Current challenges in atmospheric black carbon determination: A traceable calibration for aerosol light absorption measurement
by Dr. Jorge Saturno, Dr. Andreas Nowak, Prof. Volker Ebert, Dr. Konstantinos Eleftheriadis, Dr. Thomas Mueller

Abstract

Combustion generated aerosol particles have strong climate impacts by directly absorbing incoming solar radiation from the ultraviolet (UV) up to the infrared (IR) region, and by causing different indirect and semi-direct effects related to cloud radiative properties. Black carbon (BC), a particulate material with strong light absorption properties is produced by incomplete combustion of fossil fuels or biomass and is emitted by multiple sources. Black carbon is among the most important short-lived climate forcers (SLCFs), although often emitted together with cooling species. Even though BC measurements based on light-absorption have been implemented in air quality monitoring since decades, a reference standard material for aerosol light absorption is still lacking. Additionally, there are different systematic errors associated to the use filterbased techniques that need to be quantified in a regular basis, like the presence of nonBC particles, and particle-filter interactions. Moreover, upon atmospheric aging, BC particles become coated with other materials and therefore, BC mass absorption crosssection (MAC) and its wavelength dependence are modified upon atmospheric transport. Radiative models require BC mass concentration input calculated from aerosol light absorption coefficients –a traceable metric– and MAC values, usually calculated in dedicated studies and fixed to a constant value. Traceable aerosol absorption coefficient measurements are possible by using the extinction minus scattering technique, which employs state-of-the-art optical measurement instrumentation. However, the description of measurement uncertainties is complicated due to the non-independent calibrations of the instruments. The EMPIR 16ENV02 BC project was aimed to investigate a pathway to make aerosol absorption measurements traceable to the SI. In order to achieve this goal, a fresh-like BC reference material was established by PTB and TROPOS (host of WCCAP-GAW), which consists of a diffusion flame generator and a conditioning and a volatile particle removal system. TROPOS is using a set of optical techniques, including Cavity Attenuated Phase Shift (CAPS) analysis, photoacoustic spectrometry and nephelometry, a traceable calibration for aerosol light absorption to establish a pathway to SI unit. Finally, a robust and harmonized procedure for calibration of aerosol absorption photometers can be implemented with the developed reference materials. Establishing the necessary infrastructure, services and calibration protocols will require the cooperation between national metrology institutes, atmospheric monitoring networks like ACTRIS-ERIC and other stakeholders. During EMPIR 16ENV02 BC, NCSR Demokritos and several other project partners conducted an intercomparison between several field instruments currently providing well documented datasets for aerosol absorption coefficient across the ACTRIS research infrastructure. The data from the urban background environment in Athens are currently analyzed further in order to provide the link between aerosol absorption and BC mass concentration.