Ambient concentrations and deposition rates of selected reactive nitrogen species and their contribution to PM2.5 aerosols at three locations with contrasting land use in southwest China☆
Introduction
Global emissions of reactive forms of nitrogen (Nr) are increasing proportionally with the human population increase and had reached a total of 220 Tg N yr−1 in 2010 (Fowler et al., 2015). The dominant Nr species emitted to the atmosphere are ammonia (NH3) and nitrogen oxides (NOx), which are precursors of secondary aerosols, such as nitric acid (HNO3), particulate NH4+ (pNH4+) and particulate NO3− (pNO3-) (Delon et al., 2012). Sources of NH3 include agricultural activities (livestock and fertilizer use), mobile and stationary fuel combustion, industrial activities and biomass burning (Lee et al., 2016, Suarez-Bertoa et al., 2014). Nitrogen oxides (NOx), which include NO3− in rainwater, gaseous NO2 and HNO3, and aerosol pNO3-, are mainly emitted from fossil fuel combustion and biomass burning (Zbieranowski and Aherne, 2013). These atmospheric Nr species are deposited to the Earth's surfaces via wet or dry deposition. Wet deposition occurs due to the incorporation of aerosol particles, acting as cloud condensation nuclei, into cloud droplets; as well as below-cloud and in-cloud scavenging of soluble gases. Dry deposition is primarily the deposition of gaseous compounds (NH3, NO2 and HNO3), with aerosol making smaller contributions (Dore et al., 2015). Increased nitrogen (N) deposition rates have led to biodiversity reductions in terrestrial ecosystems (Stevens et al., 2004), damage to human health through aerosols and ozone production (Erisman et al., 2013) and indirectly contributing to the radiative forcing of our climate. With the rapid urbanization and industrialization in China, it was estimated that the current level of air pollution may have led to 400,000 premature deaths annually (Xu et al., 2016). The economic burden of this premature mortality is estimated at approximately 157 billion Yuan (1.16% of the GDP) (Zhang and Smith, 2007), and is of increasing concern for the general public, the environmental scientific community and policy makers.
Measures to reduce Nr emissions and deposition rates were successfully implemented in some countries. In Europe, for example, policies to reduce agriculture emissions (e.g. Common Agricultural Policy, Nitrates Directive and the restructuring of Eastern Europe after 1989) as well as stringent emission controls (e.g. EC Large Combustion Plants Directive) and the EUROPE standards for road transport vehicles were implemented in the 1980s (Sutton et al., 2007). As a result, in some European countries significant reductions in NH3 volatilization has been achieved over the last 20 years (Sanz-Cobena et al., 2014). Similarly, the Chinese Government has implemented mitigation methods to reduce air pollution from stationary combustion plants in 2012. This mitigation was expected to reduce average Chinese annual NO2 and PM2.5 concentrations by 24.3% and 14.7%, respectively, by 2020 (Wang et al., 2015b). However, no regulations were implemented to reduce NH3 emissions.
The Sichuan basin, located in the upper Yangtze River is an example of serious air pollution in China. Rapid population growth and economic expansion have resulted in a wide range of Nr producing anthropogenic activities, such as biomass and fossil fuel burning, industry, transport, mining, urbanization and agricultural activities. The Sichuan basin is one of the four largest regions affected by haze in China and is one of the world's region most polluted by PM2.5 (Zhang et al., 2012). In addition, large emissions of SO2 and NOx have decreased the precipitation pH (Vet et al., 2014). Although the serious air pollution problems in the Sichuan basin are recognized, scientific evidence has focused mainly on wet deposition (Liu et al., 2013). More recent studies have shown that dry deposition rates can be large (Xu et al., 2015). As it is important to investigate wet and dry deposition rates together, we have investigated these at an urban, suburban and agricultural location. Our aims were to quantify 1) seasonal and spatial patterns for Nr concentrations and ambient PM2.5 concentrations; 2) the proportion of reduced and oxidized N deposition; and 3) the contribution of Nr species to ambient PM2.5 concentration.
Section snippets
Sampling sites
Nitrogen deposition measurements were conducted at Chengdu (CD, 30°37′ N, 104°4′ E), Shifang (SF, 31°6′ N, 104°9′ E) and Yanting (YT, 30°21′ N, 105°12′ E), Sichuan Province, southwest China. These three locations represent urban, suburban and agricultural areas, respectively (Fig. 1). Nr bulk deposition of NH4+, NO3− and organic N (DON), dry deposition of the gas phase (NH3 and NO2) and the aerosol particles (PM2.5) were measured during the period January 2014 to April 2016. All compounds were
Concentrations of Nr species in rainwater
The monthly concentrations of NH4+, NO3− and DON in rainwater at the three study sites are shown in Fig. 2. Here we classify the periods of March–May, June–August, September–November and January, February and December as spring, summer, autumn and winter. At CD, both NH4+ and NO3− concentrations in winter (5.4 ± 0.8 mg N l−1 for NH4+; 10.7 ± 2.8 mg N l−1 for NO3−) were significantly higher than in the other seasons (p < 0.05). The highest and lowest DON concentrations were measured in the
Nr species concentration
Atmospheric wet deposition includes (1) rainout: within-cloud scavenging where aerosol-type air pollutants that can be transported over long distances-up to 1000 km are incorporated in cloud droplets which finally precipitate as rain, and (2) washout: below-cloud scavenging of dust and gases often occurs close to the emitting source (Zimmermann et al., 2003). As a result, seasonal patterns of NH4+ and NO3− concentrations in rainwater are strongly influenced by precipitation amount (Huang
Conclusions
Bulk and dry deposition of Nr, and PM2.5 concentrations and composition were observed in urban (CD), suburban (SF) and agricultural (YT) sites in southwest China. The estimated total N deposition rates were in the range of 19.8–49.2 kg N ha−1 yr−1. Reduced N, oxidized N and organic N (in bulk deposition) contributed 58.7%, 31.0%, 10.3%, respectively, to total N deposition. Ammonia was the main dry deposition component. Significantly higher concentrations of NH3 were measured at CD and SF than
Acknowledgements
This study was financially supported by the Natural Science Foundation of China (Grants No. 41301321, 41271321, 41371303), Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07101-001), the Chinese Academy of Sciences (‘Light of West China’ Program) and the CINAg project. We sincerely thank the staff at Yanting and Shifang experimental station for help with collecting and analyzing samples. Furthermore we wish to thank the Chinese Academy of Sciences
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This paper has been recommended for acceptance by Charles Wong.