Underestimation of sulfate concentration in PM2.5 using a semi-continuous particle instrument based on ion chromatography

Recently, a variety of ion-chromatography-based semi-continuous particle instruments such as the Dionex gas particle ion chromatograph (GP-IC), wet-annular denuder/steam-jet aerosol collector (WAD/SJAC), particle-into-liquid sampler with ion chromatograph (PILS-IC), gas and aerosol monitoring system (GAMS) have been introduced for measuring particle chemical components in the atmosphere. It has been reported that sulfate concentrations in PM2.5 measured by these semi-continuous particle instruments correlate well with those measured by other semi-continuous instruments such as the Aerodyne aerosol mass spectrometer (AMS), R&P 8400S and R&P 8400N analyzers, and Thermo model 5020 sulfate particle analyzer and at times exhibit a unity slope. However, the sulfate concentration measured by some of these semi-continuous instruments has been reported to be underestimated by 20-50%, as compared to that in PM2.5 filter samples. In this study, numerous potential causes for underestimation of the sulfate concentrations by the GP-IC were investigated. We found a 30-40% negative artifact arising from an improper calibration procedure inherent within the instrument design. An improved calibration procedure was developed and it substantially reduced the sulfate concentration difference between the GP-IC measurements and PM2.5 filter samples from 36% to 5%.