Airports and environmental sustainability: a comprehensive review

Sustainability, as defined in the United Nations' Brundtland Report, states that present society must manage and consume resources so as not to compromise future society's needs (Brundtland et al 1987). While the Brundtland definition acknowledges human activity's environmental impact, it does not offer concrete guidance for achieving sustainability. A less abstract framework is the 'triple bottom line' approach, which aims to identify solutions that balance environmental, social, and economic interests (Elkington 1994).

Sustainability indicators, or metrics, can be used to measure the 'sustainability performance' of an airport. Metrics are critical because they allow for:

• Comparing the sustainability of one airport (or one type of airport) against another;
• Identifying the weak points or opportunities for improvement in airport infrastructure;
• Measuring progress towards meeting targeted goals.

A standardized, empirical metric is also crucial for making decisions about sustainable design and operation of airport infrastructure (Longhurst et al 1996). Stakeholder involvement in developing these indicators is necessary (Upham and Mills 2005). Sustainability metrics are a component of a larger-scale sustainability plan. Ideally, formalized sustainability plans developed by airports should incorporate metrics for tracking progress towards goals.

Airport sustainability, as defined by the aviation industry, incorporates the 'triple bottom line' concept with a fourth pillar focused on operational efficiency. Airport Council International (ACI) refers to this approach to sustainability as EONS (Martin-Nagle and Klauber 2015, Prather 2016). Common subcategories of EONS are shown in table 1. An important research dimension of the airport industry is the U.S. National Academies of Sciences' Airport Cooperative Research Program (ACRP), which researches and publishes synthesis reports and guidance for current sustainability practices at airports. ACRP reports are largely compiled through literature reviews of airports' published sustainability reports and through interviews, surveys, and questionnaires with airport operators. Recent topics of ACRP reports include:

• overall sustainability (Brown 2012, Delaney and Thomson 2013, Lurie et al 2014, Prather 2016, Malik 2017);
• feasibility of on-site energy provision (Lau et al 2010, Barrett et al 2014) and microgrids (Heard and Mannarino 2018);
• GHG emission reduction strategies (ACRP, FAA, Camp, Dresser, & McKee et al 2011, Barrett 2019);
• air quality impacts (ACRP, FAA, CDM Federal Programs Corporation et al 2012a, ACRP 2012b, Lobo et al 2013, Kim et al 2014, 2015)
• water efficiency (Krop et al 2016) and stormwater management (Jolley et al 2017);
• habitat management (Belant and Ayers 2014);
• sustainable ground transport (Kolpakov et al 2018);
• sustainable construction practices (ACRP, FAA, Ricondo & Associates et al 2011);
• waste management (Turner 2018);
• climate change adaptation of airports (Marchi 2015).

The definition of environmental airport sustainability in the academic literature varies with some defining it according to multiple categories of environmental impacts (Chao et al 2017, Ferrulli 2016, Gomez Comendador et al 2019; Kilkis and Kilkis 2016) and others limiting that definition to traditional environmental aviation impacts such as emissions and noise (Lu et al 2018). Environmental sustainability is assessed using both quantitative and qualitative metrics/measures, and using both generalized, average airports (Chester and Horvath 2009) and data from operating airports (Chao et al 2017; Kilkis and Kilkis 2016, Li and Loo 2016).

In both industry and academic research, environmental impacts are often disaggregated according to the airside and landside components of the airport system boundary. Figure 1 shows a plan view schematic of the typical features included in the airport system boundary. It should be noted that energy generation, water/wastewater (WW) treatment, and waste management infrastructure can be located within airport-owned property (i.e. decentralized) or within the surrounding community of the airport (i.e. centralized). Table 2 identifies the purpose and primary stakeholders for each airport component. Understanding the scope of airport infrastructure aids in identifying the most relevant environmental impacts and the stakeholders best equipped to mitigate those impacts.

Zoom In Zoom Out Reset image size

While previous studies have examined sustainability practices of individual airports (Berry et al 2008, Prather 2016), this work represents the first comprehensive systematic review of academic and industry literature on airport environmental sustainability. The five objectives of this research are: (1) synthesize the existing literature on environmental sustainability indicators and metrics for airports; (2) review the application of sustainability indicators developed for the construction of terminals and other airport facilities at a case study airport (San Francisco International Airport also known as SFO); (3) identify gaps in the literature; (4) recommend what sustainability indicators/metrics should be employed at airports based upon the results of the literature review; (5) provide recommendations for future directions of research. Sustainability indicators are grouped according to the SFO framework: Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Materials and Resources. These five categories provide a framework for stakeholders to begin exploring the scope of relevant environmental impacts. The breadth of the five categories also highlights that sustainability encompasses more than one type of impact (e.g. GHG emissions) and underscores that airports have multiple priorities in addressing their environmental impacts. The expected outcome from this review is the identification of gaps in the existing literature and practice as it pertains to evaluating the sustainability of airport infrastructure. Recommendations for future research directions will provide those in the academic realm, as well as in the public aviation sector, a robust assessment of what metrics, practices, and methods should be applied to achieve optimal performance outcomes.

Section 2 presents the methodology for conducting the systematic review. Section 3 follows with a characterization, trend analysis, and synthesis of the reviewed literature, along with a review of the sustainability indicators used at a current SFO infrastructure project. Section 4 discusses the limitations and gaps of the existing literature, analyzes the efficacy of SFO's sustainability assessment framework, and provides guidance for future research directions. Section 5 concludes with a summary of the overall work and a recommendation for practices that airports should implement in the short term.

2.1.1. Criteria for selecting research papers

The foremost criterion in selecting peer-reviewed research articles and technical reports is that they pertain to indicators (i.e. metrics or measurements) for environmental sustainability. Although the concept of sustainability also includes economic and social factors, they are outside the scope of this review. We excluded corporate sustainability reports published by individual airports as data from these reports often appear in non-standard formats. However, individual airport sustainability practices were explored as part of the review of academic and ACRP literature. We iteratively searched for peer-reviewed research articles and technical reports in Web of Science, Google Scholar, and the National Academies of Science' ACRP database that were relevant to 'airport sustainability,' using the key terms of 'airport' and variations of 'sustainability' including 'environmental sustainability,' 'sustainable development,' and 'environmental impact.'

Searches were conducted with key terms related to the five categories of the SFO framework (i.e. Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Materials and Resources). Additional searches also included articles that incorporated life-cycle assessment (LCA), a method for assessing the 'cradle-to-grave' environmental impacts of a product, process, or project. We elected to also include search terms for Scope 1, Scope 2, and Scope 3 GHG emissions. Table 3 summarizes the definitions and examples of Scope GHG emissions.

Table 3. Summary of GHG scope emissions for airports.

	Scope 1	Scope 2	Scope 3
Definition	GHG emissions come from on-site sources that airport owns	GHG emissions come from purchase of off-site energy	GHG emissions come from on-site sources that are controlled by tenants
Examples
	• On-site natural gas combined heat and power plant	• Utility-supplied electricity	• GSE owned by airlines
	• Airport-owned vehicles		• Concessionaire activities
			• Passenger/employee transportation to and from airport

Characterizing GHG emissions according to the three Scopes aligns with airport industry practice of allocating responsibility for GHG emissions among airport stakeholders (ACA 2020). Exact search terms for all criteria are provided in table A1 in appendix A (available online at https://stacks.iop.org/ERL/15/103007/mmedia). Articles that were relevant to at least more than one of the five sustainability categories were considered as part of a Multidimensional category.

Articles that focused on sustainability indicators for the construction and operation of physical airport infrastructure were prioritized. Articles were excluded if they concentrated on aircraft, aircraft fuel, or on aircraft operations within the airport boundary such as taxiing, queuing, and the landing and take-off (LTO) cycle. The rational for this exclusion is that aircraft-related sustainability is an already extensively reviewed subject (Agarwal 2010, Blakey et al 2011, Sarlioglu and Morris 2015). However, articles pertaining to aircraft servicing operations at airports (e.g. ground service equipment or GSE, de-icing) were included. All screening criteria are listed in table A2 in appendix A. Note that the time period of 2009 to 2019 is selected to provide a meaningful analysis of the academic literature, as interest in airport environmental sustainability as a research field began in earnest at the end of the 2000s.

The searches yielded a total of 108 articles grouped according to Energy and Atmosphere (n = 22), Comfort and Health (n = 25), Water and Wastewater (n = 14), Site and Habitat (n = 16), Materials and Resources (n = 18), Multidimensional (n = 13). Common themes of sustainability indicators for each category are depicted in figure 2. A bibliography for all articles included in this systematic review is provided in appendix A (table A3). Section 3 provides a trend analysis of the articles included in the systematic review.

Zoom In Zoom Out Reset image size

A trend analysis of the reviewed articles indicates that interest in airport environmental sustainability has steadily increased over the period of 2009 to 2019 (figure 3). Article counts in each category theme (figure 4) reveal that research among the various categories is relatively balanced, with some prominent exceptions. Article counts for 'Ambient Air Quality,' 'Airfield Materials,' and 'Multidimensional' research themes are the highest. The high article counts for 'Ambient Air Quality' and 'Airfield Materials' suggests that research in the field of airport environmental sustainability largely focuses on the characteristics of an airport that are most prominent and apparent (i.e. the runway, taxiway, and apron). The high article count for the 'Multidimensional' category indicates that the research community is beginning to recognize that airport sustainability is comprised of multiple environmental impacts across multiple airport functions. In categories such as 'Waste Management' and 'Building Materials,' the small article counts imply that these specific subjects are still emerging as relevant research areas.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

3.1.1. Synthesis of research by category

3.1.2. Summary of trends in existing research

Figure 5 shows a word cloud diagram of the article titles included in each of five sustainability categories and the multidimensional category. Frequently used words appear larger relative to less frequently used words. Figure 5 provides a visual representation of the key themes for each category. A summary of key trends in the five sustainability categories and the multidimensional category include:

• Energy and atmosphere: Articles focus on investigating the efficacy of on-site renewable energy at various case study airports. Common sustainability indicators are total energy consumed and mass of GHG emissions from energy consumption. Best practices are considered as: monitoring of energy consumption; utilization of energy efficient HVAC equipment and lighting; installation of on-site renewable energy. There is some effort, particularly in the ACRP literature, to evaluate best practices from a practical perspective (e.g. addressing the safety implications of PV installations). Use of LCA in this category is limited.
• Comfort and health: Most of the research is focused on indoor comfort and health indicators like preferences for thermal and lighting conditions and concentrations of PM, VOCs, CO, and CO₂. Studies on exposure to ambient air pollutants from non-aircraft sources are limited. Most of the research on ambient air quality aggregates emissions from all sources. There is recent effort to investigate the impact from non-aircraft sources such as APUs, GSE, and GPUs and to identify possible solutions for these equipment (e.g. use of external electrical power and air conditioning units).
• Water and wastewater: Articles focusing on estimating the potential utilization of alternative water sources at airports dominate. Water quality research pertains to impacts from stormwater and de-icing fluids. A typical article in the Water and Wastewater category includes annual water consumption per passenger or flight operation. There is discussion in the literature on whether a disaggregated metric (e.g. indoor water consumption per passenger, outdoor water consumption per passenger) might be a more effective performance indicator.
• Site and habitat: This category is the least explored in the literature. Few articles offer measurable indicators, with most of the quantifiable metrics relating to land use efficiency and destruction of wildlife habitat. There is need for quantifiable indicators for research in on-site, public/private transport and for climate change adaptation practices.
• Materials and resources: Research on the environmental sustainability of airfield pavements dominates this category. LCA is the most frequently used assessment methodology, with life-cycle GHG emissions and energy consumption the most common assessment metrics.
• Multidimensional: Research that investigates airport sustainability from a multidimensional perspective is grouped according to efforts by ACRP and by the academic community. ACRP largely defines environmental sustainability across the five categories (i.e. energy and atmosphere, comfort and health, water and wastewater, site and habitat, materials and resources), but often focuses on economic and practical factors of implementing sustainability best practices. These best practices are often identified through interviewing and surveying U.S. airports. Sustainability indicators in the academic literature predominantly focus on energy consumption and GHG emissions. Sustainability is assessed with a number of methodologies (e.g. utility-based theories, cost-benefit analysis, LCA), suggesting that within the academic community there is a lack of consensus on what attributes and indicators make an airport sustainable.

Zoom In Zoom Out Reset image size

This section reviews the application of the SFO environmental sustainability framework on an existing infrastructure project at the airport.

3.2.1. Selection of case study airport

3.2.2. Development of sustainability indicators

3.2.3. Implementation of indicators

The indicators are intended to be used for the planning, design, construction, and operation/maintenance phases of airport facilities. An additional level of evaluation is applied to each 'Expanded Requirement.' Requirements are rated as 'Baseline,' 'Baseline Plus,' or 'Exceptional Project Outcome.' Per the previous 'Expanded Requirement' example, 'Baseline,' 'Baseline Plus,' or 'Exceptional Project Outcome' ratings would be given to 10%, 20%, and 30% reductions in GHG emissions, respectively. Such a rating system allows SFO to discern between project outcomes that are more 'sustainable' than others.

The results of an analysis of the projected reduction in annual GHG emissions per square meter from implementing Energy and Atmosphere 'Expanded Requirements' in SFO's Terminal 1 project are shown in figure 6. The specific 'Expanded Requirements' include practices that rely on reduced natural gas and electricity consumption in terminal buildings (e.g. energy-efficient escalators, dynamic glazing, radiant heating and cooling). It is projected that these 'Expanded Requirements' will reduce Terminal 1's energy use intensity (EUI). The EUI indicates how much natural gas and electricity is consumed by buildings. By converting the EUI to an equivalent amount of GHG emissions per square meter, it can be shown that the GHG intensity of the Terminal 1 project will be less than the average of other SFO buildings. The blue bars in figure 6 show the amount of GHG emissions per square meter, while the dotted outline indicates the amount of annual GHG savings per square meter in the Terminal 1 project. The GHG emissions account for the upstream processes related to natural gas provision and electricity generation. See appendix B for the complete methodology in producing figure 6. The savings represent an approximate 57% reduction relative to the average GHG intensity for all SFO airport building infrastructure.

Zoom In Zoom Out Reset image size

With few exceptions on airport energy (Kilkis and Kilkis 2017, Tagliaferri et al 2018), overall sustainability (Chester and Horvath 2009, 2012, Taptich et al 2016), and airfield pavements, much of the research fails to holistically analyze the environmental impacts through supply chains and regional variations. While the ACRP literature provides a sample representation of current best practices at airports, its analysis is sometimes limited by the responses it receives from case-study airports. For both the ACRP and academic literature, analysis of sustainability indicators is often limited by the scope of a case-study airport, so it is difficult to link research results with suggested practice or policy outcomes.

The literature in the Energy and Atmosphere category lacks a broader understanding of how much energy is used at different airports, what it is used for, and where it comes from. Current estimates are limited by the number of existing case-study airports. With an exception (Ozdemir and Filibeli 2014), the academic literature limits its characterization of GHG emissions according to Scope 1, Scope 2, and Scope 3. This limitation in the literature indicates that there is a slight disconnect between the academic research community and the airport industry and stakeholders as the Scope characterization is how the industry thinks about and manages GHG emissions. Research that investigates different energy sources (e.g. solar; bioenergy) and energy provision strategies (e.g. grid versus on-site storage) is just beginning, and more effort in this area is needed. Additional gaps in the research include:

• Environmental impacts of energy consumption in terms of other pollutants besides GHG emissions;
• Environmental assessment of airports and supply chains using local and regional models and data (Cicas et al 2007);
• Characterization and environmental impact assessment of energy consumption patterns for specific airport infrastructure and equipment by region (e.g. U.S. airport terminals are focused on food consumption; European/Asian airports serve as retail/recreational centers);
• Energy consumption impacts from construction of new airport expansion/retrofitting projects.

As with the Energy and Atmosphere category, research in the Comfort and Health category could be broadened to include more research and innovative and exploratory case studies. In light of COVID-19, more research is urgently needed to investigate how terminal building design and ventilation equipment might influence spread of infectious diseases. Ambient air quality research tends to aggregate sources, which makes it difficult to determine if mitigation policies are effective. Additional gaps in the research include:

• More human health-focused exposure studies related to operation of non-aircraft equipment, such as GSE, GPUs, APUs, and ground access vehicles;
• Investigation of air pollutant concentrations related to landside operations, such as passenger pick-up and drop-off;
• Research on human health impacts from airfield and terminal building maintenance, retrofit, and construction;
• Air quality impacts related to selection of different building materials and cleaning/daily maintenance procedures.

As suggested in the Water and Wastewater literature, assessing an airport's water consumption in terms of volume per day provides minimal insight. More research should be conducted to provide a thorough overview of disaggregated water consumption at the airport level so that sustainable practices can be implemented appropriately. A major gap in the literature is the complete lack of research into the linkage between water consumption, water quality, energy needed to convey, treat and heat water, and the resulting GHG and other environmental emissions and impacts. This water-energy nexus is particularly relevant in examining the environmental sustainability of using alternative sources of water at airports, especially with respect to potable versus non-potable demands and options.

Much of the literature in the Site and Habitat category lacks explicit, quantifiable sustainability indicators and there is vast room for investigation into the following gaps:

• Energy and environmental implications of constructing resilience infrastructure, such as sea walls and stormwater systems;
• Environmental impacts of onsite transportation systems, such as underground rapid transit systems;
• Overview of the types of suitable, environmentally efficient transportation modes within and outside of the airport boundary, which is dictated by airport configuration and location;
• Environmental trade-offs between site selection and terminal building orientation and layout of runways.

Research in the Materials and Resources category is predominantly focused on environmental impacts of airfield pavement construction and maintenance, with life-cycle energy consumption and GHG emissions as common metrics. Within the theme of airfield pavements, more research regarding innovative designs and maintenance techniques are warranted. There is a lack of understanding on what sustainable pavement practices can be implemented at airports of different operational capacities. Small and medium-sized airports might be good candidates for testing out innovative practices because their load or volume requirements tend to be smaller than those of larger airports. In terms of sustainable materials and design for airport buildings, research results are limited. In practice, it is more common for airports to strive for LEED certification of airport buildings. LEED, for practical purposes, is a relatively easy standard to implement, but is not sufficient for meeting quantified performance goals throughout the life cycle of airports. Additional gaps in the research include:

• Environmental impact of conventional and alternative construction materials in terminal building infrastructure;
• Sustainability impacts of supply chains and sourcing of airport construction materials;
• Deeper understanding leading to defensible actions on waste generation and waste management techniques at airports, especially in the context of waste-management policies such as 'zero-waste' and bans of single-use plastics.

A review of articles in the Multidimensional category indicates that there is no cohesive, agreed-upon definition of airport environmental sustainability. Gaps in the research include:

• Determining optimal methods for achieving overall environmental sustainability at an airport, also integrated with achieving specified city, regional-level, airline, or civil aviation targets;
• Integration of life-cycle, or holistic, thinking within a specified time horizon into decision making (e.g. should an airport implement an electricity-based strategy if the electricity is generated from fossil fuels?);
• Specifying environmental sustainability indicators in the context of airport operational safety;
• Investigating the overlap between environmental sustainability and airport resilience;
• Rigorous analysis of environmental sustainability and operational parameters;
• Integration of actions in achieving societal sustainable development (economic, environmental, social) with airport, airline, air traffic control, and in general, civil aviation goals.

A projected 57% reduction in annual GHG emissions per square meter from consuming natural gas and electricity on-site within the airport terminal buildings suggests that SFO's sustainability assessment indicators have the potential to be effective. A more meaningful expression of results would relate saved GHG emissions to the airport's level of service (e.g. GHG emissions per passenger or per revenue dollar). There are limitations to stating one airport's efforts as 'best practice.' It should be emphasized that applicability from the results of the case study are dependent upon local factors. For SFO, implementing energy-efficient strategies saves more GHG emissions because SFO's electricity is supplied from hydropower, which is less carbon-intensive relative to the state average. Utilizing low carbon-intensive energy is a key sustainability performance indicator. While post-facto analysis would be able to confirm actual GHG reductions from implementing 'Expanded Requirements,' the project is still ongoing. Some important observations can still be made regarding SFO's sustainability indicators.

In discussions with parties involved with the Terminal 1 reconstruction projects, having sustainability criteria at the outset of project development is crucial. All involved parties must be aware of their specific commitments. It is a good practice going forward for project contracts to incorporate strong sustainability performance indicators. SFO plans to integrate language more thoroughly into the Architectural and Engineering standards and guidelines that specifically align with two of SFO's guiding environmental priorities, namely climate change and human and ecological health. Regarding the former, the new contract language will explicitly require that decarbonization be reflected in project design and procurement. For example, instead of a voluntary consideration as part of an 'Expanded Requirement,' low-carbon structural steel would have to be selected as a building material.

The voluntary aspect of the framework (i.e. the 'Expanded Requirements') and the evaluation of 'Expanded Requirements' as baseline, baseline plus, and exceptional project outcome are rather subjective. Such subjectivity does not necessarily result in a completed project with the best environmental performance. Additionally, the SFO framework relies upon building codes that while they are 'state of the art' compared to building codes outside of California, represent a minimum standard. If interested in attaining a facility or project that meets a specified, quantifiable environmental outcome, the subjectivity of a rating system or checklist is not the most effective approach.

SFO's sustainability indicators do not explicitly consider the tradeoffs that potentially occur with prioritizing one criteria over the other; it is a rather static framework that could benefit from incorporating spatial and temporal factors. For example, electing to use a decentralized recycled water source (which is an 'Expanded Requirement' in the Water and Wastewater category) is sometimes an energy-intensive process which can result in increased GHG emissions while enhancing resilience. In this anecdotal example, there is a potential tradeoff between achieving water conservation and reducing GHG emissions. While the SFO framework might work well for an airport that explicitly prioritizes overarching goals (e.g. reducing GHG emissions and climate change impact), it might need to be reevaluated for airports that must equally consider sometimes conflicting environmental priorities.

The roadmap for future research of airport environmental sustainability emphasizes increased stakeholder involvement, more life cycle-based analysis, linkage of environmental impacts with operational outcomes, and addressing major challenges such as adaptation to climate change and mitigation of infectious diseases like COVID-19.

Airport environmental sustainability is often addressed at project scale. There is a need for investigating the larger role that airports have in impacting the environment, especially in the context of achieving city- and regional-level environmental outcomes that lead most directly to higher environmental quality of people and ecosystems. This ties in with stakeholder involvement because for sustainability indicators including GHG emissions, an airport only claims responsibility for Scope 1 and Scope 2 emissions. Airports often exclude ownership of Scope 3 emissions (e.g. emissions from an airline's GSE, without which there are no airports). The outcome of an airport excluding ownership of Scope 3 emissions is twofold: (1) it is more difficult to manage Scope 3 emissions, and (2) it is difficult to understand an airport's total GHG impact at the city/regional/state/national level, which is important for meeting larger-scale climate performance targets. Therefore, a broader analysis of how different stakeholders should be included in addressing environmental sustainability efforts is necessary.

Society faces important challenges such as adapting to climate change, mitigating the spread of pandemic-causing diseases, and enhancing environmental quality of people and ecosystems. An airport's role in addressing these challenges is largely undefined, but sure to be a significant one. It is imperative that thorough research on an airport's role in managing these challenges gets organized.