Photo-controllable Luminescence from Radicals Leading to Ratiometric Emission Switching via Dynamic Intermolecular Coupling.

The development of photoinduced luminescent radicals with dynamic emission color is still challenging. Herein we report a novel molecular radical system (TBIQ) that shows photo-controllable luminescence, leading to a wide range of ratiometric color changes via light excitation. The conjugated skeleton of TBIQ is decorated with steric-demanding tertiary butyl groups that enable appropriate intermolecular interaction to make dynamic intermolecular coupling possible for controllable behaviors. We reveal that the helicenic pseudo-planar conformation of TBIQ experiences a planarization process after light excitation, leading to more compactly stacked supermolecules and thus generating radicals via intermolecular charge transfer. The photo-controllable luminescent radical system is employed for a high-level information encryption application. This study may offer unique insight into molecular dynamic motion for optical manufacturing and broaden the scope of smart-responsive materials for advanced applications.

Intramolecular Cyclization: A Convenient Strategy to Realize Efficient BT.2020 Blue Multi-Resonance Emitter for Organic Light-Emitting Diodes.

Rationally tuning the emission position and narrowing the full width at half-maximum (FWHM) of an emitter is of great importance for many applications. By synergistically improving rigidity, strengthening the resonant strength, inhibiting molecular bending and rocking, and destabilizing the HOMO energy level, a deep-blue emitter (CZ2CO) with a peak wavelength of 440 nm and an ultranarrow spectral FWHM of 16 nm (0.10 eV) was developed via intramolecular cyclization in a carbonyl/N resonant core (QAO). The dominant υ0-0 transition character of CZ2CO gives a Commission Internationale de I'Éclairage coordinates (CIE) of (0.144, 0.042), nicely complying with the BT.2020 standard. Moreover, a hyper-fluorescent device based on CZ2CO shows a high maximum external quantum efficiency (EQEmax ) of 25.6 % and maintains an EQE of 22.4 % at a practical brightness of 1000 cd m-2 .

Efficient Pyrazolo[5,4-f]quinoxaline Functionalized Os(II) Based Emitter with an Electroluminescence Peak Maximum at 811†nm.

Upon fusing the pyrazinyl pyrazole entity in giving pyrazolo[3,4-f]quinoxaline chelate, the corresponding Os(II) based NIR emitter exhibited "invisible" and efficient electroluminescence with a peak maximum at 811 nm. A maximum external quantum efficiency of 0.97 % and a suppressed efficiency roll-off till a current density of 300 mA cm-2 was also exhibited.

pH-Dependent Chemical Transformations of Humic-Like Substances and Further Cognitions Revealed by Optical Methods.

Humic-like substances (HULIS) are macromolecular complex groups in water-soluble organic compounds (WSOC). pH is a crucial factor that influences the chemical transformations of HULIS in atmospheric particles, but this has been rarely investigated, especially under varying pH conditions. This study attempted to unveil the chemical transformation mechanisms of HULIS under a range of pH conditions using optical methods. The pH-dependent light absorption and fluorescence properties of HULIS were comprehensively analyzed; the acidity coefficient (pKa) of HULIS in relation to chemical structures was determined, and the hypothetical chemical transformation mechanisms of HULIS with increasing pH were analyzed by optical characterizations. The results suggested that pH greatly impacted the light absorption and fluorescence efficiencies of HULIS in both winter and summer seasons, and pKa was an important inflection point. The pKa of HULIS ranged from 3.5 to 8.0 in winter and 6.4 to 10.0 in summer. The acidic/basic groups were identified as -OH or -NH2 substituted quinolines, carboxylic aromatics, and pyridines. The pH-sensitive species accounted for about 6% and 21% of HULIS-C (carbon concentrations of HULIS) in winter and summer, respectively. The varying optical spectra with increasing pH might result from charge transfer or complex reactions with HULIS deprotonation.

Removal of Cd(II) from Micro-Polluted Water by Magnetic Core-Shell Fe3O4@Prussian Blue.

A novel core-shell magnetic Prussian blue-coated Fe3O4 composites (Fe3O4@PB) were designed and synthesized by in-situ replication and controlled etching of iron oxide (Fe3O4) to eliminate Cd (II) from micro-polluted water. The core-shell structure was confirmed by TEM, and the composites were characterized by XRD and FTIR. The pore diameter distribution from BET measurement revealed the micropore-dominated structure of Fe3O4@PB. The effects of adsorbents dosage, pH, and co-existing ions were investigated. Batch results revealed that the Cd (II) adsorption was very fast initially and reached equilibrium after 4 h. A pH of 6 was favorable for Cd (II) adsorption on Fe3O4@PB. The adsorption rate reached 98.78% at an initial Cd (II) concentration of 100 µg/L. The adsorption kinetics indicated that the pseudo-first-order and Elovich models could best describe the Cd (II) adsorption onto Fe3O4@PB, indicating that the sorption of Cd (II) ions on the binding sites of Fe3O4@PB was the main rate-limiting step of adsorption. The adsorption isotherm well fitted the Freundlich model with a maximum capacity of 9.25 mg·g-1 of Cd (II). The adsorption of Cd (II) on the Fe3O4@PB was affected by co-existing ions, including Cu (II), Ni (II), and Zn (II), due to the competitive effect of the co-adsorption of Cd (II) with other co-existing ions.

Source apportionment of VOCs in a typical medium-sized city in North China Plain and implications on control policy.

Characteristics of atmospheric VOCs (volatile organic compounds) have been extensively studied in megacities in China, however, they are scarcely investigated in medium/small-sized cities in North China Plain (NCP). A comprehensive research on possible sources of VOCs was conducted in a medium-sized city of NCP, from May to September 2019. A total of 143 canister samples of 8 sites in Xuchang city were collected, and 57 VOC species were detected. The average VOC concentrations were 42.6 ± 31.6 µg/m3, with 53.7 ± 31.0 µg/m3 and 32.1 ± 27. 8 µg/m3, in the morning and afternoon, respectively. Alkenes and aromatics contributed 80% of the total ozone formation potential (OFP). Aromatics accounted for more than 95% of secondary organic aerosol potential (SOAP). VOCs were dominated by the local emission with significant transport from the southeast direction. PMF analysis extracted 6 sources, which were combustion (33.1%), LPG usage (19.3%), vehicular exhaust & fuel evaporation (15.8%), solvent usage (15.2%), industrial (9.11%) and biogenic (7.51%), respectively and they contributed 33.4%, 17.6%, 12.9%, 18.6%, 9.28% and 8.22% to the OFP, respectively. Combustion and LPG usage were the dominant VOC sources; and combustion, solvent usage and LPG usage were the main sources of OFP in Xuchang city, which were different to megacities in China with a high contribution from vehicular exhaust, solvent usage and industry, suggesting specific control strategies on VOCs need to be implemented in medium-sized city such as Xuchang city.

Water-Soluble Organic Nanoparticles with Programable Intermolecular Charge Transfer for NIR-II Photothermal Anti-Bacterial Therapy.

Extensive recent efforts have been put on the design of high-performance organic near-infrared (NIR) photothermal agents (PTAs), especially over NIR-II bio-window (1000-1350 nm). So far, the development is mainly limited by the rarity of molecules with good NIR-II response. Here, we report organic nanoparticles of intermolecular charge-transfer complexes (CTCs) with easily programmable optical absorption. By employing different common donor and acceptor molecules to form CTC nanoparticles (CT NPs), absorption peaks of CT NPs can be controllably tuned from the NIR-I to NIR-II region. Notably, CT NPs formed with perylene and TCNQ have a considerably red-shifted absorption peak at 1040 nm and achieves a good photothermal conversion efficiency of 42 % under 1064 nm excitation. These nanoparticles were used for antibacterial application with effective activity towards both Gram-negative and Gram-positive bacteria. This work opens a new avenue into the development of efficient PTAs.

Two-Dimensional Silicon Fingerprints Reveal Dramatic Variations in the Sources of Particulate Matter in Beijing during 2013-2017.

Since the implementation of the "Air Pollution Prevention and Control Action Plan" (APPCAP) in 2013, the air quality in China has been greatly improved but still much exceeded the WHO guideline limit. Here we employed a novel approach, two-dimensional Si fingerprints, including stable Si isotopic composition (δ30Si and Si abundance (Si%), to investigate the annual variations in both primary and secondary sources of PM2.5 in Beijing during the APPCAP period (2013-2017). The δ30Si and Si% values were used as tracers to reflect the variations in primary and secondary sources, respectively. For primary sources, the mean δ30Si value of PM2.5 in 2015-2017 (>-0.5‰) was significantly more positive than that of 2013 (-1.24‰), indicating a dramatic decline in the contribution of 30Si-depleted sources (i.e., coal burning and industrial emission). For secondary sources, the mean Si% of PM2.5 increased from 1.2% in 2013 to 4.6% in 2017, suggesting a large decrease in the secondary aerosol contribution from 83% to 42%. It is worth noting that we found the 30Si-depleted sources showed a rebound trend during 2015-2017. This study reveals the responses of anthropogenic emission sources under strong regulation policies and provides a reference for future policymaking in Beijing and other polluted regions and countries.

Stokes Shift and Specific Fluorescence as Potential Indicators of Organic Matter Hydrophobicity and Molecular Weight in Membrane Bioreactors.

Hydrophobicity and molecular weight (MW) are two fundamental properties of dissolved organic matter (DOM) in wastewater treatment systems. This study proposes fluorescence Stokes shift and specific fluorescence intensity (SFI) as novel indicators of hydrophobicity and MW. These indicators originate from the energy gap and photon efficiency of the fluorescence process and can be readily extracted from a fluorescence excitation-emission matrix (EEM). The statistical linkages between these indicators and hydrophobicity/MW were explored through investigation of DOM across 10 full-scale membrane bioreactors treating municipal wastewater. Stokes shift was found to exhibit a general rule among the hydrophobicity components in the order of hydrophilic substances (HIS) < hydrophobic acids (HOA) < hydrophobic bases (HOB). The Stokes shift of 1.2 µm-1 is a critical border, above which the relative fluorescence correlated significantly with the HOA-related content (Pearson's r = 0.8). With regard to MW distribution (<1, 1-10, 10-100, and >100 kDa), SFI was found to be the most sensitive to the change of MW of <1 kDa proportion, especially at the excitation/emission wavelengths of 200-320/310-550 nm (r > 0.9). Hydrophobicity-related π conjugation and MW-dependent light exposure might be responsible for the correlations. These fluorescence indicators may be useful for convenient monitoring of DOM in wastewater treatment systems.

Two new quinoline-based regenerable fluorescent probes with AIE characteristics for selective recognition of Cu2+ in aqueous solution and test strips.

Two novel highly selective quinoline-based fluorescent probes (1 and 2) with an aggregation induced emission (AIE) feature have been designed and synthesized for the rapid analysis of Cu2+ ions in aqueous media and on paper strips with a fluorescence quenching mechanism. Moreover, probes 1 and 2 exhibit excellent sensitivity and anti-interference for Cu2+ detection, and the detection limits are as low as 1.3 × 10-8 M and 8.5 × 10-8 M, respectively, which are much lower than the allowable standard of Cu2+ (∼20 µM) in drinking water (EPA). More importantly, these two probes were successfully applied for the determination of Cu2+ in real aqueous samples and fabrication of simple device test strips for rapid and on-site detection of Cu2+ ions. Interestingly, they can also be regenerated by adding an excess of S2-. Additionally, the crystallographic structure of probe 1 was confirmed through a single crystal X-ray study. Job's plot analysis and ESI-MS spectroscopic studies reflect the 1 : 1 complexation of the 1-Cu2+ and 2-Cu2+ complexes. Furthermore, DFT/TDDFT calculations were performed in order to help in understanding the electronic properties of the complexes and the chelation-induced quenching mechanism.

Characteristic Regions of the Fluorescence Excitation-Emission Matrix (EEM) To Identify Hydrophobic/Hydrophilic Contents of Organic Matter in Membrane Bioreactors.

This study systematically investigated the correlations between fluorescence distributions characterized by the excitation-emission matrix (EEM) and hydrophobic/hydrophilic composition of dissolved organic matter (DOM) in membrane bioreactors (MBRs). On the basis of samples from 10 full-scale MBRs, we performed point-to-point comparisons among different components using an EEM fluorescence quotient (FQ) method and obtained a hydrophobic/hydrophilic fluorophore distribution map via Wilcoxon signed rank test. Hydrophobic acids/bases (HOA/HOB) concentrated in the low-wavelength region [excitation wavelength (Ex) < 235 nm], while hydrophilic substances (HIS) were enriched in the region of Ex > 235 nm [especially with emission wavelength (Em) = 300-360 nm]. Quantitatively, EEM regional contribution to whole wavelength fluorescence was found to significantly correlate with the hydrophobic/hydrophilic proportions of DOM, with Pearson's coefficients of 0.94 and 0.78 ( p < 0.01) for HOA and HIS, respectively. We established a linear regression model showing the HOA proportion as a function of the EEM regional contribution at (Ex, Em) = (200-285, 340-465 nm), with R2 = 0.876, which was validated via leave-one-out cross-validation and Monte Carlo simulation. This study shows a statistically hydrophobicity-dependent fluorescence property across different MBRs, and it might be applied to provide a quick estimation of hydrophobic/hydrophilic composition of DOM in wastewater treatment systems based on EEM monitoring.

Photochemical production of atmospheric carbonyls in a rural area in southern China.

For the first time, ambient carbonyls were measured in a rural area in southern China from August 2012 to February 2013 to investigate their distribution characteristics and sources. Formaldehyde, acetaldehyde, and acetone were the three most abundant carbonyls, which accounted for 83-95 % of total seven carbonyls identified. The O3 formation potential of carbonyls in summer (59.55 µg/m(3)) was approximately ten times greater than that (6.37 µg/m(3)) in winter, and calculated photolysis rates were significantly faster in summer than those in winter, suggesting intensive photochemical activities in summer. Seasonal and diurnal variations of carbonyls showed that (1) the concentration of total carbonyls in summer (12.62 ± 10.83 µg/m(3)) was approximately five times greater than that in winter (2.33 ± 0.90 µg/m(3)), and a similar trend applied to the three abundant carbonyls; (2) the average summer to winter (S/W) ratio of formaldehyde and acetaldehyde was 10-13, and the S/W ratio of acetone was ~2.59; and (3) the highest concentrations of the three carbonyls and total carbonyls occurred at 14:00-16:00 with high temperature and intensive sunlight, especially in summer. These variations provided direct evidence for significant photochemical production of ambient carbonyls. Average C1/C2 ratios (3.07 ± 1.62) in summer were much greater than those (1.28 ± 0.25) in winter, and average C2/C3 ratios (35.09 ± 58.67) in summer were significantly greater than those (4.75 ± 2.12) in winter, both cases indirectly implying positive photochemical productions in summer. Especially, strong correlations (R(2) = 0.63-0.98) of temperature and sunlight intensity with the three abundant carbonyls and total carbonyls were observed, indicating a similar causal source such as significant photochemical production.

Size distribution, characteristics and sources of heavy metals in haze episode in Beijing.

Size segragated samples were collected during high polluted winter haze days in 2006 in Beijing, China. Twenty nine elements and 9 water soluble ions were determined. Heavy metals of Zn, Pb, Mn, Cu, As, Cr, Ni, V and Cd were deeply studied considering their toxic effect on human being. Among these heavy metals, the levels of Mn, As and Cd exceeded the reference values of National Ambient Air Quality Standard (GB3095-2012) and guidelines of World Health Organization. By estimation, high percentage of atmospheric heavy metals in PM2.5 indicates it is an effective way to control atmospheric heavy metals by PM2.5 controlling. Pb, Cd, and Zn show mostly in accumulation mode, V, Mn and Cu exist mostly in both coarse and accumulation modes, and Ni and Cr exist in all of the three modes. Considering the health effect, the breakthrough rates of atmospheric heavy metals into pulmonary alveoli are: Pb (62.1%) > As (58.1%) > Cd (57.9%) > Zn (57.7%) > Cu (55.8%) > Ni (53.5%) > Cr (52.2%) > Mn (49.2%) > V (43.5%). Positive matrix factorization method was applied for source apportionment of studied heavy metals combined with some marker elements and ions such as K, As, SO4(2-) etc., and four factors (dust, vehicle, aged and transportation, unknown) are identified and the size distribution contribution of them to atmospheric heavy metals are discussed.

Facilely Achieving Near-Infrared-II J-Aggregates through Molecular Bending on a Donor-Acceptor Fluorophore for High-Performance Tumor Phototheranostics.

Constructing J-aggregated organic dyes represents a promising strategy for obtaining biomedical second near-infrared (NIR-II) emissive materials, as they exhibit red-shifted spectroscopic properties upon assembly into nanoparticles (NPs) in aqueous environments. However, currently available NIR-II J-aggregates primarily rely on specific molecular backbones with intricate design strategies and are susceptible to fluorescence quenching during assembly. A facile approach for constructing bright NIR-II J-aggregates using prevalent donor-acceptor (D-A) molecules is still lacking. In this study, we present a facile method that transforms D-A molecules into J-aggregates by simply bending the molecule through introducing a methyl group, enabling high-performance NIR-II phototheranostics. The TAA-BT-CN molecule exhibits hypsochromic-shift absorption upon forming H-aggregated NPs, while the designed mTAA-BT-CN with a bent structure demonstrates a bathochromic shift of over 100 nm in absorption upon forming J-aggregated NPs, leading to much enhanced NIR-II emission beyond 1100 nm. With respect to its H-aggregated counterpart with the aggregation-caused quenching (ACQ) phenomenon, the J-aggregated mTAA-BT-CN NPs exhibit a 7-fold increase in NIR-II fluorescence owing to their aggregation-induced emission (AIE) property as well as efficient generation of heat and reactive oxygen species under 808 nm light excitation. Finally, the mTAA-BT-CN NPs are employed for whole-body blood vessel imaging using NIR-II technology as well as imaging-guided tumor phototherapies. This study will facilitate the flourishing advancement of J-aggregates based on prevalent D-A-type molecules.

Highly efficient pure-blue organic light-emitting diodes based on rationally designed heterocyclic phenophosphazinine-containing emitters.

Multi-resonance thermally activated delayed fluorophores have been actively studied for high-resolution photonic applications due to their exceptional color purity. However, these compounds encounter challenges associated with the inefficient spin-flip process, compromising device performance. Herein, we report two pure-blue emitters based on an organoboron multi-resonance core, incorporating a conformationally flexible donor, 10-phenyl-5H-phenophosphazinine 10-oxide (or sulfide). This design concept selectively modifies the orbital type of high-lying excited states to a charge transfer configuration while simultaneously providing the necessary conformational freedom to enhance the density of excited states without sacrificing color purity. We show that the different embedded phosphorus motifs (phosphine oxide/sulfide) of the donor can finely tune the electronic structure and conformational freedom, resulting in an accelerated spin-flip process through intense spin-vibronic coupling, achieving over a 20-fold increase in the reverse intersystem crossing rate compared to the parent multi-resonance emitter. Utilizing these emitters, we achieve high-performance pure-blue organic light-emitting diodes, showcasing a top-tier external quantum efficiency of 37.6% with reduced efficiency roll-offs. This proposed strategy not only challenges the conventional notion that flexible electron-donors are undesirable for constructing narrowband emitters but also offer a pathway for designing efficient narrow-spectrum blue organic light-emitting diodes.

Synergistic effect of hydrogen bonding and π-π interaction for enhanced adsorption of rhodamine B from water using corn straw biochar.

Dyes adsorption to biochar via hydrogen bonding, and π-π interaction alone have attracted much research attention, however, their synergism in adsorption mechanisms remains largely unnoticed. The synergistic effects of the hydrogen bonding and π-π interaction might improve the adsorption capacity and need more understanding to prepare high-capacity biochar. In this work, we evaluated the adsorption of various dyes on biochar prepared via the activation of potassium bicarbonate and urea (named BC-KN) to explore their synergistic effects. Batch experiments indicated the BC-KN showed a high adsorption capacity to rhodamine B at 4839.0 mg/g, azure B at 4477.7 mg/g, and methylene blue at 2223.0 mg/g, respectively. The mechanism of such significant adsorption was investigated by their comparative experiments, characterizations, and computational analyses. The computational analyses suggested that the synergism of the hydrogen bonding and π-π interaction improves the adsorption energies of BC-KN/RhB system from -10.35 kcal/mol to -20.49 kcal/mol. It can be concluded that the hydrogen bonding and π-π interaction can synergize to significantly improve the adsorption by increasing the π-electron density and shortening the distance of aromatic rings, thus dyes with H-donor show significantly better adsorption capacities. The insight of hydrogen bonding being the governing factor in the synergistic system will help produce high-capacity biochar in removing aromatic dyes and suggest a sustainable technology for the efficient decolorization of dye effluent to minimize its damage to the health and environment.

Unexpectedly high levels and health risks of atmospheric polychlorinated biphenyls in modern mechanical dismantling of obsolete electrical equipment: Investigations in a large integrated e-waste dismantling industrial estate.

Large industrial estates for electrical and electronic waste (e-waste) mechanical dismantling and recycling are gradually replacing outmoded small factories and intensive domestic workshops for e-waste manual and chemical dismantling. However, the air pollution and health risks of persistent organic pollutants during the modern mechanical processing of e-waste, especially obsolete electrical equipment, still remain unclear. Here, unexpectedly high levels (409.3 ng/m3) and health risks of airborne polychlorinated biphenyls (PCBs) were found during the mechanical processing of obsolete electric equipment or parts in a large integrated dismantling industrial estate, which is comparable to or a dozen times higher than those reported during chemical processing. In contrast, the levels (936.0 pg/m3) and health risks of particulate polybrominated diphenyl ethers (PBDEs) were all lower than those of previous studies. PCB emissions (44.9-3300.5 ng/m3) varied significantly across six mechanical dismantling places specifically treating waste motors, electrical appliances, hardware, transformers, and metals, respectively. The high PCB content and mass processing number of obsolete electrical equipment probably result in the highest PCB emissions from the mechanical dismantling of obsolete motors, followed by waste electrical appliances and metals. The PCB non-cancer and cancer risks associated with inhalation and dermal exposure in different mechanical dismantling places were all above the given potential risk limits. In particular, the health risks of dismantling obsolete motor exceeded the definite risk levels. Little difference in PCB emissions and health risks between working and non-working time suggested the importance of PCB volatilization from most e-waste. Such high PCB emissions and health risks of PCBs undoubtedly posed a severe threat to frontline workers, but fortunately, they decreased significantly with the increasing distance from the industrial estate. We highlight that PCB emissions and associated health risks from obsolete electrical equipment with high PCB content during mechanical dismantling activities should be of great concern.

Anti-Quenching NIR-II J-Aggregates of Benzo[c]thiophene Fluorophore for Highly Efficient Bioimaging and Phototheranostics.

Molecular fluorophores with the second near-infrared (NIR-II) emission hold great potential for deep-tissue bioimaging owing to their excellent biocompatibility and high resolution. Recently, J-aggregates are used to construct long-wavelength NIR-II emitters as their optical bands show remarkable red shifts upon forming water-dispersible nano-aggregates. However, their wide applications in the NIR-II fluorescence imaging are impeded by the limited varieties of J-type backbone and serious fluorescence quenching. Herein, a bright benzo[c]thiophene (BT) J-aggregate fluorophore (BT6) with anti-quenching effect is reported for highly efficient NIR-II bioimaging and phototheranostics. The BT fluorophores are manipulated to have Stokes shift over 400 nm and aggregation-induced emission (AIE) property for conquering the self-quenching issue of the J-type fluorophores. Upon forming BT6 assemblies in an aqueous environment, the absorption over 800 nm and NIR-II emission over 1000 nm are boosted for more than 41 and 26 folds, respectively. In vivo visualization of the whole-body blood vessel and imaging-guided phototherapy results verify that BT6 NPs are excellent agent for NIR-II fluorescence imaging and cancer phototheranostics. This work develops a strategy to construct bright NIR-II J-aggregates with precisely manipulated anti-quenching properties for highly efficient biomedical applications.

Nonmethane Hydrocarbons in Ambient Air of Hazy and Normal Days in Foshan, South China.

A first study of nonmethane hydrocarbons (NMHCs) on hazy and normal days was performed in Foshan for providing deep insight into the local deteriorating air quality. Ethane, propane, i-pentane, ethene, propene, ethyne, benzene, and toluene were eight most abundant compounds, accounting for 71%-85% of total NMHCs. Most hydrocarbons showed much higher levels on hazy days than normal days together with hydrocarbon/ethyne ratios and diurnal variations, indicating hazy days are more dominated by vehicular emission. Correlation coefficients (R(2)) of ethane, propane, ethane, propene, benzene, and total NMHCs with ethyne were 0.62-0.83, indicating these compounds are mainly related to vehicular emission. R(2) analysis indicated that solvent usage is responsible for toluene and other aromatic hydrocarbons (e.g., ethylbezene). Benzene/toluene (B/T) ratio was 0.44±0.23 during whole sampling periods, again indicating vehicular emission is the dominant source. Lower B/T ratio (0.30±0.14) on hazy days than that (0.58±0.21) on normal days suggested that solvent usage emitted toluene.

Characteristics of atmospheric non-methane hydrocarbons during haze episode in Beijing, China.

This study firstly focused on non-methane hydrocarbons (NMHCs) during three successive days with haze episode (16-18 August 2006) in Beijing. Concentrations of alkanes, alkenes, aromatic hydrocarbons, and ethyne all peaked at traffic rush hour, implying vehicular emission; and alkanes also peaked at non-traffic rush hour in the daytime, implying additional source. Especially, alkanes and aromatics clearly showed higher levels in the nighttime than that in the daytime, implying their active photochemical reactions in the daytime. Correlation coefficients (R (2)) showed that propane, n-butane, i-butane, ethene, propene, and benzene correlated with ethyne (R (2) = 0.61-0.66), suggesting that their main source is vehicular emission; 2-methylpentane and n-hexane correlated with i-pentane (R (2) = 0.61-0.64), suggesting that gasoline evaporation is their main source; and ethylbezene, m-/p-xylene, and o-xylene correlated with toluene (R (2) = 0.60-0.79), suggesting that their main source is similar to that of toluene (e.g., solvent usage). The R (2) of ethyne, i-pentane, and toluene with total NMHCs were 0.58, 0.76, and 0.60, respectively, indicating that ambient hydrocarbons are associated with vehicular emission, gasoline evaporation, and solvent usage. The sources of other hydrocarbons (e.g., ethane) might be natural gas leakage, biogenic emission, or long-range transport of air pollutants. Measured higher mean B/T ratio (0.78 ± 0.27) was caused by the more intensive photochemical activity of toluene than benzene, still indicating the dominant emission from vehicles.