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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]

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implications at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heterogeneous conversion of N O 2 was more efficient on aerosol surfaces than on ground, highlighting the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 ± 0.26, 0.97 ± 0.47 and 1.45 ± 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (N O 2 heterogeneous reaction and particulate nitrate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 ± 0.71), (1.75 ± 1.26) and (1.82 ± 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O 3 photolysis (i.e., (0.004 ± 0.004), (0.006 ± 0.007) and (0.0035 ± 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxidation capacity and hence promote the formation of secondary aerosols and pollutants. [ABSTRACT FROM AUTHOR]