Laboratory-Based Estimation of Ammonia-Derived Secondary PM2.5 for Air Quality Assessment of Concentrated Animal Feeding Operations

Ammonia (NH3) emissions from concentrated animal feeding operations (CAFOs) are recognized contributors to secondary fine particulate matter (PM2.5) formation, yet empirically derived secondary PM2.5 emission factors applicable to livestock operations remain limited. This study investigated NH3-derived secondary PM2.5 formation under controlled laboratory conditions using a PTFE flow reactor in which NH3 was reacted with sulfur dioxide (SO2) across ammonia-rich NH3:SO2 ratios, with and without zero air. The resulting aerosols were characterized using gravimetric analysis, elemental analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), and particle size distribution (PSD) measurements. The recovered particles were dominated by inorganic ammonium–sulfur species, with FTIR and elemental trends indicating sulfite-related intermediates under no-zero-air conditions and more oxidized ammonium–sulfur products under oxygenated conditions. Accounting for both filter-collected and wall-deposited particles, unit particulate emission factors normalized to ammonia input were derived. Size-based apportionment using PSD data indicated that approximately 76.6% of the recovered particulate mass was within the PM2.5 size range. Scaling the experimentally derived unit emission factors using literature-based ammonia emission rates yielded an estimated secondary PM2.5 emission factor of 0.351 ± 0.084 g PM2.5 per animal head per day for cattle feedlots, corresponding to approximately 3–4% of reported total PM2.5 emissions. Because the experimental system isolates NH3–SO2 interactions under idealized conditions and does not represent full atmospheric chemistry, the derived values should be interpreted as screening-level estimates of NH3-derived secondary PM2.5 formation potential intended to support comparative air quality assessments of CAFOs rather than direct predictions of ambient PM2.5 concentrations.