# Direct Measurement of Aerosol Liquid Water Content: A Case Study in Summer in Nanjing, China
## 3 Results and Discussion
### Characteristics of ALWC under Different Pollution Scenarios
Moreover, since large amounts of ALWC can increase the dissolution of metal ions or other toxic and hazardous substances, this result indicates that the same toxic species might be released more effectively in polluted than in clean air. CO can be considered as a tracer of the primary combustion source, and the ratio of PM\({}_{2.5}\)/CO can be used to qualitatively infer the contribution of secondary aerosols [53; 54]. The mean value of PM\({}_{2.5}\)/CO in this experiment was 0.04 (0.004-0.190) which is consistent with that of a previous study [55]. From Figure 6c, it can be seen that the content of ALWC in general increased with the increase in the PM\({}_{2.5}\)/CO ratio (2.36-5.95 \(\mu\)g m\({}^{-3}\)), suggesting the possible presence of more accumulation-mode particles and secondary species during the polluted period, which indeed enhanced the water uptake of the aerosols [56; 57]. The air quality index (AQI) is a combined indicator considering both particular matter and the major gaseous pollutants [58; 59]. The mean AQI was on average 42.47, ranging from 12 to 142 throughout the measurement period, and only 3.57% of the AQI values were classified as \"polluted\". Also, it should be noted that the AQI is mainly governed by O\({}_{3}\) levels, owing to strong photochemical reactions between volatile organic compounds and nitrogen oxides during the summer in Nanjing, consistent with a previous analysis during the summers of 2019 and 2020 in the YRD region [60]. Here, we further explored the features of the ALWC under different AQI levels. First, the average concentrations of PM\({}_{2.5}\), gaseous pollutants, and the ALWC at different AQI categories are illustrated in Figure 7a. The O\({}_{3}\) concentrations continuously increased with the AQI, while the PM\({}_{2.5}\) increased at an AQI \(<\) 75, but remained generally stable at an AQI \(>\) 75, showing that relatively large AQI values (more polluted air) were mainly affected by O\({}_{3}\) not PM\({}_{2.5}\) in this work. The behavior of the ALWC against the AQI resembled that of PM\({}_{2.5}\) at an AQI \(<\) 75 (maximum of 5.49 \(\mu\)g m\({}^{-3}\) for an AQI of 50-75), but decreased quickly and significantly at an AQI \(>\) 75 (minimum of 0.30 \(\mu\)g m\({}^{-3}\) for an AQI of 125-150). This can be expected as high AQI values were associated with high O\({}_{3}\) levels accompanied by high temperatures, strong winds, and low RHs (the average RH dropped from 65.04% at an AQI \(<\) 75 to 42.17% at an AQI \(>\) 75), which weakened the aerosol water uptake. Figure 7b shows the variation in the mass share of the ALWC in the total PM\({}_{2.5}\) under different AQI conditions, and it can be seen that the average mass contribution of the ALWC in PM\({}_{2.5}\) was 13.27\(\sim\)15.13% at an AQI \(<\) 75, but decreased to 0.18-3.27% at an AQI \(>\) 75. Particularly, the ALWC only contributed \(\sim\)0.18% of the PM\({}_{2.5}\) mass in the polluted air (AQI \(>\) 100), indicating that the ALWC negligibly impacts the AQI, again due to the large AQI that was mainly caused by O\({}_{3}\) not PM\({}_{2.5}\) during the summer. Yet, we expect the ALWC to play an important role in air pollution events during the winter, when the AQI is mainly controlled by the PM pollution.