Identification of Major Sources of Atmospheric NH in an Urban Environment in Northern China During Wintertime.

To assess the relative contributions of traffic emission and other potential sources to high levels of atmospheric ammonia (NH) in urban areas in the wintertime, atmospheric NH and related pollutants were measured at an urban site, ∼300 m from a major traffic road, in northern China in November and December 2015. Hourly average NH varied from 0.3 to 10.8 ppb with an average of 2.4 ppb during the campaign. Contrary to the common perspective in literature, traffic emission was demonstrated to be a negligible contributor to atmospheric NH. Atmospheric NH correlated well with ambient water vapor during many time periods lasting from tens of hours to several days, implying NH released from water evaporation is an important source. Emissions from local green space inside the urban areas were identified to significantly contribute to the observed atmospheric NH during ∼60% of the sampling times. Evaporation of predeposited NH through wet precipitation combined with emissions from local green space likely caused the spikes of atmospheric NH mostly occurring 1-4 h after morning rush hours or after and during slight shower events. There are still ∼30% of the data samples with appreciable NH level for which major contributors are yet to be identified.