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Atmospheric particulates can be produced by emissions or form de novo. New particle formation usually occurs in relatively clean air. This is because preexisting particles in the atmosphere will scavenge the precursors of new particles and suppress their formation. However, observations in some heavily polluted megacities have revealed substantial rates of new particle formation despite the heavy loads of ambient aerosols. Yao et al. investigated new particle formation in Shanghai and describe the conditions that make this process possible. The findings will help inform policy decisions about how to reduce air pollution in these types of environments. Science , this issue p. 278 Atmospheric new particle formation in heavily polluted cities can occur in certain chemical environments. Atmospheric new particle formation (NPF) is an important global phenomenon that is nevertheless sensitive to ambient conditions. According to both observation and theoretical arguments, NPF usually requires a relatively high sulfuric acid (H 2 SO 4 ) concentration to promote the formation of new particles and a low preexisting aerosol loading to minimize the sink of new particles. We investigated NPF in Shanghai and were able to observe both precursor vapors (H 2 SO 4 ) and initial clusters at a molecular level in a megacity. High NPF rates were observed to coincide with several familiar markers suggestive of H 2 SO 4 –dimethylamine (DMA)–water (H 2 O) nucleation, including sulfuric acid dimers and H 2 SO 4 -DMA clusters. In a cluster kinetics simulation, the observed concentration of sulfuric acid was high enough to explain the particle growth to ~3 nanometers under the very high condensation sink, whereas the subsequent higher growth rate beyond this size is believed to result from the added contribution of condensing organic species. These findings will help in understanding urban NPF and its air quality and climate effects, as well as in formulating policies to mitigate secondary particle formation in China.

A method to estimate the representative elementary volume (REV) size for the permeability and equivalent permeability coefficient of rock mass with a radial flow configuration was developed. The estimations of the REV size and equivalent permeability for the rock mass around an underground oil storage facility using a radial flow configuration were compared with those using a unidirectional flow configuration. The REV sizes estimated using the unidirectional flow configuration are much higher than those estimated using the radial flow configuration. The equivalent permeability coefficient estimated using the radial flow configuration is unique, while those estimated using the unidirectional flow configuration depend on the boundary conditions and flow directions. The influences of the fracture trace length, spacing and gap on the REV size and equivalent permeability coefficient were investigated. The REV size for the permeability of fractured rock mass increases with increasing the mean trace length and fracture spacing. The influence of the fracture gap length on the REV size is insignificant. The equivalent permeability coefficient decreases with the fracture spacing, while the influences of the fracture trace length and gap length are not determinate. The applicability of the proposed method to the prediction of groundwater inflow into rock caverns was verified using the measured groundwater inflow into the facility. The permeability coefficient estimated using the radial flow configuration is more similar to the representative equivalent permeability coefficient than those estimated with different boundary conditions using the unidirectional flow configuration.

The permeability coefficient of rock mass is a critical parameter for groundwater flow analysis of underground water-sealed oil storages. Based on the site geological investigation, monitoring information, and data statistics, a method of determining representative permeability coefficients of original and grouted rock mass for underground water-sealed oil storage was proposed. Then, the proposed method was applied to the flow field analysis of an underground water-sealed oil storage under construction. The assessment of containment properties, the prediction of water inflow, and the analysis of grouting effect were carried out. For the similar kind of underground water-sealed oil storages, the geometric mean of the tested permeability coefficients was suggested to be the representative value of original rock mass. The relationship between the permeability reduction factors of grouting and the original permeability coefficients was obtained to predict the grouting zone permeability. Based on the proposed method, the representative original permeability coefficient of the storage zone is determined to be 1.96E−03 m/day, and the permeability coefficient of the grouted zone is conservatively suggested to be reduced to 10% of the original value. The predicted results of water inflow and grouting effect by flow simulation were compared with that by engineering analogy and theoretical analysis, which further verified the rationality of the proposed method and the reliability of the flow field analysis. The method of determining the representative permeability coefficients of rock mass and its application can help improve the accuracy of groundwater flow field analysis and offer guidelines for the construction of underground water-sealed oil storage.

Secondary organic aerosol contributes a significant fraction to aerosol mass and toxicity. Low-volatility organic vapours are critical intermediates connecting the oxidation of volatile organic compounds to secondary organic aerosol formation. However, the direct measurement of intermediate vapours poses a great challenge. Here we present coordinated measurements of oxygenated organic molecules in the three most urbanized regions of China and determine their likely precursors, enabling us to connect secondary organic aerosol formation to various volatile organic compounds. We show that the oxidation of anthropogenic volatile organic compounds dominates oxygenated organic molecule formation, with an approximately 40% contribution from aromatics and a 40% contribution from aliphatic hydrocarbons (predominantly alkanes), a previously under-accounted class of volatile organic compounds. The irreversible condensation of these anthropogenic oxygenated organic molecules increases significantly in highly polluted conditions, accounting for a major fraction of the production of secondary organic aerosol. We find that the distribution of oxygenated organic molecules and their formation pathways are largely the same across the urbanized regions. This suggests that uniform mitigation strategies could be effective in solving air pollution issues across these highly populated city clusters. The formation of secondary organic aerosol in Chinese megacities is dominated by the condensation of anthropogenic organic vapours, according to measurements across three urbanized regions.

The hydro-physical and hydro-chemical interactions between groundwater and a rock mass can lead to changes in the mineral composition and structure of the rock (e.g., generation of voids and dissolution pores and an increase in the porosity), thereby altering the macroscopic mechanical characteristics of the rock mass. Sandstone specimens were saturated with distilled water and five aqueous solutions characterized by various ion concentrations and pH values for several months, and their porosity was measured in real time. Simultaneously, the concentration and pH of each aqueous solution were monitored every 30 days. The results indicate that after immersion in the aqueous solutions for 180 days, the porosity of the sandstone specimens and the ion concentrations and pH of the aqueous solutions tended to stabilize. Then, the immersed sandstone specimens were analyzed in thin section and subjected to computerized tomography scanning. It turns out that the mineral composition and structure of the specimens had all changed to various degrees. Finally, the uniaxial compression tests were conducted on the sandstone specimens to analyze the effects of the hydro-physical and hydro-chemical alteration on the macroscopic mechanical characteristics of the rock (e.g., the stress–strain relationship, elastic modulus, and peak strength). The results of this study can serve as a reference for investigations into theories and applications of water–rock interactions and for research in related fields.

A series of laboratory experiments on water flow through rough fractures was performed using self-designed experimental devices to investigate the effect of fracture roughness on the flow behavior. Nine models of single rough fractures—with three joint roughness coefficients (JRCs) of 0–2, 8–10 and 18–20, and three apertures for each JRC—were prepared using three-dimensional printing technology. In the flow experiments, the values of Reynolds numbers ranged widely from less than 10 to around 10,000. According to the experimental data, the fracture roughness has an obvious influence on the hydraulic properties of fractures. A parametric expression for the Forchheimer equation was proposed to quantitatively describe the influence of fracture roughness on the flow behaviour in the fractures. The relations between the parameters for nonlinear flow (such as critical Reynolds number, non-Darcy effect coefficient and friction factor) and the JRCs were obtained. It was found that the critical Reynolds number decreased significantly from 566 to 67 as the JRC increased from 2 to 20. The increase in fracture roughness causes more extra energy losses and enhances the degree of flow nonlinearity in single fractures.

A series of laboratory tests were performed to examine the fatigue behavior of granite subjected to cyclic loading under triaxial compression condition. In these tests, the influences of volumetric change and residual strain on the deformation modulus of granite under triaxial cyclic compression were investigated. It is shown that the fatigue behavior of granite varies with the tendency for volumetric change in triaxial cyclic compression tests. In the stress–strain space, there are three domains for fatigue behavior of rock subjected to cyclic loading, namely the volumetric compaction, volumetric dilation with strain-hardening behavior, and volumetric dilation with strain-softening behavior domains. In the different domains, the microscopic mechanisms for rock deformation are different. It was also found that the stress level corresponding to the transition from volumetric compaction to volumetric dilation could be considered as the threshold for fatigue failure. The potential of fatigue deformation was compared with that of plastic deformation. The comparison shows that rocks exhibit higher resistances to volumetric deformation under cyclic loading than under plastic loading. The influence of residual strain on the fatigue behavior of rock was also investigated. It was found that the axial residual strain could be a better option to describe the fatigue behavior of rock than the loading cycle number. A constitutive model for the fatigue behavior of rock subjected to cyclic loading is proposed according to the test results and discussion. In the model, the axial residual strain is considered as an internal state variable. The influences of confining pressure and peak deviatoric stress on the deformation modulus are considered in a term named the equivalent stress. Comparison of test results with model predictions shows that the proposed model is capable of describing the prepeak fatigue behavior of rock subjected to cyclic loading.

Hydrogeochemical environment is of critical importance for the environment-friendly operation of underground oil storage caverns. The construction of underground oil storage caverns usually has an impact on the hydro-environment. The characterization and analysis of the hydrogeochemical environment can provide information on the relation between construction and hydro-environment. The quality of water samples was detected and analyzed to determine the chemical type in an underground oil storage cavern in China. The water samples are classified using principal component analysis and cluster analysis. The source and proportion of seepage water into the storage caverns are determined with end member mixing calculation. The results show that the chemical type of groundwater is mainly HCO3 + Cl − Na type, and the two dominant factors affecting the evolution of hydrogeochemical content are rock dissolution and groundwater seepage. All water samples can be catalogued as seepage water, water curtain water, X River water and background water. The water curtain water can fully penetrate into the ground to provide containment for the storage caverns, and the water curtain system has a good performance and can basically cover the project area. Most of the seepage water into the storage caverns comes from water curtain water and X River water, while the proportion of background water is relatively low. The construction of underground oil storage caverns affects the groundwater flow regime by changing the directions of groundwater flow around the caverns. This study showcases the use of hydrogeochemical analysis in depicting the interplay between surface water and groundwater for underground rock engineering.

Identification of various emission sources and quantification of their contributions comprise an essential step in formulating scientifically sound pollution control strategies. Most previous studies have been based on traditional offline filter analysis of aerosol major components (usually inorganic ions, elemental carbon – EC, organic carbon – OC, and elements). In this study, source apportionment of PM2.5 using a positive matrix factorization (PMF) model was conducted for urban Shanghai in the Yangtze River Delta region, China, utilizing a large suite of molecular and elemental tracers, together with water-soluble inorganic ions, OC, and EC from measurements conducted at two sites from 9 November to 3 December 2018. The PMF analysis with inclusion of molecular makers (i.e., MM-PMF) identified 11 pollution sources, including 3 secondary-source factors (i.e., secondary sulfate; secondary nitrate; and secondary organic aerosol, SOA, factors) and 8 primary sources (i.e., vehicle exhaust, industrial emission and tire wear, industrial emission II, residual oil combustion, dust, coal combustion, biomass burning, and cooking). The secondary sources contributed 62.5 % of the campaign-average PM2.5 mass, with the secondary nitrate factor being the leading contributor. Cooking was a minor contributor (2.8 %) to PM2.5 mass while a significant contributor (11.4 %) to the OC mass. Traditional PMF analysis relying on major components alone (PMFt) was unable to resolve three organics-dominated sources (i.e., biomass burning, cooking, and SOA source factors). Utilizing organic tracers, the MM-PMF analysis determined that these three sources combined accounted for 24.4 % of the total PM2.5 mass. In PMFt, this significant portion of PM mass was apportioned to other sources and thereby was notably biasing the source apportionment outcome. Backward trajectory and episodic analysis were performed on the MM-PMF-resolved source factors to examine the variations in source origins and composition. It was shown that under all episodes, secondary nitrate and the SOA factor were two major source contributors to the PM2.5 pollution. Our work has demonstrated that comprehensive hourly data of molecular markers and other source tracers, coupled with MM-PMF, enables examination of detailed pollution source characteristics, especially organics-dominated sources, at a timescale suitable for monitoring episodic evolution and with finer source breakdown.

Macroscopic nonlinear flow, which is closely related to mesoscopic flow structures such as vortices, is an important property of fluid flow and solute transport in rock fractures with a high pressure gradient. Both mesoscopic flow structure and macroscopic nonlinear flow of rock fractures are affected by fracture roughness and aperture. Therefore, the macroscopic seepage process in fractures is studied numerically by directly solving the Navier–Stokes equation. The results demonstrate that the Forchheimer equation can be used to describe the relationship between flow rate and pressure gradient of rough fractures with different apertures. The linear and nonlinear coefficients of the Forchheimer equation increase with fracture roughness and decrease with fracture aperture. Empirical formulas between the linear coefficient, the nonlinear coefficient, the fracture roughness and aperture are established. When the roughness is equal to zero, the empirical formulas can degenerate into conventional cubic law. In additions, the distribution characteristics of the mesoscopic flow structure in rough fractures are also investigated. The results show that the area and kinetic energy distribution characteristics of the mesoscopic flow structure in fractures with different roughness and apertures are similar. The frequency characteristics of the area and kinetic energy of the mesoscopic flow structure in fractures can be fitted with negative exponential and logarithmic normal functions, respectively. The effect of fracture roughness on area and kinetic energy distribution characteristics is significant, but the effect of aperture can be ignored. Empirical formulas between fracture roughness and mesoscopic flow structure characteristics are established for the first time.