Direct Measurement of Aerosol Liquid Water Content: A Case Study in Summer in Nanjing, China

Aerosol liquid water content (ALWC) affects the mass loading, optical properties, and toxicity of aerosols. However, the measurement of ALWC is very rare due to its requirement of sophisticated instruments and its high operational costs. In this work, we improved on our previous simple, low-cost method by using a combination of one real-time fine particulate matter (PM_2.5) monitor and two turbidimeters and successfully applied these for the direct measurement of ALWC in PM_2.5 in Nanjing during the summer of 2023. The average ALWC during this measurement period occupied ~1/6 of the total PM_2.5 mass, and this contribution was even greater with the elevation in the PM_2.5 concentration. The ALWC was, as anticipated, closely related to the relative humidity (RH) and PM_2.5 concentrations, but it did not always increase with the air quality index (AQI) due to the fact that polluted periods in summer were often governed by high O₃ levels, not PM_2.5 levels. The ALWC also had a great impact on visibility; it could decrease the visibility rapidly to hazy conditions when the dry PM_2.5 was not high (~30 μg m^-3) or the AQI was "good" (75~100), indicating that the air quality classified as "good" using the dry PM_2.5 concentration might actually be "lightly polluted" if the ALWC is included. We also found that the air mass originating from Northeast China had the lowest PM_2.5 mass concentration yet the highest ALWC values due to its high RH. Moreover, the quantification of ALWC levels can help us understand the solubility/bioavailability and thus the toxic effects of some specific components (for example, heavy metals or organics). Moreover, the influence of ALWC on air quality classifications should also be considered in the assessment of the health effects of air pollution and in public health early warning and protection.