Gravimetric Measurements Gravimetri

Gravimetric Measurements

Gravimetric measurements rely on measuring the increase in mass of some substrate following its exposure to an ambient airstream at a known flow rate. If the sampling efficiency of the substrate is known, the averag particle concentration in the air may then be calculated. The simplest such approach is to use a filter paper in series with the airflow, leading to the trapping of particles by impaction, diffusion electrostatic attraction and interception among other mechanisms. Filter papers are commonly formed from paper or glass fiber, although numerous other substrates exist. The efficiency with which particles are retained varies with their size -- for example, while larger particles are efficiently trapped by impaction of an airstream, and smaller particles by diffusion, intermediate sized particles may not be so readily retained; consequently, the relationship between the retained particle mass and the ambient concentration may not be straightforward. Further complications arise from the potential adsorption of gaseous species in the airflow onto the sampled particles -- for example, heterogeneous condensation of sulfuric acid and semivolatile organic -- and from the revolatilization of more volatile species species from the trapped particles back into the airstream. To minimize such changes, temperature and humidity should be regulated in some way. A degree of automation is offered by multiple sampling units, which switch between successive filter papers, mounted in cartridges, over a period of time (e.g., to make daily measurements over a period of a couple of weeks), but analysis of the weight gain must still be performed manually. A related technique is impaction, in which particles impact on a medium due to their aerodynamic-diameter-dependent inability to follow a bending airstream.

Automated Measurements: Tapered Element oscillating Microbalances

Recently, automated methods have been adopted, which permit (near) real time particle monitoring and measurement with integration times as short as a few minutes. The approach uses a tapered element oscillating microbalance (TEOM) -- a flow of ambient air, possible sampled through an aerodynamic inlet, impacts upon a small (~1 cm diameter) filter paper mounted on the tip of a lightweight rod that undergoes oscillation in an applied electromagnetic field. The resonant frequency vibration of the rod changes with the changing filter mass due to the accumulation of particles; this is assessed through the feedback response to the applied field, allowing a near-real time measure of the increasing filter mass, and hence the mass concentration of aerosol. In commercial systems, the inlet airstream is preheated to approximately 50 oC to drive off water; this also leads to the loss of some volatile components of the aerosol and so TEOMs tend to underestimate the actual aerosol mass, a correction factor (commonly a value of 1.3 is used for urban/suburban boundary layer environments) is applied in an attempt to allow for the effect. A modification, in which a screening filter that removes aerosol particles is periodically introduced into the sample airflow, provides a means to determine the true correction factor required. While the screening filter is in place, which prevents fresh particles from reaching the TEOM filter, the resulting change in mass of the oscillating filter indicates the rate of loss of volatile aerosol components (or condensation of low-volatility constituents) on the filter. This mass decrease (or increase) can then be applied to correct the measured mass increase in the absence of the screening filter.

Optical Methods

Particle number and size can also be determined throug their light-scattering properties and through the use of nephelometer-based approaches, in which a light beam, typically from a near- IR diode laser is directed into an ambient airstream and any reflected light (from suspended particles) detected perpendicularly to the incident beam. The frequency of scattering events provides an indication of the particle concentration, while the intensity of the scattered light is related to the particle size -- following assumptions about the shape and refractive index of the particles, an aerosol number and size distribution can be obtained (in practice, particle concentration in a certain number of discrete size bins is usually reported). Optical particle counters permit real time measurement of particle size distribution and (unlike gravimetric methods) include any volatile component of the aerosol; however, the assumptions required to derive the instrument calibration curves dictate that care is required when comparing optical particle counter measurements with, for example, filter or TEOM data, or in any situation where the particle population may vary significantly from the standard used to calibrate the instrument (typically monodisperse polystyrene latex spheres). Typical instrument performance is to measure particles over a diameter range 0.1-10 um, divided into 10-20 size bins. Optical particle or dust monitors are frequently handheld units, used to monitor industrial and occupational health-related aerosol exposure, in addition to performing environmental measurements.

Aerosol Composition Measurements

Until recently, aerosol composition measurement required relatively large sample sizes to permit analysis, which were acquired using high-volume samples (known as Hi-Vols), and were analyzed off-line by using techniques such as colorimetry, atomic absorption spectroscopy, and organic/elemental carbon analysis. Recently, on-line techniques have allowed the real time measurement of the average, and single-particle, aerosol composition through coupling of aerosol sampling stages to mass spectrometers. Aerosol mass spectrometers (AMS) sample ambient particles onto a heated surface, which volatilizes the aerosol; the constituent gases are ionized, and the ionic chemical composition is assessed through either a quadrupole or time-of-flight mass spectrometer. The characteristic chemical signatures can then be used to distinguish between different primary and secondary source contributions to the ambient aerosol population. The complementary aerosol time-of-flight mass spectrometer (ATOFMS) measures the composition of individual aerosol particles through UV laser volatilization and ionization, followed by time-of-flight mass spectrometry. These instruments are more commonly used within the atmospheric research communities.

Long-Term Monitoring of Global Pollutants

An alternative class of pollutants of considerable current interest are the long-lived globally mixed gases principally responsible for climate change and stratospheric ozone depletion: long-lived greenhouse gases (LLGHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fall into the former category, whereas chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) make up the latter (although most CFCs/HCFCs are also potent greenhouse gases on a molecule-for-molecule basis). These species are measured at a number of global monitoring stations, which operate under the authority of international programs such as the Advanced Global Atmospheric Gases Experiment (AGAGE) project, the World Meteorological Organization's Global Atmospheric Watch program, and various other national programs. A number of manned/semiautonomous stations make measurements of these gases in situ, and flask samples are also collected at other stations and returned to base laboratories for periodic analysis -- an approach only possible for the nonreactive, long-lived species under consideration.

CFC and HCFC species are measured using GC, following a drying stage. The GC analysis separates in dividual species, based on their retention times while flowing through the column, followed by detection by electron capture detector (ECD) for the halogenated gases, and FID for methane. Carbon dioxide is measured by nondispersive IR absorption spectroscopy, with the attenuation of filtered 4.25 um IR radiation by CO2 in the ambient airstream determined relative to that from a (known) concentration of CO2 in a reference cell. For all such measurements of trends in long-lived gases over timescales of decades and longer, quality control, and in particular, the repeatable analysis of traceable standard mixtures to validate the ambient measurements, is of paramount importance. The calibration and validation process for the measurements, involving generations of traceable primary and daughter standard mixtures, is commonly far more demanding than acquisition of the atmospheric signal.

Summary

Measurements of air pollutants are required to advise populations of current air quality levels and any specific hazardous conditions, assess compliance with regulatory controls, and monitor the changing atmospheric environment. Most pollutants are present at low levels within a complex matrix (ambient air); therefore sophisticated instrumentation, commonly targeted toward a specific species or group of species, has been developed to measure atmospheric pollution depending on the specific physicochemical properties of the pollutant in question. Common techniques incorporated in automatic monitors deployed at specific locations or monitoring stations include short-path absorption spectroscopy i the UV (ozone) and IR (sulfur dioxide), chemiluminescence (nitrogen oxides), fluorescence (carbon monoxide), whereas semiautonomous monitoring for VOCs may be conducted by GC. For PM, aerosol, recently developed instrumentation permits automated near-real time monitoring of ambient levels without recourse to time-integrated filter sampling and weighing procedures. The methods described are reliable under most ambient atmospheric conditions, and have been accepted as reference techniques for the various pollutant species, but they all have limitations, for example, detection limit

ผลลัพธ์ (ไทย) 1:

ต้องการวัด Gravimetric measurements rely on measuring the increase in mass of some substrate following its exposure to an ambient airstream at a known flow rate. If the sampling efficiency of the substrate is known, the averag particle concentration in the air may then be calculated. The simplest such approach is to use a filter paper in series with the airflow, leading to the trapping of particles by impaction, diffusion electrostatic attraction and interception among other mechanisms. Filter papers are commonly formed from paper or glass fiber, although numerous other substrates exist. The efficiency with which particles are retained varies with their size -- for example, while larger particles are efficiently trapped by impaction of an airstream, and smaller particles by diffusion, intermediate sized particles may not be so readily retained; consequently, the relationship between the retained particle mass and the ambient concentration may not be straightforward. Further complications arise from the potential adsorption of gaseous species in the airflow onto the sampled particles -- for example, heterogeneous condensation of sulfuric acid and semivolatile organic -- and from the revolatilization of more volatile species species from the trapped particles back into the airstream. To minimize such changes, temperature and humidity should be regulated in some way. A degree of automation is offered by multiple sampling units, which switch between successive filter papers, mounted in cartridges, over a period of time (e.g., to make daily measurements over a period of a couple of weeks), but analysis of the weight gain must still be performed manually. A related technique is impaction, in which particles impact on a medium due to their aerodynamic-diameter-dependent inability to follow a bending airstream.วัดอัตโนมัติ: องค์เรียวขา Microbalances Recently, automated methods have been adopted, which permit (near) real time particle monitoring and measurement with integration times as short as a few minutes. The approach uses a tapered element oscillating microbalance (TEOM) -- a flow of ambient air, possible sampled through an aerodynamic inlet, impacts upon a small (~1 cm diameter) filter paper mounted on the tip of a lightweight rod that undergoes oscillation in an applied electromagnetic field. The resonant frequency vibration of the rod changes with the changing filter mass due to the accumulation of particles; this is assessed through the feedback response to the applied field, allowing a near-real time measure of the increasing filter mass, and hence the mass concentration of aerosol. In commercial systems, the inlet airstream is preheated to approximately 50 oC to drive off water; this also leads to the loss of some volatile components of the aerosol and so TEOMs tend to underestimate the actual aerosol mass, a correction factor (commonly a value of 1.3 is used for urban/suburban boundary layer environments) is applied in an attempt to allow for the effect. A modification, in which a screening filter that removes aerosol particles is periodically introduced into the sample airflow, provides a means to determine the true correction factor required. While the screening filter is in place, which prevents fresh particles from reaching the TEOM filter, the resulting change in mass of the oscillating filter indicates the rate of loss of volatile aerosol components (or condensation of low-volatility constituents) on the filter. This mass decrease (or increase) can then be applied to correct the measured mass increase in the absence of the screening filter.Optical Methods Particle number and size can also be determined throug their light-scattering properties and through the use of nephelometer-based approaches, in which a light beam, typically from a near- IR diode laser is directed into an ambient airstream and any reflected light (from suspended particles) detected perpendicularly to the incident beam. The frequency of scattering events provides an indication of the particle concentration, while the intensity of the scattered light is related to the particle size -- following assumptions about the shape and refractive index of the particles, an aerosol number and size distribution can be obtained (in practice, particle concentration in a certain number of discrete size bins is usually reported). Optical particle counters permit real time measurement of particle size distribution and (unlike gravimetric methods) include any volatile component of the aerosol; however, the assumptions required to derive the instrument calibration curves dictate that care is required when comparing optical particle counter measurements with, for example, filter or TEOM data, or in any situation where the particle population may vary significantly from the standard used to calibrate the instrument (typically monodisperse polystyrene latex spheres). Typical instrument performance is to measure particles over a diameter range 0.1-10 um, divided into 10-20 size bins. Optical particle or dust monitors are frequently handheld units, used to monitor industrial and occupational health-related aerosol exposure, in addition to performing environmental measurements.
Aerosol Composition Measurements

Until recently, aerosol composition measurement required relatively large sample sizes to permit analysis, which were acquired using high-volume samples (known as Hi-Vols), and were analyzed off-line by using techniques such as colorimetry, atomic absorption spectroscopy, and organic/elemental carbon analysis. Recently, on-line techniques have allowed the real time measurement of the average, and single-particle, aerosol composition through coupling of aerosol sampling stages to mass spectrometers. Aerosol mass spectrometers (AMS) sample ambient particles onto a heated surface, which volatilizes the aerosol; the constituent gases are ionized, and the ionic chemical composition is assessed through either a quadrupole or time-of-flight mass spectrometer. The characteristic chemical signatures can then be used to distinguish between different primary and secondary source contributions to the ambient aerosol population. The complementary aerosol time-of-flight mass spectrometer (ATOFMS) measures the composition of individual aerosol particles through UV laser volatilization and ionization, followed by time-of-flight mass spectrometry. These instruments are more commonly used within the atmospheric research communities.

Long-Term Monitoring of Global Pollutants

An alternative class of pollutants of considerable current interest are the long-lived globally mixed gases principally responsible for climate change and stratospheric ozone depletion: long-lived greenhouse gases (LLGHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fall into the former category, whereas chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) make up the latter (although most CFCs/HCFCs are also potent greenhouse gases on a molecule-for-molecule basis). These species are measured at a number of global monitoring stations, which operate under the authority of international programs such as the Advanced Global Atmospheric Gases Experiment (AGAGE) project, the World Meteorological Organization's Global Atmospheric Watch program, and various other national programs. A number of manned/semiautonomous stations make measurements of these gases in situ, and flask samples are also collected at other stations and returned to base laboratories for periodic analysis -- an approach only possible for the nonreactive, long-lived species under consideration.

CFC and HCFC species are measured using GC, following a drying stage. The GC analysis separates in dividual species, based on their retention times while flowing through the column, followed by detection by electron capture detector (ECD) for the halogenated gases, and FID for methane. Carbon dioxide is measured by nondispersive IR absorption spectroscopy, with the attenuation of filtered 4.25 um IR radiation by CO2 in the ambient airstream determined relative to that from a (known) concentration of CO2 in a reference cell. For all such measurements of trends in long-lived gases over timescales of decades and longer, quality control, and in particular, the repeatable analysis of traceable standard mixtures to validate the ambient measurements, is of paramount importance. The calibration and validation process for the measurements, involving generations of traceable primary and daughter standard mixtures, is commonly far more demanding than acquisition of the atmospheric signal.

Summary

Measurements of air pollutants are required to advise populations of current air quality levels and any specific hazardous conditions, assess compliance with regulatory controls, and monitor the changing atmospheric environment. Most pollutants are present at low levels within a complex matrix (ambient air); therefore sophisticated instrumentation, commonly targeted toward a specific species or group of species, has been developed to measure atmospheric pollution depending on the specific physicochemical properties of the pollutant in question. Common techniques incorporated in automatic monitors deployed at specific locations or monitoring stations include short-path absorption spectroscopy i the UV (ozone) and IR (sulfur dioxide), chemiluminescence (nitrogen oxides), fluorescence (carbon monoxide), whereas semiautonomous monitoring for VOCs may be conducted by GC. For PM, aerosol, recently developed instrumentation permits automated near-real time monitoring of ambient levels without recourse to time-integrated filter sampling and weighing procedures. The methods described are reliable under most ambient atmospheric conditions, and have been accepted as reference techniques for the various pollutant species, but they all have limitations, for example, detection limit

ผลลัพธ์ (ไทย) 3:

การวัดด้วยการวัดด้วย

พึ่งวัดเพิ่มมวลของพื้นผิวและการสัมผัสกับอุณหภูมิแอร์ รีมที่เรียกว่าอัตราการไหล ถ้าศึกษาประสิทธิภาพของพื้นผิวที่เป็นที่รู้จักกัน โดยถัวเฉลี่ยความเข้มข้นของอนุภาคในอากาศนั้นอาจจะคํานวณ วิธีที่ง่ายที่สุดคือการใช้ตัวกรองเช่นกระดาษในชุดมีให้ที่นำไปสู่การดักอนุภาคด้วยไฟฟ้าสถิต และการสกัดกั้นห้องแบบดึงดูดระหว่างกลไกอื่น ๆ ตัวกรองเอกสารมักเกิดขึ้นจากกระดาษ หรือใยแก้ว แม้ว่าพื้นผิวอื่น ๆ มากมายอยู่ ประสิทธิภาพที่แตกต่างกันไป มีขนาดอนุภาคจะถูกเก็บไว้ . . . ตัวอย่างเช่น ในขณะที่อนุภาคขนาดใหญ่ สามารถติดกับห้องของแอร์ รีม ,โดยกระจายอนุภาคขนาดเล็กและกลางขนาดอนุภาคอาจไม่พร้อมสะสม ดังนั้น ความสัมพันธ์ระหว่างการสะสมอนุภาคมวลและความเข้มข้นอุณหภูมิอาจจะไม่ตรงไปตรงมา ภาวะแทรกซ้อนต่อไปเกิดขึ้นจากศักยภาพการดูดซับก๊าซชนิดของอนุภาคบนตัวอย่าง -- ตัวอย่างข้อมูลการควบแน่นของกรดซัลฟูริกและ semivolatile อินทรีย์ -- และจาก revolatilization ระเหยมากกว่าสายพันธุ์สายพันธุ์จากดักอนุภาคเข้าไปในแอร์ รีม . เพื่อลดการเปลี่ยนแปลง เช่น อุณหภูมิ และ ความชื้น ควรควบคุมในบางวิธี ระดับของระบบอัตโนมัติที่ถูกเสนอ โดยหลายตัวอย่างหน่วย ซึ่งสามารถสลับไปมาระหว่างตัวกรองเอกสารต่อเนื่อง ,ติดตั้งในตลับ , ระยะเวลา ( เช่น ให้วัดทุกวันในช่วงสองสามสัปดาห์ ) แต่การวิเคราะห์น้ำหนักยังต้องดำเนินการด้วยตนเอง เป็นเทคนิคที่เกี่ยวข้อง คือ อาหาร ซึ่งผลกระทบอนุภาคในตัวกลางเนื่องจากพวกเขาไม่สามารถที่จะปฏิบัติตามขึ้นอยู่กับขนาดเส้นผ่าศูนย์กลางอากาศพลศาสตร์ดัดแอร์ รีม

วัดอัตโนมัติ :องค์ประกอบ microbalances เรียวสั่น

เมื่อเร็ว ๆนี้วิธีการอัตโนมัติได้ถูกนำมาใช้ซึ่งอนุญาต ( ใกล้ ) เวลาจริงการตรวจสอบและการวัดอนุภาคครั้งรวมเป็นสั้นไม่กี่นาที แนวทางการใช้องค์ประกอบ microbalance เรียวสั่น ( teom ) -- การไหลของอากาศผ่านท่อหรือตัวอย่างที่เป็นไปได้ ,ผลกระทบเมื่อเล็ก ( ~ 1 ซม. ) กระดาษกรองติดตั้งอยู่บนปลายไม้ที่เบาของการแกว่งในใช้สนามแม่เหล็กไฟฟ้า จังหวะความถี่ของการสั่นสะเทือนของการเปลี่ยนแปลง มีการเปลี่ยนแปลงตัวกรองเหล็กมวลเนื่องจากการสะสมของอนุภาค ซึ่งการประเมินผ่านความคิดเห็นการใช้สนามให้ใกล้เวลาจริงมาตรการการเพิ่มตัวกรองและมวลมวล ดังนั้นความเข้มข้นของสเปรย์ . ในระบบการค้าปากน้ำแอร์ รีมเป็นเตาอบประมาณ 50 องศาเซลเซียส เพื่อขับออกจากน้ำ นี้ยังนำไปสู่การสูญเสียของบางองค์ประกอบที่ระเหยของละอองและ teoms มักจะประมาทมวลสเปรย์จริง ปัจจัยการแก้ไข ( ปกติมูลค่า 13 ใช้สำหรับเมือง / ชานเมืองชั้นขอบเขตสภาพแวดล้อม ) ใช้ในความพยายามเพื่อให้มีผล ปรับเปลี่ยน ซึ่งในการตรวจกรองที่ลบอนุภาคละอองลอยเป็นระยะ ๆเข้าไปเพื่อให้ มีวิธีการหาจริงการแก้ไขปัจจัยจำเป็น ในขณะที่การตรวจกรองที่อยู่ในสถานที่ซึ่งป้องกันไม่ให้อนุภาคสดจากการเข้าถึงตัวกรอง teom ผลเปลี่ยนแปลงในมวลของตัวกรองชนิดระบุอัตราการสูญเสียของส่วนประกอบของละอองระเหยหรือไอน้ำขององค์ประกอบความผันผวนต่ำ ) ในเครื่องกรอง นี้มวลจะลดลง ( หรือเพิ่มขึ้น ) จากนั้นจะสามารถใช้เพื่อแก้ไขวัดมวลเพิ่มในกรณีที่ไม่มีการตรวจกรองแสง

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วิธีจำนวนอนุภาคและขนาดนอกจากนี้ยังสามารถกำหนดคุณสมบัติของแสงกระจายทั่วโลกและผ่านการใช้เนฟีโลมิเตอร์ตามแนวทางในที่แสงสว่างปกติจากใกล้ - IR ไดโอดเลเซอร์กำกับ เป็นแอร์ รีมรอบข้างและการสะท้อนแสง ( จากอนุภาคแขวนลอย ) ตรวจพบดิ่งเหตุคาน