Characterization of Atmospheric Ammonia over Xi’an, China

Continuous measurements of atmospheric ammonia (NH3) between April 2006 and April 2007 were conducted at an urban site and a suburban site in Xi’an, northwest China. NH3 was collected using Ogawa passive samplers every sixth day. At the same time, NH4+ in fine particles was collected using battery-powered mini-volume samplers. The annual average concentrations of NH3 were 12.9 µg/m3 and 14.1 µg/m3, at the urban and suburban sites, respectively. The NH3 concentrations reached a maximum (~22.8-35.3 µg/m3) in June and July and were minimum (~3.0-4.7 µg/m3) in December, which was closely linked with NH3 volatilization under different ambient temperatures. The seasonal variation in NH3 was summer > spring > autumn > winter at both sites, which may be ascribed to the impact of biological emission sources such as agricultural activity. NH3 and NH4+ aerosol concentrations were weakly correlated, implying that gas-particle reactions are influenced by many factors such as sources, meteorology and removal. Average NH3/NH4+