NH3 emissions from the human body in central Tokyo decreased during the COVID-19 pandemic lockdown

Atmospheric NH3 is emitted from various sources, such as agriculture, automobile exhaust, industrial activities, and human bodies (dermal and respiratory). Although the contribution from automobile exhaust to NH3 con-centrations in urban areas is well understood, the contribution from human body emissions has not been clar-ified, even in densely populated areas far from agricultural and industrial sources. Tokyo, Japan, is the one of the most populated megacities in the world. During April and May 2020, socioeconomic activities in Tokyo were severely restricted to prevent the spread of the coronavirus disease (COVID-19), resulting in significant changes in human flow and automobile traffic. The sudden decrease in emissions presented a unique opportunity to clarify the contributions of human body emissions to NH3 levels. NH3 concentrations observed in central Tokyo from 2018 to 2021 decreased significantly, by about 20%, during the lockdown in 2020. No significant differ-ences were found in wind speed or rainfall during the same period. Air quality model simulations of NH3 con-centrations in the area were performed for 2019 and 2020. The simulation results for 2019 reproduced the observed NH3 concentrations and temporal variations well. However, the simulation results were overestimated for 2020. To evaluate the differences in emissions' contributions during the lockdown, sensitivity tests of NH3 emissions' strength were conducted for 2020. The simulation in which NH3 emissions from automobiles and human bodies were reduced by 20% and 80%, respectively, reproduced the observed results well. These results indicate that emissions from human bodies and automobile traffic contribute significantly to NH3 concentrations in central Tokyo. The contributions from human body emissions to NH3 concentrations revealed in this study should be applied to update emissions factors for human bodies; clarifying human emissions factors under various conditions will improve the accuracy of air quality simulations over densely populated megacities.