2.1. Service Quality

The importance of the service sector in the global economy cannot be overstated, and it is expected to remain the dominant force in the future. As a result, service quality has been a subject of research for nearly three decades (Stafford et al., 1999).

Some authors have defined service quality from different perspectivessuch asthe viewpoint of customers, companies, marketers and so on. Butler and Keller (1992) argue that the quality of services in the aviation industry can only be defined by customers. Fodness and Murray (2007) suggest that the quality of airport services can be evaluated based on the expectations and experiences of customers who use them. The airport industry has experienced rapid growth in recent times, with an increasing demand from users for high-quality services (Bogicevic et al., 2013; Chang & Yeh, 2002; Fodness & Murray, 2007; Prentice & Kadan, 2019; Su’arez-Aleman’ & Jim’enez, 2016; Tseng, 2020).

Passengers' perception of the physical environment is a crucial aspect of airport service quality, both on board an aircraft (Bogicevic et al., 2013) and on the ground. According to Fodness and Murray (2007), a clean and pleasant environment, security, courteous staff, and comfortable waiting areas are among the factors that contribute to passengers’ satisfaction with airport services.

The most widely used measure of service quality is SERVQUAL (Cronin & Taylor, 1992; Oh, 1999). Despite criticism from other researchers, SERVQUAL remains the most commonly used diagnostic model for evaluating service quality and developing service quality strategies. SERVQUAL has been adapted for use in various industries, products, and target markets, including airline services (Aksoy et al., 2003; Chen & Chang, 2005; Park et al., 2004).

In this study, the service quality of an airport can be defined as being measured by the performance of service delivery by comparing customer expectations and experiences.

2.2. Sustainability Practice

Sustainability integrates three pillars which are economic development, environmental protection, and social progress (Akerman et al., 2000). These pillars are interdependent and equally important (Lubk, 2017).

Numerous studies have examined airport sustainability from various perspectives, including general sustainability, energy supply, greenhouse gas emission reduction strategies, air quality impacts, water management, ground transportation, sustainable construction, and waste management (Carlucci et al., 2018). Therefore, the definition of airport environmental sustainability varies in the academic literature, with some defining it based on multiple categories of environmental impacts and others limiting this definition to the traditional environmental impacts of aviation.

Airports consume water for indoor and outdoor operations. To reduce water consumption, they can use water-efficient fixtures, reduce irrigation needs, and explore alternative water sources such as rainwater harvesting, wastewater reclamation, greywater reuse, and seawater reuse, as already explored by Zaki et al. (2021).

Airports also have social impacts on communities. The adoption of social sustainability practices varies among small hub airports and focuses on four categories of stakeholders: passengers and travellers, employees, communities and local businesses, and concessionaires and tenants (Marete et al., 2021).

Currently, various institutions propose nuanced definitions of airport sustainability, with different interpretations of priority and scope in specific contexts. However, it is worth noting that all efforts in this field are aimed in the same direction.

Based on prior research, in this study, the airport's sustainability practice was considered to provide environmental, social, and economic services without threatening the vitality of the local community's environmental and social systems within the airport's limited capacity.

In recent years, there has been an increased focus on service quality and sustainability practices in both business and academia. Chaudhary and Dey (2021) found a strong relationship between service quality and sustainability practices. The study's results demonstrated the impact of students' perception of service quality on their perception of the university's sustainability practices. This study suggests that students believe higher service quality leads to better implementation of sustainable practices at universities, including support services, waste reduction, community outreach programs, and government partnerships. As well as Syapsan et al. (2019) found that service quality had a positive and significant impact on sustainable economic growth.

Steffen et al. (2019) presented a conceptual model that explains the drivers of consumer trust and satisfaction in organic supermarkets through sustainable practices. The study confirmed that sustainable practices of producers and retailers, retailers' reputation and image, product attributes, claims, labels and brands, and peer information have a positive effect on trust. It has been confirmed that companies can build trust and satisfaction by acting on clearly communicated sustainability claims.

2.3. Brand Trust

Mogan and Hunt (1994) conceptualized that trust is considered to exist when one party has confidence in the trustworthiness of the exchange partner. Also, trust is characterized as a willingness to rely on another (Moorman et al., 1993).

According to other researchers, brand trust is the belief that consumers have in a brand, which leads them to rely on the brand of a product prior to making a decision. It has been shown that brand trust reduces uncertainty, especially when there is a lack of clarity about product information prior to purchase (Chaudhuri & Holbrook, 2001). Products with brand trust fulfil consumer expectations and provide a sense of safety in brand perception (Elena & Jose, 2001).

Various studies establish the association between brand trust and brand image. Morgan and Hunt (1994) state that trust is a key factor in the creation of a long-term commitment to a brand. Brand image can be enhanced by focusing on developing consumer-valued innovations for customers. A positive brand image plays a crucial role in building trust in the brand, which in turn increases brand equity (Khan et al., 2021; Khan, 2022). A strong brand image inspires consumers to have confidence in the quality of the products they are buying, helps them to make their decisions and allows them to feel at ease when they are doing so (Raza et al., 2020). During the process of building a brand image, brand equity can be acquired (Bello & Md Nor, 2021). Customers' positive perceptions of a brand are an indication of a high level of trust in that brand. In fact, brand trust is the principal factor influencing brand image, as it is believed to be the pathway to certain marketing benefits and consequences (Khan & Fatma, 2023b; Fatma & Rahman, 2017).

Thus, there are still many studies about band trust and brand advocacy. It is absolutely vital for an organisation to build trust with their brand to have a positive impact on customer advocacy towards the brand (Kemp & Beccera, 2014). When customers trust the brand and it leads to reformation to the customers, which is stated as brand advocacy (Bhati & Verma, 2020). Customers participate in advocacy only when they trust the brand and believe that the brand meets their perceived expectations (Fatma & Khan, 2023a).

2.4. Brand Image

Brand image is defined by Keller (1993) as the perception of brand associations retained in the consumer's memory. These associations relate to any aspect of the brand in the consumer's memory (Aaker, 1996). In addition, brand image includes the consumer's thoughts and feelings about the brand (Roy & Banerjee, 2007). In other words, brand image is the overall mental picture that consumers have of a brand and its uniqueness that distinguish it from other brands (Faircloth, 2005).

The positive brand image could be built by marketing programs which creates powerful, positive and unique associations with the brand in the consumer's mind. It can be produced through various channels, such as the direct experiences of consumers, their opinions and World-of-Mouth of other consumers, in addition to the information controlled by the marketing manager (Keller, 2003; Seo et al., 2020).

Based on the concept of brand image stated in this review, these authors are inclined to consider it as a multidimensional phenomenon in the airport industry.

When passengers pass through an airport, they have certain expectations of the infrastructure and evaluate the airport according to their perceptions-knowledge-emotions towards the products and services offered (Leisen, 2001). Similarly, airport image consists of information, beliefs, impressions, attitudes, and emotions based on an individual's experiences (Kotler et al., 1993).

Keller (1993) defines brand image as the overall impression that a consumer has of a brand, which is created by combining multiple associations related to the brand, and the perception expressed by the brand associations imprinted in the consumer's memory. In addition, brand image is a perceptual concept that a consumer has about a particular brand (Aaker, 1997) and can be expressed as the sum of the evaluations that a consumer has about a specific brand (Kotler, 1996).

2.5. Brand Advocacy

The definition of brand advocacy is the effort of brand advocates to share their experience of using a specific brand with other relevant people (Badrinarayanan & Sierra, 2018).

Brand advocacy is defined as the promotion or defence of an organisation, product or brand by one consumer to another and the highest level of endorsement of the relationship between the consumer and the brand (Walz & Celuch, 2010).

Endorsed by numerous scholars, Positive word-of-mouth and brand advocacy are considered to be two of the most effective and reliable communication tools of a brand (Keller, 2007).

Brand advocates refer to positive evaluations of the organisation's brand or product (Ahmadi & Ataei, 2024). Brand advocacy is identified as positive communication about a brand, this includes recommending it or defending it when it is attacked (Wilk et al., 2020). Brand advocacy is defined asthe proactive engagement of consumers who have had prior brand experiences, wherein they endorse a brand by recommending it to others or actively supporting the brand's initiatives (Badrinarayanan & Sierra, 2018).

Hence, brand advocacy can be characterized as a set of distinct activities aimed at supporting, defending and recommending a brand

2.6. Research Hypothesis

Based on the above factors, this empirical study examines how service quality and sustainability practices interact in the airport environment, focusing on Incheon International Airport (IIA), and their impact on brand image and brand advocacy by mediated brand trust.

Therefore, the following hypotheses are proposed:

H1: Service Quality has a positive effect on sustainability Practice.

H2: Services Quality positively influences Brand Trust.

H3: Sustainability Practice has positive effect on Brand Trust.

H4: Brand trust has a positive influence on brand image.

H5: Brand trust has a positive influence on brand advocacy.

3.1. Survey Design

Nineteen survey items were created for a quantitative approach, based on the research model presented in Figure 1 (Saunders et al., 2009). Table 1 shows the revised and supplemented itemsfrom previous studies on service quality, sustainability practice, brand trust, brand image, and brand advocacy. The questionnaires for this study were revised to use a 5-point Likert scale. The study used a non-probabilistic, self-participation sampling method based on Tarhini's guidelines (Tarhini et al., 2016). Twenty participants who had used Incheon International Airport at least once were provided with the questionnaire to participate in three times pilot tests. The questionnaires were modified to suit the topic of this study.

3.2. Data Collection and Analysis

The main survey format was collected from individuals who have previously used Incheon International Airport using the Google Survey Program. The survey was conducted over a two-week period starting from January 11, 2024, and 287 responses were obtained. SPSS version 25 was used to conduct various analyses, including descriptive statistics and internal reliability (Cronbach, 1951), as well as AMOS 23 for convergent reliability, discriminant reliability, model fit analysis, SEM path analysis, and hypothesis validation (Hair et al., 2011; Leontitsis & Pagge, 2007).

4.1. Demographic Profile

The demographic profile in Table 2 shows that out of 287 individuals, 60.3% were female and 39.7% were male. Regarding age, the largest age group of respondents was in the 30-39 range, accounting for 31.7%. The 40-49 range accounted for 30.3%, the 50-59 range accounted for 18.8%, and the 20-29 range accounted for 12.5%. In terms of the purpose of visits to Incheon International Airport, 49.1% of respondents visited one to two times annually, followed by 22.3% visiting three to five times. Departures and arrivals were the highest at about 79.1% for the purpose of visit, and business purposes were 13.6%. Incheon International Airport was the airport where the highest percentage of time was spent, with 44.3% of passengers spending two to three hours there. This was followed by 32.1% of passengers spending one to two hours, 13.6% spending more than three hours, and 10.1% spending less than one hour.

4.2. Confirmative Factor Analysis

Confirmatory factor analysis (CFA) was used to verify the factor structure among the observed variables presented in Table 2. To analyse the relationship between observed and latent variables, we used the squared multiple correlation to determine the proportion of variance accounted for in the latent variables. The squared multiple correlations ranged from 0.398 to 0.716. Furthermore, Cronbach's alpha is utilised to evaluate the internal consistency and reliability of observed variables. All Cronbach values have been deemed acceptable (Cronbach, 1951; Santos, 1999).

AVE is the measure of the amount of variance captured by a construct in relation to the amount of variance due to measurement error. The threshold values for these parameters are as follows: AVE should be 0.5. However, an AVE equal to 0.4 can be accepted if the composite reliability is higher than 0.6 for that specific construct (Fornell & Larcker, 1981; Rahman & Al-Emad, 2018). Furthermore, our measurement model assessment involves analysing convergent validity through AVE (Hair et al., 2014). In Table 4, this illustrates that the values of Cronbach’s alpha (CA) exceed the recommended threshold of 0.7 (Hair Jr et al., 2021; Henseler, 2017; Murè, 2021). The CR values are also above 0.7 or 0.8, which is considered adequate (Cachón-Rodríguez et al., 2020; Hair Jr et al., 2021; Murè, 2021).

The model fits by confirmatory factor analysis were assessed, and the results are presented in Table 5, ensuring the sustainability of this model.

As shown in Table 4, the goodness-of-fit test performed on the hypothetical SEM research model proved the measurement indices can reflect the characteristics of five latent variables. Therefore, the SEM goodness of fit which includes service quality, sustainability practice, brand trust, brand image and brand advocacy revealed that all fit indices fall within acceptable ranges.

Modifying and deleting a few observed variables, the goodness of fit of the model becomes a little bit decent. That is, CMIN/df = 3.781, meeting the standard of accepting range between 1 and 4. RMR=0.039, complying with the acceptable range of being smaller than 0.05. GFI = 0.842, close to the acceptable excellent goodness of fit 0.9 index. AFGI = 0.777, close to the acceptable excellent goodness of fit 0.9 index. RMSEA = 0.099, which is slightly above the acceptable range of being smaller than 0.08. In addition, the other index shows acceptable goodness-of-fit test results. NFI = 0.835, close to the acceptable excellent goodness of fit 0.9 index. TLI = 0.838, close to the acceptable excellent goodness of fit 0.9 index. IFI = 0.873 very close to the acceptable excellent goodness of fit 0.9 index.

Except for AGFI, RMSEA which falls slightly short of the standard value of 0.9, all other indices meet the criteria. Therefore, this research can be deemed appropriate (Bae & Park, 2021; Lee et al., 2022).

4.3. Structural Equation Modelling Analysis

Path analysis, in Figure 2 and Table 5, shows the main results of the structural equation model providing coefficient estimates, construct reliability and p-value etc. First, the hypothesis connecting Service Quality to Sustainability Practice (H1), with the values of β = 0.694 (C.R. 8.623, p<.001) was supported. Thus, it was demonstrated that service quality has a significantly positive effect on sustainability practice. Secondly, the hypothesis connecting Service Quality to Brand Trust (H2), with the values of β = 0.456(C.R. 5.251, p<.001) was supported. Therefore, it was shown that service quality has a significantly positive influence on brand trust. Thirdly, the hypothesis connecting Sustainability Practice to Brand Trust (H3), with the values of β = 0.406 (C.R. 4.917, p<.001) was supported.

Thus, it was proven that brand trust has a positive and significant influence on brand advocacy. According to hypothetical test results, among all the paths tested, the relationship between brand trust and brand image showed the highest construct reliability of 11.721.

As Figure 2 demonstrates, the effect of service quality perceived by airport users on sustainability practice and the effect of service quality and sustainability practice on brand trust were examined. As a result, the higher the perception of service quality, the more positive the sustainability practice was. In addition, both service quality and sustainability practice were found to have a significant effect on brand trust. Brand trust was found to have a positive effect on the relationship between brand image and brand advocacy. The higher the evaluation ofservice quality and sustainability practice perceived by Incheon International Airport users, the higher the brand trust, and it can be understood that the affected brand trust has a more positive effect on brand image and brand advocacy. As shown in previous studies (Chaudhary & Dey, 2021), it was confirmed that the higher the perception of service quality, the more positive the sustainability practice was.

5.1. Conclusion

This study examined the mediating role of brand trust in the relationship between service quality and sustainable practices of Incheon International Airport on brand image and brand advocacy. The purpose of this study is to analyse how service quality and sustainable practices at Incheon International Airport affect the airport's brand image and brand advocacy in the context of the aviation service distribution sector. The mediator of brand trust was employed to examine the impact of service quality and sustainable practices on airport users' perceptions. These mediators are essential to understand the relationship between service quality and airport sustainable practices and brand strategy.

The main findings of this study are as follows. First, we found that airport service quality and sustainable practices have a positive impact on airport users' brand trust. This result emphasises the importance of not only improving service quality but also managing sustainable practices within the Korean airport industry. Sustainable practices can increase the brand trust of public organisations such as Incheon International Airport when they are more actively implementing sustainability practice.

Second, increased brand credibility has a positive impact on brand image and brand advocacy. This shows that airports should demonstrate service quality and sustainable management that is differentiated from other airports and should be planned and managed in the long term in order to increase their brand image and brand advocacy. When the user's experience with the airport is enhanced with a sense of trust, it will go beyond the role of the airport and create a strong and positive national image.

Thirdly, the airport's brand image needs to be further strengthened as a representative image of Korea, and a differentiation strategy needs to be implemented in the experience of airport users. It can be seen that the quality of airport services and sustainable management of airports increase the brand trust of airports, and the brand image of Korea is strengthened and endorsement is positively affected.

In summary, by analysing and validating the impact of service quality and sustainability practices on brand trust, image and advocacy from the perspective of Incheon International Airport users, this study laid the foundation for providing practical data for airport brand improvement planning and operations. These findings will contribute to the development of related research and help to find ways to enhance positive country image and improve national brand value.

5.2. Implications

Based on these findings, we can draw several implications for both academic research and managerial practices.

This study provides a theoretical foundation for understanding the influence between brand image and advocacy through the mediation of the variables of service quality, sustainable practices, and brand trust from the perspective of airport users. Thisis an under-researched area within the Korean airport industry. This study found that social and environmental activities at airports have a positive impact on airport growth by increasing brand image and brand advocacy. Given the fact that other airports in Korea, except Incheon International Airport, are staying in their traditional roles, the authors believe that this study can provide a theoretical basis for the development of regional airports.

While many previous studies have focused on airport users' intention to return and service satisfaction, there is a lack of research that examines the impact of sustainable practices on brand advocacy in addition to airport service quality. To the best of the authors' knowledge, thisisthe first study to consider the impact of sustainable practices of airports on brand advocacy using the parameter of brand trust. The proposed conceptual framework contributes to the development of related research and fills a research gap left by existing studies.

The findings of this study hold meaningful insights for managerial practices within the airport industry.

First, airport management should understand how airport users evaluate the airport's brand and have a favourable brand image of the airport in order to prepare and manage strategic plans. In order to improve the quality of service as an airport brand, it is important to pay attention to the education and training of employees. Therefore, airports should strengthen the education of employees on service delivery skills and attitudesto improve the quality ofservice perceived by customers.

Second, it emphasizes the importance of strengthening social, environmental, and economic activities in the sustainability practices of airports. Public organisations, especially airports, should strive for long-term sustainability and demonstrate their commitment to environmental, social, and governance issues. Airports are no longer solely responsible for handling arrivals and departures, but are also evolving into cultural and social complex. This perspective views airports as a cycle of distribution. Airports are working towards creating a distribution ecosystem for the aviation industry by collaborating with SMEs (small medium size enterprises), strengthening technology, promoting a fair culture, supporting sales, and resolving regulatory difficulties for airport partners. This study could have a positive impact on the airport's image as a brand representing the country and expand its distribution network by gaining brand advocacy. Today, Incheon International Airport has a wide range of comprehensive sustainability practices, but it lacks a marketing component. Collaborating with various stakeholders, such as the local community, airlines, and environmental organisations, can further enhance the effectiveness of sustainable practices.

Such a strategy would have an impact on improving the airport's brand image and advocacy.

In summary, this study provides an understanding of how airports can achieve service excellence and sustainability practices. The experience of these airports will not only enhance the brand value of the airport, but also contribute to a positive national image.

5.3. Limitations and Future Research

This study examines the relationship among airport service quality, sustainable practices, and brand trust, image, and advocacy. However, there are limitations to the study. The focus on a specific region may restrict generalizability, neglecting diverse cultural and economic contexts.

Future studies should encompass broader contexts. Additionally, the study primarily gauged customer perspectives, overlooking organisational dimensions within airport organisations. Future research should investigate the interplay between internal processes, leadership, and organisational culture with brand performance. Further research can explore how specific service quality elements and sustainability impact brand trust to inform more targeted strategies. Furthermore, differentiating brand trust and image among airport user characteristics, such as business and leisure travellers, can guide tailored strategies. Finally, the current study does not consider the evolution of brand trust and image over time in the dynamic aviation industry. Future research should adapt to changing industry trends and explore strategies for sustained success amidst technological and environmental changes.