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

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 PM₂.₅ water-soluble ions (WSIs), and simulations from a thermodynamic equilibrium model. On average, the sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺) components (SNA) contributed ∼90% of WSI concentrations, which in turn made up about 50% of the PM₂.₅ mass. During the entire observation period, PM₂.₅ 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 PM₂.₅ increased, whereas the SO₄²⁻ 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 SO₄²⁻ (i.e. a saturated factor in visibility reduction) and excess NH₄⁺ amidst a NH₃-rich environment. As a result, PM₂.₅ 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⁻, PM₂.₅, 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 PM₂.₅ composition.