Clues for a standardised thermal-optical protocol for the assessment of organic and elemental carbon within ambient air particulate matter

Abstract

Along with some research networking programmes, the European Directive 2008/50/CE requires chemical speciation of fine aerosol (PM2.5), including elemental (EC) and organic carbon (OC), at a few rural sites in European countries. Meanwhile, the thermal-optical technique is considered by the European and US networking agencies and normalisation bodies as a reference method to quantify EC-OC collected on filters. Although commonly used for many years, this technique still suffers from a lack of information on the comparability of the different analytical protocols (temperature protocols, type of optical correction) currently applied in the laboratories. To better evaluate the EC-OC data set quality and related uncertainties, the French National Reference Laboratory for Ambient Air Quality Monitoring (LCSQA) organised an EC-OC comparison exercise for French laboratories using different thermaloptical methods (five laboratories only). While there is good agreement on total carbon (TC) measurements among all participants, some differences can be observed on the EC/TC ratio, even among laboratories using the same thermal protocol. These results led to further tests on the influence of the optical correction: results obtained from different European laboratories confirmed that there were higher differences between OCTOT and OCTOR measured with NIOSH 5040 in comparison to EUSAAR-2. Also, striking differences between ECTOT/ECTOR ratios can be observed when comparing results obtained for rural and urban samples, with ECTOT being 50% lower than ECTOR at rural sites whereas it is only 20% lower at urban sites. The PM chemical composition could explain these differences but the way it influences the EC-OC measurement is not clear and needs further investigation. Meanwhile, some additional tests seem to indicate an influence of oven soiling on the EC-OC measurement data quality. This highlights the necessity to follow the laser signal decrease with time and its impact on measurements. Nevertheless, this should be confirmed by further experiments, involving more samples and various instruments, to enable statistical processing. All these results provide insights to determine the quality of EC-OC analytical methods and may contribute to the work toward establishing method standardisation

Measurement of carbonaceous aerosol with different sampling configurations and frequencies

“…values (by about 15 %) were measured by the low-frequency samples (Fig. 4), consistent with results from a suburban site in Europe (Chiappini et al, 2014). Although the EC measurement is not susceptible to sampling artifacts, thermaloptical determination of EC could be largely complicated by the transformation of OC into char OC (Yang and Yu, 2002).…”

“…A major problem in thermal-optical methods is that a substantial fraction of organic species can be transformed into materials whose thermal and optical behaviors are quite similar to EC (these materials are typically termed char OC or pyrolysis OC), thus affecting the separation of OC and EC (Yang and Yu, 2002;Chow et al, 2004;Subramanian et al, 2006). A variety of studies have shown that thermal-optical determination of OC and EC depends strongly on the analytical procedure such as the temperature protocol and the charring correction method (Chow et al, 2001(Chow et al, , 2004Schmid et al, 2001;Schauer et al, 2003;ten Brink et al, 2004;Cavalli et al, 2010;Chiappini et al, 2014;Panteliadis et al, 2015); moreover, the dependence is significantly influenced by sample properties such as source of carbonaceous components and abundance of minerals. The spatial and temporal variations of the PM 2.5 composition in China are far from being well characterized, indicating that a national PM 2.5 speciation monitoring network is necessary.…”

Comparison of Measurement-Based Methodologies to Apportion Secondary Organic Carbon (SOC) in PM2.5: A Review of Recent Studies

“…They differ mostly in their temperature programs and optical correction types for charring based on transmittance or reflectance. The three protocols are comparable for total carbon (TC) concentrations but the results may significantly vary concerning EC-OC split [106][107][108]. Moreover, depending on the protocol used, very low EC loading can be difficult to measure, for instance in the case of samples from remote locations.…”

Multi-year ACSM measurements at the central European research station Melpitz (Germany) – Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter

“…Therefore, ACSM intercomparison workshops are regularly conducted within the framework of the European Center for Aerosol Calibration (ECAC; http://www.actris-ecac.eu, last access: 15 September 2019) at the Aerosol Chemical Monitor Calibration Center (ACMCC) in France. Data quality is ensured by determining instrumental variability between ACSMs (total mass 9 %, organics 19 %, nitrate 15 %, sulfate 28 %, ammonium 36 %; Crenn et al, 2015;Fröhlich et al, 2015a;Freney et al, 2019).…”

“…Although intercomparison exercises provide instrumental variability, a comparison between ACSM and collocated measurements remains a fundamental aspect of in situ quality control. These intercomparisons are considered in a number of publications (e.g., Fröhlich et al, 2015b;Petit et al, 2015;Parworth et al, 2015;Ovadnevaite et al, 2014;Ripoll et al, 2015;Minguillon et al, 2015;Poulain et al, 2011b, a;Huang et al, 2018;Takegawa et al, 2009;Wang et al, 2015;Crenn et al, 2015;Guo et al, 2015;Schlag et al, 2016;Sun et al, 2015). Usually, the comparisons between ACSM and collocated measurements were only performed for a few months up to 1 year.…”