Characterization of atmospheric PM_2.5 inorganic aerosols using the semi-continuous PPWD-PILS-IC system and the ISORROPIA-II

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_2.5) 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 R² ranging from 0.88 to 0.98 for inorganic ions (NH4⁺, Na⁺, K⁺, NO₃^-, SO₄^2-, and Cl^-) and 0.89 to 0.98 for precursor gases (NH₃, HNO₃, HONO, and SO₂) 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 SO₄^2- (R² = 0.99 and slope = 1.0) and NH₃ (R² = 0.97 and slope = 1.02). The correlation of the predicted results and observed data was further improved for NH₄+ and NO₃^- with the slope increasing from 0.90 to 0.96 and 0.95 to 1.09, respectively when the HNO₂ and NO₂^- were included in the total nitrate concentration (TN = [NO₃^-] + [HNO₃] + [HONO] + [NO₂^-]). The predicted HNO3 data were comparable to the sum of the observed [HNO₃] and [HONO] indicating that HONO played an important role in the thermodynamic equilibrium of ambient PM_2.5 aerosols but has not been considered in the ISORROPIA-II thermodynamic equilibrium model.