A study of the interaction of sulfate, ammonium and nitrate on the particulate matter (PM) concentration in Taiwan using WRF-CMAQ

Aerosol particles, one major air pollution, are getting serious attention not only for health concerns, but also affecting global radiation budgets. They can be directly emitted into the atmosphere (primary aerosol), including black carbon, sea salt, dust, and some organic substances, and produced by chemical reactions in the atmosphere (secondary aerosol), such as sulfate (SO­₄^2-), nitrate (NO₃^-), and ammonium (NH₄⁺). Secondary inorganic composition accounts for a significant proportion in particulate matter (PM) and controls the pH value of PM, which can further affect the secondary organic matter formation. In this study, we focus on the secondary inorganic species, SO­₄^2-, NO₃^- and NH₄⁺ due to the complex interaction of NH₃ and HNO₃ partitioning on the aerosols containing SO­₄^2- using the Community Multiscale Air Quality (CMAQ) model with Weather Research and Forecasting (WRF) model for the meteorological conditions for December 2018. With either the decrease of SO₂ or having the SO₄^2- formation pathway off, NH₄⁺ has a similar trend as SO₄^2- while NO₃^- has a minor variation. The model analysis indicates an ammonia-saturated condition for most of Western Taiwan. With such an ammonia-saturated condition, the emission adjustment results show that the reduction of either NO_X or NH₃ emission can reduce both NO₃^- and NH₄⁺ and lead to a more significant effect on PM_2.5 than changing SO₂. The simulated ammonia is significantly higher over Western Taiwan than the observation data of the ground stations, and that promotes the dissolution of available nitric acid. Furthermore, sensitivity tests suggest that the accuracy of ammonia emission plays an important role in ruling the PM concentration via the interaction with nitric acid, which will be further discussed to reveal the feasible PM reduction via emission reduction.