<p>The Winter Fog Experiment (WiFEX) was an intensive field campaign conducted at Indira Gandhi International Airport (IGIA) Delhi, India, in the Indo-Gangetic Plain during the winter of 2017–2018. Here, we report the first comparison in South Asia of the high temporal resolution measurements of NH<sub>3</sub> along with water-soluble inorganic ions in PM<sub>2.5</sub> (Cl<sup>-</sup>, NO<sub>3</sub><sup>-</sup>, SO<sub>4</sub><sup>2-</sup> and NH<sub>4</sub><sup>+</sup>) and corresponding precursor gases (HCl, SO<sub>2</sub>, HONO, and HNO<sub>3</sub>) made at the WiFEX research site, using the Monitor for AeRosols and Gases in Ambient Air (MARGA) and high-resolution simulations with Weather Research and Forecasting model coupled with chemistry (WRF-Chem). The hourly measurements were used to investigate how well the model captures the temporal variation of gaseous and particulate water-soluble species and gas-to-particle partitioning of NH<sub>3</sub>, using the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) aerosol scheme. The model frequently simulated higher NH<sub>3</sub> and lower NH<sub>4</sub><sup>+</sup> concentrations than the observations, while total NH<sub>x</sub> values/variability agreed well with the observations. Under the winter conditions of Delhi, high concentrations of hydrochloric acid (HCl) in the ambient air are found to dominate the gas-to-particle partitioning, as NH<sub>3</sub> is usually in excess. The default model set-up of WRF-Chem excludes anthropogenic HCl emissions, so sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) dominates the gas-to-particle partitioning with NH<sub>3</sub> during the simulation period. The sensitivity experiments, including HCl emissions in the model, showed that the inclusion of HCl emissions improves the simulated gas-to-particle conversion rate of ammonia by 24 % (as indicated by NH<sub>4</sub><sup>+</sup> concentrations) while reducing the bias in gas phase NH<sub>3</sub> by 10 %. Nevertheless, even with waste burning HCl emissions included, we find that WRF-Chem still overestimates sulfur dioxide (SO<sub>2</sub>) and nitrate (NO<sub>3</sub><sup>−</sup>) formation and underestimates sulfate (SO<sub>4</sub><sup>2−</sup>), nitrous acid (HONO), nitric acid (HNO<sub>3</sub>), and HCl concentration in which it interacts, thus limit the gas-to-particle conversion of NH<sub>3</sub> to NH<sub>4</sub><sup>+</sup> in the model. This indicates that modeling of ammonia requires a correct chemistry mechanism with accurate emission inventories for the industrial HCl emissions.</p>