Citation:

Chen, X., Johnt Walker, AND Chris Geron. Chromatography related performance of the Monitor for Aerosols and Gases in Ambient Air (MARGA): laboratory and field based evaluation. Atmospheric Measurement Techniques. Copernicus Publications, Katlenburg-Lindau, Germany, 10:3893-3908, (2017). https://doi.org/10.5194/amt-10-3893-2017

Impact/Purpose:

Program and regional offices need to better understand the contribution of ammonium and nitrate aerosol to total PM2.5 to assess the effectiveness of the Primary NAAQS for PM and secondary NAAQS for NOx/SOx/PM. Additionally, States need this information to develop implementation plans for addressing PM attainment. To support these needs, time-resolved simultaneous measurements of the gas and aerosol components of the ammonium-sulfate-nitrate system are required to investigate inorganic aerosol characteristics (e.g., phase partitioning, acidity) and formation processes and to quantify the dry component of nitrogen deposition. The purpose of this study is to assess the performance characteristics of a commercially available monitor (MARGA) for semi continuous measurements of speciated inorganic gases and aerosols. Though differences between the MARGA and other measurement systems have been documented, the extent to which the differences may be attributable solely to chromatogram processing methods has not been evaluated. Using laboratory and field measurements, analytical accuracy, precision, and method detection limit derived using the commercial MARGA automated software was compared to a new chromatography procedure. Our analysis revealed issues with accuracy and precision resulting from mis-identification and mis-integration of chromatograph peaks by the MARGA automated software as well as a systematic bias at low concentrations for certain compounds. We demonstrate that positively biased nitrate and sulfate measurements result in overestimation of aerosol acidity and introduce non-trivial errors to ion balances of inorganic aerosol. Our results show that calibration by multi-level liquid standards is required to adequately control analytical accuracy. Findings from this work can be used to interpret differences between the MARGA and other measurement systems observed during previous studies and will guide the operation of the MARGA in the future. Our results will assist EPA program and regional offices in the proper application of MARGA data and to better understand the comparability of aerosol measurement systems for air quality assessments and validation of chemical transport models.

Description:

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 limit derived using the commercial MARGA software was compared to an alternative chromatography procedure consisting of a custom Java script to reformat raw MARGA conductivity data and Chromeleon (Thermo Scintific Dionex) software for peak integration. Our analysis revealed issues with accuracy and precision resulting from mis-identification and mis-integration 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 new 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 related 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.

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