Identification of Major Sources of Atmospheric NH3 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 (NH3) in urban areas in the wintertime, atmospheric NH3 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 NH3 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 NH3. Atmospheric NH3 correlated well with ambient water vapor during many time periods lasting from tens of hours to several days, implying NH3 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 NH3 during ∼60% of the sampling times. Evaporation of predeposited NHx through wet precipitation combined with emissions from local green space likely caused the spikes of atmospheric NH3 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 NH3 level for which major contributors are yet to be identified.