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 NH(4)(+), SO(4)(2-), and NO(3)(-) 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 H(2)O(2) 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.

A Comparison Between a Semi-Continuous Analyzer and Filter-Based Method for Measuring Anion and Cation Concentrations in PM10at an Urban Background Site in London

“…However, a few issues arose during the first two years of operation, mainly related to saturation in the column during PM pollution episodes. It has also been shown that with the use of a concentrator in the AIM analyzer, reported ambient concentrations could be biased low by 9-26%, depending on the species (Markovic et al 2012). For these reasons, a 340 ml sample loop was installed on both anion and cation systems and used for this comparison.…”

Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010

“…A detailed description of this inlet design and the performance characteristics of the AIM-IC system can be found in Markovic et al [2012]. Briefly, particle size selection was carried out using a 2.5 micron particulate matter (PM 2.5 ) inertial impactor (URG Corp., Chapel Hill, NC) on a 3 L min À1 sample flow.…”

“…The inlet was overflowed with this gas combination for more than 8 h. NH 3(g) and HONO (g) were produced from a permeation device housed in a temperature-controlled oven [e.g., Markovic et al, 2012], with HONO (g) being generated at 50% relative humidity in a NaNO 2(s) -packed tubing fed with HCl (g) from a permeation device according to the method of Febo et al [1995]. A maximum breakthrough of 4% of gas phase HONO (g) to the particle channel was observed at the highest examined HONO (g) mixing ratio of 2 ppbv, which is above the maximum HONO (g) observed in this study.…”

“…The PM 2.5 particle impactor, parallel-plate wet denuder, and particle supersaturation chamber were placed in a mobile casing that was mounted at 4.5 m above ground level (agl) on the tower, allowing analytes to be collected at ambient temperature and relative humidity (RH). The AIM-IC was calibrated for NO 2 À using mixed anion standard solutions (Dionex Combined Seven Anion Standard, P/N 056933) containing 0.02-1.0 μg of NO 2 À , which was concentrated and analyzed as described by Markovic et al [2012].…”

Improved Time-Resolved Measurements of Inorganic Ions in Particulate Matter by PILS-IC Integrated with a Sample Pre-Concentration System

“…These techniques do not involve sampling on filters and are thus free from the above sampling artifacts. To name some, the steam-jet aerosol collector (SJAC; Khlystov et al 1995), the aerosol mass spectrometer (AMS; Jayne et al 2000;Allan et al 2003), the flash volatilization nitrate monitor (R&P8400N; Stolzenburg and Hering 2000), the ambient ion monitor-ion chromatograph (AIM-IC; Markovic et al 2012), and the particle-into-liquid sampler coupled with ion chromatograph (PILS-IC; Weber et al 2001).…”