A new on-line OCEC analvzer based on both TOT and TOR
Organic and elemental carbons are the major chemical constituents of atmospheric aerosols. Both of them have been found to be significant causes of light extinction and participate in climate change, air pollution, and human health. Since OC and EC have thermally different properties, they are typically defined operationally by the analysis protocols. Thermal/optical method is one of the classical methods and has been adapted by several laboratories for the quantification of organic and elemental carbon on quartz-fiber filter. In this method, carbon on filter substrates is made to evolve through programmed progressive heating in a controlled atmosphere. The typical two protocols based on thermal/optical method are IMPROVE/TOT (interagency monitoring of protected visual environments/thermal optical reflectance) (Chow et al., 1993) and NIOSH/TOR (national institute of occupational safety and health/thermal optical transmittance) (Turpin et al., 1990a), both of which have been applied in different two devices developed by Desert Research Institute (DRI) and Sunset Laboratory, respectively. Although both of these instruments produce OC and EC values and have a good agreement in TC (OC+EC) values, but because of differences in these protocols, such as, analysis time, temperature ramping, combustion temperatures and charring correction methods, different OC and EC values are obtained. TOR, which is dominated by the surface deposit, is less sensitive to internal charring than TOT. Moreover, it is proved that NIOSH/TOR EC was typically less than IMPROVE/TOT EC (Chow et al., 2001). So it is difficult for different laboratories owing different instruments to compare their monitoring results. A second cause of this trouble is that there is no on-line device based on TOT protocols, while Sunset Laboratory developed both on-line and off-line TOR OCEC analyzer. To obtain and compare the on-line OC and EC data both based on TOT and TOR, a new OCEC analyzer instrument has been developed. In this OCEC analyzer, the front oven is separated from back oven, which gives benefit for maintenance. When collecting particulates on a quartz-fiber filter at the flow of 16.7 L/min, a small flow of helium gas goes through back oven in an opposite direction to keep manganese dioxide in a dry atmosphere. Purge time is controlled flexibly according to CO2 concentration to ensure every analysis begin in a little O2 and CO2 atmosphere. The gases line (Figure 1) is different and gases consumption will be less than before, especially for He/CH4, up to 90% saving. When carbonaceous compositions evolve in a stable flow of 70ml/min, at certain different temperatures, the reflectance and transmittance laser (650nm) signal from the filter are monitored. Usually, both of the values decrease when OC volatilizes in the helium atmosphere owing to the pyrolysis of organic material. When oxygen is added, charring carbon and EC evolve and values increase. Comparing these data just after collecting will reduce uncertainties resulted from different instruments and artifacts which is a problem in off-line method. The time resolution of the device is much flexible, from minutes to hours. The low detection limit of analyzer mainly depends on the accuracy of CO2 sensor, but also differs with respect to sampling time and sampling flow. Modifications improve this new device capable of rapid automated on-line and continuous sampling particles and measurement of ambient aerosol organic composition.
organic carbon (OC) elemental carbon (EC) Thermal Optical Reflectance (TOR) Thermal Optical Transmittance (TOT) instrumentation on-line
Li Min Zeng Qiao Ling Liu
State Key Joint Laboratory of Environment Simulation and Pollution Control,College of Environmental Science and Engineering,Peking University,Beijing 100871,China
国际会议
International Conference on Environment Simulation and Pollution Control(第六届环境模拟与污染控制学术研讨会)
北京
英文
106-107
2009-11-01（万方平台首次上网日期，不代表论文的发表时间）