Ammonia and inorganic acid gases emitted from livestock and poultry operations, manure treatment, handling and application can affect air quality by the formation of secondary fine particles. The process of gas-to-particle conversion (GTPC) of relatively short-lived gaseous ammonia to more persistent fine particles can affect local and regional air quality far away from the agricultural sources. This study focused on understanding the phenomenon and related problems of the GTPC process between ammonia, acid gases and fine particles in the atmosphere. We discuss the knowledge of PM measurement technologies with their uncertainties and introduce the most recent aerosol models available developed to simulate the equilibrium partitioning of inorganic compounds between the gas and aerosol phases. We discuss the development and evaluate progress on ambient PM research using 3D air quality models and demonstrate the importance of the GTPC process concerning the contribution of ammonia on fine PM formation in agricultural areas.