Quantifying the equivalency factor between PM-absorbance and EC mass concentration – converting the “old” exposure proxy in large health studies to the new metric for diesel soot

The light absorbance of PM-samples on PTFE filters is often used as a measure for exposure to diesel soot in large-scale health studies. Absorbance is a synonym for the optical parameter “absorption coefficient” (AbsC). The formal exposure measure for diesel soot is the mass concentration of the light absorbing species of PM, viz. elemental carbon (EC). In the central health effects study by Janssen et al. [2011] a relation between AbsC and EC is presented, however with an overall uncertainty of 2.5. In the present study, we started with an analysis of the measuring approach of light absorption according to ISO-9835 [ISO, 1993]. Following this procedure, absorption is probed in reflection and expressed in the ratio of the intensity of light reflected from/by a clean and a loaded filter. The AbsC is the logarithm of this ratio (which is known as optical depth) scaled to the volume of air sampled and loaded filter area. We first critically reanalysed the studies used by Janssen et al. [2011] in which the equivalency factor between absorbance and EC concentration was given. We found a good linear relationship when we selected only those data points for which the optical density (OD) was within the proper limits of 0.05 and 2.0. We then analysed which methods had been used to obtain EC data in those studies (and also more recent ones) and selected only those studies where EC had been determined with an official reference approach, i.e. the USreference method NIOSH-5400. The overall relation of EC mass concentration in µg m⁻³ and AbsC in units of 10^–5 m⁻¹ was 0.8 (R² = 0.92), or 1.0 according to the EU-reference method EUSAAR2-TOT with a factor of 1.25 between US and EU reference EC values. This highly improved estimate of equivalence factors between AbsC and EC might be used to translate the results of existing health effects studies (based on AbsC) to studies using current EC monitoring data (as prescribed in EU-guidelines for air quality) to investigate the possible health effects at a given EC level.