Reply to Comment on "Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from 15N-Stable Isotope in Size-Resolved Aerosol Ammonium".

Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from N‐Stable Isotope in Size-Resolved Aerosol Ammonium” P et al. presented the isotopic measurements of sizeresolved aerosols in Beijing, summarizing that fossil fuelrelated ammonia emissions (including traffic, coal combustion and power plants ammonia slip) have overtaken agricultural activities as the dominant source of atmospheric ammonia during hazy days of 2013. This study is timely given that the important role of ammonia in terms of curbing China’s PM2.5 pollution. The study provides new and interesting insights into the topic of “missing sources of ammonia in urban atmospheres” and the results generally agree with our earlier studies. However, we have to point out that the article has a few severe problems, which may lead to wrong conclusions and therefore could potentially mislead China’s policy of ammonia reduction in the future. Different from the conclusions by Pan et al., we propose our viewpoints as follows: