The analysis of particulate carbon is carried out at the Laboratoire d’Aerologie (LA) using thermo-optical methods.
Three different methods exist for mesuring the mass of carbon present on a filter, all of which differentiate between the Organic Carbon (OC) and Black Carbon (BC) components. All three methods, thermic, optical and thermo-optical rely on the physical properties of carbonaceous aerosol. The optical method makes use of the black body properties of BC, whilst the thermal method makes use of the refractory characteristics of BC.
The LA acquired a DRI (Desert Research Institute, of Reno, USA)thermo-optical carbon analyser (2001 model) in 2003.
The IMPROVE thermo-optical method is used at the LA. The IMPROVE method – Interagency Monitoring of PROtected Visual Environments (Fung et al., 2002) – makes use of the fact that BC and OC volatilise at different temperatures. The threshold temperatures used for BC/OC speciation are shown in the figure below (showing TOR – Thermo-Optical Reflectance).
Description of Analytical Methods
- First, the sample is burned in an inert atmosphere (in pure helium, the gaseous vector) at different threshold temperatures. Organic components are liberated during this process and the « charring » products are trasnported by the helium across an oxidising oven (with a bed of MnO2 in order to form CO2. These products are then passed into a methniser in which the CO2 is reduced to CH4 by a reaction catalysed with nickel. The quantity of CH4 formed is detected by a FID (Flame Ionisation Detector). This quantity of CH4 is then converted into mass carbon using a calibration coefficient. The carbon masses detected correspond to the peaks of OC1, OC2, OC3, OC4 + « charring » on the thermogram.
- In a second step, the oxygen present in the gas in which the samples were burned in the previous step (a mix of 98% helium, 2% oxygen) allows for the BC to be burned and the « charring » to form during the first process, as just described. The quantity of carbon emitted is measured as above (oxidation in CO2, reduction in CH4, detection using FID and conversion to mass carbon. The mass of carbon detected corresponds to the peaks EC1, EC2, EC3 – « charring » on the thermogram.
- The quantity of carbon deteced is represented graphically in the form of peaks (mV) of which the area is proportional to the amount of methane detected. A formula is used to calculate this amount. The calibration curve is obtained either by the injection of a known quantity of gas containing a known quantity of methane, or by depositing a layer of sucrose on the surface of a filter. Calibration is carried out by determining the ratio between the analysis of the benchmark (gaseous or sucrose filter) and the peak of the internal benchmark.
Thermogram obtained using the DRI analyser and IMPROVE method
The DRI allows the measurement of carbon masses ranging from 0.05-750µg C/cm2 and the limits of detections are as follows: 0.41±0.2µg OC/cm2, 0.03±0.2µg EC/cm2, 0.44±0.2µg TC/cm2.