Characterization and optimization of an online system for the simultaneous measurement of atmospheric water-soluble constituents in the gas and particle phases

Abstract

In this work we present the results of extensive characterization and optimization of the Ambient Ion Monitor-Ion Chromatograph (AIM-IC) system, an instrument developed by URG Corp. and Dionex Inc. for simultaneous hourly measurements of the water-soluble chemical composition of atmospheric fine particulate matter (PM_2.5) and associated precursor gases. The sampling assembly of the AIM-IC consists of an inertial particle size-selection assembly, a parallel-plate wet denuder (PPWD) for the collection of soluble gases, and a particle supersaturation chamber (PSSC) for collection of particles, in series. The analytical assembly of the AIM-IC consists of anion and cation IC units. The system detection limits were determined to be 41 ppt, 5 ppt, and 65 ppt for gas phase NH_3(g), SO_2(g), and HNO_3(g) and 29 ng m⁻³, 3 ng m⁻³, and 45 ng m⁻³ for particle phase NH₄⁺, SO₄²⁻, and NO₃⁻ respectively. From external trace gas calibrations with permeation sources, we determined that the AIM-IC is biased low for NH_3(g) (11%), SO_2(g) (19%), and HNO_3(g) (12%). The collection efficiency of SO_2(g) was found to strongly depend on the composition of the denuder solution and was found to be the most quantitative with 5 mM H₂O₂ solution for mixing ratios as high as 107 ppb. Using a cellulose membrane in the PPWD, the system responded to changes in SO_2(g) and HNO_3(g) within an hour, however for NH_3(g), the timescale can be closer to 20 h. With a nylon membrane, the instrument response time for NH_3(g) was significantly improved, becoming comparable to the responses for SO_2(g) and HNO_3(g). Performance of the AIM-IC for collection and analysis of PM_2.5 was evaluated by generating known number concentrations of ammonium sulfate and ammonium nitrate particles (with an aerodynamic diameter of 300 nm) under laboratory conditions and by comparing AIM-IC measurements to measurements from a collocated Aerosol Mass Spectrometer (AMS) during a field-sampling campaign. On average, the AIM-IC and AMS measurements agreed well and captured rapid ambient concentration changes at the same time. In this work we also present a novel inlet configuration and plumbing for the AIM-IC which minimizes sampling inlet losses, reduces peak smearing due to sample carryover, and allows for tower-height sampling from the base of a research tower.