Bridging Knowledge Gaps and Charting Future Directions in Urban and Industrial Air Pollution Research

Air pollution remains one of the most pressing environmental challenges of our time, particularly in urban and industrial settings, where high population densities and concentrated anthropogenic activities intersect. In cities, the primary contributors to airborne pollutants include vehicular emissions, residential heating, and various combustion-related processes. As industrial areas undergo technological transformations, they continue to emit significant quantities of regulated and emerging pollutants. The complexity of these environments, combined with the intricate chemical and physical behaviour of atmospheric contaminants, underscores the necessity of ongoing and innovative research. This Special Issue Air Pollution in Urban and Industrial Areas II, along with the forthcoming Volume III, gathers diverse contributions that collectively advance our understanding of pollutant sources, chemical characterization, exposure assessments, and mitigation strategies in these dynamic contexts.

In recent years, substantial progress has been made in pollutant monitoring and exposure assessments, particularly in the realm of fine particulate matter (PM_2.5). Exposure to PM_2.5 has long been associated with cardiovascular and respiratory morbidity and mortality, as demonstrated by large-scale epidemiological studies such as Burnett et al. (2018) [1], which emphasize the global public health burden of air pollution. Yet despite the strength of these associations, significant gaps remain in our understanding of the composition of pollutants at the microscale and its differential impacts on vulnerable populations.

Technological innovations have enabled a higher spatial resolution in air quality monitoring, allowing researchers to uncover pronounced intra-urban gradients that were previously masked by sparse monitoring networks. Apte et al. (2017) [2] leveraged mobile platforms to map the street-level variations in PM concentrations across urban environments, revealing how factors such as the street canyon geometry, traffic flow, and localized emissions can generate dramatic differences in exposure even within small spatial areas. Meanwhile, the integration of satellite remote sensing and machine learning has expanded the capacity to monitor air pollution in regions with limited ground-based data. Zhang et al. (2019) [3] demonstrated how satellite-derived PM estimates can be used to track the long-term exposure trends in rapidly urbanizing areas of China, contributing to a more comprehensive global assessment of air pollution.

One particularly notable development highlighted in the current volume is growing recognition of non-exhaust vehicular emissions as a substantial source of urban air pollution. While regulatory efforts have significantly reduced tailpipe emissions, other sources such as brake and tire wear continue to release a variety of particles into the atmosphere. Ingo et al. (2022) [4] provided compelling evidence that braking systems in trafficked areas are a dominant source of inhalable airborne magnetite particles—an emerging class of pollutants with potential neurotoxic effects. These findings challenge the conventional source apportionment models and call for regulatory frameworks that account for the full spectrum of vehicular emissions.

The interplay between meteorology, chemical processes, and seasonal variability adds further complexity to urban air pollution. Chang et al. (2023) [5] investigated the secondary organic aerosol (SOA) formation in Beijing, showing how photochemical aging during different seasons affects the abundance and toxicity of atmospheric particles. Similarly, Huang et al. (2020) [6] examined the PM_2.5 components during haze episodes in Nanjing, shedding light on the rapid evolution of secondary species under stagnant conditions. Such studies not only enhance our mechanistic understanding of pollution formation but also inform predictive models and early warning systems.

Industrial emissions, while traditionally associated with legacy sectors such as steel, cement, and petrochemicals, are now also increasingly linked to novel processes related to the circular economy. As waste recycling, biomaterial production, and renewable energy systems grow in prevalence, their localized impacts on air quality remain insufficiently studied. This Special Issue reflects this emerging trend, with multiple contributions exploring the aerosol characteristics of biomass-based production, the emission profiles of recycling plants, and the potential for co-benefits in air quality and climate mitigation. The link between air pollution and climate change and its perception by citizens are analysed in Stanisci et al. (contribution 1).

Moreover, there is a discernible shift in the literature toward characterizing not just the mass of pollutants but also their toxicity and biological relevance. While the regulatory standards have traditionally focused on mass-based thresholds for pollutants like PM₁₀ and PM_2.5, recent studies emphasize the importance of particle numbers, surface area, and chemical composition in driving health outcomes. For instance, a previous work by Querol et al. (2021) [7] analysing the impacts of COVID-19 lockdowns in Spain revealed a stark reduction in traffic-related emissions, offering a natural experiment that reinforced the contribution of vehicular activity to urban PM levels. At the same time, it underscored the persistence of industrial emissions during reduced mobility periods, highlighting the need for integrated policy approaches.

Health impact assessments are also becoming more nuanced, incorporating stochastic modelling techniques to capture the variability in exposure and susceptibility. Otu et al. (contribution 2) explore the patterns of exposure to air pollutants in urban centres by innovatively integrating an air pollution exposure index and a social analysis. Ghosh et al. (contribution 6) applied Monte Carlo simulations to assessing the cancer risk posed by mobile source air toxics, accounting for the demographic and behavioural differences across population subgroups. Such approaches represent a significant methodological advance over deterministic models and provide a more realistic picture of air-pollution-related health disparities.

Another area receiving increasing attention is the characterization of semi-volatile organic compounds (SVOCs), particularly those emitted indoors from building materials, consumer products, and combustion appliances. Despite the fact that people spend the vast majority of their time indoors, indoor air quality has historically been understudied relative to outdoor pollution. The inclusion of studies of SVOCs and their transformation products in this Special Issue expands the conversation to encompass all microenvironments of exposure.

In reviewing the full range of the contributions to Volumes II and III, several thematic patterns emerge. There is notable emphasis on pollution events and episodic emissions, such as those arising from industrial fires, which pose acute health risks and require swift emergency response capabilities. This topic is reviewed in full detail by Deary and Griffiths (contribution 7). Using a similar approach, Gearhart et al. (contribution 5) report the concentration of fugitive dust emitted by scrap metal processing and its composition, while Wang et al. (contribution 3) record the volatile organic pollutants emitted during a wildfire episode.

Another trend involves monitoring secondary pollutants like ozone, particularly in mixed urban–rural zones, where the photochemical reactions differ from those in dense city centres. Petrus et al. (contribution 4) explore the ozone concentrations in an urban environment, with specific emphasis on the role of trees in mitigating pollution levels. Furthermore, studies on microplastic and fibrous emissions from synthetic turf fields and other recreational surfaces reveal novel exposure pathways that are just beginning to be understood.

Some of the most innovative contributions explore the environmental and health implications of biochar and other biomass-derived particles, linking agricultural practices, soil amendment strategies, and air quality outcomes. These studies exemplify the systems-thinking approach that is increasingly necessary to address the interconnectedness of environmental challenges. Green infrastructure is also gaining traction as both a pollution mitigation and urban adaptation strategy, with several papers critically examining its design, implementation, and co-benefits.

Despite these advancements, considerable work remains. Standardized methods for assessing the toxicity of ultrafine and nanometre-scale particles are still lacking. Moreover, the integration of indoor and outdoor exposure models remains underdeveloped, limiting our ability to estimate total personal exposure with high fidelity. Emerging industries driven by decarbonization goals may inadvertently introduce new forms of air pollution unless their emissions are carefully monitored and managed. This highlights the importance of conducting life-cycle assessments that incorporate local air quality impacts alongside global climate metrics.

Looking forward, future research must continue to bridge disciplinary boundaries. Atmospheric scientists, public health experts, engineers, and urban planners need to cooperate in designing monitoring systems, interpreting complex chemical data, and developing actionable policy recommendations. Community-based monitoring initiatives and citizen science could also play a crucial role in increasing public engagement and democratizing access to air quality data. Lastly, there is a pressing need for regulatory evolution. As our understanding of air pollution becomes more granular and sophisticated, so too must our governance frameworks evolve to reflect the complexity of the sources, chemical transformations, and human behaviours that drive exposure.

This Air Pollution in Urban and Industrial Areas series, through its second and forthcoming third volume, reflects a maturing field that is increasingly capable of dealing with these complexities. It offers a platform for high-quality, interdisciplinary research that addresses both foundational questions and emerging challenges. By fostering dialogue among researchers, policymakers, and practitioners, this body of work lays the groundwork for the next generation of air quality science—one that is data-rich, socially responsive, and globally informed.