the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Divergent Drivers of Aerosol Acidity: Evidence for Shifting Regulatory Regimes in a Coastal Region
Abstract. Aerosol acidity plays a crucial role in multiphase atmospheric chemistry, influencing aerosol composition, gas-particle partitioning, and the oxidative capacity of atmosphere. However, the mechanisms governing aerosol acidity in coastal area under extreme weather remains challenging due to its complexity of atmospheric transport. Here, we investigate aerosol pH in Shenzhen, a coastal megacity in China, by integrating field observations with multiphase buffer theory and ISORROPIA simulations. Our observations captured both a typhoon episode and typical non-typhoon periods with two contrasting regimes: during non-typhoon periods, aerosols were consistently buffered by the NH4+/NH3 pair, with relative humidity serving as the primary driver of pH variability, enabling reliable predictions using multiphase buffer theory. In contrast, during a typhoon episode, sea salt derived nonvolatile cations emerged as the dominant drivers, violating the charge balance for NH4+/NH3 buffering and leading to poor performance of buffer theory. Under these conditions, ISORROPIA simulations with constant aerosol water content reproduced the observed pH more reliably, highlighting a compositional rather than meteorological control. Our results provide the direct field-based evidence for regime shifts in aerosol acidity regulation in coastal area, and underscore the need for chemical transport models to account for composition-meteorology interactions to improve acidity predictions under extreme weather events.
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Status: open (until 17 Nov 2025)
- RC1: 'Comment on egusphere-2025-4757', Anonymous Referee #1, 20 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4757', Anonymous Referee #2, 20 Oct 2025
reply
The authors conducted observations at the coastal Xichong site in Shenzhen, integrated field data, multiphase buffer theory, and ISORROPIA simulations to comparatively analyze the buffering capacity and driving factors of aerosol pH during typhoon and non-typhoon periods. They found that during non-typhoon periods, aerosol pH is buffered by the NH4+/NH3 pair with relative humidity as the main driver, while during typhoon periods, it is dominated by sea salt-derive NVCs.
This study points out the limitations of the multiphase buffer theory in scenarios dominated by NVCs, which is innovative. The overall quality of the study is relatively high, and after addressing the following comments, I believe it meets the quality requirements of Atmospheric Chemistry and Physics.
The detailed comments are as follows.
1. What does “constent aerosol water” in the Abstract mean? Why is this parameter considered?
2. The Introduction lacks a review of how the multiphase buffer theory explains the driving factors of pH.
3. This study uses ISORROPIA to calculate the acidity of inorganic aerosols, and it is necessary to define the boundaries at an appropriate position.
4. Figure 1: It is suggested to use arrows to indicate wind direction instead of degrees.
5. Figure 2: Based on the calculations using the multiphase buffer theory, can it be indicated that the buffer peak is the range where the pH value should lie? Is there evidence from other studies to support this, or is it a coincidental result of this paper?
6. Do Figure 2 and Figure 3 contradict each other? The former indicates that the buffer capacity during non-typhoon periods is stronger, so why is the ΔpH here much larger than that during typhoon periods? This is especially considering that the difference in the standard deviation of the data reported in Figure 1 is not significant.
7. Figure 4: What previous evidence led the authors to want to fix AWC? And to what value is AWC fixed? The mean? The median?
Citation: https://doi.org/10.5194/egusphere-2025-4757-RC2
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The article titled “Divergent Drivers of Aerosol Acidity: Evidence for Shifting Regulatory Regimes in a Coastal Region” combines field observations, multiphase buffer theory, and ISORROPIA simulations to investigate aerosol pH dynamics in Shenzhen under typhoon and non-typhoon conditions. The work holds scientific value and practical relevance. Some issues need to be addressed.