Field performance of a semi-continuous monitor for ambient PM_2.5 water-soluble inorganic ions and gases at a suburban site

To reduce sampling artifacts and to improve time-resolved measurements of inorganic aerosol system, a recently commercialized semi-continuous In-situ Gas and Aerosol Composition (IGAC) monitoring system was evaluated against a reference annular denuder system (ADS; denuder/two-stage filter pack) at a suburban site over a year, during which the average PM_2.5 was 37.0 ± 24.8 μg/m³. A suite of eight ions SO₄²⁻, NO₃⁻, Cl⁻, NH₄⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ and two gases SO₂ and NH₃ were the target species. In comparison to the reference ADS method, the IGAC performed well in measuring the major ions SO₄²⁻, NO₃⁻ and NH₄⁺, and the SO₂. For those species, the linear slopes, intercepts and R² values between the two methods all passed the performance evaluation criteria outlined by earlier similar studies. The performance of IGAC on Cl⁻, Na⁺, K⁺ and NH₃ was marginally acceptable, whereas Ca²⁺ and Mg²⁺ could not be properly evaluated due to the low concentrations (<0.2 μg/m³) and hence inadequate amount of sample size. The ionic balance of the hourly IGAC samples averaged very close to unity, as did the daily ADS samples, though the former was considerably more variable than the latter. The overall performance of the IGAC has been shown to be comparable to other similar monitors and its improvements are discussed.