Evaluation of Contemporary UAV-Based Measurement Techniques for Gas Emissions Monitoring

With global environmental concerns reaching new heights, monitoring and quantifying gas emissions have become paramount. UAVs, due to their flexibility and adaptability, have emerged as a pivotal tool for this purpose. This paper endeavors to provide a comprehensive review and evaluation of the challenges and modern measurement techniques utilizing UAVs for gas emission assessments. Through a systematic study of relevant publications and technical reports, we review the current state-of-the-art systems and their applications in research and field missions. Additionally, we present a detailed discourse on the identified weaknesses and specificities of these UAV-based systems, elucidating potential avenues for enhancement.


INTRODUCTION
Amid ongoing environmental challenges such as ozone depletion, the greenhouse effect, climate change, and air pollution, concerns about environmental sustainability and air quality have gained significant attention.Accurate measurement of gas emissions is crucial for shaping policies, industrial regulations, and understanding human impact on the environment.Transportation is a major contributor to greenhouse gas emissions, accounting for approximately 29% of total U.S. emissions and a significant share in Europe as well [1].Emissions in the transportation sector have increased significantly over time.Shipping and maritime transport, while energy-efficient in terms of fuel consumption per transport project, still emit gaseous pollutants and greenhouse gases.To combat air pollution from shipping, the International Maritime Organization (IMO) and the European Union have imposed strict rules and monitoring procedures, which have a direct negative impact on the operation costs of ships creating also strong incentives to break the law when possible.
Traditional static ground-based methods for measuring emissions have limitations in terms of accessibility, coverage, and realtime monitoring.Enforcing environmental rules through on-board inspections is challenging and costly, limiting effectiveness.Aerial surveillance, particularly using Unmanned Aerial Vehicles (UAVs), offers an efficient approach to detect polluters and non-compliant vessels.UAVs, with their flexibility and precision, are transformative in environmental monitoring, capable of navigating challenging terrain and collecting data from previously inaccessible locations.
The IERAX project, co-funded by the Greek Ministry of Development and the European Commission, aims to create advanced portable exhaust and environmental sensors for UAVs.It seeks to establish efficient aerial emission monitoring systems, ensure optimal sampling conditions, and develop comprehensive systems for addressing air pollution from ships and monitoring compliance with international and European standards.
This paper provides a review of the state-of-the-art in this field and outlines the architectural design of a system that addresses key challenges and meets specific performance indicators systematically listed in the remainder of this manuscript.

METHODOLOGY
To address the objectives outlined, this paper explores recent advancements in UAV-based measurement techniques for monitoring gas emissions.The subsequent sections will offer readers a comprehensive overview of the state-of-the-art techniques, the intricacies of their application, and insights for further research and technological enhancement.This study seeks to navigate the expansive realm of UAV-based measurement techniques tailored for gas emission assessments.Primarily, our aim is to furnish a thorough review of the current methodologies, shedding light on their technical intricacies, advantages, and inherent limitations.Through examining real-world applications and field studies, this paper aims to highlight the practical challenges and advantages of UAV-assisted air pollution monitoring.It also seeks to distill key insights from existing literature to inform future research and applications.The ultimate goal is to provide a clear and useful overview of UAV-based gas emission measurement systems for both researchers and industry practitioners in today's environmental monitoring domain.
Our research methodology embraces a multi-faceted approach to comprehensively evaluate UAV-based techniques for gas emission assessments.We begin with a systematic literature review to distil knowledge on current UAV measurement systems, ensuring a solid understanding of state-of-the-art methodologies.Following this, we conduct a case study analysis on selected real-world applications and pilot projects, gaining practical insights into the challenges and benefits of deploying UAVs for environmental monitoring.
Through this rigorous exploration of UAV-based gas emission measurement techniques, we anticipate several key outcomes.Firstly, we expect to provide a comprehensive delineation of the current landscape, presenting a clear snapshot of the state-of-the-art methodologies and their practical implications.Furthermore, by examining various case studies regarding maritime drone monitoring, we aim to identify common challenges and bottlenecks, providing a realistic view on the operational aspects of UAV-based monitoring in this domain.Ultimately, this study strives to furnish actionable recommendations, catalyzing further research, and fostering advancements in UAV-assisted environmental monitoring, ensuring more accurate, efficient, and expansive gas emission assessments in the future.

CURRENT LANDSCAPE OF UAV-BASED SHIP EMISSIONS MONITORING AND COMPLIANCE
Research that mentions the use of UAVs, equipped with specialized sensors, to monitor emissions from ships has only appeared in the last few years.There are several innovative approaches to addressing this challenge, and some research efforts under consideration are described in this section.In the study of Deng et al. [2], UAVs equipped with special sensors detected Sulfur Dioxide (SO2) emissions by flying near ships.Signal processing techniques were used to analyze the results.The study concluded that realtime monitoring of ship exhaust emissions using sensor-equipped UAVs is feasible, with the aim of improving ship compliance with environmental regulations.The use of the models designed and implemented by Anand et al. [3], significantly improves the estimation of sulfur content in real time.They used an (UAV)-borne microsensor system (MSS) that is lightweight (750g) and capable of real-time measurements of various pollutants like SO2, NO2, NO, CO2, CO, and particulate matter.In the lab, the electrochemical sensor-based system showed a highly linear response to target gas pollutant concentrations.Field tests were conducted to measure the accuracy of FSC estimation, which ranged from 0.07% to 2.7%.These tests considered factors like plume dilution and air mixing due to the UAV's propeller movements.The results showed a high level of agreement between the UAV sensor and reference measurements, indicating that the system is robust.
Another research involved monitoring emissions using UAVs and diffusion models for remote estimation of sulfur content in fuel oil used by ships in Emission Control Areas (ECAs) [4].Meteorological conditions were considered to simulate how the SO2 will disperse in the atmosphere.This gives a theoretical SO2 diffusion concentration at the location of the monitoring equipment.The real-time SO2 concentration, monitored by an observation device, is then integrated with the theoretical SO2 concentration to estimate the ship's FSC.The proposed technical approach achieved a 46% improvement in accuracy to traditional methods.The method could reliably detect ships using fuel oil with an FSC greater than 0.146 %m/m.
The study of Hu et al [5], focused on the compliance rates of ship fuel content in the Pearl River Delta of China, a busy sea area vital to shipping and trade.UAVs equipped with mini-sniffing sensors to monitor air pollutants emitted by ships, specifically targeting sulfur oxides (SO2) and carbon mono/dioxide (CO/CO2).Collected data during the UAV flights showed a compliance rate of approximately 92% among monitored vessels, suggesting a promising method for monitoring compliance with emission regulations.The study also compared its findings with other studies conducted in the North Sea region of Belgium, concluding that the compliance rates were similar, thereby validating the effectiveness of the Domestic Emission Control Area (DECA) policy.
The primary aim of this study was to measure the concentrations of SO2 and CO2 to estimate the Fuel Sulfur Content (FSC) of ships.The UAV was equipped with sensors for this purpose and is flown close to the ship's funnel mouth (about 5-10 meters away) to collect data.The study was conducted at the Waigaoqiao port in the Yangtze River Delta, an ECA in China.The UAV-based measurements were compared with laboratory tests of fuel samples.The deviation in FSC estimates was generally less than 0.03% (m/m), making the method reliable for preliminary judgments.In about 10% of the cases, the UAV missed the smoke plume, impacting the measurements.One year later, according to Zhou et al. [6], UAVs were equipped with gas sensors that measured SO2 and CO2 concentrations in ship emissions from sailing ships in open water, specifically in the Yangtze River estuary, a designated Emission Control Area (ECA) in China.Various algorithms were used to process the data to assess the ship's compliance with environmental regulations.The primary finding was that UAVs enabled scalability and cost-effective monitoring, revealing that some selected vessels exceeded permitted emission limits.This study highlights the potential of UAVs to enhance regulatory enforcement.
All the aforementioned studies highlighted the potential use of UAVs to monitor ship emissions and improve compliance with environmental regulations.Differences in methodologies were observed in diffusion models, direct sampling, and signal processing techniques.Some innovative approaches have shown promising results for future estimation of fuel sulfur content based on ship emission and IERAX is exploiting an on-board mutli-sensor design and calibrated for the ship-emission measurements.

TECHNICAL, ENVIRONMENTAL, AND OPERATIONAL CHALLENGES
Evidently, UAVs emerge as invaluable assets, offering unparalleled flexibility and access to previously unreachable vantage points in environmental monitoring.However, as with all burgeoning technologies, UAVs are not without their sets of challenges.These can be broadly classified into technical, environmental, and operational domains, each presenting unique hurdles that can influence the accuracy, efficiency, and feasibility of UAV-enabled assessments.Technical limitations often revolve around the very machinery and software that empower UAVs.From sensor accuracy to battery longevity, these challenges question the reliability and endurance of UAVs in prolonged or intricate missions.On the environmental front, the very nature that UAVs aim to analyze can pose significant obstacles.Weather patterns, temperature fluctuations, and even geographical features can markedly impact UAV performance and the validity of collected data.Operational challenges, perhaps the most multifaceted, encapsulate a range of issues from flight permissions and data management to safety protocols and economic considerations.Collectively, these challenges underscore the importance of a holistic understanding and approach in harnessing the full potential of UAVs in gas emission studies.

Technical limitations
4.1.1Sensor accuracy.UAVs offer exceptional accessibility, with the data quality hinging on their sensor precision.Contemporary UAV sensors use techniques like spectroscopy [7] and tunable diode laser absorption [8] for gas detection.Nonetheless, they might not always equate the performance of ground-based sensors, particularly with intricate gas scenarios.Calibration, influenced by factors like atmospheric changes and other gas interferences [9], remains critical.
Recent technological progress has improved ship gas emission monitoring, aiding environmental protection and maritime emission compliance.A notable photoacoustic gas detector focuses on SO2 emissions, utilizing electromagnetic radiation absorption at 285 nm with a UV LED and a MEMS microphone [10].In tests, it identified up to 1 ppm, showcasing its maritime relevance.
A UAV system with a mini-sniffing sensor is introduced for monitoring in emission control areas [2], addressing challenges linked with traditional sensors.Additionally, an integrated multi-rotor UAV system offers real-time emission insights and data transfer [11].
The growth in semiconductor lasers [12] has enhanced the TD-LAS technology, becoming a top choice for various non-invasive measurements across sectors.Danfoss IXA's Marine Emission Sensor employs UV absorption spectroscopy, differentiating emissions using a digital gas "fingerprint" database [13].This progress underlines the push for optimized ship emission monitoring, vital for environmental adherence and sustainable maritime operations.
4.1.2Battery life.One of the inherent constraints of UAV technology, irrespective of its application, is the limited battery life.In the context of gas emission measurements, this limitation can pose significant challenges.UAVs, equipped with advanced sensors and communication modules, often consume considerable power, thereby reducing the effective time they can remain airborne.While a typical commercial UAV may operate for 20 to 30 minutes on a single charge, comprehensive environmental surveys often demand prolonged periods of sustained flight, especially over expansive or remote areas.This battery constraint not only limits the geographical reach and duration of a single mission but also necessitates frequent landings and battery replacements or recharges.Such interruptions can lead to data gaps, reduce operational efficiency, and increase the chances of missing transient emission events.As the push for longer, more efficient UAV missions intensifies, the development and integration of more robust and longer-lasting battery technologies, innovative hybrid propulsion systems and the overall UAV energy management [14], [15] become focal points for ensuring uninterrupted, efficient gas emission assessments.

Payload capacity.
The payload capacity of UAVs directly impacts the range of instruments and sensors they can carry, shaping the scope and depth of environmental monitoring missions.Each UAV is designed with a specific maximum payload weight, beyond which its flight dynamics, stability, and battery life can be adversely affected.When it comes to gas emission measurements, the inclusion of advanced detection sensors, data storage units, and communication modules can quickly accumulate weight.As a result, researchers and practitioners often find themselves navigating a delicate balance: equipping the UAV with the necessary tools for comprehensive data collection while ensuring it remains within its payload limits.Addressing these payload limitations necessitates a strategic approach.Modifying larger UAVs to increase their payload capacity or employing advancements in UAV and sensor technology can provide viable solutions.For instance, leveraging UAVs with greater payload capacity allows for the integration of advanced or multiple sensors, enhancing the gas monitoring mission's comprehensiveness and accuracy [16].Moreover, the inherent trade-off between payload and performance, where heavier payloads could lead to reduced flight time and range, underscores the importance of continuous technological advancements.Future trends in UAV technology aimed at improving payload capabilities while maintaining or enhancing performance metrics are imperative for extending the scope and effectiveness of gas monitoring missions in marine environments [17].

Environmental factors
4.2.1 Harsh weather conditions and temperature as a factor.A significant and often unpredictable variable in UAV-based gas emission assessments is the weather.Despite their technological advancements, they remain highly susceptible to atmospheric conditions.High winds can jeopardize the stability and flight path of a UAV, potentially leading to skewed sensor readings or, in extreme cases, damage to the equipment.Rain and fog introduce moisture-related challenges, with the former posing direct threats to electronic components and the latter obscuring sensors and reducing visibility.Additionally, temperature extremes can impact battery performance, sensor calibration, and overall UAV operability.Not only the extremes but also temperatures fluctuations can cause baseline readings and calibration of these sensors to shift, potentially leading to erroneous measurements or requirement for frequent recalibrations during a mission.The physical properties of the gases themselves can also change with temperature, impacting their detectability and the accuracy of concentration estimations.Such weather dependencies not only question the reliability of data gathered under fluctuating conditions but also limit the operational windows available for monitoring.Ensuring robust and consistent measurements necessitates meticulous planning, with contingency protocols in place for unexpected weather changes, and, in some cases, the use of UAVs designed for resilience in specific environmental conditions.4.2.2Salt Spray and Corrosion.Marine environments are often characterized by salt spray which can lead to corrosion of electronic components and sensors on UAVs.This corrosion can affect the performance and longevity of UAVs and their payloads since saltwater corrosion is a significant challenge for electronic devices and mechanical parts.Utilizing corrosion-resistant materials, protective coatings, landing pads [18] encapsulation, and regular maintenance are critical strategies to mitigate these challenges and ensure the effectiveness and reliability of UAVs in maritime gas monitoring missions.Moreover, these protective measures underline the importance of design considerations and maintenance practices to preserve UAV functionality and extend their operational lifespan in corrosive maritime environments.

High Humidity.
The high humidity typical in marine environments can cause condensation on electronic components, potentially leading to malfunctions.Furthermore, some gas sensors are sensitive to humidity changes, which may interfere with accurate gas detection.Metal Oxide Semiconductor (MOS) sensors, commonly used for detecting gases like CO and volatile organic compounds (VOC), can experience resistance changes due to humidity.Electrochemical sensors, which operate based on chemical reactions, may have their electrolyte affected by moisture.Infrared (IR) gas sensors measure gas concentrations by detecting infrared light absorption, and while generally less affected, can still have their readings influenced under certain humid conditions.Photoionization Detectors (PIDs) used for VOC detection can see variations in their ionization process due to humidity.Catalytic bead sensors, employed for combustible gas detection, might experience reduced sensitivity or sensor poisoning in humid environments.Lastly, polymer-based sensors, which utilize a polymer layer for gas absorption, can undergo changes like swelling in the presence of moisture, affecting their response.While many modern sensors incorporate humidity compensation mechanisms, it remains essential to ensure their suitability and calibration in high or fluctuating humidity environments.

Gas Dispersion Patterns.
Gas dispersion patterns can be affected by harsh maritime conditions such as wind like sea breeze or onshore breeze and big fluctuations in temperature due to sea respiration, potentially leading to inaccurate readings.Understanding [19] gas dispersion patterns in marine environments is crucial for accurate gas monitoring.

Solar
Glint.The sun glint, the phenomenon of sunlight reflecting off water surfaces, poses a significant challenge to the performance of infrared (IR) cameras utilized for ship and fume detection in maritime environments.The intense reflection can cause a high level of background radiance within the camera's field of view, potentially obscuring or overwhelming the thermal signatures of ships and their emissions, thereby leading to reduced detection sensitivity or false negatives.Additionally, solar glint can lead to image saturation, where the intensity of reflection is so high that the sensor pixels within the IR camera become saturated, rendering them incapable of providing accurate intensity information crucial for differentiating between the target and background.This interference extends to the creation of hotspots, areas of significantly higher radiance, in the image, which can mislead automated detection algorithms and hinder accurate identification and tracking of ships and emissions.Moreover, solar glint reduces the contrast between the targets, whether ships or fumes, and the water background, which is critical for detection.Lower contrast can result in lower detection probabilities and higher false.alarm rates, posing challenges to both manual and automated monitoring efforts.[20], [21].

Operational challenges
Operating drones present, among other problems, challenges like collision risks in busy environments, signal interference near electronic equipment, and compliance with flight restrictions near sensitive areas.Flight Restrictions is one of these challenges.Integrating UAVs into airspace, particularly near populated or sensitive areas, presents regulatory and safety complexities.Aviation authorities worldwide establish flight restrictions to ensure safe UAV coexistence with other aircraft and protect critical infrastructures, wildlife, and the public.These restrictions vary, from no-fly zones around ports to altitude limits that confine UAVs vertically.Prior to any UAV operation, a thorough understanding of local regulations and restrictions is essential [22].UAV regulations typically cover three key aspects: (1) airspace utilization rules, (2) operational constraints, and (3) administrative procedures for flight permissions, pilot certifications, and data collection authorization.Operational guidelines differentiate restrictions based on flight types, including visual line of sight (VLOS), extended visual line of sight (EVLOS), and beyond visual line of sight (BVLOS).Pilots must ensure compliance with local regulations during all flights.Looking forward, standardizing UAV use in environmental research may lead to regional or global harmonization of UAV regulations.For researchers and industries using UAVs for gas emission assessments, these restrictions can present logistical challenges, require meticulous planning, and lead to delays.
Dynamic Targeting is another one.Monitoring emissions from moving ships presents challenges due to their constant motion.Unlike stationary sources, ships cover vast distances, varying in speed and direction.UAVs must have advanced tracking abilities and agility to adapt to erratic movements, often using Model Predictive Control (MPC) [23].Continuous monitoring in these conditions can stress the UAV's navigation systems, and minor miscalculations may lead to data gaps or skewed readings Furthermore, Ships frequently operate in distant sea regions, far removed from land-based control stations.This remoteness poses a significant hurdle for UAVs in terms of communication range and operational endurance.Establishing and maintaining stable communication links over vast marine expanses becomes challenging, risking data loss or reduced real-time control.Additionally, the longer flight times necessitate UAVs with extended battery lives or alternative power solutions.
Operating UAVs from ships poses inherent risks due to the confined space and the ship's constant motion caused by waves and tides.These conditions make landings and takeoffs challenging, with the potential for accidents and damage to both the UAV and the ship [24].
Also, Modern ships have numerous electronic devices, including communication, navigation, and machinery controls, creating significant electromagnetic interference.This interference can disrupt UAVs, affecting their navigation, data transmission, and sensor readings, especially when operating near large vessels.
Additionally, operating UAVs near ports, shipping lanes, or naval bases can raise security worries about potential misuse for espionage or unauthorized surveillance.This may result in stricter flight restrictions, requiring additional permits or clearances, and even barring access to specific strategic areas.
Finally, navigating UAVs in busy shipping areas poses a heightened risk of collision due to tall obstacles like cranes, ship masts, and smokestacks.These structures create a complex navigation challenge, necessitating precise planning and control to prevent accidents.Port operations with moving equipment and vessels further complicate flight paths.Emissions from ships or industrial activities can impair visibility and UAV sensors, leading to navigation errors.These emissions may also interfere with the UAV's electronics or propulsion, increasing the risk of mishaps.To address these challenges, advanced planning, robust control systems, and autonomous collision-avoidance technology are essential.Safety measures like geo-fencing, real-time monitoring, and automated avoidance systems are crucial for safer UAV operations in busy maritime port environments.

RECOMMENDATIONS AND FUTURE DIRECTIONS
This section outlines the recommendations and the future prospective pathways for UAVs employed in emission monitoring within maritime domains.The recommendations underscore the imperatives of ongoing research in sensor accuracy, battery longevity, and robust design tailored for maritime adversities, alongside the integration of Artificial Intelligence (AI) for real-time data analysis and adaptive navigation.

State of the art technology
In the rapidly evolving field of UAV-based maritime emission monitoring, addressing technical challenges is of utmost importance.
The recommendation [25] to invest in research for improving sensor accuracy and enhancing battery longevity [26] stands out, given these components' critical role in UAV mission efficiency and reliability.Such advancements and their performance [27] can lead to UAVs that are more resilient, offer extended flight times, and provide higher precision in data collection [28], reinforcing the trust in the data they capture.

Artificial intelligence integration
Equipping UAVs with adaptive technologies to navigate and execute environmental missions is a forward-looking approach that leverages the power of artificial intelligence (AI) to enhance operational efficiency and data accuracy in emission monitoring tasks.By integrating AI-based systems, UAVs can in real-time predict, adapt to, and navigate through challenging weather patterns that would otherwise hinder their performance or compromise the integrity of the collected data [29] and UAV's decision making capabilities [30] Moreover, by harnessing AI, UAVs can autonomously identify, track, and analyze ship movements in real-time over vast expanses of water.The integration of machine learning algorithms [31] can facilitate the recognition of distinct ship characteristics, enabling the UAV to differentiate between various vessel types and even detect anomalies or unauthorized vessels.This AI-driven approach not only augments the efficiency [32] and scope of maritime monitoring but also frees up human resources for other critical tasks.UAV.

Robustness
Designing UAVs [33] with robustness to withstand challenging maritime weather conditions is a critical consideration for ensuring reliable operations over water.A robust design encompasses the use of corrosion-resistant materials, waterproofing components, and aerodynamic structures that can endure high wind speeds and turbulent conditions.The integration of advanced sensor technology can further allow UAVs to accurately capture emission data even in adverse conditions, thus ensuring the integrity of the monitoring process.This robust design philosophy not only extends the operational life of UAVs but also ensures that emission monitoring activities in maritime environments are carried out accurately and efficiently, regardless of the prevailing weather conditions.By prioritizing robustness in design, UAVs can be optimized to navigate the maritime environment's unique challenges, thereby contributing significantly to the broader objective of environmental monitoring and compliance in these regions.
5.4 Take-off and landing procedures.
Advanced take-off and landing procedures in constrained and moving landing zones such as ships are crucial for ensuring the safety, accuracy, and efficiency of UAV operations.These specialized procedures cater to the unique dynamics of such environments, which are characterized by limited space, motion, and at times adverse weather conditions.By leveraging advanced technologies like realtime kinematic (RTK) GPS, automation, and computer vision, these procedures facilitate precise navigational control, enabling UAVs to accurately align with the moving decks of ships during landing [34] and take-off [35].Moreover, they allow for swift response to unexpected changes in wind speed or direction, which is vital for preventing accidents.Beyond safety, these advanced procedures also extend the operational capabilities of UAVs, enabling missions in complex maritime environments, thereby contributing to a broader range of applications such as surveillance, search and rescue operations, and environmental monitoring in offshore settings.Through enhancing safety and operational flexibility, advanced take-off and landing procedures significantly contribute to maximizing the utility and effectiveness of UAV technologies in constrained and dynamic maritime environments.

CONCLUSIONS
The growing use of UAVs in gas emission monitoring highlights their role in environmental studies, particularly in maritime contexts.They provide access to hard-to-reach areas but face issues like sensor accuracy in harsh environments, battery life, and payload limitations.However, integrating AI, especially for real-time data analysis and predictive modeling, can boost the accuracy and reliability of maritime gas emission detection.With AI, UAVs can quickly process large datasets, spot irregular emission patterns, and predict emission areas, leading to proactive monitoring.Still, external factors such as weather and operational complexities pose challenges.The role of AI-enhanced UAVs in environmental monitoring is pivotal.As tech advances, it's vital for researchers to tackle these challenges and for industry professionals to create reliable UAV systems.Collaborative work and AI's potential can transform gas emission monitoring in maritime settings.Projects like IERAX, focusing on AI, are vital for progress in maritime gas emission standards and practices.