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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]

Submicron aerosol (PM1) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM1 and PM2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM1/PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM1/PM2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Plain Language Summary: Current air pollution studies rely largely upon aerosol mass spectrometers that provide real‐time measurements of submicron aerosol (PM1) species, and in many studies, PM1 aerosol species are used to validate those of PM2.5 in chemical transport models and estimate particle acidity and aerosol water content which are key parameters in studying heterogeneous reactions. However, the uncertainties in air pollution studies due to the chemical differences of PM1 and PM2.5 are poorly constrained, particularly in highly polluted environment, for example, China and India. We found that the changes in PM1/PM2.5 ratios as a function of relative humidity were largely different for primary and secondary aerosol species in highly polluted environment, which was likely driven by the changes in aerosol hygroscopicity and phase states. The chemical differences of PM1/PM2.5 ratios were also found to have negligible impacts on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Considering the rapid increases in the deployments of aerosol mass spectrometers worldwide, the results in this study are of great importance for a better understanding of the uncertainties in both modelling and observations. Key Points: We observed large chemical differences between PM1 and PM2.5 under high relative humidity periods in highly polluted environmentThe RH dependence of PM1/PM2.5 ratios of primary and secondary species was largely different due to different hygroscopicityThe chemical differences between PM1 and PM2.5 have negligible influences on particle acidity prediction, yet have a large impact on AWC [ABSTRACT FROM AUTHOR]