Yunfei Wu

Magnetoelectric CoFe2O4–Pb(Zr,Ti)O3 composite thin films derived by a sol-gel process

Magnetoelectric (ME) CoFe2O4-Pb(Zr,Ti)O-3 composite thin films have been prepared by a sol-gel process and spin-coating technique. X-ray diffraction and scanning electron microscopy reveal that there exists local aggregation or phase separation of the CoFe2O4 and Pb(Zr,Ti)O-3 phases in the films. Vibrating sample magnetometer, ferroelectric test unit, and magnetoelectric measuring device were used to characterize the magnetic and ferroelectric properties, as well as the ME effect of the films. It is shown that the films exhibit both good magnetic and ferroelectric properties, as well as a ME effect. A high initial magnetoelectric voltage coefficient for the film is observed. The ME effect of the film strongly depends on the magnetic bias and magnetic field frequency

Source apportionment of PM2.5 at urban and suburban areas of the Pearl River Delta region, south China - With emphasis on ship emissions

Daily PM2.5 samples were collected at an urban site in Guangzhou in 2014 and at a suburban site in Zhuhai in 2014-2015. Samples were subject to chemical analysis for various chemical components including organic carbon (OC), element carbon (EC), major water-soluble inorganic ions, and trace elements. The annual average PM2.5 mass concentration was 48±22μgm-3and 45±25μgm-3 in Guangzhou and Zhuhai, respectively, with the highest seasonal average concentration in winter and the lowest in summer at both sites. Regional transport of pollutants accompanied with different air mass origins arriving at the two sites and pollution sources in between the two cities caused larger seasonal variations in Zhuhai (>a factor of 3.5) than in Guangzhou (<a factor of 2.0). Positive matrix factorization (PMF) analysis identified six and five major source factors for PM2.5 in Guangzhou and Zhuhai, respectively. Ship emissions, a source factor previously ignored in making emission control policies in the Pearl River Delta region of south China, were among the top contributors to PM2.5 at both sites, accounting for >17% of PM2.5 mass concentrations.

Control of PM2.5 in Guangzhou during the 16th Asian Games period: Implication for hazy weather prevention

To evaluate the effectiveness of the integrated control measures for reducing PM2.5 (aerosol particles with an aerodynamic diameter of less than 2.5 μm) and hazy weather, day- and night-time PM2.5 samples were collected at an urban site in Guangzhou during the 16th Asian Games period in November 2010. PM2.5 samples were subject to chemical analysis for major water-soluble ions, organic carbon (OC), element carbon (EC), and biomass burning tracers-anhydrosugar levoglucosan (LG). In addition, aerosol scattering coefficient (bsp) at dry condition and aerosol absorption coefficient (bap) and visibility at ambient condition were measured. The seven major control measures were effective for reducing PM2.5 mass concentration and improving visibility during the Asian Games period. All monitored air pollutants except PM2.5 satisfied the National Ambient Air Quality Standards (NAAQS). However, daily PM2.5 concentrations still exceeded the NAAQS on 47% of the days and hazy weather also occurred on 80% of the days during this period. One factor causing the high frequency of hazy weather occurrence was the increased relative humidity during the Asian Games period. To avoid hazy weather occurrence, new PM2.5 standard was recommended based on visibility calculations using three available aerosol hygroscopic curves previously obtained for this city. The recommended PM2.5 standard was 63 μgm(-3) under dry condition and lower than 42 μg m(-3) under humid condition (RH ≥ 70%). These recommended value s were much stricter than the NAAQS value of 75 μg m(-3). To reach the new standard, more rigorous control measures for coal industries should be established in the Pearl River Delta (PRD) region.

Uncertainty assessment of source attribution of PM(2.5) and its water-soluble organic carbon content using different biomass burning tracers in positive matrix factorization analysis--a case study in Beijing, China.

Daily PM2.5 samples were collected at an urban site in Beijing during four one-month periods in 2009-2010, with each period in a different season. Samples were subject to chemical analysis for various chemical components including major water-soluble ions, organic carbon (OC) and water-soluble organic carbon (WSOC), element carbon (EC), trace elements, anhydrosugar levoglucosan (LG), and mannosan (MN). Three sets of source profiles of PM2.5 were first identified through positive matrix factorization (PMF) analysis using single or combined biomass tracers - non-sea salt potassium (nss-K(+)), LG, and a combination of nss-K(+) and LG. The six major source factors of PM2.5 included secondary inorganic aerosol, industrial pollution, soil dust, biomass burning, traffic emission, and coal burning, which were estimated to contribute 31±37%, 39±28%, 14±14%, 7±7%, 5±6%, and 4±8%, respectively, to PM2.5 mass if using the nss-K(+) source profiles, 22±19%, 29±17%, 20±20%, 13±13%, 12±10%, and 4±6%, respectively, if using the LG source profiles, and 21±17%, 31±18%, 19±19%, 11±12%, 14±11%, and 4±6%, respectively, if using the combined nss-K(+) and LG source profiles. The uncertainties in the estimation of biomass burning contributions to WSOC due to the different choices of biomass burning tracers were around 3% annually and up to 24% seasonally in terms of absolute percentage contributions, or on a factor of 1.7 annually and up to a factor of 3.3 seasonally in terms of the actual concentrations. The uncertainty from the major source (e.g. industrial pollution) was on a factor of 1.9 annually and up to a factor of 2.5 seasonally in the estimated WSOC concentrations.

Roles of regional transport and heterogeneous reactions in the PM 2.5 increase during winter haze episodes in Beijing

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations during five haze episodes in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM2.5 increased from the clean (<75μgm-3) to the light-medium pollution level (75-150μg m-3) and to levels of heavy (150-250μgm-3) and severe (>250μgm-3) pollution. In the initial twelve hours of each episode, the PM2.5 reached the light-medium level with an increase of approximately 120μgm-3. At the same time, the particle (~10-700nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM2.5 occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating the rapid increase of PM2.5 in the initial stage was caused by the regional transport from the south. Subsequently, PM2.5 elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM2.5 in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108nm to 120nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in these two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM2.5 increase, while heterogeneous reactions dominated the later elevation.

Investigation of hygroscopic growth effect on aerosol scattering coefficient at a rural site in the southern North China Plain

Aerosol optical properties and the effect of hygroscopic growth on the scattering coefficients at a rural site in the southern North China Plain were investigated based on a two-month observation conducted in the summer of 2014. The scattering coefficient of dry aerosols was high, with a mean (±standard deviation) of 338.8±209.9Mm-1 (520nm) during the observation period. A noticeable enhancement in aerosol scattering due to hygroscopic growth was observed, e.g., by a factor of 2.28±0.69 at RH of 80% (referred to as f(RH=80%)) and 3.39±1.14 at RH of 85% (f(RH=85%)). The high content of water-soluble secondary inorganic aerosols (SIAs), accounting for 53.1% of fine particulate matter (i.e., PM2.5) on average, was mainly responsible for the high hygroscopicity. f(RH=80%) increased with increasing SIA mass fraction in PM2.5. This was especially the case when SIAs were mainly in finer particulate matter, i.e., PM1. A number of considerably low f(RH=80%) values was observed due to relatively low mass fraction of SIAs in PM1 despite high fraction in PM2.5. Particle size distributions, especially those of SIAs, also played a remarkable role in the hygroscopicity of ambient aerosols. No significant difference in hygroscopicity was found between different pollution episodes due to the dominance of SIAs in all the cases. Slightly higher hygroscopic growth factors were observed during the clean episode, which were attributed to the smaller particle sizes.

Impact of size distributions of major chemical components in fine particles on light extinction in urban Guangzhou

To evaluate the impact of fine particulate matter (PM2.5) size distribution on aerosol chemical and optical properties, dominant chemical components including water-soluble inorganic ions (WSII), organic carbon (OC) and elemental carbon (EC) in PM1 and PM2.5, aerosol scattering coefficient (bsp), and aerosol absorption coefficient (bap) were collected synchronously at an urban site in Guangzhou, south China during a typical summer month in 2009 and a winter month in 2010. PM1 (sizes smaller than 1μm) constituted 77% and 63% of PM2.5 in summer and winter, respectively. From the reconstructed mass concentrations, the sum of SO42-, NO3- and NH4+ (SNA) distributed more in PM1 than in PM1-2.5 (PM2.5 minus PM1) in summer and the opposite was found in winter, while carbonaceous aerosols distributed more in PM1 in both summer and winter. With the aggravation of PM2.5 pollution, the mass fraction of PM1/PM2.5 increased for (NH4)2SO4 (AS), NH4NO3 (AN) and EC but decreased for organic matter (OM) in summer, and the opposite was found in winter. Bsp of PM1 and PM1-2.5 was estimated from the mass extinction efficiencies (MSEs) of the dominant chemical components, which showed good correlations (R2=0.99) with measured ones and those estimated using the IMPROVE formula. The fractional contributions of dominant chemical components to extinction coefficient (bext) were consistent with their respective mass size distributions, indicating the importance of chemically-resolved aerosol size distributions on aerosol optical properties and haze formation.

Comparison of aerosol and cloud condensation nuclei between wet and dry seasons in Guangzhou, southern China

Cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were measured simultaneously at an urban site of Guangzhou from July to August 2015 and in January 2016, and the seasonal variations of aerosol activated fractions (NCCN/NCN) as well as their relevant influence factors were further studied accordingly. NCN is generally higher in winter (dry season), whereas NCCN and NCCN/NCN are mostly higher in summer (wet season) instead. In particular, NCCN and NCCN/NCN are much lower at smaller supersaturation levels (SS<0.2) in winter. In spite of similar diurnal variations for NCCN and NCN, NCCN/NCN indicates an opposite tendency, relatively lower at midday, dusk and before midnight. Other than the size of particles as well as their chemical composition, some other factors, such as mass, gas precursors, pollutant transportation, meteorological conditions, etc., also contribute to the variations of NCCN and NCCN/NCN. Particles from the local source or local-oceanic combination source cast influence on CN and CCN significantly, while the pollutants originating from and crossing over distant polluted areas contribute largely to CCN/CN. NCN and NCCN are relatively higher under pollution-free conditions in summertime and polluted conditions in wintertime, but NCCN/NCN is just the opposite. On various polluted conditions, aerosol CCN activities are greatly discrepant between summer and winter, especially during mist or heavy haze periods. The results imply that anthropogenic pollutants exert critical impacts on aerosol CCN activation.

Size distribution and source of black carbon aerosol in urban Beijing during winter haze episodes

Black carbon (BC) has important impact on climate and environment due to its light absorption ability, which greatly depends on its physicochemical properties including morphology, size and mixing state. The size distribution of the refractory BC (rBC) was investigated in urban Beijing in the late winter of 2014, during which there were frequent haze events, through analysis of measurements obtained using a single-particle soot photometer (SP2). By assuming void-free rBC with a density of 1.8 g cm−3, the mass of the rBC showed an approximately lognormal distribution as a function of the volume-equivalent diameter (VED), with a peak diameter of 213 nm. Larger VED values of the rBC were observed during polluted periods than on clean days, implying an alteration in the rBC sources, as the size distribution of the rBC from a certain source was relative stable, and VED of an individual rBC varied little once it was emitted into the atmosphere. The potential source contribution function analysis showed that air masses from the south to east of the observation site brought higher rBC loadings with more thick coatings and larger core sizes. The mean VED of the rBC presented a significant linear correlation with the number fraction of thickly coated rBC, extrapolating to be ∼ 150 nm for the completely non-coated or thinly coated rBC. It was considered as the typical mean VED of the rBC from local traffic sources in this study. Local traffic was estimated to contribute 35 to 100 % of the hourly rBC mass concentration with a mean of 59 % during the campaign. Lower local traffic contributions were observed during polluted periods, suggesting increasing contributions from other sources (e.g., coal combustion and biomass burning) to the rBC. Thus, the heavy pollution in Beijing was greatly influenced by other sources in addition to the local traffic.

The Solubilization Capability of Polycyclic Aromatic Hydrocarbons Enhanced by Biosurfactant Saponin Mixed With Conventional Chemical Surfactants

Water solubilities of anthracene and phenanthrene were measured by adding a plant-derived biosurfactant Saponin and/or synthetic surfactants correspondingly below and above their critical micelle concentrations (CMCs). The results showed that the solubilities of anthracene and phenanthrene in water were greatly enhanced following a linear fashion if adding Saponin, TX100 and Brij35, respectively. The enhancing solubility capacities for Polycyclic aromatic hydrocarbons (PAHs) followed the order as: Saponin > Brij35 > TX100 at the concentrations below their CMC. It also proved that biosurfactant Saponin exhibited much higher enhancement efficiency for PAHs solubilization than the synthetic ones at the concentrations below the CMC. However, the solubility enhancement efficiencies of surfactants above the CMC turned to be the following order as Brij35 > Saponin > TX100. Furthermore, this study has confirmed that PAHs are synergistically solubilized in the mixed biologic-synthetic surfactant solutions, especially at the low surfactant concentrations. Synergistic effect of a prepared mixed-surfactant solution on different PAH compounds also appeared to be linear related to the surfactant concentrations. The synergistic power of mixed surfactants were examined as Saponin-Brij35 > Saponin-TX100 while the efficiencies of synergistic solubilization ranged from 27% to 137%. The noted solubilization for the single and mixed surfactants could be attributed to the formation of single and mixed micelles, the lower CMC of the single and mixed surfactant solutions, and the increase of the solubilization ratio or micellar partition coefficients.