Network activity 2:

Standardised measurement protocols, intercomparisons and quality assurance for carbonaceous aerosols compounds

Objectives and expected impact

The goal of this network activity is to define a standardized artefact-free procedure for determining particulate organic and elemental carbon, validated for all kinds of sites Europe. To achieve its goal, the Network Activity will:

Objective 1: deliver a sampling train for particulate carbonaceous matter, well characterised with respect to particle trapping efficiency, positive and negative artefact assessment, lifetime, running cost, maintenance, etc…
Objective 2: define a standardised thermal programme for thermo-optical analysers of OC+ EC.
Objective 3: assess the comparability of this standard method with the other methods used within the network and periodically check their consistency (intercomparisons).
Objective 4: design a standardised method for analysing the back up sorbents (volatilised OC).
Objective 5: achieve adoption of the overall standardised procedure for OC+EC determination by the EMEP TFMM (task force on measurements and modelling) and the WHO / GAW (Global Atmospheric Watch) scientific advisory group.

Description of work

Carbonaceous matter is thought to contribute on average for 20 to 35% to PM10 and to have important local (human health) to global effects (radiative forcing). However, there is currently no standardized procedure for sampling and analysing this aerosol fraction, which implies that data obtained by different laboratories at various sites are affected by unknown errors. So called semi-volatile species are split between the gas and particulate phases in the atmosphere. While sampling aerosols, gaseous species can be trapped on/in the substrate (positive artefact), and particulate matter can be lost from the substrate (negative artefact), or undergo chemical reactions (mainly oxidation). Estimates of artefact contribution to measured particulate organic carbon concentrations range from – 80 to + 100%. “Artefact-free” sampling trains including denuder(s), filter(s), and sorbent(s) have been developed and used in the USA for ca. 20 years. However, these sampling trains are not well characterised with respect to their efficiency, lifetime, etc…, so that they are not widely used at the US-EPA supersites. In Europe, various sampling techniques with insufficiently defined efficiency and/or artefacts are currently operated. Validation and harmonisation are needed.

Carbonaceous particulate matter is commonly divided into organic carbon (OC) and elemental carbon (EC). Although the different environmental impacts of these two classes may justify such a division, there is no clear borderline between very high molecular weight poly-aromatic compounds and EC which would consist of a pure graphitic backbone. Thus, OC and EC are methodologically defined, based on some of their chemical and physical properties (solubility, volatility, refractory character). As a consequence, different analytical techniques obviously lead to unacceptable discrepancies (up to 300%) in EC determination through various methods. In order to improve monitoring of OC/EC aerosols, the following activities will be implemented:

Based on the previous experience, the NA2 consortium will propose the first sampling train (denuder – filter – sorbent) to be tested.

Tests including simultaneous sampling with various denuder – filter – sorbent combinations will be performed over different seasons at the various sites of the network run by NA2 participants. Feedbacks from the test stations will lead to the definition of the best affordable sampling train, which will be fully characterised in term of efficiency, positive and negative artefacts, lifetime, running cost, maintenance, etc….

Standard Operational Procedures for the validated sampling train will be edited and disseminated by the NA2 lead participant (technical training with NA6), with a contribution from all others.

A harmonised procedure for the thermo-optical analysis of particulate organic carbon (OC) and elemental carbon (EC) will be decided upon, based on NA2 participants’ experience and crossed analysis of aerosol standards.

Periodic intercomparisons of all the OC + EC analytical methods used in the Network will be carried out by circulating selected aerosol filters and standards among NA2 participants.

A standardised analytical procedure for analysing volatilised OC from the back-up sorbent will be decided upon, based on the results of concerted investigations performed by the NA2 participants.