# Direct Measurement of Aerosol Liquid Water Content: A Case Study in Summer in Nanjing, China
###### Abstract  
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 \(\sim\)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 O\({}_{3}\) 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 (\(\sim\)30 \(\upmu\)g 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. a 2024 12 164 12024 120  
important indicator to link metals with their adverse health effects in epidemiological and toxicological studies [14]. Reactive oxygen species (ROS) produced by transition metal ions (such as cadmium and nickel) may trigger a wide range of health consequences including inflammation, DNA damage, cardiovascular disease, and acute heart disease [15; 16]. Some other non-transition metals can be quite harmful to human health too; for instance, lead and arsenic are found to be highly carcinogenic [16]. Previous studies show that soluble fractions of Fe, Cd, Pb, and Zn increased significantly under acidic solvents, suggesting that acidic aerosols with an abundant ALWC are able to dissolve more trace metals [17] and therefore make the particles more toxic and pose a greater threat to human health than dry aerosols. However, despite its critical importance, ALWC is often not routinely measured or included in reported PM\({}_{2.5}\) concentrations. Most instruments determine the (dry) aerosol mass and composition after the removal of associated aerosol water. The ALWC is typically estimated based on the measured aerosol composition and concentration by using various thermodynamic models (for examples, the extended aerosol inorganic model (E-AIM) [18] and ISORROPIA II [19]). Such a methodology assumes a thermodynamic equilibrium state of the aerosols and often does not include all aerosol components (especially organics) [20; 21]. A humidified tandem differential mobility analyzer (H-TDMA) can be used to determine the hygroscopic growth factor (HGF) of ambient aerosols under a fixed RH (typically at 90%) [22]; it can obtain the particle number size distributions [23; 24; 25] under both dry and humid conditions and then calculate the ALWC by comparing the volume changes before and after humidification [26; 27; 28]. The cloud condensation nuclei counter (CCNC) and electrodynamic balance (EDB) or optical tweezers can also be used to study the hygroscopicity (though they often are not direct measurements of ALWC) of particles [29; 30; 31].