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

Abstract

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