# 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  
According to the current China ambient air quality standard (CAAQS), we classified the sampled period into \"clean\" (24 h averaged PM\({}_{2.5}\) < 35 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)) and \"polluted\" periods (24 h averaged PM\({}_{2.5}\)\(\geq\) 35 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)). Figure 6a displays the distribution of the PM\({}_{2.5}\), ALWC, O\({}_{3}\), CO, NO\({}_{2}\), and SO\({}_{2}\) concentrations as well as the key meteorological parameters under the two scenarios. The meteorological conditions were more stagnant during the polluted period than the clean period, with a lower mean WS (0.73 vs. 1.14 m s\({}^{-1}\)), higher mean RH (65.65% vs. 63.24%), and lower mean T (30.83 vs. 31.22 \({}^{\circ}\)C). The four gaseous pollutants increased in varying degrees from the clean to the polluted periods too. The average PM\({}_{2.5}\) concentration was 17.79 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\) during the clean period and increased to 41.95 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\) during the polluted period, which was lower than the PM\({}_{2.5}\) levels in the summer of 2014 in Nanjing, indicating an improvement in air quality [47]. Correspondingly, the ALWC during the polluted period was 3.15 fold that of during the clean period (10.55 vs. 3.35 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)); the mass fraction of the ALWC to the total PM\({}_{2.5}\) mass also increased to 20.1% from 15.85%. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & \multirow{2}{*}{**Sampling Period**} & \multirow{2}{*}{**Method**} & \multicolumn{2}{c}{**PM2.5**} & **ALWC** & \multirow{2}{*}{**RH (\%)**} & \multirow{2}{*}{**Reference**} \\ \cline{3-3} \cline{5-7}  & & & & & & \\ \hline Shanghai & 23 October 2018 to 5 August 2019 & ISORROPIA–II & 38.60 & 14.80 & 70.00 & [46] \\ Huangshan & 14 September to 26 October 2012 & AIM-II & 21.28 & 9.98 & 64.50 & [47] \\ Jiaxing & 1 January to 31 December 2021 & ISORROPIA–II & 24.45 & 46.65 & 78.38 & [48] \\ Nanjing & 18 July to 26 August 2021 & E-AIM & 18.88 (PM\({}_{1}\)) & 5.33 & 69.05 & [20] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Representative ALWC levels in the YRD region, China. Figure 6: (a) Distributions of ALWC, PM\({}_{2.5}\), gaseous pollutants (O\({}_{3}\), CO, NO\({}_{2}\), SO\({}_{2}\)), and the key meteorological parameters (WS, RH, T) during the clean and polluted periods, respectively. (b) Exponentially fitted equations (y = a*eb*x) of ALWC versus RH under different PM\({}_{2.5}\) levels (marked in different colors). (c) Variation in water content over a range of PM\({}_{2.5}\)/CO ratios. Furthermore, we investigated the relationships between the ALWC and RH under different PM\({}_{2.5}\) levels, which show non-linear exponential correlations as seen in Figure 6b. At low RH levels, the ALWC increased slowly with the RH, but much rapidly in regions with a high RH. This is likely due to the fact that the key hygroscopic aerosol components like nitrate and sulfate do not uptake water at a RH that is lower than their deliquescence points, but can uptake water exponentially when the RH exceeds the deliquescent RHs [23; 25; 26], though no clear threshold RHs can be directly observed in Figure 6b since ambient aerosols are a complex mixture of hygroscopic salts and a wide range of organics, etc. (many of them have no definite deliquescent RH values). The increase in ALWC become more rapid with the elevation in the PM\({}_{2.5}\) levels, likely due to a two-way coupling effect between the ALWC and PM\({}_{2.5}\), as an increased ALWC may promote aqueous/heterogeneous oxidation and the gas-to-particle condensation of some species, therefore increasing the PM\({}_{2.5}\) mass, which in turn leads to more water uptake and further increases the ALWC [49; 50; 51; 52].