Secondary inorganic aerosol chemistry and its impact on atmospheric visibility over an ammonia-rich urban area in Central Taiwan

Li-Hao Young; Ta-Chih Hsiao; Stephen M Griffith; Ya-Hsin Huang; Hao-Gang Hsieh; Tang-Huang Lin; Si-Chee Tsay; Yu-Jung Lin; Kuan-Lin Lai; Neng-Huei Lin; Wen-Yinn Lin

doi:10.1016/j.envpol.2022.119951

Secondary inorganic aerosol chemistry and its impact on atmospheric visibility over an ammonia-rich urban area in Central Taiwan

Environ Pollut. 2022 Nov 1:312:119951. doi: 10.1016/j.envpol.2022.119951. Epub 2022 Aug 21.

Authors

Affiliations

¹ Department of Occupational Safety and Health, China Medical University, 100, Sec. 1, Jingmao Rd., Beitun Dist., Taichung, 406040, Taiwan. Electronic address: lhy@mail.cmu.edu.tw.
² Graduate Institute of Environmental Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan.
³ Department of Atmospheric Sciences, National Central University, 300, Zhongda Rd., Zhongli Dist., Taoyuan, 320317, Taiwan.
⁴ Department of Occupational Safety and Health, China Medical University, 100, Sec. 1, Jingmao Rd., Beitun Dist., Taichung, 406040, Taiwan.
⁵ Center for Space and Remote Sensing Research, National Central University, 300, Zhongda Rd., Zhongli Dist., Taoyuan, 320317, Taiwan.
⁶ NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA.
⁷ Institute of Environmental Engineering and Management, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei, 106344, Taiwan.

PMID: 36002097
DOI: 10.1016/j.envpol.2022.119951

Abstract

This study investigated the hourly inorganic aerosol chemistry and its impact on atmospheric visibility over an urban area in Central Taiwan, by relying on measurements of aerosol light extinction, inorganic gases, and PM_2.5 water-soluble ions (WSIs), and simulations from a thermodynamic equilibrium model. On average, the sulfate (SO₄^2-), nitrate (NO₃^-), and ammonium (NH₄⁺) components (SNA) contributed ∼90% of WSI concentrations, which in turn made up about 50% of the PM_2.5 mass. During the entire observation period, PM_2.5 and SNA concentrations, aerosol pH, aerosol liquid water content (ALWC), and sulfur and nitrogen conversion ratios all increased with decreasing visibility. In particular, the NO₃^- contribution to PM_2.5 increased, whereas the SO₄^2- contribution decreased, with decreasing visibility. The diurnal variations of the above parameters indicate that the interaction and likely mutual promotion between NO₃^- and ALWC enhanced the hygroscopicity and aqueous-phase reactions conducive for NO₃^- formation, thus led to severely impaired visibility. The high relative humidity (RH) at the study area (average 70.7%) was a necessary but not sole factor leading to enhanced NO₃^- formation, which was more directly associated with elevated ALWC and aerosol pH. Simulations from the thermodynamic model depict that the inorganic aerosol system in the study area was characterized by fully neutralized SO₄^2- (i.e. a saturated factor in visibility reduction) and excess NH₄⁺ amidst a NH₃-rich environment. As a result, PM_2.5 composition was most sensitive to gas-phase HNO₃, and hence NOx, and relatively insensitive to NH₃. Consequently, a reduction of NOx would result in instantaneous cuts of NO₃^-, PM_2.5, and ALWC, and hence improved visibility. On the other hand, a substantial amount of NH₃ reduction (>70%) would be required to lower the aerosol pH, driving more than 50% of the particulate phase NO₃^- to the gas phase, thereby making NH₃ a limiting factor in shifting PM_2.5 composition.

Keywords: Aerosol chemistry; Control strategy; Gas-particle partition; Inorganic salts; Light extinction; Thermodynamic equilibrium.

MeSH terms

Aerosols / analysis
Air Pollutants* / analysis
Ammonia / analysis
Ammonium Compounds*
China
Environmental Monitoring
Gases
Nitrates / analysis
Nitrogen
Particulate Matter / analysis
Sulfates / analysis
Sulfur
Taiwan
Water / chemistry

Substances

Aerosols
Air Pollutants
Ammonium Compounds
Gases
Nitrates
Particulate Matter
Sulfates
Water
Sulfur
Ammonia
Nitrogen