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<p>Atmospheric ammonium (NH<sub>4</sub><sup>+</sup>)-containing aerosols significantly impact air quality and visibility, yet formation mechanisms in ammonia-rich agricultural environments remain poorly understood due to complex humidity-dependent conversion processes. Agricultural regions exhibit ammonia-excess conditions with substantially higher NH<sub>3</sub> concentrations, creating distinct inorganic aerosol formation pathways beyond traditional acid–base neutralization mechanisms. This study investigated NH<sub>4</sub><sup>+</sup> aerosol formation mechanisms though continuous hourly measurements at an agricultural site during peak NH<sub>3</sub> emission periods. Water-soluble inorganic ions, gaseous precursors, and light scattering coefficients were measured to investigate humidity-dependent NH<sub>4</sub><sup>+</sup> formation and optical properties in ammonia-rich conditions. Agricultural environments demonstrated ammonia-excess conditions during stagnant meteorological periods, enabling investigation of NH<sub>4</sub><sup>+</sup> formation beyond stoichiometric acid–base neutralization. NH<sub>3</sub>-to-NH<sub>4</sub><sup>+</sup> conversion efficiency peaked at moderate relative humidity (60–80% RH) but declined at higher humidity levels (&gt; 80% RH), reflecting distinct chemical regimes in semi-deliquesced versus fully deliquesced aerosols. Agricultural aerosols exhibited significant residual NH<sub>4</sub><sup>+</sup> formation and enhanced light scattering efficiency, primarily due to NH<sub>4</sub>NO<sub>3</sub> prevalence and residual NH<sub>4</sub><sup>+</sup> contributions. Multivariate linear regression analysis demonstrated strong performance (R<sup>2</sup> &gt; 0.82) in quantifying aerosol optical properties. Combined contributions of NH<sub>4</sub>NO<sub>3</sub> and residual NH<sub>4</sub><sup>+</sup> accounted for approximately 40–49% of total light scattering, with dramatic enhancement during stagnant conditions. These findings indicate the humidity-dependent NH<sub>3</sub> conversion mechanisms in agricultural environments and suggest that NH<sub>3</sub> emissions controls might be particularly important in ammonia-rich regions. Further multi-site and multi-season studies are needed to validate the generalizability of these mechanisms for regional air quality management.</p>

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Humidity-dependent ammonium formation and enhanced light scattering in agricultural aerosols

  • 期刊论文

  • 发表日期：

    2025年9月24日

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  • Chien-Hao Lin,
  • Ling-Ya Chen,
  • Ting-Yu Chiang,
  • Sally C. W. Tai,
  • Shih-Yu Chang

摘要

Atmospheric ammonium (NH4+)-containing aerosols significantly impact air quality and visibility, yet formation mechanisms in ammonia-rich agricultural environments remain poorly understood due to complex humidity-dependent conversion processes. Agricultural regions exhibit ammonia-excess conditions with substantially higher NH3 concentrations, creating distinct inorganic aerosol formation pathways beyond traditional acid–base neutralization mechanisms. This study investigated NH4+ aerosol formation mechanisms though continuous hourly measurements at an agricultural site during peak NH3 emission periods. Water-soluble inorganic ions, gaseous precursors, and light scattering coefficients were measured to investigate humidity-dependent NH4+ formation and optical properties in ammonia-rich conditions. Agricultural environments demonstrated ammonia-excess conditions during stagnant meteorological periods, enabling investigation of NH4+ formation beyond stoichiometric acid–base neutralization. NH3-to-NH4+ conversion efficiency peaked at moderate relative humidity (60–80% RH) but declined at higher humidity levels (> 80% RH), reflecting distinct chemical regimes in semi-deliquesced versus fully deliquesced aerosols. Agricultural aerosols exhibited significant residual NH4+ formation and enhanced light scattering efficiency, primarily due to NH4NO3 prevalence and residual NH4+ contributions. Multivariate linear regression analysis demonstrated strong performance (R2 > 0.82) in quantifying aerosol optical properties. Combined contributions of NH4NO3 and residual NH4+ accounted for approximately 40–49% of total light scattering, with dramatic enhancement during stagnant conditions. These findings indicate the humidity-dependent NH3 conversion mechanisms in agricultural environments and suggest that NH3 emissions controls might be particularly important in ammonia-rich regions. Further multi-site and multi-season studies are needed to validate the generalizability of these mechanisms for regional air quality management.