Investigation of structural changes of 21 atmospheric aerosol samples during a thermal-optical measurement procedure (EUSAAR2)

Elemental Carbon (EC), Black Carbon (BC) and Organic Carbon (OC) contribute a large amount to atmospheric aerosols. Due to their significant influence on climate and health, a reliable measurement of these components is essential. Nevertheless, their correct determination is not trivial and results of different measurement techniques show differences by factors up to nine especially in the presence of Brown Carbon (BrC) (e.g. Reisinger et al., 2008; Hitzenberger et al., 2006; Wonaschuetz et al., 2009). EC and OC are usually measured with thermal-optical techniques: The sample is heated stepwise, first in an inert (He) atmosphere, then in an oxidizing (He+O₂) atmosphere. The darkening of the sample during the heating procedure is traced with a laser transmission/reflection signal. Based on the progress of this signal, the amount of pyrolyzed carbon is calculated and attributed to OC in the subsequent evaluation. Despite this optical correction, the pyrolyzation of OC can lead to uncertainties in the OC/EC split (Cheng et al., 2012). Especially Brown Carbon (BrC) and water soluble organic carbons (WSOC) have a high tendency to pyrolyze and therefore bias the OC/EC split. Moreover several metal salts in the atmospheric aerosol can influence the measurement process and enhance or suppress pyrolysis of OC (Wang et al., 2010). These highly complex chemical and physical reactions are not fully investigated yet but are essential for a profound understanding of the biases in thermal-optical measurement techniques. 

The aim of the present study was to investigate the structural reorganizations of the carbonaceous materials in atmospheric aerosol samples occurring during a thermal-optical heating procedure (EUSAAR2, Cavalli et al., 2010) and to set them in relation with several properties of the samples such as ionic composition, EC, OC, BC and BrC, as well as the air mass origins during sampling of the atmospheric aerosol samples.
The changes of the internal structure of the material during the heating procedure of an EC/OC analyzer (Sunset instruments) were analyzed with Raman spectroscopy, which is sensitive to C-C bonding types and to the degree of structural ordering within the sample (Ferrari and Robertson, 2000). Different types of restructuration behavior were defined depending on the temperature levels of the EUSAAR2 protocol where measurable structural changes occur. For all samples ion chromatography was performed with a Dionex Aquion system (Thermo Fisher), BrC and BC were analyzed with the Integrating Sphere method (Wonaschütz et al., 2009) and air mass back trajectories for the respective sampling days were calculated with HYSPLIT.