Supporting information

Supporting information Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts: Implying Its Key Role in Haze Formation over the North China Plain Zhijun Wu*, Yu Wang #, Tianyi Tan, Yishu Zhu, Mengren Li, Dongjie Shang, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, Yuanhang Zhang State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China # Now at Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. 2 * Corresponding author: Zhijun Wu (zhijunwu@pku.edu.cn) 1 Measurements PM 2.5 (Particles with an aerodynamic diameter less than or equal to 2.5 micrometers) samples were collected on Teflon filters (Whatman, 7592-104) using a filter sampler (Partisol-Plus 2025, Thermo Scientific Inc.) from 16 January, 2013 to 31 December 2015. The Teflon filters were collected from 7:00 am to 7:00 pm next day (Local Time). The filters were conditioned at the same conditions in a super clean laboratory with stable relative humidity (40±5% over 24 hours) and temperature (22 ±2ºC) before both the pre- and post-sampling weighings. Filters were extracted with 10 ml deionized water in ultrasonic bath for 30 min. The extraction solution was then filtered with a 0.45 μm PTFE filter. The anions (SO 2-4, NO - 3, Cl -, and F - ) and cations (Na +, K +, NH + 4, Ca 2+, and Mg 2+ ) were analyzed with the Dionex ICS-2000 and Dionex ICS-2500 Ion Chromatography systems, respectively. The size-resolved (50 nm, 75 nm, 100 nm, 150 nm, 250 nm, and 350 nm in diameters) particle hygroscopicity were measured by H-TDMA, which has been described in detail in previous publications, 1-2 and complied to the instrumental

standards and quality assurance prescribed in Massling et al.. 3 The hygroscopic growth factor (HGF) is defined as the ratio of the particle mobility diameter, Dp(RH), at a given RH to the dry diameter, Dp dry : HGF(RH) = DDDD(RRRR) DDDD dddddd [S1] The data inversion is based on TDMAinv method developed by Gysel et al.. 4 Dry scans (under RH<10%) are used to calibrate any offset between DMA1 (Deferential Mobility Analyzer) and DMA2 and define the width of the H-TDMA s transfer function. 4 The deliquescence point of pure ammonium sulfate particles was measured first to compare with the theoretical one and thus to validate the accuracy and performance of the H-TDMA. During the entire field measurements, the 100 nm ammonium sulfate particles at RH=90% were measured frequently (twice per three hours) to guarantee correct operation of the H-TDMA system. The hygroscopicity parameter (κ) can be calculated from the HGF 5 as equation [S2] and [S3]. Correspondingly, the κ-pdf was derived from the GF-PDF. κκ = (HHHHHH 3 1) A exp DDDD dddddd.hhhhhh RRRR 1 [SS2] A = 4σσ ss aam w RTTρ w [S3] where Dp dry and HGF are the initial dry particle diameter and the growth factor at 90% RH measured by H-TDMA, respectively. σ s/a is the droplet surface tension (assumed to be that of pure water, σ s/a = 0.0728 N m 2 ), M w is the molecular weight of water, ρ w is the density of liquid water, R is the universal gas constant, and T is the absolute temperature. An Aerodyne high-resolution Time-of-flight aerosol mass spectrometer (here simply referred to as AMS) 6 was operated in mass spectrum and particle-time-of-flight sub-modes for equal time periods. Due to the 600 C surface temperature of the vaporizer, the AMS can only analyze the non-refractory chemical composition of the particles. Elemental carbon, crustal material, and sea salt cannot be detected. Therefore, based on the transmission efficiency of the aerodynamic lenses

and the detected compounds, the AMS can provide the size-resolved chemical composition of the sub-micrometer non-refractory aerosol particle fraction (NR-PM 1 ). 7 Particle number size distributions (PNSD) were measured by TSI SMPS (Long-DMA3081+CPC3775) and Nano-SMPS (Nano-DMA3085+UCPC3776). The multiple charge correction, condensation particle counter (CPC) counting efficiency, and particle loss correction were carried out. An aerodynamic particle sizer (APS, TSI model 3321, TSI Inc., St. Paul, MN, USA) measured particle number size distributions between 500 nm and 10 µm (aerodynamic diameter). The APS results were transformed from aerodynamic to Stokes diameters with a particle density of 1.5 g cm -3. 2 Calculation of Aerosol liquid water content (ALWC) The dataset used in this study are listed in Table S1. Table S1: Dataset used in estimation of aerosol water liquid content Parameters Instrument Sampling Period Method SO 4 2-, NO 3 -, Cl -, NH 4 + HR-Tof-AMS June 2014 ISORROPIA-II SO 4 2-, NO 3 -, Cl -, NH 4 +, PM 2.5 filters+ Ion January, ISORROPIA-II K +, Na +, Ca 2+, Mg 2+ Chromatography 2013-December, 2015 HGFs for 50 nm, 75 nm, H-TDMA and April, size-resolved 100 nm, 150 nm, 250 SMPS&APS 2014-February, HGFs and nm, and 350 nm in 2015 PNSD diameters at RH=90% and PNSD 2.1 ALWC calculated from ISORROPIA-II Both forward and reverse modes were used to calculate the ALWC using the ISORROPIA-II thermodynamic model, and both modes output similar results, i.e.,

for the forward mode vs. reverse mode, the slope was 0.996 and the coefficient of determination (R 2 ) was 0.988. The water contributed by organic fraction was calculated using the method in 8. A brief introduction was given below. ALWC absorbed by organics (W o ) is calculated by equation [S4]. 5 WW oo = mm ooooooρρ ww ρρ oooooo κκ oooooo (1 RRRR 1) [S4] where m org represents mass concentrations of organics measured by AMS. ρ w and ρ org represent density of water (1 g.cm -3 ) and a typical organic density (1.4 g.cm -3 ). The hygroscopicity of organic aerosols (κ org ) represents the hygroscopicity of organics. AMS-positive matrix factor (PMF) analysis was performed to identify different organic aerosols (OA) factors on the basis of the high resolution mass spectra of organics. 9 Four OA components were resolved by PMF, including low-volatility oxygenated organic aerosol (LV-OOA), semi-volatile oxygenated OA (SV-OOA), hydrocarbon-like OA (HOA) and cooking OA (COA). LV-OOA and SV-OOA typically represented aged SOA and freshly formed SOA, respectively. 9 HOA and COA were both anthropogenic primary organic aerosol (POA) components. 10 The detail can be found in Wu et al.. 11 Unlike inorganic salts, κ org is not well-defined. In the literature, there were different approaches in representing κ org. Typically, κ org is assumed as a single value. Chang et al., 12 represented κ org by using the factors from the PMF analysis to group organics measured by AMS into two components: a non-hygroscopic, unoxygenated component consisting of the hydrocarbon-like organic aerosol (HOA) factor and a hygroscopic component, consisting of the oxygenated factors LV-OOA, SV-OOA, and biomass burning organic aerosol (BBOA). In our study, organic materials derived from AMS measurements were grouped into two components including secondary organic aerosols (SOA) and primary organic aerosols (POA) based on AMS-PMF analysis. SOA, including LV-OOA and SV-OOA factors, is a more oxygenated organic aerosol, thereby more hygroscopic and has a κ SOA of 0.1, which was calculated from the hygroscopic growth factor of organics at RH=90% as given in

Gysel et al.. 13 POA is the unoxygenated component consisting of the HOA and COA factors and is treated as hydrophobic material with κ POA =0. Then, κ org can be calculated as: kk oooooo = ff PPPPPP kk PPPPPP + ff SSSSSS kk SSSSSS [S5] Here, κ org is overall κ for organic aerosols. f POA and f SOA are volume fraction of POA and SOA in total organic aerosols measured by AMS. In our case, the POA/OA and SOA/OA were respectively 0.39 and 0.61. According to equation [S5], the κ org can be calculated as 0.06 assuming κ SOA =0.1. ALWC absorbed by organics was calculated with κ org =0.06 using equation [S4]. 2.2 Observationally derived ALWC from HGF-PNSD The PNSDs measured using the SMPS and APS system were initially fitted with a three-mode lognormal distribution. These three modes in Beijing are the nucleation mode (3-20 nm), Aitken mode (20-100 nm) and accumulation mode (200-1000 nm) 14. Here, we note that coarse particles (> 1000 nm) were considered hydrophobic and that the value of their particle hygroscopicity (κ) was assumed to be zero. It was assumed that particles in the same mode shared similar sources and that the value of κ for each mode was constant. Then, the κ values for certain particle diameters were calculated by combining the mode fitting results with the measured size-resolved κ values (50, 100, 150, 250 and 350 nm). Finally, the κ value for each size bin measured using the SMPS and APS system was substituted into Eq. (S2) to calculate the size-resolved HGFs at the ambient RH (20 99%). Then, the wet PNSD could be reconstructed using the HGFs under ambient conditions. The ALWC was calculated according to the differences between the dry (measured) and wet (reconstructed) PNSDs 15 : ALWC = π NN 6 ii iidd 3 dd,ii (HHHHHH(DD dd, RRRR) 3 1) ρρ ww [S6] where N i represents the number concentration of dry particles of the i th bin, D d,i is the particle diameter of that bin, and ρ w, water density, is 1 gcm 3 in this study. ALWC is given in the unit of g m 3 of air. Figure S1 displayed the comparisons between ALWC calculated from ISORROPIA-II with and without water associated with organics and ALWC calculated from the combination HGF and particle number size distributions. Without

considering the water associated with organic compounds, the ALWC calculated using ISORROPIA-II and that calculated from the HGF-PNSD agreed well with a slope of 1.06 and an R 2 value of 0.93. The results using the ISORROPIA-II model while taking water associated with organics into account were slightly higher than those calculated through the HGF-PNSD method (slope=1.14, R 2 =0.94). This is unexpected because the ALWC calculated from HGF-PNSD method may be the most close the real ALWC compared to the one calculated from ISORROPIA-II. While, we should keep in mind that there some factors could lead to a bias in HGF-PNSD method. (1) The measurement uncertainty of H-TDMA: The evaporation loss of the semi-volatile aerosol components, such as NH 4 NO 3 could take place in differential mobility analyzers during the H-TDMA measurements. As a result, the HGF measured could be low than the hygroscopicity of ambient aerosol particles. (2) The assumption in HGF-PNSD method could result in a basis in the calculation of ALWC: Here, aerosols in the same mode was assumed to have same sources, thus the κ values was assumed constant in three modes. While, the chemical composition may have a size-dependency even in the same mode. Figure S1: The comparison between aerosol liquid water content calculated from ISORROPIA-II with and without water associated with organics and ALWC calculated from the combination HGF and particle number size distributions.

3 Time series of SO 2 and NOx concentrations The SO 2 and NOx concentrations were measured by Thermo fisher SO 2 43i TL and NOx 42i TL, respectively. The NOx and SO 2 concentrations are displayed in Fig. S2. Figure S2: The time series of NOx and SO 2 concentration during two haze episodes. 4 Table S2: The data points for Fig. 2(a) Percentage of SIA RH [%] Water Mass [µg/m 3 ] PM2.5 [µg/m 3 ] in PM 2.5 24±8% 14.9 ±3.8 0.9 ±1.2 39.4 ±30.4 26±12% 24.6 ±2.7 2.1 ±2.6 62.3 ±50.3 32±16% 35.2 ±2.9 5.1 ±5.9 72.8 ±56.7 38±15% 45.2 ±3.0 10.6 ±12.0 78.6 ±61.5 45±15% 54.8 ±2.9 21.1 ±21.1 97.6 ±82.3 54±12% 65.0 ±2.9 33.3 ±31.3 98.7 ±80.1 55±13% 82.8 ±2.4 56.2 ±43.3 68.5 ±43.9 5 The percentage of SNA mass fraction vs. pollution levels The percentage of sulfate, ammonium, and nitrate mass fraction in SNA were classified into four groups corresponding to PM 2.5 mass concentrations of 0-50 µg/m 3,

50-100 µg/m 3, 100-150 µg/m 3, and larger than 150 µg/m 3. The results were displayed in Fig. S3. Figure S3: The sulfate (red), ammonium (yellow), and nitrate (blue) mass fraction in PM 2.5. 6 Correlation between ALWC and inorganic mass concentration Figure S4 displayed ALWC is as a function of sulfate, ammonium, and nitrate mass concentrations, respectively. ALWC was well-correlated with both nitrate and sulfate mass concentration.

Figure S4: The correlation of aerosol liquid water content and mass concentrations of sulfate, nitrate, and ammonium in PM 2.5. 6 References 1. Wu, Z. J.; Nowak, A.; Poulain, L.; Herrmann, H.; Wiedensohler, A., Hygroscopic behavior of atmospherically relevant water-soluble carboxylic salts and their influence on the water uptake of ammonium sulfate. Atmos. Chem. Phys. 2011, 11 (24), 12617-12626. 2. Massling, A.; Wiedensohler, A.; Busch, B.; Neusüß, C.; Quinn, P.; Bates, T.; Covert, D., Hygroscopic properties of different aerosol types over the Atlantic and Indian Oceans. Atmos. Chem. Phys. 2003, 3 (5), 1377-1397. 3. Massling, A.; Niedermeier, N.; Hennig, T.; Fors, E. O.; Swietlicki, E.; Ehn, M.; Hämeri, K.; Villani, P.; Laj, P.; Good, N.; McFiggans, G.; Wiedensohler, A., Results and recommendations from an intercomparison of six Hygroscopicity-TDMA systems. Atmos. Meas. Tech. 2011, 4 (3), 485-497. 4. Gysel, M.; McFiggans, G. B.; Coe, H., Inversion of tandem differential mobility analyser (TDMA) measurements. J. Aerosol Sci. 2009, 40 (2), 134-151. 5. Petters, M. D.; Kreidenweis, S. M., A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmos. Chem. Phys. 2007, 7 (8), 1961-1971. 6. DeCarlo, P. F.; Kimmel, J. R.; Trimborn, A.; Northway, M. J.; Jayne, J. T.; Aiken, A. C.; Gonin, M.; Fuhrer, K.; Horvath, T.; Docherty, K. S.; Worsnop, D. R.; Jimenez, J. L., Field-deployable, high-resolution, time-of-flight aerosol mass spectrometer. Anal. Chem. 2006, 78 (24), 8281-8289. 7. Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.; Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; Delia, A.; Williams, L. R.; Trimborn, A. M.; Northway, M. J.; DeCarlo, P. F.; Kolb, C. E.; Davidovits, P.; Worsnop, D. R., Chemical and microphysical characterization of ambient aerosols with the aerodyne aerosol mass spectrometer. Mass Spec. Rev. 2007, 26 (2), 185-222. 8. Guo, H.; Xu, L.; Bougiatioti, A.; Cerully, K. M.; Capps, S. L.; Hite Jr, J. R.; Carlton, A. G.; Lee, S. H.; Bergin, M. H.; Ng, N. L.; Nenes, A.; Weber, R. J., Fine-particle water and ph in the southeastern

United States. Atmos. Chem. Phys. 2015, 15 (9), 5211-5228. 9. Ulbrich, I. M.; Canagaratna, M. R.; Zhang, Q.; Worsnop, D. R.; Jimenez, J. L., Interpretation of organic components from Positive Matrix Factorization of aerosol mass spectrometric data. Atmos. Chem. Phys. 2009, 9 (9), 2891-2918. 10. Lanz, V. A.; Alfarra, M. R.; Baltensperger, U.; Buchmann, B.; Hueglin, C.; Prévôt, A. S. H., Source apportionment of submicron organic aerosols at an urban site by factor analytical modelling of aerosol mass spectra. Atmos. Chem. Phys. 2007, 7 (6), 1503-1522. 11. Wu, Z. J.; Zheng, J.; Shang, D. J.; Du, Z. F.; Wu, Y. S.; Zeng, L. M.; Wiedensohler, A.; Hu, M., Particle hygroscopicity and its link to chemical composition in the urban atmosphere of Beijing, China, during summertime. Atmospheric Chemistry and Physics 2016, 16 (2), 1123-1138. 12. Chang, R. Y. W.; Slowik, J. G.; Shantz, N. C.; Vlasenko, A.; Liggio, J.; Sjostedt, S. J.; Leaitch, W. R.; Abbatt, J. P. D., The hygroscopicity parameter (κ) of ambient organic aerosol at a field site subject to biogenic and anthropogenic influences: relationship to degree of aerosol oxidation. Atmos. Chem. Phys. 2010, 10 (11), 5047-5064. 13. Gysel, M.; Crosier, J.; Topping, D. O.; Whitehead, J. D.; Bower, K. N.; Cubison, M. J.; Williams, P. I.; Flynn, M. J.; McFiggans, G. B.; Coe, H., Closure study between chemical composition and hygroscopic growth of aerosol particles during TORCH2. Atmos. Chem. Phys. 2007, 7 (24), 6131-6144. 14. Wu, Z. J.; Hu, M.; Lin, P.; Liu, S.; Wehner, B.; Wiedensohler, A., Particle number size distribution in the urban atmosphere of Beijing, China. Atmos. Environ. 2008, 42 (34), 7967-7980. 15. Bian, Y. X.; Zhao, C. S.; Ma, N.; Chen, J.; Xu, W. Y., A study of aerosol liquid water content based on hygroscopicity measurements at high relative humidity in the North China Plain. Atmos. Chem. Phys. 2014, 14 (12), 6417-6426.