Dependence of Mixed Aerosol Light Scattering Extinction on Relative Humidity in Beijing and Hong Kong

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2013, VOL. 6, NO. 2, 117 121 Dependence of Mixed Aerosol Light Scattering Extinction on Relative Humidity in Beijing and Hong Kong LI Cheng-Cai 1, HE Xiu 2, DENG Zhao-Ze 3, Alexis Kai-Hon LAU 4, and LI Ying 4 1 Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China 2 Aviation Meteorological Center, Air Traffic Management Bureau, Civil Aviation Administration of China, Beijing 100871, China 3 Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 4 Institute for the Environment/Environmental Central Facility, The Hong Kong University of Science and Technology (HKUST), Hong Kong, China Received 17 July 2012; revised 11 January 2013; accepted 14 January 2013; published 16 March 2013 Abstract The hygroscopic properties of mixed aerosol particles are crucial for the application of remote sensing products of aerosol optical parameters in the study of air quality and climate at multiple scales. In this study, the authors investigated aerosol optical properties as a function of relative humidity (RH) for two representative metropolises: Beijing and Hong Kong. In addition to the RH data, mass concentrations of PM 10 (particulate matter up to 10 µm in diameter) and aerosol scattering extinction coefficient (σ ext ) data were used. The relationship between the mass scattering extinction efficiency (MEE, defined as σ ext /PM 10 ) and RH can be expressed by regression functions as f = 1.52x + 0.29 (r 2 = 0.77), f = 1.42x + 1.53 (r 2 = 0.58), f = 1.19x + 0.65 (r 2 = 0.59), and f = 1.58x + 1.30 (r 2 = 0.61) for spring, summer, autumn, and winter, respectively, in Beijing. Here, f represents MEE, x represents 1/(1 RH), and the coefficients of determination are presented in parentheses. Conversely, in Hong Kong, the corresponding functions are f = 1.98x 1.40 (r 2 = 0.55), f = 1.32x 0.36 (r 2 = 0.26), f = 1.87x 0.65 (r 2 = 0.64), and f = 2.39x 1.47 (r 2 = 0.72), respectively. The MEE values for Hong Kong at high RHs (RH > 70%) are higher than those for Beijing, except in summer; this suggests that aerosols in Hong Kong are more hygroscopic than those in Beijing for the other three seasons, but the aerosol hygroscopicity is similarly high in summer over both cities. This study describes the effects of moisture on aerosol scattering extinction coefficients and provides a potential method of studying atmospheric visibility and groundlevel air quality using some of the optical remote sensing products of satellites. Keywords: mass extinction efficiency, hygroscopicity, Beijing, Hong Kong Citation: Li, C.-C., X. He, Z.-Z. Deng, et al., 2013: Dependence of mixed aerosol light scattering extinction on relative humidity in Beijing and Hong Kong, Atmos. Oceanic Sci. Lett., 6, 117 121. 1 Introduction Visibility degradation has become a major public concern in most metropolises, including Beijing and Hong Kong (Song et al., 2003; Lu and Wang, 2008; Liu et al., Corresponding author: LI Cheng-Cai, ccli@pku.edu.cn 2011). The impairment of visibility is attributed primarily to light extinction (scattering and absorption) by aerosols and gases. Of these, aerosol scattering extinction is believed to be the dominant cause. The aerosol scattering extinction, which is determined by the size and composition of aerosols, is influenced considerably by relative humidity (RH). The hygroscopicity of atmospheric aerosol represents how particles will behave when exposed to varying RH. Soluble particles will take up water as RH increases, resulting in increased particle size and mass; this, in turn, affects the aerosol refractive index, lightscattering properties, and atmospheric visibility (Malm and Day, 2001; Liu et al., 2012). With increasing RH, an aerosol phase transformation from solid to solution will occur when the RH reaches the deliquescence relative humidity, which is governed primarily by the chemical composition of the aerosol particles (Winkler, 1988; Cocker et al., 2001). Swietlicki et al. (1999) proposed that the hygroscopic behavior could be attributed entirely to inorganic content of the aerosol, such as sulfate, nitrate, and ammonium ions. However, less is known about the hygroscopic characteristics of organic aerosols and mixtures of inorganic and organic aerosols. Abundant laboratory studies have been conducted to characterize the hygroscopic behavior of aerosols of known composition (Tang and Munkelwitz., 1993; Tang, 1996), and many further investigations have been performed to understand the overall hygroscopic behavior of aerosols. Tandem differential mobility analyzers (TDMAs) measure changes in the diameters of aerosol particles when exposed to increased (or decreased) RH. The growth factor (GF) of aerosols is determined as the ratio of the mean diameter of the humidified particles to that of the dry particles, GF = D P (RH)/D P (RH dry ); this is the most common approach to measuring the hygroscopic behavior of aerosols (Cocker et al., 2001). Two modes of urban aerosol that depend on the GF value have been reported: a less hygroscopic mode for GF values of 1.0 to 1.2 and a more hygroscopic mode for GF values of 1.3 to 1.8 (Busch et al., 1999; Ferron et al., 1999). Another method of measuring the hygroscopic growth of aerosols is the introduction of nephelometry. Malm and Day (2001) described an experimental method of obtaining growth factors based on the ratio between the dry and humidified

118 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 light scattering coefficients (b sp, the subscript sp denotes the scattering coefficients of particles), defined as f(rh) = b sp (RH)/b sp (RH dry ). In the study of Malm and Day (2001), the growth factor for aerosol diameter varied from 1 to 5. It should be noted that growth curves derived from TDMAs (GF) and the nephelometric method (f(rh)) are not directly comparable, because the former is a ratio of size and the latter a ratio of light scattering that represents a combined hygroscopic property related to aerosol composition and size distribution. The correlation between PM 10 and surface extinction derived from aerosol optical depth (AOD) has been shown to increase considerably when the effects of moisture on aerosol optical properties is taken into consideration (Li et al., 2005). Therefore, it is necessary to establish the relationships between mass concentrations, extinction coefficients of mixed aerosol, and the corresponding RH. Since neither the chemical compositions nor the mass scattering extinction efficiency (MEE) of each species are available for routine monitoring in China, we adopted a different estimation to quantify the relationship between hygroscopicity and optical properties in order to elucidate the relationship between aerosol optical properties and hygroscopic characteristics. Here, we assumed that the variation of aerosol chemical components and size distribution within each season were small. Under this assumption, aerosol MEE varies only with environmental relative humidity. Accordingly, we were able to derive direct quantitative functions to estimate aerosol mass from ambient extinction properties. The main benefit of this method is that it does not require the specific mass concentration of each species or the relative humidity enhancement factors for different chemical compositions. Our results supplement existing knowledge of aerosol hygroscopicity. The instrumentation and methodology used in our study are introduced in section 2. In section 3, we present the results of seasonal variations in RH, PM 10, and scattering extinction, as well as the relationship between MEE and RH. In the final section, we present some conclusions regarding the dependence of aerosol optical properties on RH. 2 Instrumentation and methodology 2.1 Site description To understand the geographic variation of aerosol hydroscopic properties, we conducted simultaneous observations in Beijing and Hong Kong throughout 2006. Beijing is surrounded to the west by the Taihang Mountains, to the north and east by the Yanshan Mountains, and to the southeast by the North China Plain. The population of Beijing was nearly 13.8 million in 2006 and exceeded 22 million by the beginning of 2010. The aerosol data were measured at Peking University (40.0 N, 116.3 E), located in the northeast of Beijing, during 2006. A tapered element oscillating microbalance (TEOM), an automatic weather station, and a visibility meter are located on the roof of the Physics Building of the university, about 30 meters above the ground. The same set of instruments, operated by the Hong Kong University of Science and Technology (HKUST) and the Environmental Protection Department of the Hong Kong government, are located at Yuen Long (22.27 N, 114.01 E), an urban area in the northwestern part of Hong Kong. Hong Kong is a densely populated city, with a population of 6.8 million in 2006, and an area of just over 1100 km 2 ; it is situated adjacent to the Pearl River Delta (PRD) region, one of the most rapidly developing and heavily industrialized regions in southern China, along the northern coast of the South China Sea. In Hong Kong, northeasterly winds prevail during the winter months, while southwesterly winds dominate in the summer months. Yuen Long is a residential area, adjacent to the border with mainland China, and has undergone rapid development in recent years. 2.2 Instruments and data The Rupprecht & Patashnick (R&P) TEOM Series 1400a continuous ambient particulate monitor was used to measure the mass concentration of particles with aerodynamic diameter less than 10 µm (PM 10 ). The mass concentration was derived from the mass difference for a given time period using the oscillating microbalance. A PM 10 inlet at a designated flow rate of 16.7 L min 1 allowed particles with diameter less than 10 µm to pass through. Then, a flow splitter divided the sample air stream into main and bypass flows. The main flow was directed to the sample filter at a rate of 3 L min 1 ; the bypass flow, at 13.6 L min 1, was directed straight to the control unit and vented to the atmosphere. Flow rate checks were performed every month and the filter was replaced when the loading was more than 40%. The air in the main stream was heated to maintain the relative humidity below 60%. Five-min averages of aerosol mass concentration were recorded. A Vaisala FD12 forward-scattering visibility meter was used to measure the ambient visibility. This instrument works on the assumption that light scattered at 42 in the forward direction may be linearly related to the total scattering extinction coefficient. This method allows automated continuous daytime and nighttime measurements in ambient atmosphere. However, the apparatus measures the scattering of only a small volume of local air. Therefore, we adopted one-hour mean values to obtain scattering extinction coefficients. MEE, which is the ratio of the hourly total light scattering extinction to hourly PM 10, was calculated from the observation data. MEE derived in this manner represents an overall property of mixed aerosols that can be affected by variations in relative humidity. Meteorological data were obtained every 12 s from an automatic weather station manufactured by Vaisala Ltd.; these data included measurements of parameters such as ind speed (WS), wind direction (WD), RH, precipitation, and pressure. However, only the RH data were employed in this article. Some of our aerosol data may have been influenced by precipitation, fog, and dust storms. Therefore, we performed data quality control to filter out the following data:

NO. 2 LI ET AL.: AEROSOL MASS EXTINCTION EFFICIENCY 119 those with RH (12 s) > 90%, PM 10 (5 min) > 1000 µg m 3 (Beijing), PM 10 (5 min) > 500 µg m 3 (Hong Kong), or aerosol scattering extinction coefficient (1 h) greater than 5000 Mm 1 (or 10 6 m 1 ). Finally, we analyzed the hourly data. In all subsequent discussion, March, April, and May are regarded as spring; June, July, and August are regarded as summer; September, October, and November are regarded as autumn; and December, January, and February are regarded as winter. 3 Results After analysis of the seasonal variation of the aerosol scattering extinction coefficient and PM 10 concentration for Beijing and Hong Kong (figures not shown in this paper), we found that the annual mean value of the aerosol scattering extinction coefficient σ ext in Beijing was about 700 Mm 1, with higher values appearing in summer, perhaps as a result of high RH due to abundant precipitation. The monthly mean mass concentration of PM 10 was in the range 110 260 μg m 3 in Beijing, with a mean value of 180 µg m 3. In general, PM 10 was high in spring and winter and low in summer and autumn. Our results are consistent with those of Lu et al. (2007), who described much higher values in winter and spring, lower values in summer, and the lowest values in autumn. Although high values of PM 10 appeared in spring in Beijing, the highest AOD values retrieved from satellite data appeared in summer (Li et al., 2003). This difference be- tween AOD and PM 10 is likely due primarily to the effects of humidity. Although PM 10 tends to be low in summer, AOD can be elevated in summer owing to the uptake of water, which can increase the light scattering ability of aerosols. In Hong Kong, both σ ext and PM 10 were much lower than the corresponding values in Beijing: the yearly mean σ ext was about 400 Mm 1, which is nearly half the yearly mean for Beijing. Furthermore, the average PM 10 was 60 µg m 3, i.e., only one third of that for Beijing. Yuan et al. (2008) reported an annual mean PM 10 of 53 μg m 3, which agrees well with our results. High values of both σ ext and PM 10 occurred in winter, while the lowest values of both occurred in summer. In winter, the dominant northeasterly winds blowing from the Asian continent bring higher aerosol concentrations than those originating from the ocean in summer (Man and Shih, 2001; Cheng et al., 2006). Meanwhile, the higher temperatures in summer induce a higher mixing height, which helps to disperse the air pollutants, and the washout effects of rainfall help to eliminate particles in summer (Lu and Wang, 2008). Moreover, the pattern of monthly variation of σ ext has been shown to be similar to that of PM 10 : Man and Shih (2001) reported a high correlation coefficient (r = 0.85) for the relationship between extinction coefficient and PM 10 mass concentration in Hong Kong. Figure 1 shows the results of MEE as a function of RH Figure 1 MEE as a function of RH in (a) spring, (b) summer, (c) autumn, and (d) winter in Beijing. The solid lines are fitted functions as shown in the figures (x=1/(1.00 RH)).

120 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 in spring, summer, autumn, and winter in Beijing. All of the curves illustrated exhibit similar shapes, in general. It can be seen that MEE increases with increasing RH, and observed MEEs are in the range 1 20 m 2 g 1 when RH is below 90%. However, some seasonal differences do exist. In spring, autumn, and winter, the correlations between the fitted line and the scattered points are pretty high, with almost all data samples fitting the curves well; that is, MEEs from different time are with small variation for a given RH. Conversely, the correlation for summer is poorer than the correlations in the other three seasons, such that there is a considerable variation in the magnitude of MEE for a given RH. This large variation of MEE for a given RH reflects the variability of chemical composition in summer (Malm and Day, 2001). High relative humidity combined with high temperature will benefit aqueous-phase reactions in the atmosphere, thus altering the chemical composition of aerosols. The rate of change in slope is different for the four fitted lines, which is reflected in the coefficients of x in the regressed functions. These functions are expressed as f = 1.52x + 0.29 (r 2 = 0.77), f = 1.42x + 1.53 (r 2 = 0.58), f = 1.19x + 0.65 (r 2 = 0.59), and f = 1.58x + 1.30 (r 2 = 0.61) for spring, summer, autumn, and winter, respectively. In these functions, f refers to MEE, x represents 1/(1 RH), and the coefficients of determination (which describe how well the regression lines fit the dataset) are presented in parentheses. Figure 2 illustrates the corresponding results for Hong Kong. The fitted solid lines represent the regression of MEE on x(1/(1 RH)) for each season; the regression lines for Beijing are also plotted in the figure (as dashed lines) for reference. As for Beijing, all of the curves exhibit similar shapes in general, with a high r 2 value for the spring, autumn, and winter regressions and a low r 2 value for the summer regression. The regressed functions are f = 1.98x 1.40 (r 2 = 0.55), f = 1.32x 0.36 (r 2 = 0.26), f = 1.87x 0.65 (r 2 = 0.64), and f = 2.39x 1.47 (r 2 = 0.72) for spring, summer, autumn, and winter, respectively. The fitted lines for Hong Kong at high RH (RH > 70%) plot above the corresponding lines for Beijing, except in summer. This indicates that aerosols in Hong Kong are more hygroscopic in spring, autumn, and winter than in Beijing during the same periods. One possible reason for this is that the chemical compositions of the aerosols in Hong Kong and Beijing are different. Yuan et al. (2006) reported that secondary sulfate and local vehicle emissions produced the largest contributions to PM 10 in Hong Kong (25% each), followed by secondary nitrate (12%). Conversely, in Beijing, dust is one of primary sources, contributing 15% 30% to the mass of PM 10 (Sun et al., 2004). Moreover, Okuda et al. (2004) reported that the primary sources of aerosols in Beijing were soil dust and coal combustion. Using the PIXE (proton-induced X-ray emission) analysis method, Lu et al. (2007) revealed that Si, an indicator of soil, is the richest element in PM 10 in Beijing. However, in summer, the aerosols in Beijing contain less Figure 2 MEE as a function of RH in (a) spring, (b) summer, (c) autumn, and (d) winter in Hong Kong. The scattered points are for Hong Kong. The solid lines are fitted functions as shown in the figures. The dashed lines are for Beijing (as in Fig. 1).

NO. 2 LI ET AL.: AEROSOL MASS EXTINCTION EFFICIENCY 121 dust owing to the washing out of frequent precipitation and the relative lack of input from the dust storms of the Northwest China. High temperature and relative humidity in Beijing cause the aerosols to be dominated by secondary hygroscopic particles in summer, leading to a hygroscopic status for these mixed aerosols that is as high as, or even higher than, that in Hong Kong. 4 Conclusions Aerosol mass concentrations, scattering extinction coefficients, and relative humidity in Beijing and Hong Kong, calculated on the basis of one year of observations in 2006, were used to study the hygroscopicity of mixed aerosols. We quantified the relationship by obtaining functions for the relationship between MEE and RH. In Beijing, these functions were f = 1.52x + 0.29 (r 2 = 0.77), f = 1.42x + 1.53 (r 2 = 0.58), f = 1.19x + 0.65 (r 2 = 0.59), and f = 1.58x + 1.30 (r 2 = 0.61) for spring, summer, autumn, and winter, respectively. In these functions, f refers to MEE and x represents 1/(1 RH). Good fit was obtained for the regression lines in all seasons. However, in summer, there is considerable variation in the magnitude of MEE for a given RH, which may reflect the variability of chemical composition in summer. In Hong Kong, the functions obtained were f = 1.98x 1.40 (r 2 = 0.55), f = 1.32x 0.36 (r 2 = 0.26), f = 1.87x 0.65 (r 2 = 0.64), and f = 2.39x 1.47 (r 2 = 0.72), respectively. The MEE values for Hong Kong at high RHs (RH>70%) are higher than those for Beijing, except in summer, indicating that aerosols in Hong Kong are more hygroscopic in spring, autumn, and winter than those in Beijing. One possible reason for this is that aerosols in Hong Kong exhibit more hygroscopic compositions, e.g., the inclusion of secondary aerosols, while aerosols in Beijing contain abundant dust. Such differences in aerosol chemical components between the two cities are supported by the existing literature (Yuan et al., 2006; Sun et al., 2004; Okuda et al., 2004; Lu et al., 2007). However, in summer time, the aerosols in Beijing are most similar in high hygroscopic properties with those in Hong Kong, for less dust existed and more secondary fine mode aerosols dominated in this season over Beijing. Our results present the effects of relative humidity on mixed aerosol scattering extinction coefficients in Beijing and Hong Kong and provide a potential method for the investigation of air quality using optical remote sensing products, such as AOD derived from satellite data, over these two regions. Acknowledgements. The study was partially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA05040000), the National Natural Science Foundation of China (Grant Nos. 40775002 and 41175020), and the National High Technology Research and Development Program of China (863 Program, Grant No. SQ2010AA1221583001). We would like to thank the two anonymous reviewers and the editors of the journal, whose useful comments have greatly improved the manuscript. References Busch, B., C. Sprengard-Eichel, K. Kandler, et al., 1999: Hygroscopic properties and watersoluble fraction of atmospheric particles in the diameter range from 50 nm to 3.0 µm during the aerosol characterization experiment in Lindenberg 1998, J. Aerosol Sci., 30, S513 S514. Cheng, Y., K. F. Ho, S. C. Lee, et al., 2006: Seasonal and diurnal variations of PM 1.0, PM 2.5, and PM 10 in the roadside environment of Hong Kong, China Particuol., 4(6), 321 315. Cocker, D. C., N. E. Whitlock, R. C. Flagan, et al., 2001: Hygroscopic properties of Pasadena, California aerosol, Aerosol Sci. Technol., 35, 637 647. Ferron, G. A., E. Karg, B. Busch, et al., 1999: Hygroscopicity of ambient articles, J. Aerosol Sci., 30, S19 S20. Li, C. C., J. T. Mao, A. K. H. Lau, et al., 2003: Research on the air pollutions in Beijing and its surroundings with MODIS aerosol products, Proc. SPIE, 4891, 419 430, doi:10.1117/12.466349. Li, C. C., J. T. Mao, A. K. H. Lau, et al., 2005: Application of MODIS satellite products on the air pollution research in Beijing, Sci. China Ser. D-Earth Sci., 48(Suppl. II), 209 219. Liu, P. F., C. S. Zhao, T. Göbel, et al., 2011: Hygroscopic properties of aerosol particles at high relative humidity and their diurnal variations in the North China Plain, Atmos. Chem. Phys., 11, 3479 3494, doi:10.5194/acp-11-3479-2011. Liu, X. G., Y. H. Zhang, Y. F. Cheng, et al., 2012: Aerosol hygroscopicity and its impact on atmospheric visibility and radiative forcing in Guangzhou during the 2006 PRIDE-PRD campaign, Atmos. Environ., 60, 59 67. Lu, S. L., L. Y. Shao, M. H. Wu, et al., 2007: Chemical elements and their source apportionment of PM 10 in Beijing urban atmosphere, Environ. Monit. Assess., 133, 79 85, doi:10.1007/s10661-006- 9561-6. Lu, W. Z., and X. K. Wang, 2008: Investigation of respirable suspended particulate trend and relevant environmental factors in Hong Kong downtown areas, Chemosphere, 71, 561 567. Malm, W. C., and D. E. Day, 2001: Estimates of aerosol species scattering characteristics as a function of relative humidity, Atmos. Environ., 35, 2845 2860. Man, C. K., and M. Y. Shih, 2001: Identification of sources of PM 10 aerosols in Hong Kong by wind trajectory analysis, J. Aerosol Sci., 32, 1213 1223. Okuda, T., J. Kato, and J. Mori, 2004: Daily concentration of trace metals in aerosols in Beijing, China, determined by using inductively coupled plasma mass spectrometry equipped with laser ablation analysis, and source identification of aerosols, Sci. Total Environ., 330, 145 158. Song, Y., X. Y. Tang, Y. H. Zhang, et al., 2003: The study of the status and degradation of visibility in Beijing, Res. Environ. Sci. (in Chinese), 16(2), 9 11. Sun, Y. L., G. S. Zhuang, Y. Wang, et al., 2004: The air-borne particulate pollution in Beijing Concentration, composition, distribution and sources, Atmos. Environ., 38, 5991 6004. Swietlicki, E., J. Zhou, O. H. Berg, et al., 1999: A closure study of sub-micrometer aerosol particle hygroscopic behavior, J. Atmos. Res., 50, 205 240. Tang, I. N., 1996: Chemical and size effects of hygroscopic aerosols on light scattering coefficients, J. Geophys. Res., 101, 19245 19250. Tang, I. N., and H. R. Munkelwitz, 1993: Composition and temperature dependence of the deliquescence properties of the hygroscopic aerosols, Atmos. Environ., 27A, 467 473. Winkler, P., 1988: The growth of atmospheric aerosol particles with relative humidity, Phys. Scr., 37, 223 230. Yuan, Z. B., A. K. K. Lau, H. Zhang, et al., 2006: Identification and spatiotemporal variations of dominant PM 10 source over Hong Kong, Atmos. Environ., 40, 1803 1815. Yuan, H., G. S. Zhuang, J. Li, et al., 2008: Mixing of mineral with pollution aerosols in dust season in Beijing: Revealed by source apportionment study, Atmos. Environ., 42, 2141 2157.