22 Apr 2022
22 Apr 2022
Status: this preprint is currently under review for the journal ACP.

Hydrochloric acid emission dominates inorganic aerosol formation from ammonia in the Indo-Gangetic Plain during winter

Pooja V. Pawar1,6, Sachin D. Ghude1, Gaurav Govardhan1, Prodip Acharja1, Rachana Kulkarni2, Rajesh Kumar3, Baerbel Sinha4, Vinayak Sinha4, Chinmay Jena5, Preeti Gunwani1, Tapan Kumar Adhya6, Eiko Nemitz7, and Mark A. Sutton7 Pooja V. Pawar et al.
  • 1Indian Institute of Tropical Meteorology (IITM), Ministry of Earth Sciences, Pune, India
  • 2Savitribai Phule Pune University, Pune, India
  • 3National Center for Atmospheric Research (NCAR), Boulder, CO, USA
  • 4Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Mohali, Punjab, India
  • 5India Meteorological Department (IMD), Ministry of Earth Sciences, Lodhi Road, New Delhi, India
  • 6Kalinga Institute of Industrial Technology (KIIT), Bhubaneshwar, India
  • 7UK Centre for Ecology & Hydrology (UKCEH), Edinburgh, UK

Abstract. 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 NH3 along with water-soluble inorganic ions in PM2.5 (Cl-, NO3-, SO42- and NH4+) and corresponding precursor gases (HCl, SO2, HONO, and HNO3) 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 NH3, using the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) aerosol scheme. The model frequently simulated higher NH3 and lower NH4+ concentrations than the observations, while total NHx 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 NH3 is usually in excess. The default model set-up of WRF-Chem excludes anthropogenic HCl emissions, so sulfuric acid (H2SO4) dominates the gas-to-particle partitioning with NH3 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 NH4+ concentrations) while reducing the bias in gas phase NH3 by 10 %. Nevertheless, even with waste burning HCl emissions included, we find that WRF-Chem still overestimates sulfur dioxide (SO2) and nitrate (NO3) formation and underestimates sulfate (SO42−), nitrous acid (HONO), nitric acid (HNO3), and HCl concentration in which it interacts, thus limit the gas-to-particle conversion of NH3 to NH4+ in the model. This indicates that modeling of ammonia requires a correct chemistry mechanism with accurate emission inventories for the industrial HCl emissions.

Pooja V. Pawar et al.

Status: open (until 08 Jun 2022)

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  • RC1: 'Comment on acp-2022-237', Anonymous Referee #1, 17 May 2022 reply

Pooja V. Pawar et al.

Pooja V. Pawar et al.


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Short summary
In this study, wintertime measurements of atmospheric ammonia (NH3), ammonium, and total ammonia are compared with the ground-based measurements (MARGA) and WRF-Chem model in the Indo-Gangetic Plain (IGP) region. Our observations and model indicate high concentrations of hydrochloric acid (HCl) in the ambient air are found to dominate the gas-to-particle partitioning of NH3. Addition of HCl emissions in the model, we report improvements found in the gas-to-particle conversion of NH3.