Investigating Chemical and Thermodynamic Conditions that Determine the Aerosol Inorganic Nitrate Size Distribution: Insights from Speciated PM2.5 and PM10 Hourly Datasets from an Urban Site

Abstract:

Nitric acid (HNO₃), the gas-phase precursor to aerosol nitrate is known to rapidly transfer to aerosols where NH₄⁺ is in excess to SO₄^2- present in the aerosol, but the HNO₃ is also subject to the slower uptake onto sea salt and dust laden particles. Understanding the competition between these routes is necessary to predict the NO₃^- distribution and impact on aerosols. In this study, we investigated the conditions leading to predominant fine or coarse mode aerosol nitrate using an hourly MARGA 2S dataset from an urban site in Hong Kong. The hourly dataset of inorganic ions (SO₄^2-, NH₄⁺, NO₃^-, Na⁺, Cl^-, Ca²⁺, K⁺, Mg²⁺) in 2 size ranges (fine, < 2.5 μm; fine+coarse, < 10 μm) and water-soluble gases (HNO₃, HCl, and NH₃) spanning more than 1 year provides a rich trove for analyzing aerosol nitrate chemistry and the underlying mechanisms that ultimately determine the fraction of NO₃^- in the fine mode. The urban site in this study is initially characterized for seasonal environmental conditions and the aerosol chemical composition. The relationship between excess NH₄⁺ and NO₃^- in the fine mode is detailed and contrasted with the influence on fine mode NO₃^- from uptake on sea salt and dust, which is typically relegated as a ‘coarse-mode’ mechanism. The distribution of NO₃^- in the fine and coarse modes is compared with the distribution of the other inorganic ions, where sea-salt ion (Na⁺, Mg²⁺) distributions yield the highest explained variability for the nitrate distributions. As a complement to that finding, the cation equivalency (excluding NH₄⁺) in the coarse mode proves to be a crucial factor in leveraging the distribution away from fine mode nitrate. The uptake potential of the water-soluble gases is used to drive a mass transfer model and compare with thermodynamic equilibrium results. In the modeling, the partitioning cycles of fine and coarse mode aerosol nitrate highlight the dynamic relationship between NO₃^- and Cl^- in both the fine and coarse modes, where the replacement of Cl^- with NO₃^- is much faster in the fine mode due to shorter mass transfer time scales and more efficient evaporation of the Cl^- in smaller particles.