Aerosol water content (AWC) significantly affects secondary aerosol formation and atmospheric visibility. Most ambient AWC values are obtained from models because direct measurement is challenging. In this study, the sequential aerosol-water measurement system (SAWMS) was applied to measure AWC at the Xiaogang air-quality monitoring site of the Taiwan Environmental Protection Administration, located in an industrialized seaport city. The relative humidity (RH) was set at 90% in the SAWMS during measurement, and the PM2.5 AWC was 39.0 ± 14.3 µg m–3 on average, which was 140% higher than the monitored PM2.5 average concentration. Water-soluble inorganic ions (WSIIs) were analyzed offline and used in ISORROPIA II to model the AWC. The modeled and measured AWC was well-correlated (R2 = 0.85; n = 39, p < 0.05), indicating that WSIIs contributed the most to AWC. During high AWC periods, NO3– concentrations were relatively higher, suggesting that NO3– was the predominant species contributing to AWC. Humidographs were constructed to analyze the AWC values under varying RH levels during the humidification and dehumidification processes for the three selected samples. The humidification results revealed a significant difference between the measured and modeled AWC within 60–80% RH. This might be due to deviations of aerosol combination types and the mixing state from atmospheric conditions. The modeled AWC was close to the measured AWC when the RH was over 80%. The nitrogen oxidation ratio and the AWC were well-correlated (R2 = 0.60) throughout the sampling period, implying that the measured AWC was beneficial to NO3– formation in the urban area. In summary, significant differences between modeled and measured AWC appeared during the humidification and dehumidification processes when the RH was below 80%, indicating that direct measurement of AWC under varying RH levels is still necessary.