The impact of ammonia (NH3) emissions from animal agriculture on the secondary formation of inorganic fine particulate matter (i.e., iPM2.5) has become of great public concern. The formation of iPM2.5 from NH3 is known as the gas–particle partitioning of gaseous NH3 and aerosol ammonium (NH4+), which is assumed to be in a thermodynamic equilibrium. This research aimed to gain an in-depth understanding of the impact of ambient NH3 on secondary iPM2.5 by analyzing the PM2.5 mass closure, atmospheric chemical conditions, and the gas particle partitioning of NH3-NH4+ in the near field of a poultry production unit in North Carolina. Samples of precursor gases (i.e., NH3, SO2, NO2) to iPM2.5 and PM2.5 were taken on this poultry production unit at four sampling stations in four wind directions through summer, autumn and winter seasons to determine gas concentrations and PM2.5 chemical compositions. It was discovered that this rural site contained low ambient concentrations of iPM2.5 precursor gases, and PM2.5 composition was dominated by organic carbon (OC) (80% to 94%) while iPM2.5 fraction was insignificant (0% to 2%). Low availability of H2SO4 and HNO3 gases (from SO2 and NO2 conversions) limited NH3 neutralization potential and iPM2.5 formation; moreover, high OC fraction may inhibit NH4+ formation. With the field measurements of ambient temperature, humidity, precursor gases and PM2.5 chemical speciation data, the ISORROPIA-II thermodynamic equilibrium model was used to conduct the sensitivity analysis, and we found that iPM2.5 was the most sensitive to increasing total HNO3 (gas + aerosol) at low temperatures. The formation potential of iPM2.5 at this rural site was at its highest during the wintertime when SO2 was extremely low.
Stratton et al. (Wed,) studied this question.