Filter Light Attenuation as a Surrogate for Elemental Carbon

Abstract

Light attenuation (b _att) measured from filter light transmission is compared with elemental carbon (EC) measurements for more than 180,000 collocated PM_2.5 (particulate matter [PM] ≤ 2.5 µm in aerodynamic diameter) and PM₁₀ (PM ≤ 10 µm in aerodynamic diameter) samples from nearly 200 U.S. locations during the past 2 decades. Although there are theoretical reasons for expecting highly variable relationships between b_att and EC (such as the effects of “brown carbon” and iron oxides in PM_2.5), reasonable correlations are found. These correlations are not a strong function of season or location (e.g., rural vs. urban). Median EC concentrations can be predicted from filter transmittance measurements to within ±15–30%. Although EC predicted from b_att shows larger uncertainties (30–60%), especially at concentrations less than 0.3 µg/m³, the consistent mass absorption efficiency (σ_att) derived from the regression analysis demonstrates the feasibility of using b _att as a surrogate for EC. This study demonstrates that a constant factor of 0.1 g/m² (equivalent to the 10 m²/g σ_att used in the Interagency Monitoring of Protected Visual Environments chemical extinction formula) can be used to estimate EC concentrations from b_att through a Teflon-membrane filter sample. Greater accuracy is achieved with site-specific σ_att derived from a period with collocated EC measurements.