The measurement of particles in air characterised as black carbon is important both for its role in climate change and as a measure of combustion products associated with health effects. Measurements are made very widely, and compact, precise, real-time, relatively inexpensive instruments are available. Although it is conceptually a simple measure of the light absorbing properties of airborne particles, the metric does not currently have SI traceability, with consequences for the comparability and interpretation of data. The project will provide a workable solution to this major problem, with widespread benefits across the worlds of both climate change and air quality.
The quantity of airborne particles loosely described as black carbon has been widely measured by various optical methods since the early 20th century, because instruments for this are relatively simple and reliable. These carbonaceous particles continue to receive high levels of attention from the scientific community [1] and policy makers, with the related parameter Elemental Carbon included in the EU Ambient Air Quality Directive, because of their role in both climate change and health effects. The dominant sources have changed over the decades, from domestic and industrial coal burning to vehicle combustion emissions, with more recent contributions from wood-burning.
Black carbon has been identified as the second most important climate forcing agent [2] behind CO2, contributing an amount of radiative forcing nearly 30 % that of current CO2 concentrations. Because black carbon has a much shorter atmospheric lifetime than CO2, black carbon mitigation strategies could rapidly slow down the rate of climate change, by up to 40 % within 20 years [3].
Airborne particles have serious human health effects across Europe and worldwide. In 2011, about 430,000 premature deaths in the EU were attributed to fine particulate matter (PM). Studies of short-term health effects suggest that black carbon is a better indicator of harmful particulate substances from combustion sources than PM mass concentration [4].
Although black carbon measurement is in principle a simple optical measurement of absorption, characterised by the aerosol light absorption coefficient, traceability is hampered by the fact that routine monitors determine the absorption of particulate matter collected on a fibrous filter. While the optical absorption measurement itself can be done accurately, the presence of the filter has a large effect, due to internal scattering within the filter, which can increase absorption by a factor of five, and to shadowing effects as the filter accumulates material. These effects are currently handled with generic correction factors. It is the need to replace these correction factors with properly determined calibration factors (which are expected to depend strongly on various particle properties), that is at the heart of this project. These correction factors are often incorporated into the mass absorption coefficient that is used to convert the light absorption coefficient, which is typically in units of Mm-1, into a particle mass concentration, typically in μg/m3.
The lack of a metrological framework, or even standardisation, for black carbon was highlighted in 2013 by the European Environment Agency (EEA) [5], as well as by the BIPM-GAWG Particulate Workshop in April 2015. At present, different types of measurement instrument give results differing by up to 30 % [6], and there is no mechanism to provide the traceability that would define the correct result. This has led to the development of the BIPM-GAWG roadmap, on which this project is based. Traceability will make standardisation of black carbon simple. Standardisation without traceability is far less valuable.
This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the Project.
