Chloride (HCl ∕ Cl ⁻) dominates inorganic aerosol formation from ammonia in the Indo-Gangetic Plain during winter: modeling and comparison with observations

The Winter Fog Experiment (WiFEX) was an intensive field campaign conducted at Indira Gandhi International Airport (IGIA) Delhi, India, in the Indo-Gangetic Plain (IGP) during the winter of 2017–2018. Here, we report the first comparison in South Asia of high-temporal-resolution simulation of ammonia (NHinline-formula₃) along with ammonium (NHinline-formula $M6inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg3fe22ea21bb8c3940d1d54b092ea883d acp-23-41-2023-ie00004.svg8pt15ptacp-23-41-2023-ie00004.png ) and total NHinline-formula_x (i.e., NHinline-formula₃+ NHinline-formula $M9inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimge480ef7ec07e999cc23d83cf2e7b4868 acp-23-41-2023-ie00005.svg8pt15ptacp-23-41-2023-ie00005.png ) using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) and measurements made using the Monitor for AeRosols and Gases in Ambient Air (MARGA) at the WiFEX research site. In the present study, we incorporated the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) aerosol scheme into WRF-Chem. Despite simulated total NHinline-formula_x values and variability often agreeing well with the observations, the model frequently simulated higher NHinline-formula₃ and lower NHinline-formula $M12inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg9ae7496f73369cba691a3bcfb358d1a5 acp-23-41-2023-ie00006.svg8pt15ptacp-23-41-2023-ie00006.png concentrations than the observations. Under the winter conditions of high relative humidity (RH) in Delhi, hydrogen chloride (HCl) was found to promote the increase in the particle fraction of NHinline-formula $M13inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimgf1ca5762abf079d28af10bf21d382d4c acp-23-41-2023-ie00007.svg8pt15ptacp-23-41-2023-ie00007.png (which accounted for 49.5 % of the resolved aerosol in equivalent units), with chloride (Clinline-formula⁻) (29.7 %) as the primary anion. By contrast, the absence of chloride (HCl inline-formula $M15inlinescrollmathml/$ 8pt14ptsvg-formulamathimg7572a9d7afeaa92ba0e8bb6f686362bd acp-23-41-2023-ie00008.svg8pt14ptacp-23-41-2023-ie00008.png  Clinline-formula⁻) chemistry in the standard WRF-Chem model results in the prediction of sulfate (SOinline-formula $M17inlinescrollmathml{}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 13pt17ptsvg-formulamathimg31655fb078684da776f2b5262b6b028b acp-23-41-2023-ie00009.svg13pt17ptacp-23-41-2023-ie00009.png ) as the dominant inorganic aerosol anion. To understand the mismatch associated with the fraction of NHinline-formula_x in the particulate phase (NHinline-formula $M19inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg139efb60169e4a095469ebc19a8eceb3 acp-23-41-2023-ie00010.svg8pt15ptacp-23-41-2023-ie00010.png  inline-formula $M20inlinescrollmathml/$ 8pt14ptsvg-formulamathimg78c74280a32911099c6aadbec3864e34 acp-23-41-2023-ie00011.svg8pt14ptacp-23-41-2023-ie00011.png  NHinline-formula_x), we added HCl inline-formula $M22inlinescrollmathml/$ 8pt14ptsvg-formulamathimg31e788933a21fe22f46ea9f18ad5813e acp-23-41-2023-ie00012.svg8pt14ptacp-23-41-2023-ie00012.png  Clinline-formula⁻ to the model and evaluated the influence of its chemistry by conducting three sensitivity experiments using the model: no HCl, base case HCl (using a published waste burning inventory), and 3 inline-formula× base HCl run. We found that 3 inline-formula× base HCl increased the simulated average NHinline-formula $M26inlinescrollmathml{}_{\mathrm{normal\; 4}}^{+}$ 8pt15ptsvg-formulamathimg1641bbb75bb2b5e9dfa9abfc72a9cd25 acp-23-41-2023-ie00013.svg8pt15ptacp-23-41-2023-ie00013.png by 13.1 inline-formulaµg minline-formula⁻³ and NHinline-formula_x by 9.8 inline-formulaµg minline-formula⁻³ concentration while reducing the average NHinline-formula₃ by 3.2 inline-formulaµg minline-formula⁻³, which is more in accord with the measurements. Thus HCl inline-formula $M35inlinescrollmathml/$ 8pt14ptsvg-formulamathimg07d7a1dd05fd1823c5d4d7644fe5a26c acp-23-41-2023-ie00014.svg8pt14ptacp-23-41-2023-ie00014.png  Clinline-formula⁻ chemistry in the model increases total NHinline-formula_x concentration, which was further demonstrated by reducing NHinline-formula₃ emissions by a factor of 3 (inline-formula−3 inline-formula× NHinline-formula₃_EMI) in the 3 inline-formula× base HCl simulation. Reducing NHinline-formula₃ emissions in the 3 inline-formula× base HCl simulation successfully addressed the discrepancy between measured and modeled total NHinline-formula_x. We conclude that modeling the fate of NHinline-formula₃ in Delhi requires a correct chemistry mechanism accounting for chloride dynamics with accurate inventories of both NHinline-formula₃ and HCl emissions.