Reply To Comment On "Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources During Severe Haze Episodes: Evidence From N-15-Stable Isotope In Size-Resolved Aerosol Ammonium"

The reduction of ammonia (NH3) emissions is urgently needed due to its role in aerosol nucleation and growth causing haze formation during its conversion into ammonium (NH4+). However, the relative contributions of individual NH3 sources are unclear, and debate remains over whether agricultural emissions dominate atmospheric NH3 in urban areas. Based on the chemical and isotopic measurements of size-resolved aerosols in urban Beijing, China, we find that the natural abundance of N-15 (expressed using delta N-15 values) of NH4+ in fine particles varies with the development of haze episodes, ranging from -37.1 parts per thousand to -21.7 parts per thousand during clean/dusty days (relative humidity: 40%), to -13.1 parts per thousand to +5.8 parts per thousand during hazy days (relative humidity: 70-90%). After accounting for the isotope exchange between NH3 gas and aerosol NH4+, the delta N-15 value of the initial NH3 during hazy days is found to be -14.5 parts per thousand to -1.6 parts per thousand, which indicates fossil fuel based emissions. These emissions contribute 90% of the total NH3 during hazy days in urban Beijing. This work demonstrates the analysis of delta N-15 values of aerosol NH4+ to be a promising new tool for partitioning atmospheric NH3 sources, providing policy makers with insights into NH3- emissions and secondary aerosols for regulation in urban environments.