the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jun 2025
Atmospheric chemical processing dictates aerosol aluminum solubility: insights from field measurement at two locations in northern China
Abstract. Deposition of mineral dust aerosol into open oceans significantly impacts marine biogeochemistry and primary production, and the deposition rates can be constrained using widely measured dissolved aluminum (Al) in surface seawater as a tracer. However, aerosol Al solubility, a critical parameter used in this method, remains highly uncertain. This work investigated seasonal variations of aerosol Al solubility for supermicron and submicron particles at two locations (Xi’an and Qingdao) in northern China. Aerosol Al solubility was found to be very low at Xi’an and much higher at Qingdao. Furthermore, seasonal variability of Al solubility, its correlation with relative abundance of sulfate and nitrate, and its dependence on relative humidity (RH), are all different at the two locations. We suggest that all the features observed for aerosol Al solubility at the two locations can be well explained by the effects of atmospheric chemical processing. Mineral dust transported to Xi’an (an inland city in Northwest China) was still not significantly aged and thus chemical processing had little effects on aerosol Al solubility. After arriving at Qingdao (a coastal city in the Northwest Pacific), mineral dust was substantially aged by chemical processing, leading to substantial enhancement in aerosol Al solubility. Our work further reveals that aerosol liquid water and acidity play vital roles in the dissolution of aerosol Al by atmospheric chemical processing. We suggest that spatial variation of aerosol Al solubility should be taken into account so that oceanic dust deposition can be better constrained using dissolved Al concentrations in surface seawater.
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RC1: 'Comment on egusphere-2025-2235', Anonymous Referee #1, 07 Jul 2025
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This manuscript provides a concise and informative comparison of aerosol aluminum (Al) solubility across seasons and sites (Xi’an and Qingdao), highlighting notable differences resulted from atmospheric aging. The use of <1um and >1um size fractions adds important size segregation to the analysis, and the interpretation grounded in source proximity and dust aging processes is scientifically sound and well-discussed.
Please see below for my detailed comments:
1. in the abstract, "Furthermore, seasonal variability of Al solubility, its correlation with relative abundance of sulfate and nitrate, and its dependence on relative humidity (RH), are all different at the two locations." it would be better if authors explain in detail how different these parameters are.
2. authors used the phrase “significantly”/"significant" suggesting statistical analysis, but no mention is made of the statistical test used (e.g., t-test, ANOVA). Please clarify the method and significance level (e.g., p < 0.05) and indicate the results either in graphs in main text or in supplemental tables.
3. in line 468, authors said "Aerosol Al solubility at Xi’an showed no significant correlation with relative abundance of sulfate or nitrate", but in Table S8, the pearson r values were significant for >1 um particles at Xi'an in autumn and winter. please provide explanation.
4. in Figure 8, usually ascending order is used, such as "<2.5", "2.5-3.0"...
5. foggy weather might promote aluminum complexation reactions with organics. It's interesting that dissolved Al and Al solubility increased a lot in fog conditions in Table S7. Authors can consider adding a paragraph discussing this.
Citation: https://doi.org/10.5194/egusphere-2025-2235-RC1 -
RC2: 'Comment on egusphere-2025-2235', Anonymous Referee #2, 08 Jul 2025
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The paper systematically investigates the seasonal variations in aerosol aluminum solubility in two northern Chinese cities (Xi'an and Qingdao), revealing the critical influence of atmospheric chemical processes on aluminum solubility. The research topic holds certain scientific significance, as the study of aerosol aluminum solubility is expected to provide key parameters for accurately estimating mineral dust deposition fluxes in the context of marine biogeochemistry and global climate change. However, the paper devotes a significant portion of its content to presenting test data without in-depth analysis or detailed interpretation, resulting in insufficient depth and scientific value. Many conclusions are drawn at a speculative level, lacking solid evidence to substantiate them, which undermines the credibility of the article. The core conclusions of the paper currently carry considerable uncertainty and require further refinement; they should not be hastily drawn. Extensive revisions are recommended, and my suggestions and comments are as follows.
The abstract is overly vague and generalized. For instance, it should explicitly summarize the seasonal variation patterns of aluminum solubility in the two regions, the correlation with the relative abundance of sulfate and nitrate, and how their dependence on relative humidity (RH) differs between the two locations, rather than merely stating that they are “different.”
It is unclear why the authors claim that spatial differences in aerosol aluminum solubility must be fully considered when constraining oceanic dust deposition using dissolved aluminum concentrations in surface seawater. Although the solubility of aluminum in dust differs between Xi'an and Qingdao, and the authors attribute this to varying degrees of aging, dust transported from the same source region to the same oceanic area should undergo the same aging process, resulting in consistent aluminum solubility. It is unclear how the authors arrived at this conclusion.
The Introduction fails to clearly focus on the current major controversies and sources of uncertainty regarding the range of aluminum solubility variations and their influencing mechanisms. Some studies suggest a significant correlation between aluminum solubility and acidic components, while others hold opposing views. The Introduction should more clearly summarize these conflicting findings and unresolved mechanisms, identifying the specific “gaps” or “contradictions” the current study aims to address, thereby strengthening the research motivation.
How were the interferences from locally resuspended dust aluminum in Xi'an and Qingdao excluded? How is it proven that the dust observed in Xi'an and Qingdao originates from the same source and differs only in aging?
The study claims that its findings can be generalized to the “North Pacific dust pathway” or even the “global dust-ocean interface.” However, the current design is based on only two sampling sites (one inland and one coastal), lacking gradient observations (e.g., multi-point trajectory analysis) or broad representativeness. The observed differences may be dominated by the unique characteristics of the sampling sites themselves (e.g., urban pollution, local humidity). How can such significant uncertainty be explained?
Why is aluminum concentration highest in winter? Mineral dust is not commonly observed in Xi'an during winter. Does this indicate that the source of aluminum in Xi'an is not mineral dust?
Lines 170–173: All the sites the authors compare are island observations, which are not strongly comparable to Xi'an. There are numerous observational results from inland China—why are these not mentioned for comparison?
Lines 184–193: The comparative data presented here merely show that aluminum concentrations are higher at sites closer to dust source regions—a conclusion that is obvious and lacks significant scientific value. Could the authors supplement the discussion with differences in aluminum content (μg/g) in dust aerosols at sites at varying distances from dust sources? Analyzing changes in aluminum content during transport and their underlying mechanisms would be more scientifically valuable. The same applies to the analysis of soluble aluminum. Readers would prefer to see variations in aluminum content rather than just absolute concentration changes related to distance from dust sources.
The study presents aluminum concentrations in supermicron and submicron particles but merely displays the data without explaining its scientific significance. Why do aluminum concentrations differ between particle sizes? What mechanisms underlie these differences? How do the seasonal variation characteristics of aluminum content differ between particle sizes, and what causes these differences? The authors' data analysis needs strengthening; it should not be limited to simple data presentation.
Lines 220–224: What drives the seasonal variation in aluminum solubility? Why do differences in aluminum solubility exist between particle sizes? How do seasonal variations in aluminum solubility differ between particle sizes, and why? If there are no differences between particle sizes, then studying size-dependent features is unnecessary.
Line 233: Here, the difference in aluminum solubility between Xi'an and Qingdao is attributed to transport distance. What evidence supports this claim, or is it merely speculation? How large is the uncertainty of this speculation, and how can it be validated?
The authors attribute the differences in aluminum solubility between the two cities to aging during transport. However, it should be noted that the distance from Xi'an to the Taklamakan Desert exceeds 3,000 km, while the distance from Xi'an to Qingdao is about 1,000 km. In other words, transport from Xi'an to Qingdao increases aging time by only about 30%, which is not a substantial difference. Without solid evidence proving the significance of this 1,000 km aging process, the core conclusion is difficult to accept. At present, this conclusion appears to be speculative.
Line 239: Why is the difference in aluminum solubility between the two cities minimal in spring? Spring is precisely the season when dust events are most significant in inland China. If aluminum solubility does not exhibit differences during this critical dust period, does this suggest that the tested aluminum solubility differences are not attributable to dust-derived aluminum?
On the other hand, the impact of aluminum deposition on marine ecosystems should be evaluated from the perspective of dust load weighting. If, as the study shows, aluminum solubility is lowest during spring—when dust loads are highest—then the vast majority of annual dust-derived aluminum has poor solubility, calling into question its contribution to marine ecosystems. Moreover, does this imply that aluminum solubility in Xi'an and Qingdao is largely similar for most of the year, with differences only appearing in the less dusty seasons (summer, autumn, and winter), thus contributing minimally to annual totals? The authors should scientifically assess these differences in light of total dust transport.
Line 252: What are the concentrations of dust aerosols under different weather conditions? On clean or haze days, there is likely little dust transported from deserts, with local resuspended dust dominating. If aluminum solubility is similar under these conditions, does this suggest that solubility differences between desert dust and local resuspended dust in Xi'an are negligible, making it unreliable as a source indicator?
Line 258: If the aluminum solubility in Xi'an's dust is very close to that at the source region, how can the impact of aging over nearly 3,000 km of transport on aluminum solubility be explained? Conversely, why does the 1,000 km transport from Xi'an to Qingdao have such a pronounced effect on aluminum solubility?
Line 283: How is it proven that dust sources are consistent across haze days, fog days, and clean days? How is it confirmed that aluminum solubility at the origin is identical and that the observed differences arise primarily from varying atmospheric chemical processes?
Line 285: The authors vaguely speculate that liquid-phase reactions enhance aluminum solubility. First, dust is a hydrophobic aerosol, and even on haze days, aerosol water content does not increase significantly. Can the authors provide data on how much aerosol liquid water content actually increased during their observations? Furthermore, what specific liquid-phase reactions promote aluminum dissolution? What triggers these reactions? Why do such reactions not occur on haze days in Xi'an?
Based on the authors' analysis, the observed increase in aluminum solubility in Qingdao is more likely due to local atmospheric chemical processes (if their speculation is correct) rather than aging during transport. On haze days in Qingdao, it is unlikely that the entire dust transport pathway experiences haze conditions; instead, these are more likely dust days, representing cases with minimal aging and the least increase in aluminum solubility. Dust days are the most critical cases for annual dust transport from source regions to Xi'an and Qingdao. This suggests that large amounts of dust transported from deserts to Xi'an and Qingdao do not undergo significant aging to increase aluminum solubility. Instead, local haze in Qingdao may enhance aluminum solubility, but this accounts for only a small fraction of annual dust, which is not the dominant component.
Figure 5 should include the p-values of the fits. Has the fit passed significance testing? The correlation appears weak, making it difficult to support the authors' claim of an inverse relationship.
Line 344: On rapidly transported dust days, dust in Xi'an may exhibit external mixing with acidic components, whereas on haze days, over 95% of dust is internally mixed with acidic components. The authors could separately analyze aluminum solubility's relationship with acidic components on haze days and dust days to validate their explanation.
In Figure 6, “r” is used, while Figure 5 uses “r²,” and Figure 6 includes p-values. It is recommended to standardize the plotting conventions.
Line 377: The analysis here focuses only on the relationship between local RH variations and aluminum solubility in Xi'an. If high RH promotes liquid-phase reactions that increase aluminum solubility, then aluminum solubility should also rise during high-RH conditions in Xi'an. The role of local chemistry should not depend on the distance from the desert.
Line 379: It is difficult to understand why high RH in Qingdao increases aluminum solubility but not in Xi'an. Atmospheric chemical processes should be similar—are there other controlling factors influencing this aluminum solubility mechanism?
Line 410: I recommend categorizing samples into dust days and haze days to examine the impact of mixing state.
Line 416: I disagree with labeling dust in Xi'an as “fresh dust,” given that it originates over 3,000 km away.
Citation: https://doi.org/10.5194/egusphere-2025-2235-RC2
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