Ml204t

Water uptake was measured gravimetrically. A 0.75-inch diameter hole-punch was used to cut each hydrated film. The mass of each hydrated film, $W_s$ , was measured after quickly blotting them with tissue paper. The films were then dried under a vacuum at 50 °C for 24 h and the mass of each dried film, $W_d$ , measured [29 (link)]. The water uptake, ${\omega }_w$ , was calculated as follows: ${\omega }_w=\frac{W_s-W_d}{W_d}\times 100\%$
where $W_s$ is the mass of the swollen film and $W_d$ is the mass of the dried film.
Film density was measured by the buoyancy method with a density kit (ML-DNY-43, Mettler Toledo) coupled with a scale (ML204T, Mettler Toledo) [48 (link)]. The density, ρ_p, was calculated as follows: ${\rho }_p=\left({\rho }_L-{\rho }_0\right)\left(\frac{W_0}{W_0-W_L}\right)+{\rho }_0$
where ρ_L is the density of water (997.8 kg/m³ at 22 °C), ρ₀ is the density of air (1.225 kg/m³), W₀ is the weight of the dried film in air, and W_L is the weight of the film in water.
Water volume fraction, ϕ_w, was calculated as follows: ${\varphi }_w=\frac{\left(W_s-W_d\right)/{\rho }_p}{\left(W_s-W_d\right)/{\rho }_L+W_d/{\rho }_p}$

PM_2.5 sampling was conducted on the rooftop (~25 m high) of a 6-storey teaching building in Tianjin University, located at 39.11° N, 117.16° E in the center of Tianjin, which represents a typical urban environment (Figure 1) during the winter (domestic heating period) of three consecutive years, i.e., 2017, 2018, and 2019 (Table S1). PM_2.5 samples collection membranes were pre-combusted when collected (450 °C for 6 h). Quartz filters (23 × 18 cm) using a high-volume air sampler (TE-PM2.5HVP-BL, TISCH, Cleves, OH, USA) at a flow rate of 1.05 m³ min⁻¹ during 8:00–18:00 (local time) for daytime and from 18:00 to next morning 8:00 for nighttime in both the seasons, and stored at −20 °C. The membranes were weighed before and after sampling using a microelectronic balance (Mettler Toledo ML 204T, Switzerland) after a 48-h equilibration in a drying vessel. One blank filter was also collected in each campaign. The samples were classified into clean (PM_2.5 ≤ 75 μg m⁻³) and polluted (PM_2.5 > 75 μg m⁻³) periods based on the PM_2.5 mass concentration. Temperature (T), relative humidity (RH), and wind speed (WS) were measured at sampling points using a meteorological station (Gill MetPak, Gill Instruments, Lymington, Hampshire, UK). Gaseous pollutant (O₃, SO₂ and NO₂) concentrations collected from the Tianjin Air Quality Monitoring Station data, located 1.7 km away from the sampling site.