Toward a standardised thermal-optical protocol for measuring atmospheric organic and elemental carbon: the EUSAAR protocol
Toward a standardised thermal-optical protocol for measuring atmospheric organic and elemental carbon: the EUSAAR protocol
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DOI：
10.5194/amt-3-79-2010
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发表时间：
2010-01-01
影响因子：
3.8
通讯作者：
Putaud, J. -P.
中科院分区：
文献类型：
--
作者：
Cavalli, F.;Viana, M.;Putaud, J. -P.
热光学分析是一种常规的方法，用于确定含碳气溶胶部分，并将其分为有机碳，OC和元素碳，EC。不幸的是，使用中的不同热演化方案可导致元素碳与总碳的变化高达五倍。在欧洲，目前没有确定含碳气溶胶部分的标准程序，这意味着来自不同地点不同实验室的数据的准确性未知，不能被认为是可比的。在欧盟项目EUSAAR（欧洲大气气溶胶研究超级站点）的框架内，进行了一项全面的研究，以确定使用不同热演化协议测量的EC差异的原因;从而隔离并最大限度地减少影响热光学分析的主要正负偏差，以确定适合欧洲气溶胶的优化协议。我们的方法，以提高OC和EC之间的歧视的准确性基本上是基于四个目标。首先，炭化校正依赖于错误的假设-例如，在整个分析过程中，热解碳被认为在天然EC之前完全演变-因此，我们通过有利于OC的挥发将热解减少到最低限度。其次，我们已经最大限度地减少了EC测定中的潜在负偏差，这是由于在He模式下在较高温度下光吸收碳物种的早期演变，包括天然EC以及天然EC和热解碳的组合，其可能具有不同的特定衰减截面值。第三，我们已经最大限度地减少了EC测定中的潜在正偏差，这是由于在He模式期间OC的不完全演变导致的，然后在He/O-2模式期间演变，可能在分裂点之后。最后，我们通过在He/O-2-模式中引入多个解吸步骤，最大限度地减少了由于OC/EC分离点在FID响应曲线上的位置所引起的不确定性。基于欧洲遇到的不同类型的碳质PM，我们已经定义了一个优化的热演化协议，EUSAAR_2协议，如下：步骤1在He中，200 ℃，120 s;步骤2在He 300 ℃，150 s;步骤3在He 450 ℃，180 s;步骤4在He 650 ℃，180 s。对于He/O-2中的步骤1-4，条件分别为500 ℃持续120 s、550 ℃持续120 s、700 ℃持续70 s和850 ℃持续80 s。
Thermal-optical analysis is a conventional method for determining the carbonaceous aerosol fraction and for classifying it into organic carbon, OC, and elemental carbon, EC. Unfortunately, the different thermal evolution protocols in use can result in a wide elemental carbon-to-total carbon variation by up to a factor of five. In Europe, there is currently no standard procedure for determining the carbonaceous aerosol fraction which implies that data from different laboratories at various sites are of unknown accuracy and cannot be considered comparable. In the framework of the EU-project EUSAAR (European Supersites for Atmospheric Aerosol Research), a comprehensive study has been carried out to identify the causes of differences in the EC measured using different thermal evolution protocols; thereby the major positive and negative biases affecting thermal-optical analysis have been isolated and minimised to define an optimised protocol suitable for European aerosols. Our approach to improve the accuracy of the discrimination between OC and EC was essentially based on four goals. Firstly, charring corrections rely on faulty assumptions - e.g. pyrolytic carbon is considered to evolve completely before native EC throughout the analysis -, thus we have reduced pyrolysis to a minimum by favoring volatilisation of OC. Secondly, we have minimised the potential negative bias in EC determination due to early evolution of light absorbing carbon species at higher temperatures in the He-mode, including both native EC and combinations of native EC and pyrolytic carbon potentially with different specific attenuation cross section values. Thirdly, we have minimised the potential positive bias in EC determination resulting from the incomplete evolution of OC during the He-mode which then evolves during the He/O-2-mode, potentially after the split point. Finally, we have minimised the uncertainty due to the position of the OC/EC split point on the FID response profile by introducing multiple desorption steps in the He/O-2-mode. Based on different types of carbonaceous PMencountered across Europe, we have defined an optimised thermal evolution protocol, the EUSAAR_2 protocol, as follows: step 1 in He, 200 degrees C for 120 s; step 2 in He 300 degrees C for 150 s; step 3 in He 450 degrees C for 180 s; step 4 in He 650 degrees C for 180 s. For steps 1-4 in He/O-2, the conditions are 500 degrees C for 120 s, 550 degrees C for 120 s, 700 degrees C for 70 s, and 850 degrees C for 80 s, respectively.