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

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.