This study utilises data from intensive field observations conducted using the mobile facilities of the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) (Fig. 1) at Minqin (38.607° N, 102.959° E, 1373 m a.s.l.) from April to June 2010 and at Dunhuang (40.492° N, 94.955° E, 1061 m a.s.l.) from April to June 2012 in the Hexi Corridor region. The mobile facilities were equipped with a suite of instruments to measure key atmospheric parameters, including atmospheric radiation, aerosol scattering and absorption coefficients, aerosol optical depth, mass concentration, and aerosol vertical distribution. Dunhuang is located at the westernmost end of the Hexi Corridor, adjacent to the Kumtag Desert to the west, approximately 450 km downwind of the Taklimakan Desert, bordering the Badain Jaran Desert to the east, and close to the Qilian Mountains to the south. Owing to its unique geographical location, distinctive underlying surface characteristics, extreme aridity, and strong prevailing winds, dust storms frequently occur in this region during spring. The observation site was located approximately 45 km northeast of Dunhuang City and was surrounded by farmland, Gobi Desert, and saline–alkali land. Minqin is located in the northeastern part of the Hexi Corridor, at the convergence zone of the Badain Jaran and Tengger Deserts, and is close to the Qilian Mountains to the southwest, making it highly susceptible to aeolian processes. The region is characterised by a temperate continental hyper-arid climate. Dust events also occur frequently during springtime. The observation site was surrounded by sand dunes interspersed with small patches of farmland, representing a typical desert–oasis transition zone (Zhou et al., 2018).

First, the meteorological background during both intensive observation periods is shown in Fig. 2. It can be seen that both sites are influenced by the westerlies at the 500 hPa level. At the 700 and 850 hPa levels, the influence of complex topography becomes evident. The wind direction at both sites aligns with the orientation of the Qilian Mountains, with airflow predominantly from northwest to southeast. Areas surrounding the Qilian Mountains and the Hexi Corridor exhibit relatively high potential temperatures, suggesting enhanced thermal buoyancy. Under weakly stable or potentially unstable stratification, such thermodynamic conditions can favour the development of vertical motion (Houze, 2004; Li et al., 2017). At the Dunhuang site, wind speeds are relatively high, generally exhibiting a north-to-south flow pattern. In contrast, wind speeds at the Minqin site are lower, and the low-level flow field near the site shows weak convergence that may favour ascent. Overall, the mean meteorological fields suggest that parts of the Hexi Corridor experienced conditions favourable for upward motion and aerosol transport during the campaigns.

The complex mountainous terrain also generates distinct patterns in the local distribution of wind speed and direction. Concurrently, the extensive surrounding desert surfaces serve as major aerosol sources, profoundly influencing local aerosol composition and speciation. As shown in Fig. 3, northeasterly and southwesterly winds prevail at the Dunhuang site, with an average wind speed of 3.5 ms-1. Aerosols are likely derived mainly from dust originating from the Gobi Desert and sandy areas surrounding the observation site, as well as from the Kumtag Desert to the southwest (Luo et al., 2022). At the Minqin site, southeasterly winds dominate, with an average wind speed of 3.2 ms-1. Aerosols are likely primarily derived from dust originating from the Tengger Desert to the southeast or the Badain Jaran Desert to the northwest (Yang et al., 2024). The prevailing wind directions of both sites not only exhibit higher frequencies of occurrence but are also associated with relatively higher wind speeds, which are closely related to wind channelling effects induced by topographic undulations (Dang et al., 2024).

The evolution of aerosol vertical distribution and column properties over Dunhuang is presented in Fig. 4. In Fig. 4a and b, high normalised relative backscatter and high depolarization ratios indicate elevated dust aerosol loading, including dust events (Zhou et al., 2021, 2024). Dust events frequently occurred before 4 May. Thereafter, although dust events still occurred, their intensity and duration were significantly reduced. Instead, more distinct diurnal variations in aerosol vertical distribution are observed. These vertical distributions exhibit high normalised relative backscatter but a broader range of depolarization ratios, indicating that not all observations correspond to dust particles. The maximum normalised relative backscatter is located at approximately 1 km, indicating the presence of elevated aerosol layers. According to the method described in Sect. 2.2.1, the occurrence frequency of elevated aerosol layers is approximately 82 % during the experimental period, excluding dust events. As shown in Fig. 4c, most of these events occurred after 10 May. Owing to the limited availability of CE-318 observations during the experiment, aerosol column properties were derived by column-integrating the aerosol extinction coefficient retrieved from lidar to calculate AOD, following the approach used in our previous study (Zhou et al., 2021). The correlation coefficient between the two AOD datasets is 0.86, with a root-mean-square error of 0.18. This indicates the reliability of AOD retrieved from lidar in the absence of CE-318 observations. AOD values during elevated aerosol layer episodes were considerably lower than those during dust events, and the variability in AOD during each episode remained relatively consistent. To a certain extent, the depolarization ratio can characterise the coarse- and fine-mode particle fractions. For example, on 5 and 11 April, lower depolarization ratios correspond to higher Ångström exponent (AE) values, indicating that the elevated aerosol layers contain a larger fraction of fine-mode particles (Huang et al., 2025). Similar patterns are observed on 14 May and 7–8 June. However, relatively high depolarization ratios observed on 18 and 23 May, indicate the presence of mixed aerosol types containing dust components.

Similarly, Fig. 5 provides an overview of aerosol evolution over Minqin. A notable contrast is observed compared with Dunhuang, despite the consistency of observation season. High dust loading was also observed at Minqin; however, dust events were less intense but more frequent and temporally dispersed than those at Dunhuang, occurring intermittently throughout the experiment, with the highest frequency in May. In contrast to the lower values observed at Dunhuang, depolarization ratios remained consistently high at Minqin. This indicates that concentrations of coarse-mode particles (e.g., dust aerosols) were comparatively higher at Minqin during the observation period. For elevated aerosol layers (excluding dust days), the occurrence frequency at Minqin was only 44 %, with such events primarily concentrated in June (Fig. 5c). Moreover, the overall height of elevated aerosol layers at Minqin was lower than that at Dunhuang, and their intensity was also weaker. The correlation coefficient between AOD values retrieved by lidar and CE-318 was 0.70, with a root-mean-square error of 0.06. Notably, the AE was relatively high in June, when elevated aerosol layer events were frequent. In particular, the mean AE reached 0.6 during elevated aerosol layer events, indicating the dominance of fine-mode aerosols during this period.

To provide a more intuitive representation of the diurnal variations in aerosol vertical structure at the two sites, hourly averaged extinction coefficient profiles at specific times during the experimental periods are presented in Fig. 6. Within the vertical range of 0.6–2 km a.g.l. at Dunhuang, the aerosol extinction coefficient exhibited a weak decreasing trend from 22:00 to 12:00 BJT the following day. However, from 14:00 to 20:00, a distinct nose-shaped structure appeared, with a peak near 1.26 km a.g.l., reaching a maximum value of approximately 0.27 km-1 at around 18:00 BJT. Even when dust-influenced profiles were included in the averaging, a pronounced nose-shaped vertical distribution pattern was still observed. This indicates that elevated aerosol layers occurred more frequently and with greater intensity during the afternoon period. In contrast, the low frequency of elevated aerosol layers, coupled with the influence of dust events, resulted in the mean aerosol extinction coefficient profile at Minqin not exhibiting a distinct nose-shaped structure. Nevertheless, diurnal variation characteristics were still evident: the extinction coefficient was significantly higher from 10:00 to 18:00 than at night below 2 km a.g.l., particularly between 10:00 and 12:00, when the extinction coefficient in the lower altitude exceeded 0.4 km-1, indicating a significant increase in aerosol loading within 2 km. The depolarization ratio in the lower altitude exhibited a similar trend, remaining mostly below 0.3 at both sites, with slightly higher values at Minqin. This earlier occurrence of the peak at Minqin may be partly related to geographical differences between the two regions, including the longitudinal difference and the resulting local-time effect, which can influence the timing of boundary layer development. Overall, these averaged profiles indicate that daytime boundary layer evolution plays an important role in regulating the vertical redistribution of aerosols, providing the basis for the representative case analyses discussed below.

Furthermore, two representative cases at both sites are presented to illustrate the diurnal variation of aerosol vertical distribution and the associated thermal and dynamical factors, particularly during periods when elevated aerosol layers occur. As shown in Fig. 7a and b, except during the period of full daytime boundary layer development (approximately 14:00–20:00 BJT), the aerosol extinction coefficient at Dunhuang was relatively low (mean: 0.14 km-1), while the depolarization ratio remained high (mean: 0.21), indicating relatively low aerosol loading with a substantial dust contribution. When elevated aerosol layers were present from 15:00 to 20:00 BJT, a high aerosol extinction coefficient (∼0.27 km-1) was observed at approximately 1 km, while the depolarization ratio decreased to 0.17, suggesting an increased contribution of non-spherical dust mixed with finer or less-depolarizing particles. During this period, AOD at 440 nm ranged between 0.15 and 0.26, and AE at 440–870 nm ranged between 0.3 and 0.5. Meanwhile, SSA exhibited a distinct wavelength dependence characteristic of dust-dominated aerosols (Dubovik et al., 2002; Tutsak and Koçak, 2020), with lower values at 440 nm and higher values at longer wavelengths (not shown here). In addition, the weakened potential temperature gradient within the daytime boundary layer indicates reduced static stability and enhanced vertical mixing, which favoured the upward transport of aerosols from the lower atmosphere. The stable stratification near the boundary layer top further limited upward dispersion, thereby promoting aerosol accumulation and the formation of the elevated aerosol layer. As illustrated in Fig. 7c, the prevalence of southwesterly winds during elevated aerosol layer episodes suggests that aerosols were likely associated with desert source regions to the southwest. This is consistent with the backward trajectories of air masses presented in Fig. 1. In Fig. 7d, elevated aerosol layer episodes were accompanied by a rapid increase in surface temperature, reaching the daily maximum, while pressure and relative humidity decreased sharply. These concurrent changes in aerosol vertical structure and near-surface meteorological conditions indicate that daytime thermodynamic processes played an important role in the formation of the elevated aerosol layer.

Along the HYSPLIT backward trajectories, vertical cross-sections of relative humidity and potential temperature reveal that the elevated transport and accumulation of aerosols over the Dunhuang region were closely associated with daytime thermodynamic evolution within the boundary layer. For the case shown in Fig. 8, three representative time points were selected to examine the evolution of the thermodynamic structure, representing the pre-event (10:00 BJT), during-event (16:00 BJT) and post-event (22:00 BJT) stages of the elevated aerosol layer episode. During these three stages, relative humidity in the lower atmosphere remained approximately 10 % (Fig. 8a–c), consistent with the surface observations shown in Fig. 7d. Such dry lower-atmospheric conditions favoured enhanced daytime sensible heating, while the strong surface thermal contrasts between the low-altitude desert areas and the surrounding terrain further promoted thermally driven vertical motion and the upward transport of aerosols (Mazza and Chen, 2025; Pal et al., 2025; Wen et al., 2025). To better characterise the elevated aerosol layer, the meteorological field near 650 hPa is analysed (Fig. 8d–f). At this level, westerly winds persisted, and the geopotential height field showed a relatively smooth north–south gradient. No prominent synoptic disturbance or strong convective system directly affected the Dunhuang region during this event. These characteristics indicate that the upper part of the boundary layer environment was relatively stable. Under such meteorological conditions, stable stratification can effectively suppress vertical air motion and limit the further upward dispersion of aerosols (Haeffelin et al., 2024; Hu et al., 2024; Zhong et al., 2018).

Before the elevated aerosol layer episode, although the potential temperature at 800 hPa in the vicinity of the Dunhuang region remained relatively low, a thermal gradient had already developed between the desert areas to the west of the site and those to the east (Fig. 8g). In addition, the potential temperature gradient over the Dunhuang region was large, with gently sloping isentropic surfaces indicating stable stratification, which impeded the upward transport of aerosols and confined them to the lower atmosphere (Li et al., 2021b). As time progressed, intense daytime solar radiation strongly heated the surface in low-altitude areas, where the low heat capacity of the underlying surface allowed rapid warming. This caused substantial increases in surface potential temperature, resulting in upward-bulging isentropic surfaces and a sharp decrease in the potential temperature gradient. Meanwhile, ascending airflows developed under the influence of thermal buoyancy (De Wekker et al., 2018), facilitating the upward transport of aerosols in the lower atmosphere. During the formation of the elevated aerosol layer, the lower atmosphere transitioned from stable to unstable or weakly stable stratification, with enhanced vertical mixing observed at mean heights of approximately 0.3–1.3 km a.g.l. (Fig. 8b), although the thickness of this layer varied considerably (Jin et al., 2022). This thermodynamic structure favoured the upward transport of aerosols. Upon reaching the overlying stable stratification, the lifted air parcels encountered negative buoyancy and tended toward hydrostatic equilibrium (Serafin et al., 2018), thereby inhibiting further vertical motion. As a result, aerosols accumulated within the altitude range of 0.6–2 km a.g.l. and tended to concentrate near the base of the stable stratification. This finding is consistent with those reported in other regions (Su et al., 2020a; Meng et al., 2025). Following the elevated aerosol layer episode, surface cooling occurred as solar heating diminished. Despite the persistence of relatively high potential temperatures at this moment, isentropic surfaces gradually flattened and atmospheric stratification progressively returned to a stable state (Fig. 8c). In combination with relatively strong wind speeds within this altitude layer (Fig. 8i), these meteorological conditions facilitated the rapid dissipation of the elevated aerosol layer.

Furthermore, as illustrated in Fig. 8g–i, terrain-induced local circulations near the Qilian Mountains may modulate the horizontal diffusion and transport of elevated aerosol layers. During daytime, differential heating between low-altitude desert areas and the surrounding mountainous terrain can induce upslope or valley winds, favouring aerosol transport from low-altitude regions toward the mountains under suitable background wind conditions (Cacciani et al., 2018; Serafin et al., 2018). However, for the case illustrated in Fig. 8, southwesterly winds prevailed during the formation of the elevated aerosol layer, indicating that the transport pathway was not directed toward the Qilian Mountains. Therefore, the influence of this particular EAL event on the Qilian Mountains was likely limited. This case-specific result does not exclude the possibility of regional impacts under other wind regimes. Nevertheless, as shown in Fig. 3, northeasterly winds also occurred frequently at the Dunhuang site. Under such conditions, daytime mountain–valley circulations may facilitate the transport of elevated aerosol layers from the Dunhuang region toward the Qilian Mountains. This process may contribute to localized pollution and has potential implications for snow/glacier environments in the Qilian Mountains (Hou et al., 2018).

Similar to the Dunhuang case shown in Fig. 7, a distinct elevated aerosol layer was observed at Minqin on 14 June 2010 (Fig. 9). Consistent with the observations at Dunhuang, the average aerosol extinction coefficient at Minqin remained relatively low (∼0.08 km-1) for most of the observation period, except during the fully developed boundary layer stage (14:00–20:00 BJT), when it increased significantly to 0.23 km-1. However, the depolarization ratio at Minqin showed minimal variation throughout the observation period and remained low, with a value of approximately 0.09. This indicates that the elevated aerosol layer was primarily composed of spherical or weakly depolarizing particles, suggesting a larger contribution from fine-mode and/or anthropogenic aerosols than in the Dunhuang case. Based on the optical properties, during the elevated aerosol layer episode, AOD at 440 nm ranged from 0.08 to 0.14, and AE at 440–870 nm ranged from 0.65 to 0.75. In parallel, SSA exhibited a spectral pattern more consistent with fine-mode and anthropogenic aerosol contributions than with dust-dominated aerosols (Dubovik et al., 2002; Tutsak and Koçak, 2020): values at all four wavelengths (440, 675, 870, and 1020 nm) were comparable (0.86–0.92), with no pronounced gradient between short and long wavelengths (not shown here). Together with the low depolarization ratio and relatively higher AE, this SSA feature suggests that the Minqin elevated aerosol layer was more strongly influenced by fine-mode and anthropogenic aerosols than the Dunhuang case. Similar to Fig. 7, the case presented here represents only one example and does not imply that elevated aerosol layers at both sites are dominated by a single aerosol type. Combined with Figs. 4 and 5, it can be observed that both dust-influenced and fine-mode/anthropogenic-aerosol-influenced elevated layers occurred at both sites during the experimental periods, with differences likely arising from variations in dust aerosol content. Although not exclusively dominated by a single type, certain aerosol types exhibit higher occurrence frequencies and greater representativeness in each region. Compared with the Dunhuang case, the Minqin case therefore provides a contrasting example in terms of aerosol optical properties, while showing a broadly similar daytime thermodynamic evolution. The weakened potential temperature gradient within the daytime boundary layer indicates reduced static stability and enhanced vertical mixing, favouring the upward transport of aerosols. As shown in Fig. 9c, easterly winds prevailed during the aerosol layer episode. This transport pathway is consistent with the backward trajectories of air masses presented in Fig. 1. A similar pattern of the diurnal variations in temperature, humidity, and pressure is observed in Fig. 9d, suggesting that daytime thermodynamic processes also contributed to the formation of the elevated aerosol layer at Minqin. Overall, the Minqin case supports the common uplift–accumulation mechanism identified at Dunhuang, while highlighting site-dependent differences in aerosol optical characteristics under the broadly similar modulation of mountain–valley circulations.

Consistent with the approach adopted for the Dunhuang thermodynamic analysis in Fig. 8, three time points were selected to represent the evolution of the thermodynamic structure, corresponding to the pre-event (09:00 BJT), during-event (16:00 BJT) and post-event (23:00 BJT) stages of the elevated aerosol layer episode in Fig. 10. This selection of time points is based on the earlier initiation and later dissipation of the elevated aerosol layer in the Fig. 9 case compared with that in Fig. 7. In general, elevated aerosol layers at Minqin were also associated with daytime thermal forcing, upward aerosol transport, and terrain-induced circulation, suggesting a broadly similar thermodynamic coupling mechanism to that identified at Dunhuang. In contrast to Dunhuang, a pronounced abrupt change in relative humidity occurred at the upper boundary of the unstable stratification, with a larger vertical extent and nearly constant height, where a persistent high-relative-humidity zone was present (Fig. 10a–c). This feature indicates that the upper boundary of the unstable layer at Minqin was closely associated with a humid and relatively stable layer. Rather than directly implying that water vapour itself inhibited aerosol uplift, the coexistence of enhanced relative humidity and stable stratification suggests that this layer likely acted as a cap, limiting further upward dispersion of aerosols (Zhong et al., 2018; Wang et al., 2020).

Analysis of the corresponding height layer at the 600 hPa level (Fig. 10d–f) shows that, before the elevated aerosol layer episode, potential temperature was relatively low, with cold air masses dominating. The wind direction was predominantly northerly, and geopotential height increased from northeast to southwest. Although a cyclone was located to the southwest of the region, no prominent large-scale synoptic system or strong convective activity directly affected the Minqin site. This indicates that atmospheric stratification at this level was relatively stable, favouring aerosol confinement below this layer. During elevated aerosol layer episode, the site was situated near the boundary between cold and warm air masses (Fig. 10e). This frontal transition zone led to enhanced relative humidity at this level (Zhang et al., 2007). During this process, warm, moist air ascended over the colder air mass, likely forming a frontal inversion that suppressed vertical motion and inhibited atmospheric mixing (Feng et al., 2023). Following the elevated aerosol layer episode, warm and moist air masses became more influential over the site, and the high-relative-humidity layer persisted near the upper boundary of the unstable layer. Additionally, the mountain–valley circulation near the Qilian Mountains was more evident in the Minqin case (Fig. 10g–i). This terrain-induced circulation, together with the daytime thermal forcing, may have facilitated the horizontal transport of elevated aerosols toward the Qilian Mountains under favourable background winds. Therefore, Fig. 10 indicates that the Minqin case shared the common daytime uplift–accumulation mechanism observed at Dunhuang, but with a stronger influence from the humid stable layer and terrain-induced circulation.

As shown in Fig. 11, during cases when elevated aerosol layers occurred at Dunhuang and Minqin, the potential temperature below 3 km at both sites exhibited a positive anomaly of 3–5 K (Fig. 11a–c and m–o), indicating a warmer-than-average thermal state compared with the average boundary layer potential temperature (∼310 K) at the two sites during the experiment period. Meanwhile, relative humidity below this altitude was generally 5 %–10 % lower than the average value (∼30 %) (Fig. 11d–f and p–r), indicating distinctly dry and warm anomalous meteorological conditions in the lower atmosphere. Notably, during cases at Minqin, positive relative humidity anomalies of 5 %–15 % appeared above approximately 3 km, consistent with those shown in Fig. 10a–c. This suggests that the region may be influenced by the interaction of cold and warm air masses under some synoptic conditions. This vertical humidity distribution is consistent with previous findings that high-humidity conditions near the top of the boundary layer favour the retention and accumulation of aerosols within the boundary layer (Altstädter et al., 2018). In contrast, when elevated aerosol layers did not occur, the potential temperature at both sites showed a negative anomaly of 1–6 K (Fig. 11g–i and s–u), and relative humidity was 5 %–25 % higher than the average state (Fig. 11j–l and v–x). To further support the composite anomaly analysis, daily samples were statistically compared under conditions with and without elevated aerosol layers within the main height range and occurrence period of elevated aerosol layers (Fig. S3 in the Supplement). The results show that daily potential temperature anomalies were generally higher, whereas daily relative humidity anomalies were generally lower, under conditions with elevated aerosol layers at both Dunhuang and Minqin sites. This is consistent with the dry and warm anomalies revealed by the composite analysis. The non-parametric test results indicate statistically distinguishable differences between the two groups, with significance levels varying between variables and sites. These daily statistics provide additional support that the dry and warm thermodynamic conditions associated with elevated aerosol layers are also evident in day-to-day variability. Based on these observations, the occurrence of elevated aerosol layers was closely associated with boundary layer temperature and humidity anomalies. Most elevated aerosol layer events occurred under the anomalously dry and warm conditions, characterised by positive potential temperature anomalies and negative relative humidity anomalies. These lower-atmospheric conditions were conducive to enhanced sensible heating, reduced static stability, and stronger vertical mixing within the daytime boundary layer. Consequently, daytime thermal mixing and local thermally driven circulations can promote the upward transport of aerosols from the lower atmosphere. Meanwhile, the relatively low-humidity air is characterized by a higher lifting condensation level (Lal et al., 2024), which indicates that aerosol uplift under these conditions is more closely related to dry thermal mixing than to moist convective development. When the lifted aerosols reach the stable stratification near the boundary layer top, further vertical dispersion is suppressed, thereby favouring aerosol accumulation and persistence aloft.

Overall, the representative cases and composite anomalies indicate that elevated aerosol layer formation at both sites was associated with a common daytime uplift–accumulation mechanism. Positive potential temperature anomalies and negative relative humidity anomalies within the daytime boundary layer favoured sensible heating, reduced static stability, and enhanced vertical mixing, while stable stratification near the boundary layer top limited further upward dispersion and promoted aerosol accumulation aloft. Terrain-induced local circulations further modulated the horizontal transport of the elevated aerosol layers. These results suggest that the similar thermodynamic evolution observed in the Dunhuang and Minqin cases reflects a common physical process rather than two independent case-specific phenomena. Based on these findings, a simplified conceptual prediction model for elevated aerosol layers can be constructed, with key early-warning indicators being positive anomalies in boundary layer potential temperature and negative anomalies in boundary layer relative humidity. When both indicators are satisfied simultaneously, this can be identified as a meteorological condition favourable for the occurrence of elevated aerosol layers, although additional long-term observations and model simulations are still needed to further evaluate its predictive applicability.

During both experimental periods, PM₁₀ mass concentration observations were available only at Dunhuang. Here, the conversion method described in Sect. 2 was used to assess the air pollution levels of elevated aerosol layers. As shown in Fig. 12, PM₁₀ concentration at the near-surface and at the peak of elevated aerosol layers are presented. The average near-surface PM₁₀ concentration across all observations was 129.6 ± 212.0 µgm-3 during the entire experimental period. During elevated aerosol layer episodes, the near-surface average PM₁₀ concentration decreased to 40.8 ± 30.4 µgm-3, whereas the average PM₁₀-equivalent mass concentration estimated from the aerosol extinction coefficient at the peak of the elevated aerosol layers reached 203.3 ± 106.6 µgm-3. The peak PM₁₀-equivalent concentration is nearly five times higher than that at the near-surface. This estimated concentration approaches levels observed during severe haze events. For instance, previous studies reported that a severe haze event in Beijing, China recorded an average PM₁₀ concentration of 265.2 ± 157.1 µgm-3 (Wang et al., 2015). PM₁₀ concentrations during haze and dust events in South Korea reached 163.9 ± 25.0 µgm-3 and 211.3 ± 57.5 µgm-3, respectively (Jung et al., 2017; Seo et al., 2017). This clearly indicates that even when surface pollution is low or negligible, a highly concentrated pollutant aerosol layer may exist in the upper boundary layer. Although PM₁₀ concentration observations are only available from Dunhuang, the periods of elevated aerosol layer occurrence during daytime and their associated thermodynamic conditions are similar at both sites, as shown in Figs. 7, 9 and 11. Therefore, it can be inferred that such elevated aerosol layers may occur throughout the Hexi Corridor region under similar meteorological conditions, exerting a significant impact on the regional environment. These findings highlight that, when assessing air quality and developing pollution control strategies, reliance solely on surface-level monitoring is insufficient. Instead, elevated aerosol layers within the boundary layer should be comprehensively considered, as their formation and dispersion are closely related to surface pollutant level (Dong et al., 2022).

The presence of elevated aerosol layers not only increases air pollution levels over the Hexi Corridor region but also exerts significant regional radiative effects. Simulated shortwave radiative forcing results for two elevated aerosol layer events – observed on 21 April 2012 at Dunhuang and on 14 June 2010 at Minqin – are presented (Fig. 13a). At the surface (SFC), the average shortwave radiative forcing at Dunhuang was -22.66 Wm-2, with a significantly larger magnitude than that at Minqin (-13.60 Wm-2). This indicates a stronger surface cooling effect induced by elevated aerosol layers at Dunhuang. In addition, these effects reduce solar radiation reaching the surface during daytime and can directly affect the efficiency of photovoltaic power generation. This value exceeds the shortwave radiative forcing at the SFC reported for a moderate dust event (-19.5 Wm-2) (Li et al., 2025) and is comparable to the -23 Wm-2 observed during dust events (Marsli et al., 2025). The value at Minqin is stronger than that for a light dust event (-12.0 Wm-2) reported by Li et al. (2025), and is comparable to mineral dust events (-13.65 Wm-2) reported by Barragan et al. (2020). Radiative forcing at the top of atmosphere (TOA) was negative at both sites, averaging -8.92 Wm-2 at Dunhuang, with a larger magnitude than -4.49 Wm-2 at Minqin, which is close to the -4 Wm-2 reported for dust events (Marsli et al., 2025). This value is comparable to a mineral dust event (-9.1 Wm-2) observed over the Iberian Peninsula (Barragan et al., 2020) but significantly higher than -2.7 and -4.0 Wm-2 during two other dust events at Dunhuang recorded by Bi et al. (2014). These findings demonstrate that elevated aerosol layers exert a cooling effect in both regions. Elevated aerosol layers at Dunhuang exert a more pronounced effect on increasing the planetary albedo, thereby producing a stronger suppression of the surface radiation budget. Additionally, the average atmospheric (ATM) shortwave radiative forcing at Dunhuang is 13.74 Wm-2, higher than 9.10 Wm-2 at Minqin. This suggests that elevated aerosol layers at Dunhuang are more efficient at intercepting and redistributing solar radiation, with atmospheric radiative forcing higher than 5.1 Wm-2 reported for a dust event (Bran et al., 2018), but lower than 16.94 Wm-2 reported for a mineral dust event over the Iberian Peninsula (Barragan et al., 2020). Atmospheric (ATM) shortwave radiative forcing reflects the energy absorbed by aerosols and converted into heat within the atmosphere. The peak shortwave heating rate (Jangid et al., 2024) reached 0.74 Kd-1 at Dunhuang and 0.44 Kd-1 at Minqin, with the value at Dunhuang comparable to the peak heating rate (∼0.7 Kd-1) reported for a light dust event (Li et al., 2025). The mean shortwave heating rates (SHR) at both sites (0.23 Kd-1 at Dunhuang and 0.14 Kd-1 at Minqin) were higher than those observed during several dust events at Dunhuang (Bi et al., 2014) (e.g., 0.1 and 0.14 Kd-1 on 9 and 10 April 2012, respectively), as well as those recorded over Issyk-Kul Lake (0.08 Kd-1) (Gan et al., 2026) and southern Portugal (0.11 Kd-1) (Valenzuela et al., 2016). This indicates that elevated aerosol layers absorb solar shortwave radiation, leading to significant atmospheric heating. This may influence regional weather processes and climate feedback mechanisms by altering the local thermal structure and vertical stability (Huang et al., 2009).

Considering snow cover conditions in the Qilian Mountains, seasonal snow is primarily distributed at altitudes of 3–4 km (equivalent to 1.5–2.5 km a.g.l.) (Guo et al., 2021), which coincides with the altitude range of elevated aerosol layers. After deposition in the Qilian Mountains, these pollutants significantly reduce snow albedo, enabling the surface to absorb more solar radiation. Simultaneously, as indicated by the SHR of the elevated aerosol layer, the layer itself maintains a relatively high temperature due to its absorption of shortwave radiation, and the resulting heat further accelerates snowmelt. This combined effect may accelerates snowmelt and glacier retreat. Therefore, radiative forcing associated with elevated aerosol layer may induce regional warming in the Qilian Mountains, similar to the warming effect of brown clouds over the Indian Peninsula, which accelerates snowmelt in the Himalayas (Ramanathan et al., 2007). This is of considerable concern, as it is directly linked to regional water security and storage. In addition, the Hexi Corridor is also an important solar energy base, the elevated aerosol layer weakens the solar radiation reaching the surface during daytime, thereby directly affecting photovoltaic power generation efficiency (Wang et al., 2026).