Updating vehicle VOCs emissions characteristics under clean air actions in a tropical city of China.

In the context of clean air actions in China, vehicle emission limits have been continuously tightened, which has facilitated the reduction of volatile organic compounds (VOCs) emissions. However, the characteristics of VOC emissions from vehicles with strict emission limits are poorly understood. This study investigated the VOC emission characteristics from vehicles under the latest standards based on tunnel measurements, and identified future control strategies for vehicle emissions. The results showed that the highest percentage of VOCs from vehicle consisted of alkanes (80.9 %), followed by aromatics (15.8 %) and alkenes (3.1 %). Alkanes had the most significant ozone formation potential due to their high concentrations, in contrast to the aromatics that have been dominant in previous studies. The measured fleet-average VOC emission factor was 71.3 mg·km-1, including tailpipe emissions of 39.6 mg·km-1 and evaporative emissions of 31.7 mg·km-1. The VOC emission factors of the subgroups were obtained. The emission of evaporated VOCs accounted for 44.5 % of the total vehicle VOC emissions, which have increased substantially from previous studies. In addition, the emission characteristics of vehicles that are under the latest emission threshold values have changed significantly, and the mixing ratio of toluene/benzene (T/B) has been updated to 3:1. This study updates the VOCs emission factors of vehicles under clean air actions and highlights the future mitigation policies should focus on reducing evaporative VOC emissions.

The Homogeneous Gas-Phase Formation Mechanism of PCNs from Cross-Condensation of Phenoxy Radical with 2-CPR and 3-CPR: A Theoretical Mechanistic and Kinetic Study.

Chlorophenols (CPs) and phenol are abundant in thermal and combustion procedures, such as stack gas production, industrial incinerators, metal reclamation, etc., which are key precursors for the formation of polychlorinated naphthalenes (PCNs). CPs and phenol can react with H or OH radicals to form chlorophenoxy radicals (CPRs) and phenoxy radical (PhR). The self-condensation of CPRs or cross-condensation of PhR with CPRs is the initial and most important step for PCN formation. In this work, detailed thermodynamic and kinetic calculations were carried out to investigate the PCN formation mechanisms from PhR with 2-CPR/3-CPR. Several energetically advantageous formation pathways were obtained. The rate constants of key elementary steps were calculated over 600~1200 K using the canonical variational transition-state theory (CVT) with the small curvature tunneling (SCT) contribution method. The mechanisms were compared with the experimental observations and our previous works on the PCN formation from the self-condensation of 2-CPRs/3-CPRs. This study shows that naphthalene and 1-monochlorinated naphthalene (1-MCN) are the main PCN products from the cross-condensation of PhR with 2-CPR, and naphthalene and 2-monochlorinated naphthalene (2-MCN) are the main PCN products from the cross-condensation of PhR with 3-CPR. Pathways terminated with Cl elimination are preferred over those terminated with H elimination. PCN formation from the cross-condensation of PhR with 3-CPR can occur much easier than that from the cross-condensation of PhR with 2-CPR. This study, along with the study of PCN formation from the self-condensation 2-CPRs/3-CPRs, can provide reasonable explanations for the experimental observations that the formation potential of naphthalene is larger than that of 1-MCN using 2-CP as a precursor, and an almost equal yield of 1-MCN and 2-MCN can be produced with 3-CP as a precursor.

Simulation Verification of Barrierless HONO Formation from the Oxidation Reaction System of NO, Cl, and Water in the Atmosphere.

Nitrous acid (HONO) is a major source of hydroxyl (OH) radicals, and identifying its source is crucial to atmospheric chemistry. Here, a new formation route of HONO from the reaction of NO with Cl radicals with the aid of one or two water molecules [(Cl) (NO) (H2O)n (n = 1-2)] as well as on the droplet surface was found by Born-Oppenheimer molecular dynamic simulation and metadynamic simulation. The (Cl) (NO) (H2O)1 (monohydrate) system exhibited a free-energy barrier of approximately 0.95 kcal mol-1, whereas the (Cl) (NO) (H2O)2 (dihydrate) system was barrierless. For the dihydrate system and the reaction of NO with Cl radicals on the droplet surface, only one water molecule participated in the reaction and the other acted as the "solvent" molecule. The production rates of HONO suggested that the monohydrate system ([NO] = 8.56 × 1012 molecule cm-3, [Cl] = 8.00 × 106 molecule cm-3, [H2O] = 5.18 × 1017 molecule cm-3) could account for 40.3% of the unknown HONO production rate (Punknown) at site 1 and 53.8% of Punknown at site 2 in the East China Sea. This study identified the importance of the reaction system of NO, Cl, and water molecules in the formation of HONO in the marine boundary layer region.

The homogeneous gas-phase formation mechanisms of PCPTs/PCDTs/PCDFs from the radical/radical cross-condensation of 2-CPR and 2-CTPR: a theoretical, mechanistic and kinetics study.

Polychlorinated phenoxathiins (PCPTs) are one group of dioxin-like compounds, which can be considered to be one-oxygen-substituted polychlorinated thianthrene (PCTA) compounds or one-sulfur-substituted polychlorinated dibenzo-p-dioxin (PCDD) compounds. Owing to their high toxicity and wide distribution, clarifying the formation and emission of PCPTs due to combustion and thermal processes can deepen our understanding of the dioxin formation mechanism and allow reduced-emission and dioxin-control strategies to be established. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCPT formation. In this paper, the homogeneous gas-phase formation mechanisms of PCPTs, as well as polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzothiophenes (PCDTs), from the cross-condensation of 2-chlorophenoxy radicals (2-CPRs) and 2-chlorothiophenoxy radicals (2-CTPRs) under thermal and combustion conditions were investigated theoretically using a density functional theory (DFT) method. The reaction priorities and effects of water molecules on the formation mechanisms were discussed. The rate constants of crucial elementary steps were calculated from 600-1200 K. The acute and chronic toxicities of the main products were predicted at three trophic levels. This study shows that routes starting with oxygen-carbon condensation are favored over those starting with sulfur-carbon condensation for PCPT formation, and routes ending with Cl loss can occur more easily than those ending with H loss. Water molecules have a negative catalytic effect on CH-S H-transfer steps but a positive catalytic effect on CH-O H-transfer steps.

Mechanism and kinetic properties for the complete series reactions of chloro(thio)phenols with O(3P) under high temperature conditions.

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs) are two groups of dioxin-like compounds with oxygen and sulfur substitution, respectively. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCDD/F and PCDT/TA formation. The formation of chlorophenoxy radicals (CPRs) and chlorothiophenoxy radicals (CTPRs) from chlorophenols (CPs) and chlorothiophenols (CTPs) with O(3P) is an important initial step for the formation of PCDD/Fs and PCDT/TAs, respectively. In this paper, the formation of CPRs/CTPRs from the complete series reactions of 19 CP/CTP congeners with O(3P) was studied using the density functional theory (DFT) method. The rate constants of each reaction were calculated using canonical variational transition state (CVT) theory along with a small-curvature tunneling (SCT) contribution over a wide temperature range of 600-1200 K. The effect of the chlorine substitution pattern on the structural parameters, thermochemical properties and rate constants in both CPs and CTPs was discussed. This study shows that the reactions between CPs and O(3P) can be affected by the chlorine substitution at the para-position, and the reactions between CTPs and O(3P) are mostly influenced by both ortho-substitutions. The thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more likely to occur than the phenoxyl-hydrogen abstraction from CPs by O(3P). Comparison of the reactivity of CP/CTPs with O(3P) with our previous work on CP/CTPs with H and OH shows that the order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H, and the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH.