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This study introduces an innovative outdoor nephelometer system designed to monitor the dynamic hygroscopic behavior of aerosols in ambient air. Field measurements conducted in the Pearl River Delta region of China unveil significant roles of relative humidity (RH) swings on aerosol phase states. Highlighting the occurrence of aerosol crystallization in the afternoon followed by gradual deliquescence as RH increases, particularly when minimum afternoon RH drops below 35%, with no such behavior observed when it exceeds 40%. Emphasizing that aerosol phase states are shaped not only by RH and chemical composition but also by their RH history, illustrating that RH levels of 70% in the morning or evening may correspond to fully dissolved metastable or partially dissolved metastable states of ambient aerosols. These findings underscore the need to account for RH history when predicting aerosol phase states and have broader implications for comprehending aerosol behavior and its atmospheric impacts. Plain Language Summary: This research introduces a new device to study how tiny particles called atmospheric aerosols in the air behave when they encounter moisture. We tested it in a region of China and discovered that relative humidity fluctuations from morning to evening impact significantly on phase state of atmospheric aerosols. In the afternoon, some of the particles might become solid, like tiny crystals, and later turn into liquid gradually as humidity increases. This phenomenon occurs when the afternoon minimum relative humidity is low, below 35%, but does not take place when it exceeds 40%. Suggesting that relative humidity history matters for aerosol phase. For example, relative humidity of 70% in the morning or evening, particles may be in a liquid or semi‐liquid state. This study helps us understand phase changes of atmospheric particles and their effects on our environment. Key Points: Innovative outdoor nephelometer system was developed to monitor aerosol hygroscopic behavior in ambient airAerosol crystallization in the afternoon followed by gradual deliquescence observedRelative humidity history plays significant roles in determining aerosol phase state [ABSTRACT FROM AUTHOR]

The commercial continuous‐flow streamwise thermal‐gradient cloud condensation nuclei (CCN) counter is widely used in aerosol CCN activity measurements which are critical for investigating aerosol‐cloud interactions. In CCN measurements, a critical threshold is needed for distinguishing interstitial aerosols and activated droplets, and a default threshold of 0.75 μm was set in CCN counter. Theoretically, interstitial aerosols could also grow larger than 0.75 μm in CCN counter at low supersaturations, thus theoretical thresholds derived from the Köhler theory were suggested. Here, we report that droplet growth in the CCN counter is kinetically limited and CCN‐active droplets does not reach their theoretical diameters under low supersaturations. Neglecting the kinetic growth limitation in CCN identification would count less CCN and underestimate the aerosol hygroscopicity parameter κ $\kappa $ by up to 50% for supersaturations lower than 0.1%. We recommend that thresholds considering kinetic limitations should be used as new criteria for CCN identification under low supersaturations. Plain Language Summary: Accurate Cloud Condensation Nuclei (CCN) predictions in climate and regional models are crucial for the assessment of climate effects of aerosol‐cloud interactions. Direct measurements of CCN activity, varied by aerosol size, hygroscopicity, and supersaturation, are important for model verifications and improvements. The commercial continuous‐flow streamwise thermal‐gradient CCN counter (CCNC) is widely used in CCN activity measurements. We found that the droplet growth in the CCNC is kinetically limited under low supersaturations (<0.14%), thus not reaching the theoretical critical activation diameter, which significantly affect CCN identification. Kinetic model results revealed that the wet diameter thresholds of CCN identification that took the kinetic limitations of droplet growth into account were independent of dry aerosol size and hygroscopicity thus could be directly used as new criteria for CCN identification. Neglecting the kinetic growth limitation in CCN identification would result in significant bias in CCN activity measurements and thus supersaturated aerosol hygroscopicity retrievals. Findings of this research have significant impacts on CCN activity measurements and supersaturated aerosol hygroscopicity investigations. Key Points: Limited hygroscopic growth of cloud condensation nuclei (CCN) in CCN counter is recognized and evaluated based on model analysisKinetic limitations in CCN counter impacts significantly on size‐resolved CCN activity measurementsKinetic limitations in CCN counter impacts significantly on aerosol hygroscopicity measurements under low supersaturation [ABSTRACT FROM AUTHOR]

The hygroscopicity parameter (κ) has been widely used in studies of aerosol hygroscopic growth and cloud condensation nucleus activation, but the effect of relative humidity (RH) variations on ambient aerosol hygroscopicity is rarely discussed and is often ignored in the application of κ‐Köhler theory. In this study, an improved humidified nephelometer system was used to measure the scattering enhancement (f(RH)) of bulk aerosols and the RH‐dependent κ was derived. It was found that the κ of both pure inorganic water‐soluble electrolytes and ambient aerosols is significantly RH‐dependent. The particle size had a great influence on f(RH), but κ had no obvious size dependence. Compared with using the RH‐dependent κ, the scattering coefficient of ambient aerosols with the fixed κ value would produce a relative error of −10%–25%. Parameterization schemes of RH‐dependent κ of ambient aerosols were established, which could serve as a reference for the future research. Plain Language Summary: A large proportion of environmental aerosols is hygroscopic substances. In the case of high RH, aerosol particles will absorb liquid water and will be in a state of deliquescence. The water uptake ability of aerosol is its hygroscopicity which is a very important parameter in the studies of climate effect, environmental impact, formation, and evolution process of aerosols. The κ‐Köhler theory was established and a hygroscopicity parameter (κ) was proposed in 2007 to simplify the calculation of hygroscopic growth or liquid water content for aerosol particles. The κ is commonly treated as a constant value for a certain particle or a specific mass of bulk ambient aerosols. In this study, it is found that the κ of ambient aerosol is significantly RH‐dependent, and this feature cannot be ignored in the calculation of aerosol hygroscopic growth. Key Points: A humidified nephelometer system was improved and used to measure the aerosol hygroscopicity under different relative humidityThe hygroscopicity of ambient aerosols showed significant relative humidity dependenceThe parameterization schemes of relative humidity‐dependent κ of ambient aerosols were established [ABSTRACT FROM AUTHOR]