize the industry's commitment to a sustainable future.

Aviation, as is the case for virtually all sectors, has thus far failed to develop a scientifically credible emission pathway toward a 2 °C future. If such a pathway is not forthcoming in the next few years, it will be evident that the sector either rejects the international community's 2 °C commitment, or has judged itself too important to make its full contribution, relying instead on the untenable assumption that other sectors will compensate. The aviation industry cannot be isolated from the dialogue on climate change, and as a mature industry it is incumbent on it to be clear as to its position on 2 °C, carbon budgets, and the mitigation challenge.

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Adams, P., Bows-Larkin, A., Gilbert, P., Hammond, J., Howard, D., Lee, R., McNamara, N., Thornley, P., Whittaker, C., and Whitaker, J. (2013) Understanding Greenhouse Gas Balances of Bioenergy Systems, Supergen Bioenergy Hub.

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Chapter 2

Global Atmospheric Chemistry and Impacts from Aviation

Marcus O. Köhler

School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK

1 Introduction

2 Emissions

3 Chemical Processes in the Troposphere

4 Chemical Processes in the Stratosphere

5 Impacts of Aircraft Emissions

6 Conclusions

Related Articles

References

1 Introduction

In the natural atmosphere a large number of chemical processes take place. Mostly they involve reactions between atmospheric trace gases – gaseous compounds that exist in small concentrations relative to the main constituents of air, nitrogen and oxygen. The part of the atmosphere extending from the surface to approximately 10 km altitude is called the troposphere and its chemistry forms an important part of the global biogeochemical cycles. Its processes occur on a wide range of scales in space and time. As a result of human activities, the composition of the atmosphere is altered to an increasing extent by emitting pollutants and by transforming the land surface.

The combustion of fossil fuel in aircraft engines leads to emissions of a range of pollutants. Some of these substances interfere with the natural chemical processes in the atmosphere. These chemical changes due to aircraft engine emissions can be loosely grouped into three categories: First, the chemistry of plume processes describes the rapid interactions and chemical transformation of substances in the wake of an aircraft that occurs within few minutes after emission (Hayman and Markiewicz, 1996; Meijer et al., 1997; Kraabøl et al., 2000). Second, local air quality deals with pollution near the ground due to air traffic activity. This describes an issue of regional scale that usually applies to the vicinity of airports. Third, global atmospheric chemistry deals with the impact of aircraft emissions on chemical processes that occur over large spatial scales and over time periods from weeks to many decades. In this context, the impact of aircraft emissions on the abundance of greenhouse gases and their subsequent impact on the climate system are of relevance. This chapter deals with the third aspect of global atmospheric chemistry and related changes due to emissions from global aviation. The other two aspects are dealt with elsewhere. Here global atmospheric chemistry processes are outlined as far as they are relevant for chemistry involving aircraft engine emissions. More comprehensive descriptions of atmospheric chemistry can be found in textbooks such as Jacob (1999), Finnlayson-Pitts and Pitts (2000), or Seinfeld and Pandis (2006).

The impact of aviation on atmospheric composition was first investigated in the early 1970s when concerns were raised about stratospheric ozone depletion due to the prospect of a large fleet of supersonic passenger aircraft (Johnston, 1971). While the envisaged fleet of high-speed civil transports eventually did not become a reality, it was acknowledged that an accurate quantification of the impact of aircraft emissions on atmospheric chemistry was limited by the large uncertainties associated with the knowledge of the global distribution of key chemical compounds, primarily of the tropospheric budget of nitrogen oxides. Since the 1950s, global air traffic has consistently grown and this is expected to continue for the foreseeable future during the twenty-first century. It has therefore become essential to understand the present and potential future impact from global air traffic on atmospheric chemistry in the context of global environmental change. In the 1990s, assessment reports and reviews were published based on research programs in the United States (Friedl, 1997) and Europe (Schumann, 1997; Brasseur et al., 1998) with a focus on chemical and other environmental impacts from aircraft emissions. The Intergovernmental Panel on Climate Change published a Special Report on Aviation and the Global Atmosphere at the end of the previous century (IPCC, 1999). Since then further advances in atmospheric modeling and in the understanding of atmospheric chemistry have led to the continuing publication of updated reviews such as Lee et al. (2010).

2 Emissions

In order to determine the impacts of aviation on global atmospheric chemistry, detailed knowledge on the nature of aircraft engine emissions, specifically about their local composition and global geographical distribution, is required. Global data on aircraft engine emissions are compiled into emission inventories that serve as input into numerical models capable of simulating chemical processes in the atmosphere. These inventories contain emission flux data in a grid format, with the spatial grid typically having a horizontal resolution of the order of 1° in longitude and latitude and a vertical resolution of approximately 1000 ft. Seasonal variability in the emissions is mostly expressed in the form of monthly totals for each emitted species at each grid point. Recent inventories that represent contemporary emissions from global aviation are, for example, available from the EU-funded project REACT4C (Grewe et al., 2014), as well as from the US Federal Aviation Administration's AEDT (Roof et al., 2007; Olsen, Wuebbles, and Owen, 2013). Some of these inventories also include emission forecasts for the near future (e.g., 2025) or emission scenarios for the more distant future (e.g., 2050). Aircraft emissions consist mainly of water