Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects

Lin, Zikai; Zhou, Tian; Li, Zhengpeng; Gu, Yonghong; Wu, Dongsheng; Zhou, Ping; Zhang, Keyu; Gao, Qili; Li, Xingran; Huang, Zhongwei; Bi, Jianrong; Yang, Lili; Wang, Lina

doi:10.5194/egusphere-2026-2659

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https://doi.org/10.5194/egusphere-2026-2659

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29 May 2026

| 29 May 2026

Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects

Zikai Lin, Tian Zhou, Zhengpeng Li, Yonghong Gu, Dongsheng Wu, Ping Zhou, Keyu Zhang, Qili Gao, Xingran Li, Zhongwei Huang, Jianrong Bi, Lili Yang, and Lina Wang

Abstract. The cyclic processes of aerosol evolution have a profound impact on regional climate, water resources and ecosystems. However, studies on diurnal variations of aerosol vertical distribution in typical ecologically sensitive areas remain scarce due to limited availability of high-resolution profiles. This study first identifies elevated aerosol layer (EAL) within daytime mixed layer over the Hexi Corridor, northwest China, defined as a high-concentration layer above surface but within mixed layer. Based on intensive observation campaigns conducted in 2010 and 2012, we analyze the diurnal variation, formation mechanisms, air quality and radiative effects of EALs. The results show EALs frequently occur at altitudes of 0.6–2km during daytime. Excluding dust storms, occurrence frequencies reaches 81% (37%) in Dunhuang (Minqin) region , dominated by dust aerosol (anthropogenic pollutants). Driven by a thermodynamic coupling effect characterized by positive anomalies in potential temperature and negative anomalies in relative humidity, aerosols accumulate at the bottom of the stable stratification. Thus, a simplified conceptual model for prediction is proposed. The peak PM10 concentration within EAL (203.3±106.6μg/m³) at ~1.2km is five times higher than that at surface (40.8±30.4μg/m³). Furthermore, EALs enhance the atmospheric shortwave heating rate (up to 0.7K/day), and form a daily-scale heating pump that may accelerate snowmelt/glacier retreat in the region. These findings suggest that aerosol vertical evolution should receive greater consideration in air pollution prevention, ecosystem protection, water resource management, and wind/solar energy utilization in ecologically sensitive areas-particularly over complex terrains-rather than focusing solely on surface air pollution.

Received: 08 May 2026 – Discussion started: 29 May 2026

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Zikai Lin, Tian Zhou, Zhengpeng Li, Yonghong Gu, Dongsheng Wu, Ping Zhou, Keyu Zhang, Qili Gao, Xingran Li, Zhongwei Huang, Jianrong Bi, Lili Yang, and Lina Wang

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-2659', Anonymous Referee #1, 26 Jun 2026
The manuscript titled “Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects” addresses an important and relatively understudied topic: the high-resolution diurnal vertical structure of aerosols over the complex terrains of Northwest China (specifically the Hexi Corridor). The identification of Elevated Aerosol Layers (EALs) during the daytime and their quantification via lidar observations provides valuable insights into regional air quality budgets, boundary layer dynamics, and localized radiative forcing. The paper falls well within the scope of Atmospheric Chemistry and Physics. However, the manuscript still needs revised and improved; there are several critical issues that should be addressed before publication.
Major Comments
The Introduction contains much general background on aerosol-related studies, while the discussion of current research bottlenecks remains insufficient. I would suggest placing more emphasis on the key knowledge gaps in existing studies. In addition, the novelty of this work is not clearly highlighted. The authors should more explicitly state the main innovation and scientific contribution of this study.

Section 3.2 contains excessive descriptive narration of meteorological evolution. Much of the text provides step-by-step qualitative descriptions of temperature, potential temperature, wind, and humidity fields, resembling a case report rather than a concise mechanism analysis. Additionally, the two case studies presented for Dunhuang and Minqin show highly similar thermodynamic evolution, yet both are described in extensive detail. Although Section 3.2 aims to explain formation mechanisms, much of the discussion remains case-specific. Some summary conclusions and key points should be provided.

Given that the observational data used in this study were collected more than a decade ago (2010–2012), their representativeness for present-day atmospheric conditions may be limited. Changes in emissions, aerosol composition, and regional atmospheric environments over the past decade could affect the occurrence and characteristics of elevated aerosol layers. I suggest the authors discuss this limitation in the conclusion.

In the abstract, the authors claim that these EALs form a daily-scale heating pump that transports pollutants via mountain-valley breezes to the Qilian Mountains, accelerating glacier melt. However, in Section 3.2 (Line 430), the authors explicitly state: “the Dunhuang site is located far from the Qilian Mountains. Consequently, the elevated aerosol layer exerted a negligible influence on the Qilian Mountains in this case.” This seems contradictory.

Line 155: “This classification is based on the gradient characteristics of the vertical profiles. If a continuous positive gradient is observed and the corresponding proportion of observations exceeds 20%, an elevated aerosol layer is considered to be present.” What does positive grade refer to here. This is currently ambiguous.

Minor Comments
Line 85: supply literatures.

Line 150: “following the method of (Zhou et al., 2021; Zhou et al., 2018)”, this expression seems to be non-standard.

Line 165: “PM10”, indicate the subscripts correctly. Is there any reference source for Equation 1? Provide more information.

Line 225: Were the formulas 2-6 developed by the authors, or are they based on existing references?

Line 300: The legend for Figure 5 is not standardized; provide a detailed description. Similar cases include Fig. 9, Fig 10.

The fonts in the figures are inconsistent. For example, Figure 1 seems to be Times New Roman, Figure 2 seems to be Calibri.

Some references appear inconsistent or incomplete. Please carefully check all references.
Citation: https://doi.org/10.5194/egusphere-2026-2659-RC1
RC2:
'Comment on egusphere-2026-2659', Anonymous Referee #2, 29 Jun 2026
Review of "Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects" by Zikai Lin et al.
The manuscript "Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects" first identifies the elevated aerosol layer within the daytime mixed layer over the Hexi Corridor, northwest China, defined as a high-concentration layer above the surface but within the mixed layer. In general, the paper is well written and presented in a logical way. It is a timely and important piece of work, and of general interest for elevated aerosol layers. I therefore recommend publication of this paper in Atmospheric Chemistry and Physics after major revisions. My comments are listed as follows:
Major Comments:
The study identifies frequent elevated aerosol layers within the daytime mixed layer over the Hexi Corridor and further suggests that their formation is jointly controlled by positive potential temperature anomalies and negative relative humidity anomalies. However, the current EAL identification method remains relatively empirical, especially regarding the physical meaning of the “continuous positive gradient” and the 20% threshold. The authors are encouraged to provide a sensitivity test of different threshold combinations.

The authors suggest that EAL occurrence is associated with positive potential temperature anomalies and negative relative humidity anomalies, which is physically plausible. However, the current evidence is mainly based on composite anomaly fields, and a daily-sample statistical comparison between EAL and non-EAL days is still lacking. I suggest that the authors provide simple site-specific statistics, such as the mean differences, bootstrap 95% confidence intervals, and non-parametric significance tests for potential temperature and relative humidity anomalies within the main EAL height range and daytime period. A boxplot or scatter-boxplot showing the day-to-day variability would also be helpful. This would provide stronger statistical support for the proposed thermodynamic signature and help clarify possible site-dependent differences.

The authors establish an empirical relationship between near-surface lidar extinction coefficients and ground-level PM10 concentrations, and then use this relationship to estimate PM10 concentrations within the EAL. Since the conversion from extinction coefficient to mass concentration can be affected by particle size distribution, relative humidity, and aerosol type, the authors are encouraged to provide a clearer presentation of the paired PM10–extinction samples and the fitted relationship. This would help readers assess the applicability of the empirical conversion. In addition, the estimated PM10 concentrations within the EAL should be described more cautiously as empirical estimates to avoid over-quantitative interpretation.

The discussion of radiative effects and potential impacts on snow and glaciers in the Qilian Mountains is scientifically interesting, but the current conclusions appear somewhat overextended. The radiative results are mainly based on two representative SBDART case simulations and are insufficient to demonstrate that EALs generally form a “heating pump” or significantly accelerate regional glacier/snow melting. The authors are encouraged to revise this part as a potential implication.

Specific Comments:
In the abstract, the phrase “This study first identifies...” should be weakened. EALs are not a completely new aerosol phenomenon. The novelty of this study would be more appropriately described as the first systematic analysis of the diurnal variation and potential mechanisms of EALs based on high-resolution ground-based observations in ecologically sensitive areas of the Hexi Corridor.

Lines 150–160 should provide more details on the EAL identification method. The authors should specify the variable used for the “continuous positive gradient”, the minimum vertical thickness, minimum duration, smoothing method, and the criteria for excluding clouds and dust contamination.

Lines 164–168 present the empirical regression between PM10 and aerosol extinction coefficient. The authors should provide units, scatter plots. Since the regression equation has a negative intercept, the treatment of possible negative concentrations under low-extinction conditions should also be explained.

Lines 357–360 state that a weakened potential temperature gradient indicates increased atmospheric stability. This physical interpretation should be reconsidered. In general, a stronger positive vertical gradient of potential temperature indicates stronger static stability. If the authors refer to enhanced local stable stratification or suppressed vertical motion, the sentence should be rewritten with clearer evidence.

Figures 7 and 9 are key case studies. The authors should clarify whether the time is Beijing Time or local solar time. Since Dunhuang and Minqin are located in northwest China, the difference between local solar time and Beijing Time may affect the interpretation of the diurnal peak.

In Lines 540–555, when comparing PM10 concentrations within EALs with severe haze or dust events in other regions, the authors should emphasize that these values are equivalent mass concentrations estimated from extinction coefficients rather than directly measured concentrations.

The discussion on snow, glaciers, and water resources in the Qilian Mountains in Lines 599–610 should be moderated. Without deposition fluxes, snow-sample black carbon/dust concentrations, or snow albedo observations, “accelerate snowmelt/glacier retreat” should be revised to “may have potential implications for snow and glacier energy balance”.

The phrase “heating pump” in the abstract and conclusion is vivid but not sufficiently rigorous. It is suggested to replace it with “enhanced shortwave heating within the aerosol layer” or “a daily-scale atmospheric heating effect”.

The manuscript should use either “depolarisation ratio” or “depolarization ratio” consistently throughout the text.

The linear depolarization ratio shown in Figure 6 is inconsistent with the linear volume depolarization ratio indicated in the figure caption.
Citation: https://doi.org/10.5194/egusphere-2026-2659-RC2

Zikai Lin, Tian Zhou, Zhengpeng Li, Yonghong Gu, Dongsheng Wu, Ping Zhou, Keyu Zhang, Qili Gao, Xingran Li, Zhongwei Huang, Jianrong Bi, Lili Yang, and Lina Wang

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Short summary

This study examined high-altitude airborne particles over the Hexi Corridor using field observations. Such particles commonly accumulate 0.6–2 kilometres aloft. Dunhuang features natural dust, while Minqin is dominated by human pollutants. Upper-level pollution is five times higher than that on the ground. These trapped particles warm the air and may accelerate glacier melting. This work suggests that high-altitude pollution should not be overlooked to better protect fragile local environments.


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