Quantitative uncertainty and post-processing for micro-aethalometers measuring black carbon

Abstract

Aethalometers measure black carbon mass concentrations by monitoring light attenuation through a particle filter as it becomes laden with aerosols. As the uncertainties in the resulting measurements are not easily quantified via a bottom-up traceable approach, there is a need for inter-device comparisons to provide operationally defined uncertainties. The present work compared five micro-aethalometers to known mass concentrations of laboratory-generated soot, formed using an inverted ethylene flame and a Centrifugal Particle Mass Analyzer-Electrometer Reference Mass Standard (CERMS). Uncertainties were found to scale with mass concentration, with contributions from Gaussian, Poisson, and multiplicative components. The multiplicative errors between devices are approximately 10 % in the best case of long sampling times and/or high mass concentrations. This represents a minimum uncertainty estimate, for an aerosol of constant composition. A quantitative expression is provided for this uncertainty as a function of mass concentration, sampling interval, and flow rate. An open-source algorithm is also provided for the unsupervised reanalysis of aethalometer or other filter photometer data over varying periods to reach a specified target uncertainty.