On the use of an explicit chemical mechanism to dissect peroxy acetyl nitrate formation.

Peroxy acetyl nitrate (PAN) is a key component of photochemical smog and plays an important role in atmospheric chemistry. Though it has been known that PAN is produced via reactions of nitrogen oxides (NOx) with some volatile organic compounds (VOCs), it is difficult to quantify the contributions of individual precursor species. Here we use an explicit photochemical model--Master Chemical Mechanism (MCM) model--to dissect PAN formation and identify principal precursors, by analyzing measurements made in Beijing in summer 2008. PAN production was sensitive to both NOx and VOCs. Isoprene was the predominant VOC precursor at suburb with biogenic impact, whilst anthropogenic hydrocarbons dominated at downtown. PAN production was attributable to a relatively small class of compounds including NOx, xylenes, trimethylbenzenes, trans/cis-2-butenes, toluene, and propene. MCM can advance understanding of PAN photochemistry to a species level, and provide more relevant recommendations for mitigating photochemical pollution in large cities

Observations of aerosol optical properties at a coastal site in Hong Kong, South China

Temporal variations in aerosol optical properties were investigated at a coastal station in Hong Kong based on the field observation from February 2012 to February 2015. At 550 nm, the average light-scattering (151 +/- 100Mm(-1) / and absorption coefficients (8.3 +/- 6.1Mm(-1) / were lower than most of other rural sites in eastern China, while the single-scattering albedo (SSA = 0.93 +/- 0.05) was relatively higher compared with other rural sites in the Pearl River Delta (PRD) region. Correlation analysis confirmed that the darkest aerosols were smaller in particle size and showed strong scattering wavelength dependencies, indicating possible sources from fresh emissions close to the measurement site. Particles with D-p of 200-800 nm were less in number, yet contributed the most to the light-scattering coefficients among submicron particles. In summer, both Delta BC / Delta CO and SO2 / BC peaked, indicating the impact of nearby combustion sources on this site. Multi-year backward Lagrangian particle dispersion modeling (LPDM) and potential source contribution (PSC) analysis revealed that these particles were mainly from the air masses that moved southward over Shenzhen and urban Hong Kong and the polluted marine air containing ship exhausts. These fresh emission sources led to low SSA during summer months. For winter and autumn months, contrarily, Delta BC / Delta CO and SO2 / BC were relatively low, showing that the site was more under influence of well-mixed air masses from long-range transport including from South China, East China coastal regions, and aged aerosol transported over the Pacific Ocean and Taiwan, causing stronger abilities of light extinction and larger variability of aerosol optical properties. Our results showed that ship emissions in the vicinity of Hong Kong could have visible impact on the light-scattering and absorption abilities as well as SSA at Hok Tsui.Peer reviewe

High loadings and source strengths of organic aerosols in China

Nation-wide studies of organic aerosols were conducted on a molecular level in 15 Chinese cities. The results showed strikingly high levels of organic compounds (e.g., annual concentrations of polycyclic aromatic hydrocarbons, phthalates, sugars and diacids are 110, 370, 400 and 830 ng m−3, respectively), especially in the mid-west region during winter (up to 125 μg m−3 organic carbon). Fossil fuel combustion and/or biomass burning products are 3−30 times more abundant in winter than in summer. In contrast, significant quantity of phthalates (168−2200 ng m−3) was detected in summer. Concentrations of the pollutants are generally 1−3 orders of magnitude higher than those in developed countries. Their source strengths are characterized in winter by fossil fuel combustion, followed by secondary oxidation, plant wax emissions and biomass burning, whereas in summer by secondary oxidation, followed by fossil fuel combustion and plastic emissions

Surface Characterization of Secondary Organic Aerosols from Ozonolysis of Monoterpene and the Effects of Acute Lung Injury in Mice

Monoterpene is a biogenic volatile organic compound commonly found in cleaning products and air fresheners. It can react rapidly with indoor oxidants, such as ozone (O-3), to produce secondary organic aerosols (SOAs) in indoor environments, and the reactions are potentially influenced by ammonia (NH3). This study simulated the reactions of O-3 and monoterpene with and without the presence of NH3 in an environmental chamber and investigated the surface characterization (elemental components and carbon states) of the PM2.5 generated by these reactions. We found that the generated particles possessed a higher content of nitrogen-containing organic compounds when NH3 was present. Unsubstituted aromatic carbon and aliphatic carbon were the main carbon structures, exhibited by over 60% of the carbon-containing compounds. Additionally, in the presence of NH3, more amide carbon and carboxylic carbon formed during the reactions. We also examined acute lung injury in mice caused by new particle formation under different reaction conditions. Oxidative stress was observed in the bronchoalveolar lavage fluid of the mice, as evidenced by a decrease in antioxidant enzymes (superoxide dismutase) and antioxidants (glutathione) as well as an increase in malondialdehyde. Moreover, the SOAs generated in the presence of NH3 lowered glutathione levels, indicating a rise in oxidative stress. Hence, fine particles formed by indoor oxidative reactions may trigger acute lung injury in humans, potentially causing further respiratory disease

Improved Oxygen Activation over a Carbon/Co3O4 Nanocomposite for Efficient Catalytic Oxidation of Formaldehyde at Room Temperature

Oxygen activation is a key step in the catalytic oxidation of formaldehyde (HCHO) at room temperature. In this study, we synthesized a carbon/Co3O4 nanocomposite (C-Co3O4) as a solution to the insufficient capability of pristine Co3O4 (PCo3O4) to activate oxygen for the first time. Oxygen activation was improved via carbon preventing the agglomeration of Co3O4 nanoparticles, resulting in small particles (approximately 7.7 nm) and more exposed active sites (oxygen vacancies and Co3+). The removal efficiency of C-Co3O4 for 1 ppm of HCHO remained above 90%, whereas P-Co3O4 was rapidly deactivated. In static tests, the CO2 selectivity of C-Co3O4 was close to 100%, far exceeding that of P-Co3O4 (42%). Various microscopic analyses indicated the formation and interaction of a composite structure between the C and Co3O4 interface. The carbon composite caused a disorder on the surface lattice of Co3O4, constructing more oxygen vacancies than P-Co3O4. Consequently, the surface reducibility of C-Co3O4 was improved, as was its ability to continuously activate oxygen and H2O into reactive oxygen species (ROS). We speculate that accelerated production of ROS helped rapidly degrade intermediates such as dioxymethylene, formate, and carbonate into CO2. In contrast, carbonate accumulation on P-Co3O4 surfaces containing less ROS may have caused P-Co3O4 inactivation. Compared with noble nanoparticles, this study provides a transition metal-based nanocomposite for HCHO oxidation with high efficiency, high selectivity, and low cost, which is meaningful for indoor air purification

Improved photocatalytic activity of BaTiO3/La2Ti2O7 heterojunction composites via piezoelectric-enhanced charge transfer

The separation efficiency of photogenerated carriers in photocatalysis is one of the key factors limiting the overall performance. The polarization field modification of piezoelectric materials is considered to be an effective strategy to regulate their photocatalytic performance. In this study, novel heterojunction photocatalysts that combine piezoelectric BaTiO3 nanorods and La2Ti2O7 nanosheets were synthesized and the polarization electric field promoted the bulk charge separation, while regulating the migration and separation of the carriers at the heterojunction interface. Under the combined action of visible light and ultrasonic, the piezo-photocatalytic degradation efficiency of CIP over BaTiO3/La2Ti2O7 composites increased by 24% and 12.5% compared with its photocatalytic and piezocatalytic degradation efficiency, respectively. Furthermore, the finite element simulation shows that BaTiO3/La2Ti2O7 heterojunctions have a stronger piezoelectric potential difference compared to pure BaTiO3. Our findings provide insights into the development of piezo-photocatalysts that can effectively degrade pollutants under visible light

Increasing trend of primary NO 2 exhaust emission fraction in Hong Kong

Despite the successful reduction in roadside NO x levels, no such decrease has been detected in roadside NO 2 concentration in Hong Kong. One underlying cause could be the rising primary NO 2 fraction of the total emission of NO x. Primary NO 2 can be particularly detrimental to Hong Kong because a large fraction of the population are exposed to the traffic-related primary pollutants in the street canyons formed by congested high-rise buildings. In this study, hourly mean concentration data for roadside nitrogen oxides (NO x), nitrogen dioxide (NO 2), and background ozone (O 3) were used to estimate the mean primary NO 2 fraction from vehicle exhausts in Hong Kong. An overall increasing trend was observed for the primary NO 2 fraction (f-NO 2) values in all the three roadside air monitoring sites. The primary NO 2 as a fraction of total NO x (f-NO 2) increased approximately from 2% in 1998 to 13% in 2008 in Hong Kong. The two particular periods of rising f-NO 2 coincided with the two implementation periods of the diesel retrofit programs for the light-duty vehicles and heavy-duty vehicles. Future vehicle emission control strategies should target not only total NO x but also primary NO 2. Health benefit or disease burden estimates should be taken into account and updated in the process of policy planning and evaluation. © 2011 Springer Science+Business Media B.V.link_to_subscribed_fulltex