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, NHL₄⁺ 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 NH ₃ concentrations reached a maximum (∼22.8-35.3 M-g/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 NFL₄⁺ aerosol concentrations were weakly correlated, implying that gas-particle reactions are influenced by many factors such as sources, meteorology and removal. Average NH₃/NH ₄⁺ ratios varied from 0.1 to 25.3, with an annual average of 4.0. High NH₃ concentrations at Xi'an had a significant influence on atmospheric acidity and the formation of secondary NH₄⁺ aerosol.