| Summary: | A semi-continuous monitoring system, a parallel plate wet denuder and particle into liquid sampler coupled with ion chromatography (PPWD-PILS-IC), was used to measure the hourly precursor gases and water-soluble inorganic ions in ambient particles smaller than 2.5 µm in diameter (PM<sub>2.5</sub>) for investigating the thermodynamic equilibrium of aerosols using the ISORROPIA-II thermodynamic equilibrium model. The 24-h average PPWD-PILS-IC data showed very good agreement with the daily data of the manual 5 L/min porous-metal denuder sampler with R<sup>2</sup> ranging from 0.88 to 0.98 for inorganic ions (NH<sub>4</sub><sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, and Cl<sup>−</sup>) and 0.89 to 0.98 for precursor gases (NH<sub>3</sub>, HNO<sub>3</sub>, HONO, and SO<sub>2</sub>) and slopes ranging from 0.94 to 1.17 for ions and 0.87 to 0.95 for gases, respectively. In addition, the predicted ISORROPIA-II results were in good agreement with the hourly observed data of the PPWD-PILS-IC system for SO<sub>4</sub><sup>2−</sup> (R<sup>2</sup> = 0.99 and slope = 1.0) and NH<sub>3</sub> (R<sup>2</sup> = 0.97 and slope = 1.02). The correlation of the predicted results and observed data was further improved for NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup> with the slope increasing from 0.90 to 0.96 and 0.95 to 1.09, respectively when the HNO<sub>2</sub> and NO<sub>2</sub><sup>−</sup> were included in the total nitrate concentration (TN = [NO<sub>3</sub><sup>−</sup>] + [HNO<sub>3</sub>] + [HONO] + [NO<sub>2</sub><sup>−</sup>]). The predicted HNO<sub>3</sub> data were comparable to the sum of the observed [HNO<sub>3</sub>] and [HONO] indicating that HONO played an important role in the thermodynamic equilibrium of ambient PM<sub>2.5</sub> aerosols but has not been considered in the ISORROPIA-II thermodynamic equilibrium model.
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