Time-resolved inorganic chemical composition of fine aerosol and associated precursor gases over an urban environment in western India: Gas-aerosol equilibrium characteristics

- Time-resolved aerosol composition and trace gas concentrations were measured.
- We examine gas-aerosol equilibrium characteristics using ISORROPIA II model.
- NH3-NH4+ are in equilibrium with measured particulate and gas composition.
- HCl and HNO3 deviate from thermodynamic equilibrium possibly due to uptake by dust.

Inorganic ionic constituents (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−) of PM2.5 and associated trace gases (NH3, HNO3 and HCl) were measured simultaneously by Ambient Ion Monitor - Ion Chromatograph (AIM-IC) system with a time resolution of one hour at an urban location in semi-arid region of western India during summer and winter. The average NH3, HNO3 and HCl concentrations were 11.6 ± 5.0, 2.9 ± 0.8 and 0.15 μg m−3, respectively, during winter. During summer, NH3 and HNO3 concentrations were of similar magnitude, whereas HCl concentration was less than ∼0.03 μg m−3. NH3 concentration exhibited a distinct diurnal variation during both seasons. However, HNO3 did not show a specific diurnal trend during the observation period in both seasons. The data obtained were used to study gas-aerosol equilibrium characteristics using a thermodynamic equilibrium model, ISORROPIA II. The results suggest that NH3 exists in equilibrium between measured fine-mode particle and gas phase with a systematic bias of ∼14%, whereas HCl and HNO3 deviate significantly from the modelled data. These observations have implications on thermodynamic equilibrium assumptions used for estimating various aerosol parameters such as liquid water content, pH, etc., thus causing significant bias in chemical transport model results over the study region.