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 (NO_x 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 PM_2.5 was primarily associated with SO₄^−2 at the rural site.