The Asian Tropopause Aerosol Layer: Spatio-Temporal Characteristics, Trends, and Climate Effects

Abstract

During the South Asian Summer Monsoon (SASM), intense large-scale uplift and strong deep convective activity over South Asia lead to the formation of a high aerosol concentration zone in the Upper Troposphere and Lower Stratosphere (UTLS), known as the Asian Troposphere Aerosol Layer (ATAL), which appears from June to August. ATAL not only influences the exchange processes of material and energy between the troposphere and stratosphere, but also affects the global climate by altering radiation, cloud formation, and precipitation processes. Therefore, examining the spatiotemporal distribution and climate impacts of ATAL is essential for understanding climate change and evaluating the feasibility of geoengineering. This study systematically reviews research progress on the three-dimensional spatiotemporal distribution, trends, sources, and climatic effects of ATAL. Findings reveal a prominent aerosol layer at the top of the Asian troposphere, and the SASM region potentially serving as a critical conduit for constituents of the boundary layer to reach the stratosphere. However, simulated ATAL components differ significantly across models, particularly in terms of vertical distribution patterns. The precise three-dimensional structure and long-term evolution of ATAL remain unclear, presenting challenges for assessing its climate impact. To advance the understanding of the roles of ATAL in climate change, three future research directions are proposed.

1. Introduction

Since the Industrial Revolution, human society has experienced rapid development. The expansion of industrial activities, along with the growth of the transportation and energy sectors, has led to increasingly severe pollution problems [1,2]. Substantial emissions of pollutants have formed aerosols in the atmosphere, which not only harm human health and environmental quality [3,4,5,6], but also play a notable role in the climate system and global climate change [7,8,9,10,11]. The formation, transport, and transformation of aerosols contribute to the occurrence of haze events [12] and are considered significant short-lived climate forcers that contribute to global warming [13,14]. This impact is especially evident in climate-sensitive regions such as the Polar regions and the Qinghai-Tibet Plateau (QTP), where aerosols severely affect the glacial hydrological environment of the QTP and its surrounding areas [15,16].

Anthropogenic aerosols not only lead to severe pollution within the boundary layer, but can also be transported upward into the middle and upper troposphere and even into the stratosphere [17]. These pollutants are then distributed globally through long-range transport, exerting substantial effects on global climate change [18,19,20,21,22,23]. High concentrations of anthropogenic pollutants, such as black carbon aerosols and trace gases, have been detected in the Upper Troposphere and Lower Stratosphere (UTLS) regions [24].

Rapid economic development in South Asia has significantly worsened regional atmospheric pollution [25,26]. This region lies within the influence of the Asian Summer Monsoon (ASM) and is characterized by a complex surface and unique climatic conditions. In particular, the QTP, with its distinctive terrain, serves as a crucial channel for stratosphere–troposphere exchange and plays a key role in the upward transport of aerosols into the UTLS region [17,27]. The QTP profoundly influences atmospheric circulation through its strong thermal and dynamic effects, leading to a sharp descent of the tropopause along the northern side of the plateau [28,29]. This unique combination of topography and atmospheric circulation facilitates the entrainment of aerosols from the plateau into the UTLS. During the South Asian Summer Monsoon (SASM), strong vertical motions transport surface-level pollutants into the UTLS. The “trapping effect” of the South Asian High leads to the accumulation of these pollutants, effectively forming a central hub for aerosol distribution [30,31].

Asian Tropopause Aerosol Layer (ATAL) was discovered in the early 21st century [32]. Subsequent studies have revealed that anthropogenic emissions originating at the surface are transported upward into the UTLS region, where they accumulate to form the ATAL. Aerosols that enter the stratosphere have longer residence times and are more prone to dispersion across larger areas. Through their interaction with ultraviolet, visible, and infrared radiation, they play a significant role in the Earth’s radiative budget and climate change [33]. Simulations conducted for stratospheric geoengineering using sulfate aerosols or black carbon aerosols have shown that continuous injection of aerosols into the lower stratosphere can lead to significant temperature changes in the UTLS [34,35]. Evidently, the ATAL plays a crucial role in atmospheric circulation, radiation processes, and atmospheric chemistry, exerting a profound influence on the global climate [34,36].

The discovery of the ATAL has attracted considerable research attention, leading to extensive studies on its formation mechanisms and composition. Findings indicate that during the SASM, the combined effects of the South Asian Summer Monsoon Anticyclone (SASMA) and frequent deep convection, near-surface air is lifted upward, then influenced by the trapping effect of SASMA [37,38,39]. Research on ATAL composition mainly relies on in situ and sounding observations, as well as modeling studies. However, despite substantial efforts to understand its formation and composition, significant discrepancies remain among research findings, and considerable debate persists regarding the specific components of ATAL. Moreover, due to the limited availability of high-altitude observations and the pronounced vertical variability of aerosols, the three-dimensional structure and long-term evolution of ATAL remain poorly understood. Additionally, the reliability of reanalysis data in characterizing the spatiotemporal distribution of ATAL has yet to be validated, contributing to uncertainties regarding its climatic impact [40].

Therefore, it is imperative to clarify the spatiotemporal distribution characteristics of ATAL, explore its impact on the global climate, and assess possible future changes in ATAL and its interactions with the climate. Therefore, it is essential to clarify the spatiotemporal distribution of ATAL, examine its impact on the global climate, and assess potential future changes and interactions with the climate system. A deeper understanding of its three-dimensional spatiotemporal distribution and climatic effects is critical for formulating effective emission reduction strategies to mitigate climate change [41]. This study systematically reviews research progress on the three-dimensional spatiotemporal distribution, trends, sources, and climatic effects of ATAL. The purpose of this study is to clarify the mechanisms of formation, spatiotemporal distribution characteristics, and climate impacts of ATAL in both historical and contemporary contexts, highlight current research gaps, and offer directions for future studies. These efforts aim to provide scientific references and data support for predicting and responding to climate change.

2. Spatiotemporal Distribution Characteristics and Components of ATAL

In situ observations of upper tropospheric aerosols are typically conducted using aircraft and balloon platforms [40]. Compared to observations in the lower troposphere, these are more challenging due to limited data availability, making the analysis of vertical distribution characteristics more complex [42]. These limitations complicate efforts to study the spatiotemporal distribution of ATAL. Nevertheless, early researchers have identified key features of ATAL. Using Stratospheric Aerosol and Gas Experiment II (SAGE II) satellite data from 1985 to 1993, Li et al. [43] identified a significant aerosol maximum region near 100 hPa during summer, stretching from the Bay of Bengal to the southeastern QTP, providing early evidence for the presence and concentration patterns of ATAL. Kim et al. [44] later confirmed the existence of ATAL through ground-based lidar observations over Lhasa in the summer of 1999, detecting aerosol concentrations significantly higher than background values within the 14–19 km altitude range. This provided direct observational support for ATAL. Zhou et al. [45] subsequently analyzed the vertical distribution and spatiotemporal variation characteristics of aerosols over the QTP using Halogen Occultation Experiment (HALOE) satellite data from 1991 to 2005. They found a peak in aerosol number density near the tropopause, with summer concentrations significantly higher than those in other seasons. This study not only further confirms the existence of ATAL but also reveals its seasonal variation characteristics. These early studies consistently showed that during the SASM, large quantities of aerosols are transported into the upper troposphere, forming ATAL. These findings laid a foundation for subsequent investigations.

ATAL spans a broad geographical area, extending from the eastern Mediterranean to western China, and is primarily observed during summer at altitudes between 12 and 18 km, as initially noted by Vernier et al. [46] (Figure 1). Vernier et al. further emphasized that ATAL forms through mechanisms distinct from those governing the stratospheric aerosol layer, which is typically influenced by volcanic eruptions. In contrast, ATAL is a persistent feature of the Asian summer monsoon that is unrelated to volcanic activity. Numerous studies have since confirmed the existence of ATAL using various datasets. Thomason and Vernier identified ATAL in the SAGE II satellite observations [47], while, Vernier et al. [48] and Yu et al. [49] provided additional validation by conducting in situ soundings in India and China. These observations also pointed to vertical transport channels in the UTLS over the QTP, which were later corroborated by modeling studies. Their analyses revealed that during the SASM, significant vertical distributions of aerosols extend up to 12 km above the tropopause across a wide region, including India, the northern Arabian Sea, the southeastern QTP, and the northern Bay of Bengal.

Satellite remote sensing has allowed many researchers to confirm the presence of ATAL, and subsequent sounding observations have helped elucidate its spatiotemporal variations. However, high-altitude soundings are costly and offer limited spatial and temporal coverage. In contrast, reanalysis data and numerical models provide long-term, high-resolution datasets and are widely used to investigate the spatiotemporal distribution, composition, and radiation effects of ATAL. Wu et al. [50] compared Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis data with CALIPSO observations, and found that MERRA-2 effectively captures the spatiotemporal distribution characteristics of ATAL. Liu et al. [17] compared and analyzed the long-term three-dimensional spatiotemporal variation characteristics of ATAL using multi-source (CALIPSO, MERRA-2 and SAGEII) data, indicating that ATAL began forming in the mid-to-late 1990s and intensified until 2010, after which its upward trend plateaued (Figure 2).

In terms of the composition of ATAL, research findings vary considerably due to differences in models, observational methods, and parameters considered. Using the CESM1 model coupled with CARMA, Yu et al. [51] suggested that ATAL primarily consists of secondary organic aerosols, sulfate aerosols, and primary organic aerosols. Fairlie et al. [52], employing the GEOS-Chem chemical transport model, highlighted the significant role of nitrate, along with sulfate, ammonium, and organic aerosols, consistent with the earlier findings of Gu et al. [53]. Fadnavis et al. [54] used the ECHAM6-HAM global aerosol-climate model and identified large quantities of carbonaceous aerosols in the ATAL region. Using the ECHAM/MESSy model combined with GMXe, Ma et al. [55] proposed that mineral dust and water-soluble compounds like nitrates and sulfates dominate aerosol types over the QTP. Zhang et al. [56] conducted a comprehensive study integrating experimental and modeling data to characterize ATAL and found that they are primarily submicron in size (mostly ≤ 0.25 µm), with peak number concentrations at the tropopause and notable vertical gradients.

More recently, Bossolasco et al. [57] used the CESM1.2 model coupled with the Community Atmosphere Model 5 (CAM5) and Modal Aerosol Model 7 (MAM7) to simulate the composition and long-term trends of ATAL, revealing a typical “bimodal” vertical distribution structure primarily composed of mineral dust with no significant interannual variation. The discrepancies in these research results stem from various factors. First, diverse models and observational techniques introduce inherent biases in simulation outputs. Second, the composition and spatiotemporal distribution of ATAL are influenced by multiple factors, including atmospheric circulation, climate variability, and anthropogenic emissions. Notably, the dynamic variations in the SASM strongly affects the formation, distribution, and composition of ATAL. Variations in SASM strength and spatial reach, as well as interannual differences in monsoon activity directly influence ATAL characteristics.

Additionally, the SASM exhibits intraseasonal oscillations, particularly between the Iranian Plateau and the QTP, typically on a quasi-biweekly timescale. These fluctuations introduce further uncertainty into the understanding of the composition and spatiotemporal distribution of ATAL. Consequently, despite extensive efforts, substantial uncertainties remain regarding the composition of ATAL, posing challenges for assessing its climate impacts. To improve accuracy in determining the composition, spatiotemporal distribution, and climatic impacts of ATAL, future studies must integrate multiple factors and adopt advanced, high-precision observational technologies, simulation models, and analytical approaches to reduce uncertainty and improve the robustness of research outcomes.

3. Formation Mechanism and Sources of ATAL

The SASM provides a significant pathway for transporting water vapor and atmospheric constituents from the boundary layer to the stratosphere. Both natural and anthropogenic pollutants that ascend through this channel can influence regional and even global climates through a range of atmospheric chemical, physical, and radiative processes. The vertical transport of pollutants during the SASM is primarily governed by the SASMA, which emerges in June and dissipates by September. The SASMA exhibits distinct intra-seasonal variability in its horizontal structure and displays a characteristic bimodal pattern with east–west movement [24]. Satellite observations show that the South Asian Summer Monsoon Anticyclone or the South Asian High, centered over the QTP, forms a closed circulation region in which the concentrations of atmospheric components can increase or decrease significantly [30]. Gettelman et al. [58], through observations and simulations, demonstrated that water vapor concentrations in the lower tropical stratosphere are approximately 60% higher in the Northern Hemisphere summer compared to winter, with approximately 75% of the water vapor entering the global stratosphere during summer originating from the Asian monsoon region.

Simulations using three-dimensional global chemical transport models further reveal that most pollutants reaching the UTLS region, such as sulfur dioxide (SO₂), HCN, and CO, are closely linked to SASM-driven transport [24,59]. Additional studies have shown that ATAL concentrations are significantly higher during the monsoon season than during the pre-monsoon period [60,61,62]. Collectively, these findings highlight the central role of the SASM in shaping the composition and distribution of the atmosphere, particularly in the UTLS region, and its subsequent impacts on regional and global climate systems. Numerous researchers have emphasized that the intense upward airflows and deep convection associated with the SASM are key drivers of ATAL formation [31,51,63,64,65]. Vernier et al. [32] highlighted the distinction between the ATAL and stratospheric aerosol layers, noting that the latter are mainly influenced by volcanic eruptions, whereas the ATAL is a stable, recurring feature during the SASM with no apparent volcanic connection.

Consistent evidence from satellite observations and model simulations confirms that the SASM circulation provides an important channel for the upward transport of near-surface pollutants into the stratosphere. The transport pathways of ATAL have also been widely explored among researchers. Due to its significant dynamic lifting capacity, the QTP is often considered a major ATAL transport route [30,61,63,66,67]. Lau et al. [61] emphasized the roles of the Himalaya-Ganges Plain on the southern edge of the QTP and the Sichuan Basin in eastern China as channels for aerosol uplift via deep convection. These findings improve the understanding of atmospheric dynamics during the SASM and highlight the need for continued monitoring to evaluate the impacts of these pollutants on regional and global climate systems.

The formation mechanism of ATAL involves complex interactions among various processes. The “Elevated Heat Pump” (EHP) hypothesis proposed by Lau in 2006 provides an important framework for understanding this mechanism. This hypothesis emphasizes the coupling between the monsoon system and aerosol transport, especially the role of absorptive aerosols such as black carbon and dust. During mixing or encapsulation, these aerosols enhance their absorptive capacity and heat the surrounding atmosphere by absorbing radiation. This heating effect significantly influences the static feedback of atmospheric water vapor. As the SASM intensifies, large volumes of water vapor are transported to the southern slopes of the QTP. Upon encountering the barrier of the QTP, this water vapor is forced upward, resulting in frequent convective activity over northern India and the Himalayan foothills during the early onset of the SASM. These convective processes not only lift water vapor into the UTLS region but also transport substantial quantities of aerosols. Moreover, by absorbing shortwave radiation, aerosols heat the lower and middle layers of the atmosphere. This heating contributes to the development of an inversion layer through dry convection in the mid-to-upper atmosphere, which further promotes the vertical transport of aerosols into the UTLS. Simultaneously, rising warm air masses draw in additional heat and water vapor from the Indian subcontinent, thus facilitating the earlier onset of the SASM.

These interconnected physical and chemical processes jointly contribute to the formation of the ATAL. Notably, the ATAL formation is not only influenced by the monsoon system, aerosol transport, and radiative heating but also by terrain features, atmospheric circulation patterns, and water vapor availability. Therefore, a comprehensive understanding of the ATAL formation mechanism requires further in-depth and systematic research. Numerous studies have identified India, southwestern China, and southeastern China as major sources of ATAL [52,58,63,65,68,69]. According to the numerical simulation results by Zhang et al. [56], ATAL primarily originates from two key surface sources. The first is the region south of the Himalayas, characterized by intense convective activity capable of vertically transporting near-surface pollutants to UTLS altitudes. The second is the spiral upward motion along isentropic surfaces, which facilitates the transport of aerosols from the upper troposphere to the lower stratosphere within the SASM region. These findings suggest that aerosols present in the upper troposphere across the SASM region may all contribute to the development of ATAL.

4. Climate Effects of the ATAL

As a key factor influencing the radiative budget of the Earth-atmosphere system, cloud formation, and precipitation processes, aerosols exert substantial impacts on the climate system and global change. Moreover, they are among the most uncertain and complex variables at different meteorological scales [70]. The climatic effects of aerosols are typically categorized into two aspects. The first is the direct effect, referring to aerosol–radiation interactions, in which aerosols modify the radiative budget at the surface and in the atmosphere by scattering and absorbing solar radiation [71]. Studies have shown that since the 1960s, surface incident shortwave radiation in China has declined, particularly in the QTP region [72,73]. However, cloud cover has not changed significantly during this period [74]. This trend is primarily attributed to increased pollutant emissions. Different aerosol types affect the radiative budget in different ways. For instance, black carbon, brown carbon, and certain mineral dust particles absorb solar radiation directly [75,76], whereas sulfates, nitrates, and most organic carbon and dust aerosols primarily scatter solar radiation [77,78]. The reduction in surface incident radiation may lead to temperature changes. From 1960 to 1990, as aerosol concentrations increased rapidly in China, most regions experienced rising temperatures. However, in heavily polluted areas such as eastern China, temperatures exhibited a decreasing trend. The overall increase in temperature is closely linked to climate warming. Research suggests that in eastern China, the radiative forcing from increased aerosol concentrations can be several times greater than that from increased CO₂ [79]. Nevertheless, aerosols have a much shorter residence time compared to greenhouse gases. Therefore, when considering a longer time frame, China generally exhibits a warming trend. Aerosol-induced changes in radiative forcing can also influence atmospheric stability and circulation patterns [7].

The second aspect of aerosol climatic effects is their role as cloud condensation nuclei, influencing cloud formation and precipitation, also known as the indirect effect of aerosols [80,81,82,83]. Studies have indicated that increased aerosol concentrations alter the energy balance between the Earth’s surface and the atmosphere and impact the global water cycle [84,85]. Ramanathan et al. [86] pointed out that black carbon aerosols, organic carbon, and dust aerosols in the Asian monsoon region contribute to weakened summer monsoon rainfall. However, Lau et al. [76] argued that due to the high surface albedo in the high-altitude regions of the QTP, absorptive aerosols can strengthen the Indian summer monsoon through an intensified heat pump effect. A strong SASM circulation means a strong upward branch of the circulation located in the southern foothills of the QTP. Strong upward movement can transport aerosols near the surface upwards to the UTLS region. These contrasting findings reflect the considerable uncertainty in current research on the climatic effects of aerosols. Given the complexity and variability of aerosol behavior, it is crucial to conduct further research, both experimental and through numerical simulations, to better understand their role in climate change and global environmental systems. Such research would aid in improving our ability to predict and mitigate the potential negative impacts of aerosols on the climate and weather patterns.

Similarly to aerosols in the troposphere, those in the mid-to-upper atmosphere also influence the radiative energy budget by scattering and absorbing solar radiation. Scattering aerosols contribute to atmospheric cooling, whereas absorbing aerosols warm the atmosphere through aerosol-radiation interactions. However, if aerosols exist in the upper atmosphere, especially the absorbing ones, the impacts will be different. For instance, the presence of scattering aerosols at high altitudes is expected to cause cooling at the surface, while absorbing aerosols may induce warming. Furthermore, there are differences between tropospheric aerosols and stratospheric aerosols. Stratospheric aerosols exhibit longer residence times and greater dispersion, significantly affecting the temperatures of the UTLS and altering surface precipitation patterns, thereby affecting regional and global climates [34,35,63]. Additionally, the vertically inhomogeneous distribution of aerosol types creates variability in incoming shortwave and upward surface radiation, posing a significant challenge in studying aerosol radiative effects [87]. This contributes to an estimated uncertainty of 0.5 W m⁻² in global radiative forcing [88].

In Central and East Asia, rapid population growth and industrialization have led to increased emissions of tropospheric aerosols and precursors, suggesting that ATAL may continue to intensify. Aerosols within ATAL can be transported into the tropical lower stratosphere via southward airflow on the eastern flank of the SASMA, thereby affecting the global stratospheric aerosol content. Vernier et al. [46] estimated that ATAL caused an average radiative forcing of approximately −0.1 W m⁻² at the local atmospheric top between 1996 and 2013. However, variations in aerosol type and study period may yield different results. Gao et al. [71] found that ATAL increased incoming shortwave radiation at the top of the atmosphere by 0.15 W m⁻² and reduced surface incoming shortwave radiation by 0.72 W m⁻². Given the complexity and varying impacts of aerosols in different atmospheric layers, further investigation is essential to clarify their behavior and interactions with the environment. This includes understanding how aerosols are transported and transformed, as well as their long-term effects on climate systems. Such research is crucial for improving climate model accuracy and enhancing projections of weather patterns and global temperature trends.

Aerosols such as ammonium nitrate and ammonium sulfate may serve as ice nuclei, facilitating inhomogeneous ice formation and affecting the radiative properties of cirrus clouds in the upper troposphere [89]. This, in turn, has implications for climate. Satellite data analyses indicate that rising surface pollutant emissions in Asia enhance water vapor flux into the stratosphere during summer [90]. Satellite observations indicate that, compared to cleaner regions, polluted areas at the tropical tropopause exhibit smaller ice particle radii, higher temperatures, and elevated water vapor content within cirrus clouds. Based on these observations, Su et al. [90] speculated that increased aerosol concentrations elevate the ice particle number density in cirrus clouds, reducing their size and prolonging their residence in the upper troposphere. Furthermore, the enhanced radiative heating from increased cirrus cloud cover subsequently raises temperatures near the tropopause, increasing water vapor and promoting upward motion. This process further boosts water vapor flux into the stratosphere, ultimately affecting the global radiative budget.

Conversely, variations in the chemical composition and aerosol concentrations in the UTLS region exert profound influences on near-surface climate through radiative interactions. For instance, interdecadal fluctuations in stratospheric water vapor levels have been shown to significantly affect surface temperatures [91]. This complex interaction between aerosols, clouds, and radiation across atmospheric layers highlights the ongoing need for comprehensive research to understand and predict their cumulative impacts on the global climate.

5. Conclusions and Discussion

The interaction between the stratosphere and troposphere is currently a frontier and hot research area. In recent years, with the increasing availability of satellite observation data and the rapid development of atmospheric chemical climate models, including those addressing stratospheric processes, have significantly enhanced research efforts in this field and yielded important preliminary findings. In the Asian monsoon region, rapid development in countries such as China, India, and Southeast Asia has led to severe tropospheric pollution. The SASM may serve as a crucial pathway for transporting tropospheric pollutants into the stratosphere. By analyzing the distribution, transport and chemical transformation of pollutants, researchers can better understand their effects on the stratosphere and, conversely, how stratospheric processes influence the troposphere. This research is particularly important for advancing the understanding of the global climate system and assessing the impacts of anthropogenic activities. A deeper understanding of stratosphere–troposphere interactions may enhance our ability to predict and mitigate climate change, especially regarding how tropospheric pollution influences the stratosphere and the global radiative balance. In summary, investigating the interaction between the stratosphere and troposphere—especially in the context of tropospheric pollution in the Asian monsoon region—represents a valuable and timely research direction.

To date, most studies have relied heavily on satellite observations and numerical simulations, while in situ observational data from the Asian monsoon region remain limited. Additionally, the performance of satellite-derived products in this region often lacks validation through ground-based or in situ measurements. To better understand the distribution and variability of aerosol components near the Asian tropopause and their impacts on global climate and the environment, there is an urgent need for expanded in situ observations of ATAL constituents. This includes balloon-borne measurements and aircraft-based observations to supplement and validate satellite and model-based findings.

6. Recommendations

Due to limitations in traditional observational capabilities, the current understanding of ATAL remains incomplete. Future research should focus on the following key directions:

(1) Atmospheric chemical processes and aerosol composition within ATAL: Given the diverse aerosol types present in ATAL and the substantial variability in their radiative and climatic effects, clarifying the chemical composition of these aerosols is essential for accurately assessing their climate impacts. Moreover, the aging effect of aerosols during atmospheric transport—driven by heterogeneous chemical and photochemical reactions—can alter their properties and composition. These transformations warrant further investigation.

(2) Impact of ATAL on the climate and implications for geoengineering: What are the specific climate impacts of ATAL? How can the climatic signals attributed to ATAL be isolated from a broader climate system? What implications might these findings hold for solar radiation management within geoengineering frameworks? Addressing these questions requires a precise understanding of the three-dimensional structure of ATAL. Research into ATAL’s climatic effects could provide valuable insights for evaluating the feasibility and risks of artificial climate interventions.

(3) Improvement of numerical simulation results of ATAL: Numerical modeling remains a crucial tool for investigating climate processes. However, the performance of existing models in simulating ATAL remains uneven and requires further evaluation. Therefore, enhancing the performance of climate models and accurately simulating the climate effects of ATAL using observational data represents a key direction for future research.

(4) Future prediction of ATAL changes and global climate responses: In the context of ongoing global climate change, what future trends can be expected in ATAL under different socioeconomic development trajectories? How do shifts in ATAL characteristics influence the climate system? As societies intensify efforts to combat climate change, ATAL is likely to evolve differently under varying pollutant emission scenarios, leading to distinct climate responses. These questions underscore the complexity of ATAL–climate interactions and highlight the need for deeper, targeted research.