# Direct Measurement of Aerosol Liquid Water Content: A Case Study in Summer in Nanjing, China
## 3 Results and Discussion
### Relationship between ALWC and Visibility  
As is well known, ALWC is an important factor influencing atmospheric visibility.  
Here, we further plotted the variations in visibility against the ALWC at different PM\({}_{2.5}\) and AQI levels, respectively, in Figure 8a,b. It is very clear that the visibility degraded rapidly with the increase in PM\({}_{2.5}\) levels and the exponential coefficients also increased with the PM\({}_{2.5}\) levels, indicating the same amount of ALWC can lower the visibility even more significantly when the PM\({}_{2.5}\) pollution is greater. As the ALWC increases, the aerosol size increases due to the uptake of water and can lead to a large increase in the aerosol extinction coefficient [61; 62], which has been observed in various sites including Guangzhou, China and Delhi, India [63; 64]. Overall, when the PM\({}_{2.5}\) concentration was higher than 30 \(\upmu\)g m\({}^{-3}\), in most cases, the association with a very small amount of ALWC could lower the visibility to less than 10 km, i.e., hazy conditions; when the PM\({}_{2.5}\) mass loading was between 20 and 30 \(\upmu\)g m\({}^{-3}\), the ALWC increased to \(\sim\)15 \(\upmu\)g m\({}^{-3}\) and the visibility decreased to near 10 km, approaching hazy conditions. In other cases, the ALWC decreases the visibility but does not induce hazy conditions. This result is similar to that of a previous study, which shows that aerosols may have a great impact on visibility in summer when the PM\({}_{2.5}\) concentration reaches a certain threshold [65].  
On the other hand, Figure 8b shows the variation in visibility against the ALWC at different AQI levels. The visibility shows an even faster decrease with the increase in AQI levels, as can be roughly determined from the exponential coeffici  
Figure 8: (**a**) Variation in visibility with ALWC for different PM\({}_{2.5}\) concentrations; and (**b**) Variation in visibility with ALWC for different AQI values. The fitting equation used is y = a*xb.  
Figure 7: (**a**) Average concentrations of different gaseous pollutants (CO, NO\({}_{2}\), SO\({}_{2}\), O\({}_{3}\)) as well as PM\({}_{2.5}\) and ALWC at different AQI ranges. (**b**) Distribution of mass fractions of ALWC to total PM\({}_{2.5}\) at different AQI ranges.  
visibility can decrease to <10 km even at an AQI of 75-100, which means that the air quality is categorized as good (\"clean air\"), but when the ALWC is taken into consideration, the air can become hazy (\"polluted air\") in many cases, highlighting the importance of the ALWC. This finding has important implications, as the inclusion of ALWC can affect air quality classifications, which should be taken into account in public health early warning and protection systems. Another interesting result is that the visibility sometimes was not less than 10 km when the AQI was larger than 100. This result was due to the reason explained earlier: large AQI values in the summer are mainly attributed to large O\({}_{3}\) not PM\({}_{2.5}\) levels; in addition, only a small portion of the data fell into the AQI > 100 range, making the fitting less robust compared to that of other cases.