Cyclic Organosilicon Compounds in Ambient Air in Guangzhou, Macau and Nanhai, Pearl River Delta
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Applied Geochemistry 16 (2001) 1447–1454 www.elsevier.com/locate/apgeochem Cyclic organosilicon compounds in ambient air in Guangzhou, Macau and Nanhai, Pearl River Delta X.M. Wang a, S.C. Lee b,*, G.Y. Sheng a, L.Y. Chan b, J.M. Fu a, X.D. Li b, Y.S. Min a, C.Y. Chan b aSKLOG, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China bDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong Abstract Silicon is present in the Earth’s atmosphere as a consequence of several processes including the release of Si com- pounds from anthropogenic sources, but little information is available on airborne Si compounds of anthropogenic origins. In this study ambient air samples from Guangzhou, Macau and Nanhai in the Pearl River Delta, South China were collected by sorbent tubes for the determination of concentrations of volatile organosilicon compounds. Samples were analysed by thermal desorption followed by a GC–MSD technique for compound identification and quantitative analysis. Hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasilo-xane (D4) were found to be the two dominant organosilicon compounds in the air. In Guangzhou, higher total D3 and D4 concentrations were observed in the industrial area, landfill and waste water treatment plant, while the lowest levels occurred in suburb forest. Two types of linear correlation between D3 and D4 were found in Guangzhou samples, indicating different sources of these orga- nosilicon compounds. Samples in Macau and Nanhai showed different D3 and D4 relationships from the samples in Guangzhou. # 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Due to their unique properties such as thermal stabi- lity, low surface tension and water repellence, silicon Silicon is present in the Earth’s atmosphere as a con- polymers, a dominant product of which is poly dime- sequence of several processes including the suspension thylsiloxane (PDMS) fluids (Fig. 1), have wide use in of silicon containing particles of inorganic and biologi- industrial applications and in consumer products. There cal origin from the Earth’s surface, the release of silicon are many pathways for organosilicon compounds to compounds from anthropogenic sources (e.g. industry enter the atmosphere. Volatile organosilicon compounds and fuel consumption) to the atmosphere and the consisting of –(CH3)2SiO– structural units in linear or introduction of silicon radionuclides from cosmic ray cyclic arrangements are widely used as components of interactions in the upper atmosphere. Little information cosmetic and personal products, and as penetrating oils, is available on the anthropogenic and cosmic sources cleaning agents and lubricants. These applications of compared to the sources of particle suspension. Pelliz- volatile organosilicon compounds lead to their emission zari et al. (1976) reported silane compounds identified at to the atmosphere (Graedel et al., 1986). Study of trans- trace concentrations in air, Graedel (1978) and Graedel formation of PDMSfluids in soil by Buch and Ingeb- et al. (1986) summarized the occurrence of organic sili- rigton (1979) indicated that PDMSderived products are con compounds in the lower atmosphere, these organo- present in the atmosphere. Recent work has shown that silicons were presumed to be derived from industrial ubiquitous PDMShydrolysis in soils generates dime- processes. thylsilanediol and other volatile linear siloxane diols and cyclic dimethylsiloxanes that would enter the atmosphere through volatilization (Lehmann et al., 1994; Carpenter et * Corresponding author. Tel.: +852-2766-6011; fax: +852- al., 1995; Lehmann and Miller, 1996; Varaprath and Leh- 2334-6389. mann, 1997). Another pathway for organosilicon com- E-mail address: [email protected] (S.C. Lee). pounds to enter the atmosphere is thermal decomposition 0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00044-0 1448 X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 Fig. 1. Dimethylsilanediol and examples of linear and cyclic PDMS. of PDMS, which produces highly volatile cyclic dime- source inputs and distribution of atmospheric organosi- thylsiloxanes (Kleinert and Weschler, 1980). licons. As a category of chemicals, organosilicon compounds A few investigations into indoor organosilicon con- have generated substantial scientific and regulatory interest centrations revealed some interesting results. Shields over the last few years. Octamethylcyclotetrasiloxane (D4), and Weschler (1992) measured volatile organic com- as an important intermediate in the synthesis of silicon pounds at a telephone switch center. Shields et al. (1996) polymers and as a constituent in many personal care and reported the comparison of volatile organic compounds consumer products (deodorants, antiperspirants, skin and in 3 types of UScommercial buildings with different hair care products, etc.), was the first silicon compound occupant densities, they found that D4 and D5 were considered for regulatory review. A number of studies present in higher concentrations when there were more focused on its environmental fate and effects in the aquatic occupants, indicating that they were markers of occu- environment (Kent et al., 1994, 1996; Hobson, 1995; pant density. Table 1 summaries the physical properties Fackler et al., 1995; Mueller et al., 1995, Sousa et al., 1995). of the 3 cyclic organosilicons. Previous studies of the atmospheric chemistry of The present study aims to investigate the ground levels organosilicons emphasized their reaction kinetics with of organosilicon compounds in 3 typical cities (Guangz- OH, NO3 radicals and O3. From the study of Atkinson hou, Macau and Nanhai) situated in the Pearl River (1991), the calculated lifetimes of the selected volatile Delta, South China (Fig. 2). The Pearl River Delta region organic compounds ranged from 10 days for dec- has become China’s most populated and economically amethylcyclopentasiloxane (D5) to 30 days for hexam- developed region since early the 1980s. There are increas- ethylcyclotrisiloxane (D3) in the troposphere due to ing potential emission sources of organosilicons in the chemical reaction with the OH radical. The results sug- region. Moreover, the annual average temperature in the gested that their lifetimes are sufficiently long to undergo region is high (>20C) (see Table 2), thus making it easy long-range transport and to be regionally and globally for volatile organosilicons to enter the atmosphere via distributed. Using a smog chamber–mass spectrometer volatilization. The ambient concentrations, characteristics system, Sommerlade et al. (1993) analyzed the products and possible sources of volatile organosilicon com- and kinetics of the gas-phase reactions of selected orga- pounds in the 3 cities were investigated in this study. nosilicon compounds with OH radicals. Markgraf and Wells (1997) investigated OH radical reaction rate con- stants and atmospheric reaction products of 3 siloxanes, 2. Methodology and found that some unusual cyclic siloxane products were formed. Although organosilicon compounds in the The basic information for the 3 cities is summarized in atmosphere are considered to be released primarily from Table 2. In the screening of ambient levels of organosili- human activities, there is little data on the concentrations, con compounds in these cities, the chosen sampling X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 1449 Table 1 Summary of physical properties of selected cyclic organosilicon compoundsa,b Compound Formula MW mp bp vp Density LogKow (amu) (C) (C) (25C) mm Hg (20C) (g/cm3) D3 C6H18O3Si3 222 64.5 134 3.53 1.1200 4.47 D4 C8H24O4Si4 296 17.5 176 1.05 0.9561 5.10 D5 C10H30O5Si5 370 À44 210 0.20 0.9985 5.20 a MW, molecular weight; mp, melting point; bp, boiling point; vp, vapor pressure; Kow, octanol–water partition coefficient. b Lide (1995). Fig. 2. Locations of the 3 cities and sampling areas in the Pearl River Delta, South China. Table 2 Summary of basic information about the 3 cities Cities Guangzhou (1996) Macau (1995) Nanhai (1996) Population 3,901,840 (urban) 410,500 (peninsular) 7,168 (Taipa) 1,043,186 2,658,668 (non-urban) 3,146 (Coloane) 1,443.6 (urban) 7.0 (peninsular) Area (km2) 4.1 (Taipa) 1152.7 5990.8 (non-urban) 7.2 (Coloane) Population density (person/km2) 2703 (urban) 58,643 (peninsular) 1,748 (Taipa) 905 444 (non-urban) 437 (Coloane) Annual average temperature (C) 21.6 (urban) 23.1 21.8 1450 X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 times were 9:00 a.m. to 2:00 p.m. at each site on the pling sites. In Taipa the sampling site was in the Uni- same day. Sorbent tubes and passivated canisters were versity of Macau campus, and the sampling site in used for sampling volatile organic compounds (VOCs) Coloane was at the coastal beach. in air (Rudolph et al., 1990; Seeley and Broadway, In Nanhai, 4 sites were selected for a 3 consecutive 1994). In this study, multi-bed sorbent tubes were day sampling from 10 to 12 July 1996. Three of 4 sam- adopted for sampling. The commercial 177.8Â6.35 mm pling sites were located in towns (Dali, Shuitou, Yayao) I.D. multi-bed stainless steel sampling tubes (Tekmar with booming privately owned small factories or manu- Company, USA) were packed with silica gel, carbon- facturers. The other sampling site was near a vegetable sieve and charcoal. Before use, the sampling tubes were growing farmland in the rural area (Tanbian). conditioned by heating at 180C for 24 h while purging By coupling a thermal desorption (TD) system (Tek- with He. After each use or storage, they were recondi- mar 6000 AeroTrap) to a Hewlett Packard 5972 Gas tioned in the same way for 2 h. Samples were all col- Chromatography/Mass Selective Detector (HP 5972 lected at 1.2 m above the ground level. Volatile organic GC/MSD), the analysis of VOCs was performed by a compounds were sampled by drawing air through the TD-GC/MSD technique. A HP-5MS (30 mÂ0.25 mm sampling tube with battery-operated timing pumps.