A formal intercomparison of fine particle ammonium was conducted involving three real-time semi-continuous measurement systems and an integrated filter measurement. The three semi-continuous sampling techniques are 1) the Particle-into-Liquid Sampler-Ion Chromatography (PILS-IC), 2) the Quadropole-Aerosol Mass Spectrometer (Q-AMS), which is able to identify the particle size distribution and chemical composition alternately, and 3) the Wet Scrubbing-LOng Path Absorption Photometer (WS-LOPAP) that is based on aqueous scrubbing through a glass frit impactor with analysis by long path absorption spectrophotometry. The 24 hour co-located filter based measurements are from EPA Speciation Trends Network (STN) protocol samplers with 1-in-3 day sampling. The measurements were made at the rural site of Pinnacle State Park (PSP) in upstate New York from July 18 through August 8 in 2004 and the urban site of Queens College (QC) in New York City from January 9 through February 6 in 2004. Intercomparison of hourly averaged particulate ammonium measurements from the three semi-continuous instruments shows correlations with recoveries (forced zero intercept) between 0.71 and 1.04 with all R2 values above 0.82 at PSP. The comparable recovery at QC is 1.00 between Q-AMS and PILS-IC with R~2 of 0.77.While there is an overall agreement among the methods, some systematic and episodic differences are also noted. Sample handling, collection efficiency and seasonal differences are considered as possible reasons for the observed differences.
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