Consistency of long-term elemental carbon trends from thermal and optical measurements in the IMPROVE network

Abstract. Decreasing trends of elemental carbon (EC) have been reported at US Interagency Monitoring of PROtected Visual Environments (IMPROVE) network from 1990 to 2004, consistent with the phase-in of cleaner engines, residential biomass burning technologies, and prescribed burning practices. EC trends for the past decade are examined due to an upgrade of IMPROVE carbon instruments and the thermal/optical analysis protocol since 2005. Filter reflectance (τ_R) values measured as part of the carbon analysis were retrieved from archived data and compared with EC for 65 sites with more complete records within 2000–2009. EC–τ_R relationships suggest minor changes of EC quantified by the original and upgraded instruments for most IMPROVE samples. EC and τ_R show universal decreasing trends across the US. The EC and τ_R trends are correlated, with national average downward rates (relative to the 2000–2004 baseline medians) of 4.5% yr⁻¹ for EC and 4.1% yr⁻¹ for τ_R. The consistency between independent EC and τ_R measurements adds to the weight of evidence that EC reductions are real rather than an artifact of changes to the measurement process.