Evaluating IMPROVE PM_2.5 element measurements.

Area of Science:

• Environmental Science
• Analytical Chemistry
• Atmospheric Science

Background:

• The Interagency Monitoring of PROtected Visual Environments (IMPROVE) network has analyzed airborne particulate matter (PM) elemental composition since 1988.
• X-ray fluorescence (XRF) is a key technique used for chemical speciation of PM samples.
• A transition to new XRF instruments occurred in 2011, necessitating performance evaluation.

Purpose of the Study:

• To evaluate the performance of new XRF instruments against older ones within the IMPROVE network.
• To identify and quantify discrepancies in elemental measurements due to the instrumentation change.
• To inform data users about potential impacts on longitudinal air quality analyses.

Main Methods:

• Archived PM samples from three IMPROVE sites were re-analyzed using the new XRF instruments.
• Measurements from new and old XRF instruments were compared for various elements.
• XRF sulfur measurements were compared with collocated ion chromatography sulfate data.
• Elemental ratios were compared to expected values for soil-derived PM.

Main Results:

• Good agreement (slopes within 10%) was observed for most elements above detection limits (e.g., S, Cl, K, Ti, V, Mn, Fe, Cu, Zn, Se, Pb).
• Significant differences were found for light elements (Na, Mg) at a coastal site due to sea salt interference and old correction methods.
• New instruments show improved precision for sulfur but a slight underestimation; they better resolve Al and Si peaks.
• Site-specific particle compositions revealed different measurement interferences.

Conclusions:

• The change in XRF instrumentation within the IMPROVE network has implications for longitudinal data analysis.
• Light elements (Na, Mg) and sulfur measurements are particularly affected by the method change.
• Users should consider these findings when interpreting data related to sea salt and crustal sources, especially for historical trend analyses.