Characteristics of atmospheric ammonia over Beijing, China
Abstract. Continuous measurements of atmospheric ammonia (NH3) were conducted using Ogawa passive samplers from February 2008 to July 2010 at an urban site and from January 2007 to July 2010 at a rural site in Beijing, China. NH4+ in fine particles was also collected at the rural site during 2008–2009. The field comparison between the Ogawa passive samplers and an active analyzer for NH3 conducted at the urban site assures the quality and accuracy of the measurements. The concentrations of NH3 at the urban site ranged from 0.7 to 85.1 ppb, with the annual average of 18.5 ± 13.8 and 23.5 ± 18.0 ppb in 2008 and 2009, respectively. The NH3 concentrations at the rural site were lower than those at urban site, and varied from 0.8 to 42.9 ppb, with the annual average of 4.5 ± 4.6, 6.6 ± 7.0 and 7.1 ± 3.5 ppb in 2007, 2008 and 2009, respectively. The data showed marked seasonal variations at both sites. The results emphasized traffic to be a significant source of NH3 concentrations in winter in urban areas of Beijing. This was illustrated by the strong correlations of NH3 with the traffic related pollutants (NOx and CO) and also by the bimodal diurnal cycle of NH3 concentrations that was synchronized with traffic. Similar patterns were not observed during the summer, suggesting other non-traffic sources became more important. At the rural site, the daily NH4+ concentrations ranged from 0.10 to 36.53 μg m−3, with an average of 7.03 μg m−3 from June 2008 to December 2009. Monthly NH3 were significantly correlated with NH4+ concentrations. Average monthly NH3/NH4+ ratios varied from 0.13 to 2.28, with an average of 0.73. NH4+ in PM2.5 was primarily associated with SO4−2 at the rural site.