Diurnal variations of fossil and nonfossil carbonaceous aerosols in Beijing

https://doi.org/10.1016/j.atmosenv.2015年09月05日5 Get rights and content

Highlights

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    Diurnal variations of fossil and nonfossil total carbon were observed in Beijing.
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    Contribution of fossil secondary organic aerosol (SOA) increased in daytime.
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    This diurnal behavior was consistent with previous studies in other urban sites.
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    Organic carbon was better reproduced by a VBS model than by a SOA yield model.

Abstract

For the source apportionment of carbonaceous aerosols, measurement of 14C is an effective technique. In particular, diurnal variations of fossil and nonfossil carbonaceous aerosols provide information about the sources and production pathways of elemental carbon (EC) and organic carbon (OC). In this study, concentrations of fine-mode aerosols, including EC, OC, and 14C, were observed with a time resolution of 6 h near the urban center of Beijing in June 2010. The observations indicate that the contribution of fossil secondary organic aerosol (SOA) increased during daytime. This finding is consistent with previous studies in other urban sites, suggesting that SOA production from anthropogenic sources is important in urban areas. We evaluated the performance of two chemical transport models (CTMs) with different SOA models on simulations of EC and OC. Both models well reproduced the concentration and diurnal variation of EC. OC concentrations were underestimated by more than half by the CTM with a SOA yield model, and were better reproduced by the CTM with a volatility basis set (VBS) model. However, even the VBS model underestimated both fossil and nonfossil total carbon (EC + OC), and possible reasons for this underestimation are discussed.

Introduction

Atmospheric aerosols have large impacts on human health (Baltensperger et al., 2008, Pope and Dockery, 2006) and the radiation budget of the atmosphere (Maria et al., 2004, Matthes, 2008). In particular, fine particles with diameters less than 2.5 μm (PM2.5) are efficiently deposited in the human respiratory system and thus present relatively high risks to human health. Concentrations and ambient exposures of PM2.5 are especially high in East Asia (van Donkelaar et al., 2010). The megacities of China, including Beijing and Shanghai, have extremely high concentrations of PM2.5 and other atmospheric pollutants (Gurjar et al., 2008, van Donkelaar et al., 2010, van Donkelaar et al., 2014), reflecting high emissions of atmospheric pollutants (Kurokawa et al., 2013, Ohara et al., 2007, Zhang et al., 2009). To devise effective control strategies to reduce ambient PM2.5 concentrations, accurate knowledge of the contributions of various sources of PM2.5 is crucial.
Beijing has been the site of many recent source apportionment studies. For example, contributions of anthropogenic and natural sources to PM2.5 have been estimated from classical receptor models, such as chemical mass balance model and positive matrix factorization (Song et al., 2006, Zheng et al., 2005). As carbonaceous aerosols (i.e., total carbon (TC) = elemental carbon (EC) + organic carbon (OC)) are major contributors to PM2.5 (e.g., 36–38% in Beijing (Yang et al., 2005)), accurate source apportionment of carbonaceous aerosols is important and state-of-the-art techniques have been applied to help understand the sources of EC and OC in Beijing. For example, measurement of organic tracers (Wang et al., 2009, Zheng et al., 2005) and high-resolution time-of-flight aerosol mass spectrometry (He et al., 2010, Huang et al., 2010) have revealed the important role of biomass burning and cooking emissions in organic aerosols in Beijing. Measurements of 14C have shown that nonfossil sources make important contributions (40–50%) to carbonaceous aerosols in Beijing in summer (Sun et al., 2012, Yang et al., 2005).
Among these techniques, 14C measurements yield particularly robust results as 14C represents the total organic carbon, and thus extrapolation from small tracer concentrations is not necessary (Hallquist et al., 2009). Previous studies have indicated that 14C measurements of aerosol samples collected with high time resolution (6 h) are effective for revealing the sources of carbonaceous aerosols and for evaluating the performance of chemical transport models (CTMs) (Fushimi et al., 2011, Morino et al., 2010). It has been suggested that diurnal variations of carbonaceous aerosols provide information about the respective contributions of primary and secondary particles in Beijing (e.g., Lin et al., 2009). Thus, diurnal variations of 14C may provide information about both the sources and production pathways of EC and OC.
CTMs are also a useful tool for estimating source contributions of PM2.5. Several CTM studies have estimated the contributions of emission sources inside and outside Beijing to aerosols in Beijing (Chen et al., 2007, Matsui et al., 2009, Streets et al., 2007). Simulation using CTMs has the advantage that it can estimate the spatial distribution of source contributions on a sector basis. However, to our knowledge, modeling studies focusing on sector-based source contributions in Beijing are very limited. In general, CTM results include large uncertainties because of problems with their input data (e.g., meteorological, boundary, and emissions data), the parameterization of each process, and missing scientific elements. Evaluations of CTM performance using well-designed observational datasets are critically necessary and comparison with source-resolved observational data is particularly effective.
In this study, we measured 14C content in carbonaceous aerosols collected at a sampling interval of 6 h near the urban center of Beijing: we found diurnal variations in fossil and nonfossil TC. In addition, from diurnal variations of EC, OC, and 14C, we assessed the sources of carbonaceous aerosols and compared our results with those of previous studies in other urban areas. We also evaluated the performance of two CTMs with different secondary organic aerosol (SOA) models for apportioning EC and OC from fossil and nonfossil sources.

Section snippets

Field measurement and chemical analysis

Filter sampling of PM2.5 was done using a high-volume Andersen air sampler (GUV-15HBL-1 with G1200-41, Thermo Scientific, USA) at a site near the urban center of Beijing (Fig. 1) on 24–28 June 2010. This site, the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (40.00°N, 116.38°E), is about 1 km from the Third Ring, one of Beijing's major perimeter roads. During the measurement period, concentrations of PM2.5 at the measurement site agreed well

Meteorological data

In Beijing in the summer, air masses stagnate when anticyclones stall around northeastern China, whereas concentrations of atmospheric pollutants become low when midlatitude cyclones pass, advecting clean air from surrounding areas (Takegawa et al., 2009). During the measurement period of this study, as shown in Fig. A1, wind speed was moderate with a clear diurnal pattern, suggesting that meteorological conditions were stagnant.
For validation of the meteorological model (WRF), we used

Conclusions

Concentrations of PM2.5, including EC, OC, and 14C, were observed with a time resolution of 6 h near the urban center of Beijing in June 2010. The diurnal variations of EC concentrations were distinct with the highest concentrations during nighttime, whereas diurnal variations of OC were unclear. In addition, fossil TC showed little or no diurnal variation whereas nonfossil TC showed distinct diurnal variations. An analysis in terms of pMC and the EC/TC ratio showed that the contribution of

Acknowledgment

This research was funded by Japan Science and Technology project, "Strategic International Research Cooperative Program: Cooperative Research Projects with NSFC" (Project No. 20921140095) and the Environment Research and Technology Development Fund (5–1408 and S12-1) of the Ministry of the Environment, Japan.

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