Uncovering the crystal defects within aragonite CaCO3.

Knowledge of deformation mechanisms in aragonite, one of the three crystalline polymorphs of CaCO3, is essential to understand the overall excellent mechanical performance of nacres. Dislocation slip and deformation twinning were claimed previously as plasticity carriers in aragonite, but crystallographic features of dislocations and twins have been poorly understood. Here, utilizing various transmission electron microscopy techniques, we reveal the atomic structures of twins, partial dislocations, and associated stacking faults. Combining a topological model and density functional theory calculations, we identify complete twin elements, characters of twinning disconnection, and the corresponding twin shear angle (∼8.8°) and rationalize unique partial dislocations as well. Additionally, we reveal an unreported potential energy dissipation mode within aragonite, namely, the formation of nanograins via the pile-up of partial dislocations. Based on the microstructural comparisons of biogenic and abiotic aragonite, we find that the crystallographic features of twins are the same. However, the twin density is much lower in abiotic aragonite due to the vastly different crystallization conditions, which in turn are likely due to the absence of organics, high temperature and pressure differences, the variation in inorganic impurities, or a combination thereof. Our findings enrich the knowledge of intrinsic crystal defects that accommodate plastic deformation in aragonite and provide insights into designing bioengineering materials with better strength and toughness.

Heterogeneous iodine-organic chemistry fast-tracks marine new particle formation.

The gas-phase formation of new particles less than 1 nm in size and their subsequent growth significantly alters the availability of cloud condensation nuclei (CCN, >30-50 nm), leading to impacts on cloud reflectance and the global radiative budget. However, this growth cannot be accounted for by condensation of typical species driving the initial nucleation. Here, we present evidence that nucleated iodine oxide clusters provide unique sites for the accelerated growth of organic vapors to overcome the coagulation sink. Heterogeneous reactions form low-volatility organic acids and alkylaminium salts in the particle phase, while further oligomerization of small α-dicarbonyls (e.g., glyoxal) drives the particle growth. This identified heterogeneous mechanism explains the occurrence of particle production events at organic vapor concentrations almost an order of magnitude lower than those required for growth via condensation alone. A notable fraction of iodine associated with these growing particles is recycled back into the gas phase, suggesting an effective transport mechanism for iodine to remote regions, acting as a "catalyst" for nucleation and subsequent new particle production in marine air.

Autocombustion Route Derived Zinc Ferrite Nanoparticles as Chemiresistive Sensor for Detection of Alcohol Vapors.

Prolonged exposure to alcohol vapors can have detrimental effects on human health, potentially leading to eye irritation, dizziness, and in some cases, damage to the nervous system. The present article aims to provide a comprehensive understanding on the synthesis and characterization of zinc ferrite (ZnFe2O4) nanoparticles, as well as their interactions with a range of alcohol vapors, including methanol, ethanol, n-propanol, and isopropanol. These alcohols differ in their molecular weight, boiling points, diffusivity, and other properties. The study reveals the semiconducting ZnFe2O4 nanoparticulate sensor's capability for reversible, repeatable, and sensitive detection of alcohol vapors. The sensor exhibits the highest response to ethanol within operating temperature range (225-300 °C). An attempt is made to establish a correlation between the properties of the target analytes and the observed sensing signals. Additionally, the response conductance transients of ZnFe2O4 under the exposure to the studied alcohol vapors are modeled based on the Langmuir-Hinshelwood adsorption mechanism. The characteristic time constants obtained from this modeling are justified with respect to the properties of the analytes.

Unveiling the Function of Oxygen Vacancy on Facet-Dependent CeO2 for the Catalytic Destruction of Monochloromethane: Guidance for Industrial Catalyst Design.

Secondary pollution remains a critical challenge for the catalytic destruction of chlorinated volatile organic compounds (CVOCs). By employing experimental studies and theoretical calculations, we provide valuable insights into the catalytic behaviors exhibited by ceria rods, cubes, and octahedra for monochloromethane (MCM) destruction, shedding light on the elementary reactions over facet-dependent CeO2. Our findings demonstrate that CeO2 nanorods with the (110) facet exhibit the best performance in MCM destruction, and the role of vacancies is mainly to form a longer distance (4.63 Å) of frustrated Lewis pairs (FLPs) compared to the stoichiometric surface, thereby enhancing the activation of MCM molecules. Subsequent molecular orbital analysis showed that the adsorption of MCM mainly transferred electrons from the 3σ and 4π* orbitals to the Ce 4f orbitals, and the activation was mainly caused by weakening of the 3σ bonding orbitals. Furthermore, isotopic experiments and theoretical calculations demonstrated that the hydrogen chloride generated is mainly derived from methyl in MCM rather than from water, and the primary function of water is to form excess saturated H on the surface, facilitating the desorption of generated hydrogen chloride.

Surface-Phosphorylated Ceria for Chlorine-Tolerance Catalysis.

An improved fundamental understanding of active site structures can unlock opportunities for catalysis from conceptual design to industrial practice. Herein, we present the computational discovery and experimental demonstration of a highly active surface-phosphorylated ceria catalyst that exhibits robust chlorine tolerance for catalysis. Ab initio molecular dynamics (AIMD) calculations and in situ near-ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS) identified a predominantly HPO4 active structure on CeO2(110) and CeO2(111) facets at room temperature. Importantly, further elevating the temperature led to a unique hydrogen (H) atom hopping between coordinatively unsaturated oxygen and the adjacent P═O group of HPO4. Such a mobile H on the catalyst surface can effectively quench the chlorine radicals (Cl•) via an orientated reaction analogous to hydrogen atom transfer (HAT), enabling the surface-phosphorylated CeO2-supported monolithic catalyst to exhibit both expected activity and stability for over 68 days during a pilot test, catalyzing the destruction of a complex chlorinated volatile organic compound industrial off-gas.

Socio-environmental Opportunities for Organic Material Management in California's Sustainability Transition.

Contemporary resource management is doubly burdened by high rates of organic material disposal in landfills, generating potent greenhouse gases (GHG), and globally degraded soils, which threaten future food security. Expansion of composting can provide a resilient alternative, by avoiding landfill GHG emissions, returning valuable nutrients to the soil to ensure continued agricultural production, and sequestering carbon while supporting local communities. Recognizing this opportunity, California has set ambitious organics diversion targets in the Short-Lived Climate Pollutant Law (SB1383) which will require significant increases (5 to 8 million tonnes per year) in organic material processing capacity. This paper develops a spatial optimization model to consider how to handle this flow of additional material while achieving myriad social and ecological benefits through compost production. We consider community-based and on-farm facilities alongside centralized, large-scale infrastructure to explore decentralized and diversified alternative futures of composting infrastructure in the state of California. We find using a diversity of facilities would provide opportunity for cost savings while achieving significant emissions reductions of approximately 3.4 ± 1 MMT CO2e and demonstrate that it is possible to incorporate community protection into compost infrastructure planning while meeting economic and environmental objectives.

Characterizing PM2.5 Emissions and Temporal Evolution of Organic Composition from Incense Burning in a California Residence.

The chemical composition of incense-generated organic aerosol in residential indoor air has received limited attention in Western literature. In this study, we conducted incense burning experiments in a single-family California residence during vacancy. We report the chemical composition of organic fine particulate matter (PM2.5), associated emission factors (EFs), and gas-particle phase partitioning for indoor semivolatile organic compounds (SVOCs). Speciated organic PM2.5 measurements were made using two-dimensional gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC×GC-HR-ToF-MS) and semivolatile thermal desorption aerosol gas chromatography (SV-TAG). Organic PM2.5 EFs ranged from 7 to 31 mg g-1 for burned incense and were largely comprised of polar and oxygenated species, with high abundance of biomass-burning tracers such as levoglucosan. Differences in PM2.5 EFs and chemical profiles were observed in relation to the type of incense burned. Nine indoor SVOCs considered to originate from sources other than incense combustion were enhanced during incense events. Time-resolved concentrations of these SVOCs correlated well with PM2.5 mass (R2 > 0.75), suggesting that low-volatility SVOCs such as bis(2-ethylhexyl)phthalate and butyl benzyl phthalate partitioned to incense-generated PM2.5. Both direct emissions and enhanced partitioning of low-volatility indoor SVOCs to incense-generated PM2.5 can influence inhalation exposures during and after indoor incense use.

Phosphotungstic Acid as a Dechlorination Agent Collaborates with CeO2 for Synergistic Catalytic Elimination of NOx and Chlorobenzene.

The development of efficient technologies for the synergistic catalytic elimination of NOx and chlorinated volatile organic compounds (CVOCs) remains challenging. Chlorine species from CVOCs are prone to catalyst poisoning, which increases the degradation temperature of CVOCs and fails to balance the selective catalytic reduction of NOx with the NH3 (NH3-SCR) performance. Herein, synergistic catalytic elimination of NOx and chlorobenzene has been originally demonstrated by using phosphotungstic acid (HPW) as a dechlorination agent to collaborate with CeO2. The conversion of chlorobenzene was over 80% at 270 °C, and the NOx conversion and N2 selectivity reached over 95% at 270-420 °C. HPW not only allowed chlorine species to leave as inorganic chlorine but also enhanced the BroÌ·nsted acidity of CeO2. The NH4+ produced in the NH3-SCR process can effectively promote the dechlorination of chlorobenzene at low temperatures. HPW remained structurally stable in the synergistic reaction, resulting in good water resistance and long-term stability. This work provides a cheaper and more environmentally friendly strategy to address chlorine poisoning in the synergistic reaction and offers new guidance for multipollutant control.

Coprecipitation of Fe/Cr Hydroxides at Organic-Water Interfaces: Functional Group Richness and (De)protonation Control Amounts and Compositions of Coprecipitates.

Iron/chromium hydroxide coprecipitation controls the fate and transport of toxic chromium (Cr) in many natural and engineered systems. Organic coatings on soil and engineered surfaces are ubiquitous; however, mechanistic controls of these organic coatings over Fe/Cr hydroxide coprecipitation are poorly understood. Here, Fe/Cr hydroxide coprecipitation was conducted on model organic coatings of humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA). The organics bonded with SiO2 through ligand exchange with carboxyl (-COOH), and the adsorbed amounts and pKa values of -COOH controlled surface charges of coatings. The adsorbed organic films also had different complexation capacities with Fe/Cr ions and Fe/Cr hydroxide particles, resulting in significant differences in both the amount (on HA > SA(-COOH) ≫ BSA(-NH2)) and composition (Cr/Fe molar ratio: on BSA(-NH2) ≫ HA > SA(-COOH)) of heterogeneous precipitates. Negatively charged -COOH attracted more Fe ions and oligomers of hydrolyzed Fe/Cr species and subsequently promoted heterogeneous precipitation of Fe/Cr hydroxide nanoparticles. Organic coatings containing -NH2 were positively charged at acidic pH because of the high pKa value of the functional group, limiting cation adsorption and formation of coprecipitates. Meanwhile, the higher local pH near the -NH2 coatings promoted the formation of Cr(OH)3. This study advances fundamental understanding of heterogeneous Fe/Cr hydroxide coprecipitation on organics, which is essential for successful Cr remediation and removal in both natural and engineered settings, as well as the synthesis of Cr-doped iron (oxy)hydroxides for material applications.

Recent advances of application of bentonite-based composites in the environmental remediation.

Bentonite-based composites have been widely utilized in the removal of various pollutants due to low cost, environmentally friendly, ease-to-operate, whereas the recent advances concerning the application of bentonite-based composites in environmental remediation were not available. Herein, the modification (i.e., acid/alkaline washing, thermal treatment and hybrids) of bentonite was firstly reviewed; Then the recent advances of adsorption of environmental concomitants (e.g., organic (dyes, microplastics, phenolic and other organics) and inorganic pollutants (heavy metals, radionuclides and other inorganic pollutants)) on various bentonite-based composites were summarized in details. Meanwhile, the effect of environmental factors and interaction mechanism between bentonite-based composites and contaminants were also investigated. Finally, the conclusions and prospective of bentonite-based composites in the environmental remediation were proposed. It is demonstrated that various bentonite-based composites exhibited the high adsorption/degradation capacity towards environmental pollutants under the specific conditions. The interaction mechanism involved the mineralization, physical/chemical adsorption, co-precipitation and complexation. This review highlights the effect of different functionalization of bentonite-based composites on their adsorption capacity and interaction mechanism, which is expected to be helpful to environmental scientists for applying bentonite-based composites into practical environmental remediation.

Composition Distribution of the Thermal Soluble Organics from Naomaohu Lignite and Structural Characteristics of the Corresponding Insoluble Portions.

With cyclohexane (CH), benzene (BE), and ethyl acetate (EA) as solvents, Naomaohu lignite (NL, a typical oil-rich, low-rank coal) from Hami, Xinjiang, was thermally dissolved (TD) to obtain three types of soluble organics (NLCH, NLBE, and NLEA) and the corresponding insoluble portions (NLCH-R, NLBE-R, and NLEA-R). Ultimate analysis, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG-DTG), and gas chromatography-mass spectrometry (GC/MS) were used to characterize NL and its soluble and insoluble portions. Results showed that, compared with NL, the C element in NLCH-R, NLBE-R, and NLEA-R increased, while the O element decreased significantly, indicating that thermal dissolution is a carbon enrichment process and an effective deoxidation method. The GC/MS results showed that oxygen-containing organic compounds (OCOCs) are dominant in NLCH, NLBE, and NLEA. NLCH is mainly composed of ketones (11.90%) and esters (19.04%), while NLBE and NLEA are composed of alcohols (12.18% and 2.42%, respectively) and esters (66.09% and 84.08%, respectively), with alkyl and aromatic acid esters as the main components. Among them, EA exhibits significant selective destruction for oxygen-containing functional groups in NL. XPS, FTIR, and TG-DTG results showed that thermal dissolution can not only affect the macromolecular network structure of NL, but also improve its pyrolysis reactivity. In short, thermal dissolution can effectively obtain oxygen-containing organic compounds from NL.

Intensive Treatment of Organic Wastewater by Three-Dimensional Electrode System within Mn-Loaded Steel Slag as Catalytic Particle Electrodes.

Developing a green, low-carbon, and circular economic system is the key to achieving carbon neutrality. This study investigated the organics removal efficiency in a three-dimensional electrode reactor (3DER) constructed from repurposed industrial solid waste, i.e., Mn-loaded steel slag, as the catalytic particle electrodes (CPE). The CPE, a micron-grade material consisting primarily of transition metals, including Fe and Mn, exhibited excellent electric conductivity, catalytic ability, and recyclability. High rhodamine B (RhB) removal efficiency in the 3DER was observed through a physical modelling experiment. The optimal operating condition was determined through a single-factor experiment in which 5.0 g·L-1 CPE and 3 mM peroxymonosulfate (PMS) were added to a 200 mL solution of 10 mM RhB under a current intensity of 0.5 A and a 1.5 to 2.0 cm distance between the 2D electrodes. When the initial pH value of the simulated solution was 3 to 9, the RhB removal rate exceeded 96% after 20 min reaction. In addition, the main reactive oxidation species in the 3DER were determined. The results illustrated that HO• and SO4•- both existed, but that the contribution of SO4•- to RhB removal was much lower than that of HO• in the 3DER. In summary, this research provides information on the potential of the 3DER for removing refractory organics from water.

Performance of a polymerization-based electrochemically assisted persulfate process on a real coking wastewater treatment.

Industrial wastewater should be treated with caution due to its potential environmental risks. In this study, a polymerization-based cathode/Fe3+/peroxydisulfate (PDS) process was employed for the first time to treat a raw coking wastewater, which can achieve simultaneous organics abatement and recovery by converting organic contaminants into separable solid organic-polymers. The results confirm that several dominant organic contaminants in coking wastewater such as phenol, cresols, quinoline and indole can be induced to polymerize by self-coupling or cross-coupling. The total chemical oxygen demand (COD) abatement from coking wastewater is 46.8% and the separable organic-polymer formed from organic contaminants accounts for 62.8% of the abated COD. Dissolved organic carbon (DOC) abatement of 41.9% is achieved with about 89% less PDS consumption than conventional degradation-based process. Operating conditions such as PDS concentration, Fe3+ concentration and current density can affect the COD/DOC abatement and organic-polymer yield by regulating the generation of reactive radicals. ESI-MS result shows that some organic-polymers are substituted by inorganic ions such as Cl-, Br-, I-, NH4+, SCN- and CN-, suggesting that these inorganic ions may be involved in the polymerization. The specific consumption of this coking wastewater treatment is 27 kWh/kg COD and 95 kWh/kg DOC. The values are much lower than those of the degradation-based processes in treating the same coking wastewater, and also are lower than those of most processes previously reported for coking wastewater treatment.

Biotoxicity dynamic change and key toxic organics identification of coal chemical wastewater along a novel full-scale treatment process.

It is particularly important to comprehensively assess the biotoxicity variation of industrial wastewater along the treatment process for ensuring the water environment security. However, intensive studies on the biotoxicity reduction of industrial wastewater are still limited. In this study, the toxic organics removal and biotoxicity reduction of coal chemical wastewater (CCW) along a novel full-scale treatment process based on the pretreatment process-anaerobic process-biological enhanced (BE) process-anoxic/oxic (A/O) process-advanced treatment process was evaluated. This process performed great removal efficiency of COD, total phenol, NH4+-N and total nitrogen. And the biotoxicity variation along the treatment units was analyzed from the perspective of acute biotoxicity, genotixicity and oxidative damage. The results indicated that the effluent of pretreatment process presented relatively high acute biotoxicity to Tetrahymena thermophila. But the acute biotoxicity was significantly reduced in BE-A/O process. And the genotoxicity and oxidative damage to Tetrahymena thermophila were significantly decreased after advanced treatment. The polar organics in CCW were identified as the main biotoxicity contributors. Phenols were positively correlated with acute biotoxicity, while the nitrogenous heterocyclic compounds and polycyclic aromatic hydrocarbons were positively correlated with genotoxicity. Although the biotoxicity was effectively reduced in the novel full-scale treatment process, the effluent still performed potential biotoxicity, which need to be further explored in order to reduce environmental risk.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation.

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

Near Infrared Light-to-Heat Conversion for Liquid-Phase Oxidation Reactions by Antimony-Doped Tin Oxide Nanocrystals.

Effective utilization of the sunlight for chemical reactions is pivotal for dealing with the growing energy and environmental issues. So far, much effort has been focused on the development of semiconductor photocatalysts responsive to UV and visible light. However, the near infrared and infrared (NIR-IR) light occupying ∼50 % of the solar energy has usually been wasted because of the low photon energy insufficient for the band gap excitation. Antimony doping into SnO2 (ATO) induces strong absorption due to the conduction band electrons in the NIR region. The absorbed light energy is eventually converted to heat via the interaction between hot electrons and phonons. This Concept highlights the photothermal effect of ATO nanocrystals (NCs) on liquid-phase oxidation reactions through the NIR light-to-heat conversion. Under NIR illumination even at an intensity of ∼0.5 sun, the reaction field temperature on the catalyst surface is raised 20-30 K above the bulk solution temperature, while the latter is maintained near the ambient temperature. In some reactions, this photothermal local heating engenders the enhancement of not only the catalytic activity and selectivity but also the regeneration of catalytically active sites. Further, the photocatalytic activity of semiconductors can be promoted. Finally, the conclusions and possible subjects in the future are summarized.

Fate of Naturally Dissolved Organic Matter and Synthetic Organic Compounds Subjected to Drinking Water Treatment Using Membrane, Activated Carbon, and UV/H2O2 Technologies.

Organic pollutants are toxic and are present in drinking water. The conventional processes of most water plants can basically meet the discharge standard. However, based on the improvement of the objective of organic pollutants control and the constant change of water characteristics, the results may not be ideal. This study evaluates the effectiveness of different treatments such as microfiltration, nanofiltration, reverse osmosis, activated carbon, and ultraviolet irradiation/H2O2 in terms of the removal of organic pollutants. Among the DOM results, nanofiltration, reverse osmosis, and activated carbon showed optimal performance due to the characteristics of processes and the compound properties. However, the risks of low-molecular-weight organic residue and byproduct formation are still present. Thirty-nine species of synthetic organic compounds (SOC) were qualitatively and semiquantitatively analyzed. Different technologies showed varying removal capabilities for SOC based on their properties and many substances coexisted leading to abnormal removal performances. These residual organics showed the characteristics of lower molecular weight, more hydrophilicity, further unknown impacts, and with risk of DBPs. Based on the above insights, possible methods can be rationally chosen for on-demand decontamination of organics in unconfined aquatic environment and long-time impact on water characteristics and human health also should be considered.

Polymerization State of Vanadyl Species Affects the Catalytic Activity and Arsenic Resistance of the V2O5-WO3/TiO2 Catalyst in Multipollutant Control of NOx and Chlorinated Aromatics.

The conventional V2O5-WO3/TiO2 catalyst suffers severely from arsenic poisoning, leading to a significant loss of catalytic activity. The doping of Al or Mo plays an important role in promoting the arsenic resistance on NH3 selective catalytic reduction (NH3-SCR), but their promotion mechanism remains in debate and has yet to be explored in multipollutant control (MPC) of NOx and chlorinated organics. Herein, our experimental characterizations and density functional theory (DFT) calculations confirmed that arsenic species preferentially adsorb on both Al and Mo to form arsenate, thereby avoiding bonding to the catalytically active V sites. More importantly, Al doping partially converted the polymeric vanadyl species into monomeric ones, thereby inhibiting the near-surface and bulk lattice oxygen mobility of the V2O5-WO3/TiO2 catalyst, while Mo doping resulted in vanadyl polymerization with an enriched V5+ chemical state and exhibited superior MPC activity and COx selectivity. Our work shows that antipoisoning catalysts can be designed with the combination of site protection and occurrence state modification of the active species.

Real-World Emission Characteristics of Full-Volatility Organics Originating from Nonroad Agricultural Machinery during Agricultural Activities.

Nonroad agricultural machinery (NRAM) emissions constitute a significant source of air pollution in China. Full-volatility organics originating from 19 machines under 6 agricultural activities were measured synchronously. The diesel-based emission factors (EFs) for full-volatility organics were 4.71 ± 2.78 g/kg fuel (average ± standard deviation), including 91.58 ± 8.42% volatile organic compounds (VOCs), 7.94 ± 8.16% intermediate-volatility organic compounds (IVOCs), 0.28 ± 0.20% semivolatile organic compounds (SVOCs), and 0.20 ± 0.16% low-volatility organic compounds (LVOCs). Full-volatility organic EFs were significantly reduced by stricter emission standards and were the highest under pesticide spraying activity. Our results also demonstrated that combustion efficiency was a potential factor influencing full-volatility organic emissions. Gas-particle partitioning in full-volatility organics could be affected by multiple factors. Furthermore, the estimated secondary organic aerosol formation potential based on measured full-volatility organics was 143.79 ± 216.80 mg/kg fuel and could be primarily attributed to higher-volatility-interval IVOCs (bin12-bin16 contributed 52.81 ± 11.58%). Finally, the estimated emissions of full-volatility organics from NRAM in China (2021) were 94.23 Gg. This study provides first-hand data on full-volatility organic EFs originating from NRAM to facilitate the improvement of emission inventories and atmospheric chemistry models.

Remarkable nucleation and growth of ultrafine particles from vehicular exhaust.

High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem.