Characterization of aerosol optical properties, chemical composition and mixing states in the winter season in Shanghai, China
Introduction
Atmospheric aerosols have a “direct effect” on climate by scattering or absorbing solar radiation and an “indirect effect” by acting as cloud condensation and ice nuclei (Pöschl, 2005). The magnitude of these effects has still considerable uncertainties, especially the indirect effect (Forster et al., 2007, Schiermeier, 2010). In order to estimate the direct and indirect effect of aerosols, further understanding of aerosol optical properties is urgently needed.
The optical properties of aerosols govern their interaction with sunlight and are important parameters for estimating radiative forcing in modeling studies (Fischer et al., 2011). Studies have shown that aerosol optical properties are determined by particle size distribution, chemical components, and mixing state (Seinfeld and Pandis, 2006), with the latter being the most challenging to understand (Cappa et al., 2012, Huang et al., 2013). Different chemical species, especially soot and brown carbon (Lack and Cappa, 2010), can be mixed within a single particle (internal mixing) or across different particle types (external mixing); the exact mixing state changes aerosol optical properties and ultimately radiative forcing (Fuzzi et al., 2006, Ramanathan and Carmichael, 2008). Core–shell Mie theory has been widely used to calculate the theoretical aerosol optical properties (Jacobson, 2001). The coating shell on the cores often acts as a lens, enhancing the particle's absorbing and scattering ability, especially on black carbon cores (Lack and Cappa, 2010). However, field studies have shown that the theoretical model does not fit all cases (Cappa et al., 2012), and more work still needs to be done to investigate the influence of particle mixing state on aerosol optical properties.
Measurements of optical properties of aerosols along with mixing state within a single particle have been crucial in estimating aerosol radiative forcing (Pratt and Prather, 2010, McMeeking et al., 2011). Single particle mass spectrometers, such as the Aerosol Time-of-Flight Mass Spectrometer (ATOFMS), soot particle aerosol mass spectrometer (SP-AMS), and Single-Particle Soot Photometer (SP2), can provide information on the size-resolved chemical composition and internal mixing state of particles (Schwarz et al., 2006, Onasch et al., 2012, Pratt and Prather, 2012). Since SP-AMS and SP2 only measure material that is sufficiently light absorbing at 1064 nm (i.e., soot particles, and only the non-refractory aerosol component with SP-AMS), the ATOFMS is preferred for full chemical measurements in regions where sea salt or mineral dust is an important contributor to the aerosol.
In different regions of China, the majority of studies have been focused on the effect of the chemical mass concentration, aerosol physical properties and water content on the optical properties (Che et al., 2009, Guo et al., 2009, Jung et al., 2009a, Yang et al., 2009b, Yu et al., 2009, Eck et al., 2010, Huang et al., 2010, Wang et al., 2010). Jung et al. (2009b) found that under polluted conditions in the urban area of Beijing, ammonium sulfate, ammonium nitrate, and organic carbon contributed to the increases of single scattering albedo. Yao et al. (2010) analyzed atmospheric light extinction properties and chemical speciation of fine particulates in Shenzhen and concluded that the organic matter in PM1 contributed about 45% to the observed aerosol light extinction. Huang et al. (2011) measured black carbon (BC) mass loadings, size distributions and mixing state information in the Pearl River Delta region and estimated the potential contribution of BC mass to the radiative forcing. But studies of aerosol optical properties and their relationship with aerosol mixing state are very limited.
Cavity ring down spectroscopy (CRDS) has recently been used for measuring aerosol extinction and absorption coefficients in field and laboratory studies (Butler et al., 2007, Dinar et al., 2008, Zhang et al., 2008, Khalizov et al., 2009, Xue et al., 2009, Li et al., 2011, Li et al., 2013). Compared with filter based techniques like the Aethalometer and the Particle Soot Absorption Photometer (Bond et al., 1999, Sheridan et al., 2005), CRDS offers rapid real-time measurement of absorption coefficients (Busch and Busch, 1999, Pettersson et al., 2004, Bulatov et al., 2006). Huang et al. (2013) employed ATOFMS and CRDS to study the evolution of aerosol chemical and optical properties during a period in Shanghai when pollution was highly present. They found that organic carbon coatings could dramatically change aerosol optical properties, suggesting that this combination of measurements is ideal for elucidating the impact of particle mixing states on aerosol optical properties.
In this study, a Cavity-Ring-Down Aerosol Extinction Spectrometer (CRD-AES) (Li et al., 2011) and a nephelometer were used in Shanghai from 22 to 28 December, 2009 for the measurements of extinction and scattering coefficients, respectively. Simultaneously, an ATOFMS was used to obtain both positive and negative mass spectra from individual particles, and thus provide information on aerosol mixing state (Murphy, 2007). Examining these measurements concurrently, the sources of particles and the aerosol optical properties as a function of particle size distribution, chemical composition and mixing states are investigated.
Section snippets
Experimental
Measurements were carried out in the laboratory building of the Department of Environmental Science and Engineering at Fudan University (31°17′47.14″N, 121°30′14.94″E) in Shanghai from December 22 to December 28, 2009 (24 hr per day). This site is near residential, traffic, and construction emissions sources and represents a typical urban area. Ambient air was drawn from a height of about 5.5 m above the ground through a half-inch diameter, six-meter long, stainless steel tube at a flow rate of
Meteorology and optical measurement
The temporal profiles of temperature, relative humidity (RH), wind speed, and wind direction during the sampling period are shown in Fig. 1a. The time series of the aerosol optical properties at 532 nm, including the aerosol extinction, scattering and absorption coefficients are shown in Fig. 1b. The scattering Ångström exponent (å450/700, SAE) and single scattering albedo (SSA) are shown in Fig. 1c covering the entire sampling period. The backward air trajectories at 10 m height were determined
Conclusions
Measurements of aerosol optical and chemical properties were conducted from December 22 to December 29, 2009 in Shanghai, China. The aerosol optical properties were found to be dependent on the wind direction, and thus, are dependent on the source of the particles as well. The westerly wind usually brought particles with high extinction coefficient to the sampling site. The wind speed-resolved polar contour plots of PM10 and scattering Ångström indicated that particles originating in the
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21177027, 41275126), the Science & Technology Commission of Shanghai Municipality (Nos. 12DJ1400100, 14XD1400600), and the Jiangsu Provincial Collaborative Innovation Center of Climate Change.
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