Wei-Nai Chen

Case study of the Asian dust and pollutant event in spring 2006: Source, transport, and contribution to Taiwan

Surface measurements and a regional dust model were used to analyze the source, transport, and contribution of a dust event transporting with aerosol pollutant over Taiwan from 16 to 19 March, 2006. During the event, the hourly aerosol concentrations reached close to 400 μg m(-3) in northern Taiwan, and approximately 300 μg m(-3) in other areas of the island. Trajectory and regional dust models show that the dust event originated in eastern Mongolia and northern China, and the dust layer can descend from 2 to 3 km in the source area to below 1.5 km over Taiwan. On the other hand, model results show that pollution was transported near the surface from coastal China to Taiwan. During this dust event, polluted aerosol was first observed over northern Taiwan right after a frontal passage, and the concentration was strongly enhanced following the passage of the light rainfall 12h later. The descent of dusty air from the free troposphere lagged the arrival of polluted air by 7h, and was partially mixed with polluted aerosol when the transport decelerated over Taiwan. During the event, dust particles accounted for up to 60% of observed particulate matter less than 10 μm (PM10) over Taiwan, but decreased to less than 35% for particulate matter less than 2.5 μm (PM2.5) over most areas of the island. On the other hand, the long-range transport of non-dust aerosols, mainly anthropogenic pollutants, accounted for close to 30% of observed PM10 concentration in northern and western Taiwan prior to dust arrival, and the contribution of PM2.5 increased to close to 40% over the same areas. Local emission of aerosols accounted for less than 25% of PM10 concentrations in northern Taiwan, but was about 60% for PM2.5 in central and southern Taiwan because these areas are less influenced by long-range transport.

The real-time method of assessing the contribution of individual sources on visibility degradation in Taichung

Visibility degradation caused by air pollution has become a serious environmental problem in megacities in Northeast Asia. In general, aerosol chemical compositions are measured by a conventional method of time integrated filter sampling for off-line analysis, which cannot represent temporal and spatial variations in the real atmosphere. The in situ air composition measuring equipment, OCEC carbon aerosol analyzer and a long-path visibility transmissometer-3 were used to collect hourly measurements of the soluble ions, organic/elemental carbon, and ambient visibility, respectively. During the observation, two types of weather conditions were identified: transport and stagnant. Because PM2.5 was identified as the predominant species of light extinction, the sources of PM2.5 were determined and investigated using a positive matrix factorization (PMF) analysis. The PMF outputs characterized the six main emission sources (marine/crustal aerosols, secondary nitrate, secondary sulfate, direct vehicle exhaust, coal/incinerator combustion, and local sewage emission) and reconstructed the PM2.5 mass concentrations of each pollutant source in two weather conditions. In addition, the light extinction (bext) was reconstructed using a multivariate linear regression analysis with hourly-reconstructed PM2.5 mass concentrations to determine the contributions of each source to bext. The primary results showed that the extinction coefficient was proportional to the PM2.5 with high value in stagnant weather conditions. The secondary sulfate was the most abundant source of bext contribution during the sampling period. In addition, the bext contributions of direct vehicle exhaust and coal/incinerator combustion significantly increased in the stagnant weather condition. According to the results of hourly measurements, this work further emphasized that the sources of direct vehicle exhaust and coal/incinerator combustion in PM2.5 were the important sources of visibility degradation in the stagnant weather conditions, which suggests that the pollutants derived from direct vehicle exhaust and coal/incinerator combustion should be controlled first to improve visibility in Taichung.

Numerical investigation of the coagulation mixing between dust and hygroscopic aerosol particles and its impacts

A statistical-numerical aerosol parameterization was incorporated into the Community Multiscale Air Quality modeling system to study the coagulation mixing process focusing on a dust storm event that occurred over East Asia. Simulation results show that the coagulation mixing process tends to decrease aerosol mass, surface area, and number concentrations over the dust source areas. Over the downwind oceanic areas, aerosol concentrations generally increased due to enhanced sedimentation as particles became larger upon coagulation. The mixture process can reduce the overall single-scattering albedo by up to 10% as a result of enhanced core with shell absorption by dust and reduction in the number of scattering particles. The enhanced dry deposition speed also altered the vertical distribution. In addition, the ability of aerosol particles to serve as cloud condensation nuclei (CCN) increased from around 107 m−3 to above 109 m−3 over downwind areas because a large amount of mineral dust particles became effective CCN with solute coating, except over the highly polluted areas where multiple collections of hygroscopic particles by dust in effect reduced CCN number. This CCN effect is much stronger for coagulation mixing than by the uptake of sulfuric acid gas on dust, although the nitric acid gas uptake was not investigated. The ability of dust particles to serve as ice nuclei may decrease or increase at low or high subzero temperatures, respectively, due to the switching from deposition nucleation to immersion freezing or haze freezing.

Prepulse controlled splitting of relativistically self-guided channel and suppression of Raman forward scattering instability

The effects of laser prepulse on splitting of a relativistically self-guided channel and suppression of Raman forward scattering instability in the propagation of an intense ultrashort laser pulse in an underdense plasma are studied. They are resolved by using probing interferometry, shadowgraphy, and spectrometry. By changing the prepulse intensity, the propagation of the laser beam can be controlled to show self-guiding or channel splitting. The effect of prepulse on Raman forward scattering instability shows that the instability is significantly reduced if the gas target is preionized, identifying ionization wakefield as the seeding mechanism for this process.

Trends in Monthly Tropopause Characteristics Observed over Taipei, Taiwan*

AbstractThis study presents monthly trends in the cold-point tropopause (CPT), calculated using three decades of radiosonde data from 1981 to 2010 over Taipei, Taiwan (25°01′N, 121°27′E). Multivariate regression analysis has been used to suppress the effect of natural variations, such as quasi-biennial oscillation (QBO), ENSO, solar cycle, and volcanic eruptions. From the continuous time series, statistically insignificant heating and a decrease in the height of CPT are observed. However, the trends estimated using individual monthly time series revealed new features with statistically significant increasing trends in CPT temperature at a rate of approximately 0.03°C yr−1 and statistically significant decreasing trends in CPT height at a rate of approximately 4.7 m yr−1 during summer months. An enhanced heating rate in the upper troposphere along with a suppressed cooling rate in the lower stratosphere observed over Taipei might have caused the tropopause heating trend during summer. The possible relation...

Intercomparison of Number Concentrations by CPCs using Generated Particles in Chamber

Abstract Two butanol-based Condensation Particle Counters(CPC 3022, CPC 3025), three water-based CPCs(CPC3781,CPC3785×2), a Gardner Counter, a Fast Mobility Particle Sizer (FMPS), and an Aerosol Electrometer (AE) weredeployed to measure the number concentrations from atomized aerosol under six different conditions. Comparisonsof particle number concentrations measured by the CPCs, FMPS, and AE were conducted to evaluate the perfor-mance of the each CPCs using laboratory generated artificial particles such as NaCl, succinic acid (C 4 H 6 O 4 ), andparticles generated by propane torch & heat gun in the chamber. Good correlation between the CPC3025 and FMPSwas observed for the total particle number concentrations in the size range 15 nm to 90 nm. In addition, this papersuggests that photometric mode in water-based CPC3785 could not be used as quantitative of number concentra-tions for CPC3785. Key words : CPC, FMPS, Aerosol Electrometer, Size distribution, Comparison *Corresponding author.Tel : +82-(0)61-450-2485, E-mail : minsbae@hotmail.com

Correlation between aerosol optical depth derived from CIMEL sunphotometer and surface particulate concentration in Northern and Southern Taiwan

Three years (2003-2005) of aerosol optical depths (AOD) measured with CIMEL sunphotometer were compared to the surface concentrations of PM10 and PM2.5 in Northern (i.e. Taipei) and Southern (i.e. Tainan) Taiwan. The correlation between AOD and PM10 is higher in Taipei than in Tainan. Additional chemical compositions of PM10 and PM2.5 in these two sites, including ionic component, sea salt, OC/EC and crustal components, were also examined to find their relationship with the AOD. These analyses indicate that aerosols in Taipei are mainly composed of fine aerosols, whereas in Tainan more crustal material and OC exist in the coarse mode, which partially explain the higher correlation of PM10 and AOD in Taipei. Closure calculations are carried out by combining data from lidar, sunphotometer, nephelometer, aethalomter, SMPS and APS size spectrometers, as well as chemical analyses of aerosols from PM10 and PM2.5 samplers and MOUDI impactor to investigate their consistency. The observed surface single scattering albedo was also compared to that retrieved by CIMEL sunphotometer, with additional discussion on the possible explanation to the discrepancies of the comparisons. Then, lidar measurement is applied to relate aerosols at the surface to those aloft.

Application of lidar in the observation of atmospheric particulate pollutants in Taipei

For the assessment of climatic impact of aerosols, the knowledge of both the temporal and spatial distributions of aerosol is essential. Laser radar, more popularly known as Lidar, has becoming one of the most powerful techniques for active detection of aerosols in the atmosphere. Lidar can provide vertically resolved of extinction and backscatter coefficients, and thereby the height of the planetary boundary layer or the nighttime residual layer. As the long-term changes in the structure and dynamics of the lower and middle troposphere is now becoming a priority, a pulsed Nd:YAG Lidar system is applied for measuring the vertical distribution of aerosol properties in the metropolitan Taipei. Two years (2004-2005) of aerosol optical depth (AOD) measured by Lidar, Cimel Sunphotometer and MODerate resolution Imaging Spectroradiometer (MODIS) were compared. The AOD shows strong seasonal variation with maximum values (AODLidar > 1, AODCimel > 1 and AODMODIS > 0.39) occurred in April. AODMODIS shows significant underestimation. AODLidar has good correlation with AODCimel, but the Lidar measurement is biased toward lower values as presented by the 0.725 slope in the linear regression. This bias is mostly caused by the Lidar blind distance at the lowest part of the atmosphere. The R-squared of AODCimel and surface PM2.5 concentration is about 0.44. This reflects the fact that the atmospheric boundary layer is often not well-mixed, so aerosols there cannot represent the total AOD value. Particles in the free troposphere also need to be concerned. Further comparison of our Lidar data with the CALIPSO measurements is intended.

Determination of the vertical profile of aerosol chemical species in the microscale urban environment

This study developed a lightweight air composition measuring equipment (ACME) mounted in unmanned aerial vehicles (UAVs) to measure the vertical distribution characteristics of PM2.5 chemical species in the micro-scale urban environment for the first time. 212 samples collected from 0 to 350 m above ground level were analyzed for water-soluble ions. The concentrations of most ions on the above ground level were higher than that on the ground surface during the sampling period. The measurements of the total ion concentrations were approximately 54 to 26% of the PM2.5 mass concentrations on the ground surface. The concentrations of NH4+ and NO3- decreased with increases in the height from the ground, which may be related to the influence of the vehicle emissions and human activities. NO2- and SO42- both had a peak concentration on the higher vertical altitude at night in the sea-land wind system. In the southern wind system, the emissions of sea salts, dust, and stationary pollution, might be transported by the regional prevailing airflow from the southern coastal area, were the major pollutant sources above the boundary layer. The vertical distribution of ionic concentrations and wind field provided information concerning changes in pollutant transport and source regions that affect the local air quality. The ACME mounted in UAVs is the feasible and convenient method to fast understand the vertical distributions of aerosol chemical species. It provides important information about the accumulation and diffusion effects by the boundary layer variation to aerosol characteristics, which is difficulty observed from the conventional ground-based measurements. In future, this technology is the useful application for investigating the pollutant species emitted from the smokestack and the sudden pollution accident.