Experimental characterization of sulfur interference in IMPROVE aluminum and silicon XRF data

► XRF is used to analyze Si and Al concentrations in ambient PM2.5 filter samples. ► Pure ammonium sulfate fine particles are added to the ambient filters. ► Si masses from XRF re-analysis are systematically higher. Al has large changes. ► Implication: IMPROVE Si and Al values have large uncertainties when S/Fe is high. ► Impacts 50% of IMPROVE data from 2002 to 2010.