下垫面高度梯度对城市灰尘分布特性及其重金属污染的影响

下垫面高度梯度对城市颗粒污染物的地球化学循环有重要的影响,不同高度下垫面上累积的灰尘可能对城市地表径流与大气颗粒物污染有重要指示作用.本文以不同高度梯度的下垫面,如路面(0 m)、公交站台(3 m)、天桥(5 m)、楼顶(20 m)上的灰尘为研究对象,揭示高度梯度对灰尘颗粒物单位面积质量、粒径组成、重金属含量和粒径污染负荷比的影响.结果表明,随着高度梯度变化,不同下垫面上灰尘颗粒物的单位面积质量之间存在显著差异(p0.05):路面((33.0±27.1)g·m~(-2))楼顶((20.6±11.7)g·m~(-2))天桥((4.4±4.0)g·m~(-2))公交站台((2.3±1.4)g·m~(-2));高度梯度对细灰尘(粒径105μm)的质量比的影响为:公交站台(49.5%)楼顶(40.8%)路面(38.6%)天桥(37.6%);6种重金属在不同高度下垫面的含量不同,其中,Cr、Cu和Ni在公交站台含量最高,Cd和Pb在楼顶含量最高,6种金属(除Cu外)均在路面含量最低.细灰尘(105μm)所占颗粒物污染负荷比为:公交站台(67.4%)楼顶(49.6%)天桥(49.2%)路面(48.1%).临时性管控措施(限行、限产等)和常规性清扫(人工清扫和机械清扫)均能减少灰尘质量,使细灰尘质量比增加,重金属污染负荷有明显削减,但对城市管理来说,后者更经济实用.     

Online single particle measurement of fireworks pollution during Chinese New Year in Nanning

Time-resolved single-particle measurements were conducted during Chinese New Year in Nanning, China. Firework displays resulted in a burst of SO_2, coarse mode, and accumulation mode(100–500 nm) particles. Through single particle mass spectrometry analysis, five different types of particles(fireworks-metal, ash, dust, organic carbon-sulfate(OC-sulfate), biomass burning) with different size distributions were identified as primary emissions from firework displays. The fireworks-related particles accounted for more than70% of the total analyzed particles during severe firework detonations. The formation of secondary particulate sulfate and nitrate during firework events was investigated on single particle level. An increase of sulfite peak(80SO_3~-) followed by an increase of sulfate peaks(97HSO_4~-+ 96SO_4~-) in the mass spectra during firework displays indicated the aqueous uptake and oxidation of SO_2 on particles. High concentration of gaseous SO_2, high relative humidity and high particle loading likely promoted SO_2 oxidation. Secondary nitrate formed through gas-phase oxidation of NO_2 to nitric acid, followed by the condensation into particles as ammonium nitrate. This study shows that under worm, humid conditions, both primary and secondary aerosols contribute to the particulate air pollution during firework displays.     

Case study of dust event sources from the Gobi and Taklamakan deserts: An investigation of the horizontal evolution and topographical effect using numerical modeling and remote sensing

A severe dust event occurred from April 23 to April 27, 2014, in East Asia. A state-of-the-art online atmospheric chemistry model, WRF/Chem, was combined with a dust model, GOCART, to better understand the entire process of this event. The natural color images and aerosol optical depth (AOD) over the dust source region are derived from datasets of moderate resolution imaging spectroradiometer (MODIS) loaded on a NASA Aqua satellite to trace the dust variation and to verify the model results. Several meteorological conditions, such as pressure, temperature, wind vectors and relative humidity, are used to analyze meteorological dynamic. The results suggest that the dust emission occurred only on April 23 and 24, although this event lasted for 5 days. The Gobi Desert was the main source for this event, and the Taklamakan Desert played no important role. This study also suggested that the landform of the source region could remarkably interfere with a dust event. The Tarim Basin has a topographical effect as a “dust reservoir” and can store unsettled dust, which can be released again as a second source, making a dust event longer and heavier.     

南京市、苏州市及南通市城区道路扬尘排放特性研究

在江苏南京、苏州和南通三个城市中选取10个采样点,以采集的扬尘样品为研究对象,分析了灼烧减量和扬尘组分.结果表明:南京、南通与苏州元素占比排列基本一致,排列前几位的为Si、Ca、Fe、Al;扬尘化学组分在不同地区有较大差别,南通和苏州扬尘的有机挥发性物质含量较高,城市扬尘主要来源于土壤沙尘的微小颗粒物;同一城市不同用地类型采样点也呈现出一定的规律性,施工区域的SiO2含量最高,商业区和住宅区Al比其他区域高.     

城市道路行道树树池裸地扬尘排放特征

本研究以北京市西城区为例,研究城市道路行道树树池裸地(以下简称树池裸地)扬尘排放特征.利用GIS技术获取西城区分道路类型里程空间分布,对展览路街道树池裸地进行全口径调查,得到西城区道路树池裸地活动水平,采用便携式风洞(PI-SWERL)实测各种道路树池裸地扬尘PM2.5排放因子,估算西城区2016年树池裸地扬尘排放清单.结果表明:(1)快速路辅路、主干路、次干路和街坊路单位面积树池裸地扬尘PM2.5年排放因子分别为47.9、7.9、14.9和29.9 g·(m2·a)-1,2016年降水过程对树池裸地PM2.5排放因子的削减率为30.3%;(2)快速路辅路、主干路、次干路、支路和街坊路单位里程树池裸地扬尘PM2.5年排放因子分别为2.57、2.33、4.04、7.31和5.44 kg·(km·a)-1,支路是街坊路、次干路、快速路辅路和主干路的1.3、1.8、2.8和3.1倍,以次干路为例,冬季排放因子分别是春夏秋季的1.3、7.3和8.7倍;(3)北京市西城区树池裸地全年PM2.5排放量为1.60 t·a-1,排放清单的不确定性范围为-143%~184%,冬季排放量为0.68 t·a-1,分别是春夏秋季的1.1、4.2和5.1倍,快速路辅路、主干路、次干路、支路和街坊路占总排放量的5.6%、8.7%、23.2%、4.1%和58.4%.建议尽快对全市树池裸地采取不影响树木生长的覆盖措施,减少风蚀扬尘排放.     

华东区域高山背景点PM10 和PM2.5 背景值及污染特征

为了解华东森林及高山背景区域大气中PM10和PM2.5质量浓度的变化特征,选取国家大气背景监测福建武夷山站2011年3月～2012年2月PM10、PM2.5为期1 a的监测数据,研究其浓度变化特征及其影响因子,并利用后向轨迹模式探讨区域输送对背景区域PM10和PM2.5质量浓度的影响.结果表明:华东森林及高山区域现阶段PM10和PM2.5背景浓度分别为(23±16)μg·m-3和(18±12)μg·m-3;PM10和PM2.5浓度具有相同的季节变化特征,即春季>秋季>冬季>夏季,2011年春季武夷山背景点因受沙尘远距离输送影响,PM10和PM2.5浓度明显高于其它三季;武夷山背景地区主要以细粒子为主,PM2.5/PM10年平均比值为0.76;PM10和PM2.5浓度与气体污染物均有较好的相关性,表明PM10和PM2.5可能来源于区域人为污染源的输送和二次粒子转化;2011年4月的污染事件与北方沙尘输送有关,而9月的污染事件主要与华东高污染负荷区的污染物输送有关.     

Experiment-based investigations of magnesium dust explosion characteristics

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000-5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.     

CFD-based simulation of dense gas dispersion in presence of obstacles

Quantification of spatial and temporal concentration profiles of vapor clouds resulting from accidental loss of containment of toxic and/or flammable substances is of great importance as correct prediction of spatial and temporal profiles can not only help in designing mitigation/prevention equipment such as gas detection alarms and shutdown procedures but also help decide on modifications that may help prevent any escalation of the event.The most commonly used models - SLAB (Ermak, 1990), HEGADAS (Colenbrander, 1980), DEGADIS (Spicer & Havens, 1989), HGSYSTEM (Witlox & McFarlane, 1994), PHAST (DNV, 2007), ALOHA (EPA & NOAA, 2007), SCIPUFF (Sykes, Parker, Henn, & Chowdhury, 2007), TRACE (SAFER Systems, 2009), etc. - for simulation of dense gas dispersion consider the dispersion over a flat featureless plain and are unable to consider the effect of presence of obstacles in the path of dispersing medium. In this context, computational fluid dynamics (CFD) has been recognized as a potent tool for realistic estimation of consequence of accidental loss of containment because of its ability to take into account the effect of complex terrain and obstacles present in the path of dispersing fluid.The key to a successful application of CFD in dispersion simulation lies in the accuracy with which the effect of turbulence generated due to the presence of obstacles is assessed. Hence a correct choice of the most appropriate turbulence model is crucial to a successful implementation of CFD in the modeling and simulation of dispersion of toxic and/or flammable substances.In this paper an attempt has been made to employ CFD in the assessment of heavy gas dispersion in presence of obstacles. For this purpose several turbulence models were studied for simulating the experiments conducted earlier by Health and Safety Executive, (HSE) U.K. at Thorney Island, USA (Lees, 2005). From the various experiments done at that time, the findings of Trial 26 have been used by us to see which turbulence model enables the best fit of the CFD simulation with the actual findings. It is found that the realizable k-? model was the most apt and enabled the closest prediction of the actual findings in terms of spatial and temporal concentration profiles. It was also able to capture the phenomenon of gravity slumping associated with dense gas dispersion.     

Does your facility have a dust problem: Methods for evaluating dust explosion hazards

The hazards of dust explosions prevailing in plants are dependent on a large variety of factors that include process parameters, such as pressure, temperature and flow characteristics, as well as equipment properties, such as geometry layout, the presence of moving elements, dust explosion characteristics and mitigating measures. A good dust explosion risk assessment is a thorough method involving the identification of all hazards, their probability of occurrence and the severity of potential consequences. The consequences of dust explosions are described as consequences for personnel and equipment, taking into account consequences of both primary and secondary events.While certain standards cover all the basic elements of explosion prevention and protection, systematic risk assessments and area classifications are obligatory in Europe, as required by EU ATEX and Seveso II directives. In the United States, NFPA 654 requires that the design of the fire and explosion safety provisions shall be based on a process hazard analysis of the facility, process, and the associated fire or explosion hazards. In this paper, we will demonstrate how applying such techniques as SCRAM (short-cut risk analysis method) can help identify potentially hazardous conditions and provide valuable assistance in reducing high-risk areas. The likelihood of a dust explosion is based on the ignition probability and the probability of flammable dust clouds arising. While all possible ignition sources are reviewed, the most important ones include open flames, mechanical sparks, hot surfaces, electric equipment, smoldering combustion (self-ignition) and electrostatic sparks and discharges. The probability of dust clouds arising is closely related to both process and dust dispersion properties.Factors determining the consequences of dust explosions include how frequently personnel are present, the equipment strength, implemented consequence-reducing measures and housekeeping, as risk assessment techniques demonstrate the importance of good housekeeping especially due to the enormous consequences of secondary dust explosions (despite their relatively low probability). The ignitibility and explosibility of the potential dust clouds also play a crucial role in determining the overall risk.Classes describe both the likelihood of dust explosions and their consequences, ranging from low probabilities and limited local damage, to high probability of occurrence and catastrophic damage. Acceptance criteria are determined based on the likelihood and consequence of the events. The risk assessment techniques also allow for choosing adequate risk reducing measures: both preventive and protective. Techniques for mitigating identified explosions risks include the following: bursting disks and quenching tubes, explosion suppression systems, explosion isolating systems, inerting techniques and temperature control. Advanced CFD tools (DESC) can be used to not only assess dust explosion hazards, but also provide valuable insight into protective measures, including suppression and venting.     

Evaluating the potential for overpressures from the ignition of an LNG vapor cloud during offloading

Ignition of natural gas (composed primarily of methane) is generally not considered to pose explosion hazards when in unconfined and low- or medium-congested areas, as most of the areas within LNG regasification facilities can typically be classified. However, as the degrees of confinement and/or congestion increase, the potential exists for the ignition of a methane cloud to result in damaging overpressures (as demonstrated by the recurring residential explosions due to natural gas leaks). Therefore, it is prudent to examine a proposed facility’s design to identify areas where vapor cloud explosions (VCEs) may cause damage, particularly if the damage may extend off site.An area of potential interest for VCEs is the dock, while an LNG carrier is being offloaded: the vessel hull provides one degree of confinement and the shoreline may provide another; some degree of congestion is provided by the dock and associated equipment.In this paper, the computational fluid dynamics (CFD) software FLACS is used to evaluate the consequences of the ignition of a flammable vapor cloud from an LNG spill during the LNG carrier offloading process. The simulations will demonstrate different approaches that can be taken to evaluate a vapor cloud explosion scenario in a partially confined and partially congested geometry.