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.