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Field trials for black carbon instrumentation help improve measurement accuracy

Burning fire

Harmonised measurements of black carbon will help refine climate models and protect human health

The project

Black carbon, emitted from sources such as diesel engines and wood burning, is a major contributor to climate change, second only to carbon dioxide, and a primary component and key indicator of particulate matter, which is known to cause respiratory problems to hundreds of thousands of people across Europe each year. There are over 300,000 annual deaths in Europe associated with particulate matter pollution, which are caused by ischemic heart diseases, strokes, and respiratory cancers. The World Health Organization classifies diesel engine exhaust as carcinogenic to humas. 

However, prior to the work of this project the different instruments used for measuring the amount of black carbon in the atmosphere gave results that differed by up to 30 % and there was no way to link measurements to the International System of Units (SI).

Recently completed EMPIR project Metrology for light absorption by atmospheric aerosols (16ENV02, Black Carbon) has developed and evaluated traceability and calibration mechanisms for black carbon measurements for the first time, improving their accuracy and comparability. The improved measurements resulting from this work will be used to refine climate change models and mitigation proposals, and to improve the quality of conclusions from population studies investigating the health effects of air pollution.

The Field Campaigns

The large measurement uncertainties that black carbon measurements are subject to, are due both to the instruments' internal inaccuracies and their sensitivity to various aerosol properties such as particle size and state of mixing. To assess the uncertainties and better understand the instruments’ response for different types of absorbing aerosols (such as black carbon), under real-world conditions, two field campaigns were organised: in Athens in winter with high levels of particulates, and summer in Pallas - an extremely clean environment in Finland.

The two field campaigns gave insight into the sensitivity, uncertainties, and overall limitations of two different techniques for measuring absorbing aerosols in different atmospheric environments.

1. Filter-based absorption instruments

Filter-based measurements are globally the most common method to monitor the absorbing aerosols in long-term. It was found that the filter-based absorption instruments provide a robust and sensitive method for measuring absorption at places with extremely low absorbing aerosol concentrations. The sensitivity of the filter-based techniques was adequate for measuring aerosol light absorption coefficients down to around 0.01 Mm−1 levels. This is important when evaluating the BC climate impacts in clean environments, such as for example the Arctic, and for estimating the global baseline concentrations for modelling purposes. Intercomparability of the filter-based instruments was also evaluated and found satisfactory in the field, both in low and high concentration environments.

2. Indirect techniques:

The indirect techniques to determine aerosol absorption (i.e. extinction minus scattering method) appeared to be less sensitive than filter-based techniques; an absorption coefficient on the order of >1 Mm−1 was estimated as the lowest limit, to reliably distinguish the signal from the noise. The benefit of the method is high accuracy making it a suitable technique for laboratory standard and calibration measurements.

Conclusions

The results acquired through the laboratory and field campaigns clearly showed that different instruments’ responses were well correlated against the same type of aerosol (for absorption coefficients > 1 Mm-1). These experiments demonstrated that it is possible to achieve traceability in aerosol absorption measurements in the field. However, further targeted experimental work, combining laboratory and field tests, is needed to clarify the instruments' responses for different aerosol types, to carefully describe the measurement uncertainties and to define the standard practises for black carbon mass field measurements.

Project Participant Eija Asmi from the National Centre for Scientific Research Demokritos in Greece said ‘Black carbon is a threat for health, and for climate. It is urgent that we agree on how to monitor it and limit the exposure. This is what we work for.’

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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