Characterization of organic aerosols and their precursors in southern China during a severe haze episode in January 2017.

The rapid industrialization and economic development in the Pearl River Delta (PRD) region of southern China have led to a substantial increase in anthropogenic emissions and hence frequent haze pollution over the past two decades. In early January 2017, a severe regional haze pollution episode was captured in the PRD region, with a peak PM2.5 concentration of around 400µgm-3, the highest value ever reported at this site. During the haze episode, elevated concentrations of oxygenated volatile organic compounds (OVOCs, 33±16 ppbv) and organic matter (41±15µg m-3) were observed, indicating the enhanced roles of secondary organic aerosols (SOAs) in the formation of haze pollution. Water-soluble organic carbon (WSOC, 12.8±5.5µg C m-3) dominated the organic aerosols, with a WSOC/OC ratio of 0.63±0.12 and high correlation (R=0.85) with estimated secondary organic carbon (SOC), suggesting the predominance of a secondary origin of the measured organic aerosols during the haze episode. Four carboxylic acids (oxalic, acetic, formic, and pyruvic acids) were characterized in the aerosols (1.30±0.38µgm-3) and accounted for 3.6±1.2% of WSOC in carbon mass, with oxalic acid as the most abundant species. The simultaneous measurements of volatile organic compounds (VOCs), OVOCs, and organic acids in aerosols at this site provided an opportunity to investigate the relationship between the precursors and the products, as well as the potential formation pathways. Water-soluble aldehydes and ketones, predominantly produced via the oxidation of anthropogenic VOCs (mainly propane, toluene, n-butane, and m, p-xylene), were the main contributors of the organic acids. The formation of OVOCs is largely attributed to gas-phase photochemical oxidation, whereas the WSOC and dicarboxylic acids were produced from both photochemistry and nocturnal heterogeneous reactions. These findings provided further insights into the oxidation and evolution of organic compounds during the haze pollution episode.

Adsorption and transformation mechanism of NO2 on NaCl(100) surface: A density functional theory study.

To understand the heterogeneous reactions between NO2 and sea salt particles in the atmosphere of coastal areas, the absorption of an NO2 molecule on the NaCl(100) surface, the dimerization of NO2 molecules and the hydrolysis of N2O4 isomers at the (100) surface of NaCl are investigated by density functional theory. Calculated results show that the most favorable adsorption geometry of isolated NO2 molecule is found to reside at the bridge site (II-1) with the adsorption energy of -14.85 kcal/mol. At the surface of NaCl(100), three closed-shell dimers can be identified as sym-O2N-NO2, cis-ONO-NO2 and trans-ONO-NO2. The reactions of H2O with sym-O2N-NO2 on the (100) surface of NaCl are difficult to occur because of the high barrier (33.79 kcal/mol), whereas, the reactions of H2O with cis-ONONO2 and trans-ONONO2 play the key role in the hydrolysis process. The product, HONO, is one of the main atmospheric sources of OH radicals which drive the chemistry of the troposphere.

Chemical conversion pathways and kinetic modeling for the OH-initiated reaction of triclosan in gas-phase.

As a widely used antimicrobial additive in daily consumption, attention has been paid to the degradation and conversion of triclosan for a long time. The quantum chemistry calculation and the canonical variational transition state theory are employed to investigate the mechanism and kinetic property. Besides addition and abstraction, oxidation pathways and further conversion pathways are also considered. The OH radicals could degrade triclosan to phenols, aldehydes, and other easily degradable substances. The conversion mechanisms of triclosan to the polychlorinated dibenzopdioxin and furan (PCDD/Fs) and polychlorinated biphenyls (PCBs) are clearly illustrated and the toxicity would be strengthened in such pathways. Single radical and diradical pathways are compared to study the conversion mechanism of dichlorodibenzo dioxin (DCDD). Furthermore, thermochemistry is discussed in detail. Kinetic property is calculated and the consequent ratio of k add/k total and k abs/k total at 298.15 K are 0.955 and 0.045, respectively. Thus, the OH radical addition reactions are predominant, the substitute position of OH radical on triclosan is very important to generate PCDD and furan, and biradical is also a vital intermediate to produce dioxin.