State Key Joint Laboratory of Environment Simulation and Pollution Control

Membrane distillation crystallization (MDCr) has emerged as a promising technology to produce inorganic crystals from various feed solutions. MDCr leverages the benefits of membrane distillation (MD), including reduced membrane fouling, operation under waste heat or solar energy while facilitating high-purity crystal formation. This study provides a comprehensive operational assessment of MDCr and the governing mechanisms. The investigation emphasized the influence of temperature gradient, membrane characteristics and feed composition on the processes of supersaturation, nucleation and crystal growth. Attention was focused on the formation of the crystals, and the polymorphs' selectivity influenced by process configuration. Practical applications are discussed across inorganic materials, food and pharmaceutical industries, where MDCr shows potential for recovery of functional crystals. Despite promising application, MDCr remains affected by membrane fouling, wetting, and energy demand, which impact fouling, supersaturation and crystal growth dynamics. The role of in-situ monitoring, modelling and process control is identified to ensure reproducibility and scale-up. The findings show that process optimization and coupling of heat and mass transfer promote the formation of tailored crystalline materials from complex feed solutions. However, this process requires standardized methods, real-time analysis and techno-economic assessment to accelerate the MDCr deployment in the current industrial operations. • Mechanistic understanding of supersaturation, nucleation and crystal grow in MDCr. • Systematic evaluation of fouling, wetting and membrane material limiting MDCr's sustainability. • Risk identification of MDCr application linked to fluorinated membrane materials. • Defining the MDCr future research directions including AI-assisted process and smart membranes

The ambient fine particle (PM2.5) pollution in China has declined over the past decade, as a consequence of stringent clean air actions from 2013 to 2023. The sustained policy interventions have not only reduced the magnitude of PM2.5 concentrations but also reshaped the source profile, i.e., the contribution from different emission sources. PM2.5 toxicity varies markedly across emission sources, complicating the identification of dominant contributors and the robust quantification of associated health risks. The existing health risk assessments tend to rely primarily on PM2.5 mass concentrations and therefore neglect toxicity differences by emission source. By contrast, oxidative potential (OP), which reflects the capacity of particles to induce oxidative stress, may provide a more mechanistically grounded metric for toxicity assessment. Here, we developed a method to estimate the OP of PM2.5 in China, integrating direct ambient sample measurements with the GEOS-Chem model simulations and satellite observations, enabling nationwide, long-term reconstruction of aerosol toxicity at high spatiotemporal resolution. Using population-weighted mean exposure estimate (i.e., PWM-PM2.5 concentrations and PWM-OP) as the exposure metric, we systematically track the evolution of PM2.5 toxicity across China under policy-driven changes in emission sources and air pollution situations. The preliminary results show that, PWM-PM2.5 between 2013 and 2023 declined from 62.8 to 33.9 μg m-3 (−46%). Over the same period, PWM-OP declined from 2.48 to 1.17 nmol min-1 m-3 (−53%). Reductions were primarily attributed to control of coal combustion, underscoring the importance of energy-structure transitions and pollution control in reducing population health risks. In contrast, meteorological variability exerted a comparatively minor influence on these improvements; over 2013-2023, meteorological changes increased PWM-PM2.5 by 0.4 μg m-3 and PWM-OP by 0.62 nmol min-1 m-3, only partially offsetting the benefits of emission reductions. Our findings suggest that air quality improvements cannot be understood solely through PM2.5 mass concentrations and that toxicity metrics can offer additional insights relevant to health. The future clean air policies need to shift from concentration-oriented targets to toxicity-oriented emission source control strategies, prioritizing the reduction of high-risk sources to achieve great health benefits. Keywords: PM2.5; Oxidative potential; Policy-driven; Coal combustion; Health benefits

Abstract. Sulfuric acid (H2SO4) is a key precursor in atmospheric new particle formation and cluster early growth. However, long-term measurement of it is only available at a few sites. Although several proxies for estimating H2SO4 concentration have been proposed, they are always site-specific. Therefore, both reliable H2SO4 measurement and proxies with wider application are highly needed. Here, we conducted a long-term H2SO4 measurement in urban Beijing during 2019–2021, and derived three H2SO4 proxies based entirely on its formation and loss pathways. Results show that daytime H2SO4 concentration is 2.0–7.4×106 molec. cm−3 and shows an overall decline with an average annual decrease of 14 %. This decline is mainly due to the ongoing SO2 emission controls. Daytime H2SO4 shows a clear seasonal variation that tracks UVB. Nighttime H2SO4 concentration is 1.6–6.3×105 molec. cm−3, with higher levels in warmer seasons due to stronger sources and lower condensation sink (CS). The diurnal variations of H2SO4 across seasons follow those of photo-oxidation-related parameters, such as UVB, OH radical, and photolysis rate of NO2 (J(NO2)). All of the three proxies can reproduce H2SO4 concentration during 10:00–14:00 LT. Importantly, they can estimate H2SO4 concentration at a boreal forest site in Hyytiälä, Finland, suggesting their applicability to sites with diverse environments. Furthermore, the parameters used in UVB-PM2.5 based proxy are available at most observational sites. Further application of this proxy could provide H2SO4 concentrations covering many regions worldwide, which may further facilitate research on atmospheric nucleation and secondary aerosol growth of these sites.