Ion balance and acidity of size-segregated particles during haze episodes in urban Beijing
Introduction
Atmospheric particulate matter (PM) is an important pollutant that has been linked to adverse health effects, visibility reduction and climate change (Liang et al., 2016, Zhang et al., 2015). Water-soluble ions constitute a dominant fraction of PM (Huang et al., 2016). To a large extent, these ions determine the acidity of PM, and aerosol acidity is one of the most important parameters that affects atmospheric chemistry through the influence on many heterogeneous reactions (Hu et al., 2014). Acidic aerosols can also lead to severe degradation of ecosystems through both wet and dry deposition (Vet et al., 2014).
Aerosol acidity is difficult to measure due to its low water content; it is generally assessed using three different parameters, in situ acidity, strong acidity and ion-balanced acidity (He et al., 2012, and references therein). In situ aerosol acidity, which is the concentration of free H+ (pH) in deliquesced particles under the ambient conditions, is most likely to influence the chemical behavior of aerosols. Strong acidity, measured from aqueous extracts of aerosol samples, represents the absolute acidity of the aerosols, but it does not reflect in situ characteristics because of the large amount of excess water. Ion balanced acidity involves estimating of H+ concentration by subtracting the cations other than H+ from anions. This value is widely used to indicate the neutralizing level based on the ratio of cations to anions.
Previous studies of aerosol acidity have mainly focused on spatial and seasonal variability and investigated aerosol acidity in bulk aerosol samples or fine versus coarse aerosol fractions (Wang et al., 2016, Zhang et al., 2013). Field observations of aerosol acidity in various megacities suggest a general pattern of higher acidity in southern relative to northern China. The spatial and seasonal variability of PM2.5 (particulate matter with aerodynamic diameter lower than 2.5 μm) acidity in Beijing and Chongqing were studied, providing insight into the formation of secondary inorganic aerosols and discussing the factors that determined their spatial and temporal variability (He et al., 2012). Summertime PM2.5 ionic species in Beijing, Shanghai, Guangzhou, and Lanzhou, as well as nitrate formation under both ammonia-rich and ammonia-deficient conditions were also investigated (Pathak et al., 2009). However, such low resolution studies of particle size are usually insufficient to elucidate the physical and chemical atmospheric processes that affect aerosol properties during haze episodes because they cannot consider the acidity and ionic composition of size-segregated particles (Tian et al., 2016a). The few examples of investigations conducted with a relatively high particle size resolution are based on limited data or do not focus on the difference between hazy and non-hazy days. For example, chemical composition and acidity of size-fractionated inorganic aerosols in Shanghai during hazy days (n = 13) and non-hazy days (n = 5) of 2013–14 winter were studied based only on 18 sets of samples (Behera et al., 2015). Ion balances of size-resolved aerosol samples at both the remote and the urban sites were studied to shed light on how the aerosol acidity changes with particle size and what implications this might have for the atmospheric processing of aerosols (Kerminen et al., 2001).
Several studies have reported a high ammonium salts content in PM2.5 in Beijing. However, the role of ammonia and the neutralization mechanisms were less understood. The present study uses various acidity proxies to determine mechanisms of the aerosol neutralization in Beijing, focusing on size-resolved PM. In this work, we investigated both the ion balance and acidity, specifically the cation-to-anion ratio (RC/A) and in-situ pH (pHIS), of size-segregated aerosol samples collected during a continuous haze episode and during long-term sampling between January 2013 and February 2014.
Section snippets
Sampling site and sample collection
The experiment was performed at the Institute of Atmospheric Physics, Chinese Academy of Sciences (39°58′N, 116°22′E). Samplers were placed on the roof of a building at approximately 8 m above the ground. The sampling site was located in northwest Beijing and situated between the 3rd and 4th ring roads. The air quality at the measurement site with its typical urban atmosphere is influenced by local human activities and urban vehicular traffic.
A 9-stage sampler (Andersen Series 20–800, USA) with
Description of haze episode and ionic composition
In January and February 2013, several of the most severe haze events on record swept across many east-central cities and covered a quarter of the total land area in China. In this study, we focused on samples collected during the period from January 24 to February 1, which represents an extreme haze pollution episode with a clear periodic cycle of 8 days. Fig. 1, Fig. 2 show the size-resolved mass concentration of PM based on filter samples, the hourly concentrations of PM1, PM2.5 and PM10 in
Conclusion
We studied the ion balance and acidity of size-segregated particles during a continuous haze episode, and from long-term observations based on the calculated pHIS, RA/(S + N) and RC/A. The results showed that RC/A and RA/(S + N) were good indicators of aerosol acidity in fine mode. RC/A values were typically close to unity over most of the accumulation period and significantly increased in larger or smaller particle size fractions. In the coarse size range, the high RC/A values were partially
Acknowledgements
This study was supported by the National Key Research and Development Program of China (No. 2016YFC0201802) and the National Natural Science Foundation of China (Nos. 41405144, 41230642 and 41321064).
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