Chromatography related performance of the Monitor for AeRosols and GAses in ambient air (MARGA): laboratory and field based evaluation
Xi Chen, John T. Walker, and Chris Geron
National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, 27711, USA
Received: 11 Mar 2017 – Accepted for review: 06 Apr 2017 – Discussion started: 18 Apr 2017
Abstract. Evaluation of the semi-continuous Monitor for AeRosols and GAses in ambient air (MARGA, Metrohm Applikon B.V.) was conducted with an emphasis on examination of accuracy and precision associated with processing of chromatograms. Using laboratory standards and atmospheric measurements, analytical accuracy, precision, and method detection limits derived using the commercial MARGA software were compared to an alternative chromatography procedure consisting of a custom Java script to reformat raw MARGA conductivity data and Chromeleon (Thermo Scientific Dionex) software for peak integration. Our analysis revealed issues with accuracy and precision resulting from misidentification and misintegration of chromatograph peaks by the MARGA automated software as well as a systematic bias at low concentrations for anions. Reprocessing and calibration of raw MARGA data using the alternative chromatography method lowered method detection limits and reduced variability (precision) between parallel sampler boxes. Instrument performance was further evaluated during a one-month intensive field campaign in the fall of 2014, including analysis of diurnal patterns of gaseous and particulate water soluble species (NH3, SO2, HNO3, NH4+, SO42− and NO3−), gas-to-particle partitioning, and particle neutralization state. At ambient concentrations below ~ 1 µg/m3, concentrations determined using the MARGA software are biased +30 % and +10 % for NO3− and SO42−, respectively, compared to concentrations determined using the alternative chromatography procedure. Differences between the two methods increase at lower concentrations. We demonstrate that positively biased NO3− and SO42− measurements result in overestimation of aerosol acidity and introduce non-trivial errors to ion balances of inorganic aerosol. Though the source of the bias is uncertain, it is not corrected by the MARGA online single-point internal LiBr standard. Our results show that calibration and verification of instrument accuracy by multi-level external standards is required to adequately control analytical accuracy. During the field intensive, the MARGA was able to capture rapid compositional changes in PM2.5 due to changes in meteorology and air mass history relative to known source regions of PM precursors, including a fine NO3− aerosol event associated with intrusion of arctic air into the southeast U.S.
Chen, X., Walker, J. T., and Geron, C.: Chromatography related performance of the Monitor for AeRosols and GAses in ambient air (MARGA): laboratory and field based evaluation, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2017-68, in review, 2017.