JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/jes An intensive study on aerosol optical properties and affecting factors in Nanjing, China Fenping Cui 2,3,4, Mindong Chen 1,2,3,, Yan Ma 1,2,3, Jun Zheng 1,2,3, Yaoyao Zhou 1,2,3, Shizheng Li 1,2,LuQi 1,2, Li Wang 1,2 1. School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 2144, China 2. Jiangsu Key Laboratory of Atmospheric Environment Monitoring & Pollution Control, Nanjing 2144, China. E-mail: cuifenping@nuist.edu.cn 3. Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing 2144, China 4. School of Physics & Opto-electronic Engineering, Nanjing University of Information Science & Technology, Nanjing 2144, China ARTICLE INFO Article history: Received 3 April 215 Revised 11 August 215 Accepted 17 August 215 Available online 17 December 215 Keywords: Aerosol optical properties Single scattering albedo Ångström exponent Hygroscopic growth factor ABSTRACT The optical properties of aerosol as well as their impacting factors were investigated at a suburb site in Nanjing during autumn from 14 to 28 November 212. More severe pollution was found together with lower visibility. The average scattering and absorption coefficients (B sca and B abs ) were 375.7 ± 29.5 and 41.6 ± 18.7 Mm 1, respectively. Higher Ångström absorption and scattering exponents were attributed to the presence of more aged aerosol with smaller particles. Relative humidity (RH) was a key factor affecting aerosol extinction. High RH resulted in the impairment of visibility, with hygroscopic growth being independent of the dry extinction coefficient. The hygroscopic growth factor was 1.8 ± 1.2 with RH from 19% to 85%. Light absorption was enhanced by organic carbon (OC), elemental carbon (EC) and EC coatings, with contributions of 26%, 44% and 75% (532 nm), respectively. The B sca and B abs increased with increasing N 1 (number concentration of PM 2.5 with diameter above 1 nm), PM 1 surface concentration and PM 2.5 mass concentration with good correlation. 215 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. Introduction As is well-known, aerosol can be emitted by anthropogenic and natural sources and strongly perturbs the Earth's energy budget. Aerosol particles interact with solar radiation through scattering and absorption, which are referred to as direct radiative forcing. The Intergovernmental Panel on Climate Change (IPCC) 213 (Stocker et al. 213) reported that scattering aerosol such as ammonium sulfate, ammonium nitrate and sea salt tends to cool the climate system, while absorbing aerosol has the opposite effect. Many studies have focused on local radiative effects from anthropogenic and natural aerosol (Anderson et al. 23; Cappa et al. 212). However, the uncertainty in determining the radiative forcing of atmospheric aerosol, especially black carbon (BC), is still large (Anderson et al. 23; Stocker et al. 213), which may be related to the great temporal and spatial variation of aerosol optical properties. In addition, aerosol extinction (scattering and absorption) leads to the degradation of atmospheric visibility and the formation of haze. The optical parameters of aerosols display large differences between clean and polluted days (Kang et al. 212; Noh et al. 29; Zhang et al. 213b). Aerosol scattering, absorption and extinction coefficients (B sca, B abs and B ext ) represent the ability of aerosols to interact with solar radiation. The aerosol scattering coefficient has usually been measured based on Mie theory. The traditional instruments to measure absorption coefficients are associated with significant uncertainties. Recent methods using photoacoustic and extinction-minus-scattering techniques are Corresponding author. E-mail: chenmd@nuist.edu.cn (Mindong Chen). http://dx.doi.org/1.116/j.jes.215.8.17 11-742/ 215 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
36 JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 considered more accurate (Moosmüller et al. 29; Stocker et al. 213). Aerosol optical properties are most affected by the aerosol size distribution, chemical components and mixing state. The Ångström scattering exponent (A sca )foratmospheric aerosol varies, with lower values for larger particles and vice versa (Redmond et al. 21). Cao et al. (212) found that ammonium sulfate and organic matter were the main contributors to light extinction in Xi'an, China. Concentrations of secondary inorganic aerosols were found to be higher during haze days. Meteorological factors such as relative humidity (RH), wind speed and direction have close correlation with aerosol scattering and absorption. Wind from the west brought higher aerosol loading than those from the Northeast and Southeast in Shanghai (Li et al. 213). Liu et al. (212) found that the atmospheric extinction coefficient increased by ~51% at ambient conditions at Guangzhou. The aerosol direct radiative forcing increased by around a factor of three compared to that under dry conditions. China is situated in the eastern part of Asia, with large population, agriculture, production and consumption. In recent decades, environmental pollution has become more and more severe and results in more frequent haze episodes (Che et al. 27). Many studies have been carried out focusing on aerosol optical properties in some regions such as the Beijing Tianjin Hebei economic band (Cheng et al. 211; He et al. 29; Ma et al. 211), Yangtze River Delta (Deng et al. 211; Huang et al. 214; Kang et al. 212; Li et al. 213) and Pearl River Delta (Lin et al. 213; Liu et al. 212; Tao et al. 212). Nanjing is one of the most important cities in the Yangtze River Delta region. With the influence of human activities, air pollution is very serious. The overall annual haze days have showed a rising trend year by year. In the past 2 years, the aerosol optical properties in Nanjing have been studied (Deng et al. 211; Shen et al. 214; Zhuang et al. 214), but there have been few studies focusing on the optical properties and their impact factors (Deng et al. 211). In this study, a field observation campaign was designed to monitor optical properties, size distributions and chemical compositions of aerosols in autumn in Nanjing, China. The effect of RH, organic carbon (OC), elemental carbon (EC), EC coatings and particle size distribution on optical properties was investigated. 1. Methodology 1.1. Measurement period and location The campaign was carried out in autumn from 14 to 28 November, 212 (15 days). The monitoring station (118.7 E, 32.2 N, see Fig. 1) is situated on the campus of NUIST (Nanjing University of Information Science & Technology) in the northern suburb of Nanjing about 15 km away from the city center. Investigations of aerosol optical properties, chemical components and size distributions were conducted simultaneously inside a trailer. A steel factory is situated 2 km to the east of the sampling site, a chemical industry park is about 1 km to the northeast, and there are residential areas and farmlands to the west and north of the campus. 1.2. Sampling collection and meteorological parameters APM 2.5 cyclone (URG-2-3EH, Chapel Hill Inc., USA) was used to collect aerosol particles at a cutoff size of 2.5 μm at a rate of 16.7 L/min. Using a medium-volume PM 2.5 sampler (Model: HY-1 PM 2.5 1 L/min, Qingdao Hengyuan S.T. Development Co., Ltd., China), daily 12-hr integrated PM 2.5 (from about 7: to 19: (local time) and 19: to 7: for daytime and nighttime samples, respectively) samples were collected on quartz micro 9 mm fiber filters (QMA, Whatman, UK). Meteorological parameters, including visibility (Vis), RH, wind direction (WD), wind speed (WS) and temperature were acquired from an automatic weather station close to the observation trailer. Visibility was measured by a forward scattering visibility meter (CJY-1A, CAMA Measurements & Controls Co., Ltd., China). S H A N D O N G Xuzhou Suqian A N H U I Y E L L O W S E A Lianyungang Gaogou Batan J I A N G S U Huai an Yancheng Xinghua Dongtai Zhenjiang Nanjing Nantong Changzhou Wuxi Suzhou 1.3. Instrumentation 1.3.1. Aerosol scattering and absorption coefficient The B sca and B abs were simultaneously measured by a three-wavelength photo-acoustic soot spectrometer (PASS-3, DMT Inc., USA) with 45, 532, and 781 nm diode lasers, with the time resolution of 2 sec. A scattering sensor with a photomultiplier tube (PMT) detector gives B sca. B abs can be measured based on an advanced photoacoustic technique. The B sca and B abs data were calibrated with high concentration ammonium sulfate for scattering and nitrogen dioxide for absorption before the campaign. A bypass flow through the zero-air filter every 5 min measured the background values. More detailed description on PASS-3 has been given in previous publications (Arnott et al. 25; Flowers et al. 21; Lan et al. 213; Moosmüller et al. 29). Fig. 1 Schematic diagram of the observation location. 1.3.2. The measurement of chemical components A.523 cm 2 sample filter was analyzed by a thermal/optical carbon analyzer (DRI21A, Desert Research Institute, USA) for OC and EC mass concentration. With heating in pure
JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 37 helium at different temperature, four OC fractions (OC1, OC2, OC3 and OC4) and three EC fractions (EC1, EC2 and EC3) are produced. Organic carbon partly turns into the pyrolyzed carbon fraction (OPC) during the heating period. Hence, OC is practically defined as OC1 + OC2 + OC3 + OC4 + OPC and EC as EC1 + EC2 + EC3 OPC. The analyzer was calibrated every day using He/CH 4. Half of each filter was cut into thin strips and added to 2 ml deionized water with 6 min ultrasonic oscillation. The solution was analyzed for water soluble ions by ion chromatography (ICS2 and ICS3, Dionex, USA). Three anions (SO 4 2,NO 3 and Cl ) and five cations (Na +,NH 4 +,K +,Mg 2+ and Ca 2+ ) were determined. The mass concentration of black carbon (BC) was determined by a multiwavelength Aethalometer (Model AE-31, Magee Scientific Company, USA). The Aethalometer measures the light attenuation by the aerosol particles at seven different wavelengths (37, 47, 52, 59, 66, 88 and 95 nm). The black carbon (BC) mass concentration was obtained every 5 min based on the decrease of light transmission through the filter. The BC mass concentration was calculated in the 88 nm channel with the attenuation cross-section of 16.6 m 2 /g according to the manufacturer (Gadhavi and Jayaraman 21; Wu et al. 29). The BC mass was not calibrated because the diurnal variation of BC would be analyzed in this study. Nitrogen dioxide (NO 2 ) was measured by a NO NO 2 NO x analyzer (Model 17i, Thermo Fisher Scientific Inc., USA). A blank sample was measured every day. 1.3.3. Particle number size distribution The particle number size distributions were measured with two instruments, a scanning mobility particle sizer (SMPS) (Model 3936, TSI Inc., USA) and an aerodynamic particle sizer (APS) (Model 3321, TSI Inc., USA) spectrometer. The SMPS includes an electrostatic classifier (Model 38, TSI Inc., USA) with a long differential mobility analyzer (DMA, Model 381, TSI Inc., USA) and an ultrafine Condensation particle counter (CPC, Model 3376, TSI Inc., USA). The Long DMA offers classification in the range from 1 to 1 nm in diameter. The particles are separated according to their electrical mobility, which is inversely related to particle size and proportional to the number of charges on the particles. The number concentration of particles was measured by the CPC. The APS 3321 can obtain the aerodynamic diameter of aerosol based on the real-time measurement of particle time-of-flight velocimetry. The particle size range spanned by the Model 3321 is from.5 to 2 μm. In order to produce a wide range distribution of PM 2.5, Data Merge software (Model 3969, TSI Inc., USA) was used. The SMPS and APS data files were merged into a distribution covering 1 nm to 2.5 μm. The APS particle aerodynamic diameters were converted to actual diameters with a reference particle density of 1.6 g/cm 3 (Flowers et al. 21). 2. Results and discussion 2.1. Time series of optical properties and meteorological factors Time series of optical properties with 1 h averaging including visibility, single scattering albedo (SSA), Ångström scattering exponent (A sca ), Ångström absorption exponent (A abs ), B sca and B abs are shown in Fig. 2. During the whole observation period, the B sca and B abs fluctuated greatly with apparent diurnal variation, andshowedhighervaluesduring14 21 November, and then the aerosol coefficients remained stable with lower values. This may be related to meteorological factors (Fig. 3). Temperature decreased with high wind speed after 21 November. Increasing wind speed favors the horizontal diffusion of pollutants. The average dry B sca (±standard deviation) were 479.4 ± 258.2, 375.7 ± 29.5 and 176. ± 13.7 Mm 1 while B abs were 68.4 ± 53.5, 41.6 ± 18.7 and 28. ± 18.7 Mm 1 at 45, 532 and 781 nm, respectively. The maximum B sca and B abs (at 45 nm) reached 1616.7 and 491.2 Mm 1.ThemeanB sca (at 532 nm) in autumn was lower than the results in Shouxian (581 Mm 1, at 55 nm) and Guangzhou (473 ± 222 Mm 1, at 55 nm) (Fan et al. 21; Tao et al. 214). The mean B abs was also lower than the coefficient of a Peking university site (67 ± 53 Mm 1, at 532 nm) (He et al. 29). Both B sca and B abs in our sampling site were much larger than the values (215.8 ± 222.9 Mm 1, at 525 nm and 6.73 ± 11.6 Mm 1,at 532nm)oftheShangdianziregionalbackgroundstation(SDZ)of China (Yan et al. 28). Visibility is a visual indicator of air quality. When RH < 8% and simultaneous visibility <8 km, haze is considered to occur (Bian 211). The hourly averaged Vis varied from.6 to 15.5 km with an average of 4.3 ± 2.3 km (Fig. 2), indicating that severe pollution frequently occurred during the sampling period. It was lower than the results of the other large Chinese cities such as Xi'an, Guangzhou and Beijing (Cao et al. 212; Jung et al. 29; Liu et al. 214). RH increased with the mean value >7%, accompanied by low visibility and lower dry B sca and B abs (Fig. 3) after 21 November. This indicates that the hygroscopic growth leads to aerosol scattering and absorption enhancement (Liu et al. 212). Single scattering albedo (SSA) is an important parameter for an atmospheric radiation model. SSA is defined as the ratio of the scattering to the extinction coefficients of aerosol. The average SSA was.88 ±.4,.9 ±.3 and.85 ±.6 at blue, green and red light wavelengths, respectively, indicating aerosol light absorption accounted for around 1% of atmospheric extinction. However, the proportion of absorption at 781 nm increased to 15%, suggesting that black carbon was the major light absorbing component. The Ångström exponent represents the spectral variation of aerosol scattering and absorption. The B sca and B abs at 45 and 781 nm were used to calculate the Ångström scattering and absorption exponents (A sca and A abs ) based on the same formula used in previous studies (Flowers et al. 21; Gyawali et al. 212). AsshowninFig.2,themeanA sca and A abs during the sampling period were 1.58 ±.21 and 1.31 ±.4, respectively. The higher A sca suggests that smaller size particles were dominant in the autumn of Nanjing. The Ångström absorption exponent strongly depends on the chemical components. The observed higher A abs may be attributable to the aging of aerosols. Because the sampling site is located at a suburb of Nanjing, the aerosol from agriculture residue burning increased in autumn as the harvest season (Fan et al. 21). During the haze days from 14 to 21 November, the hourly average A sca was lower and then increased after 21 November. This may be due to high relative humidity (Zhuang et al. 214). The haze plume was dominated by larger size particles (Zhang et al. 213a).
38 JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 Vis (km) Angstrom exp. SSA B abs (Mm -1 ) B sca (Mm -1 ) 18 9 2 A sca 1 1..8.6 5 25 18 12 Visibility A abs 45 nm 532 nm 781 nm 6 14 16 18 2 22 24 26 28 Date Fig. 2 Time series of optical properties from 14 to 28 November 212. Vis: visibility; A sca : the Ångström scattering exponent; A abs : the adsorption exponent; SSA: single scattering albedo; B abs : the adsorption coefficient; B sca : the average scattering coefficients. 2.2. Diurnal variations of optical properties and meteorological parameters Fig. 4 shows the diurnal variations of B sca, B abs, A sca, A abs, SSA, Vis, wind speed, RH and mass concentration of BC. Two peaks were observed at 7: and 22: for B abs,532 (B abs at 532 nm) and BC mass concentration. The B sca maintained a higher value from 3: to 11: in the morning and from 19: to 22: in the evening, with the minimum value at about 16:. The diurnal variation of Vis is in contrast to the diurnal variation of B sca and B abs. Two minima were found at about 6: and 22:. The A sca reached a lower value in the morning and were higher in the afternoon. These diurnal cycles were likely associated with meteorological conditions and human activities. The B abs was dominated by black carbon emitted from combustion processes. The apparent peak at 7: was related with the fresh soot emitted from vehicles and cooking residues. This can be evidenced by the valley of A abs at 7:. Yang et al. (29) have reported that the Ångström absorption exponent was lower for fresh plumes. SSA 532 (SSA at 532 nm) shows a valley at 7: due to high light absorption. In the morning, the lower mixing layer depth (MLD) and wind speed brought higher aerosol loading. The B sca and B abs increased and the visibility decreased. Hygroscopic growth was enhanced with the relative humidity increasing in the morning and evening. Accordingly, the particle size increased with lower A sca and enhanced light scattering. 2.3. Impacts on optical properties 2.3.1. Relative humidity Under ambient conditions, relative humidity (RH), as a key factor in visibility impairment, affects light extinction through hygroscopic growth of particles. As RH increases, hygroscopic particles such as (NH 4 ) 2 SO 4 and NH 4 NO 3 increase the scattering cross section and reduce the atmospheric visibility with water uptake (Deng et al. 211; Tsai and Cheng 1999). To gain a preliminary insight into the influence of RH on aerosol optical properties, the visibility plotted against dry extinction, scattering and absorption coefficient (B ext, B sca and B abs ) of PM 2.5 colored by RH is presented in Fig. 5, respectively. It can be clearly seen in Fig. 5 that when RH is lower than 7%, visibility is inversely related to the dry extinction and scattering coefficient (R 2 >.7), and when RH is higher than 8%, visibility is mostly lower than 5 km, being independent of the dry extinction and scattering coefficient. The hygroscopic growth factor (f(rh)) is one of the most important parameters in calculating the direct radiative forcing of aerosols. f ext (RH) = B ep (ambient) / B ep (dry, RH < 4%) is used 36 Wind direction Wind speed 1 Wind direction 24 12 1 RH Temperature 5 2 Wind speed (m/sec) RH (%) 5 1 Temperature ( o C ) 14 16 18 2 22 24 26 28 Date Fig. 3 Temporal variations of meteorological factors during the sampling period in November 212. RH: relative humidity.
JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 39 to represent the hygroscopic growth factor of aerosol extinction. The influence of RH on aerosol extinction is generally reflected by the parameter, f ext (RH). The B ext (Bsca + B abs )measuredbypass-3atdryconditionsisusedasb ep (dry, RH < 4%). The B ep (ambient), as the extinction coefficient of PM 2.5 at ambient conditions, can be calculated by the following equation: B ep ðambientþ ¼ B ext ðambientþ B sg B ag B spðcm where, B ext (ambient) is calculated by the equation B ext (ambient) = 3/Vis based on the manual of the visibility meter. The transfer coefficient (532/55) is used for wavelength correction. The B sg is referred to as Rayleigh scattering (approximately 1 Mm 1 ). The B ag derives mainly from the absorption of NO 2.TheequationB ag =.33 [NO 2 ] (Liu et al. 28) is used to calculate the absorption of NO 2 measured by a NO x analyzer. The B sp(cm) represents the extinction of coarse particles. The B sp(cm) is negligible because coarse particles usually correspond to low extinction (Hand et al. 211). The time series of the calculated B ep (ambient) and B ext (dry) are shown in Fig. 6 colored by ambient RH. We can see that B ep (ambient) and the difference between B ep (ambient) and B ext (dry) increased with increasing RH. Higher RH results in the degradation of visibility. Meanwhile, the hygroscopic growth factor f ext (RH) (Fig. 6) strongly enlarged with theincreaseinrh.theaveragef ext (RH) was 1.8 ± 1.2 with RH ranging from 29% to 85% during the sampling period. The f ext (RH = 85%) in Nanjing is close to the values measured at Lin'an (1.7 ± 2.) by Xu et al. (22) in the Yangtze delta region, China, but is lower than that in Guangzhou (2.4 2.68) (Liu et al. 28). The degradation of visibility with lower dry B sca and B abs after 21 November (Fig. 2 ) can be attributed to the hygroscopic growth of aerosol particles. Þ Fig. 7 shows the variation of hygroscopic growth factor f ext (RH) with ambient RH. To investigate the relationship between the f(rh) and RH, a two-parameter function is used for the best curve fitting: fðrhþ ¼ 1 þ a RH b : 1 The fitting parameters a and b are 3.62 ±.93 and 3.97 ±.78, which are close to the values of mixed aerosol (Liu et al. 28). 2.3.2. Chemical composition Organic carbon (OC) emitted from combustion processes may contain brown carbon (Moosmüller et al. 29). Brown carbon absorbs solar radiation at ultraviolet (UV) and visible wavelengths (Flowers et al. 21). The fresh aerosols can undergo a series of physical and chemical transformation processes in the atmosphere, such as heterogeneous transformation, gas particle conversion, coagulation, and photochemical oxidation. These aging processes can lead to significant changes in aerosol properties. Lack and Cappa (21) found that coating on the BC surface could act as a lens and lead to BC absorption enhancement. To investigate the brown carbon mass absorption efficiency and coating effects, the method of Flowers et al. (21) was applied. The total aerosol mass absorption efficiencies MAC total were 3.6 ±.3, 2.1 ±.2 and 1.4 ±.1 at 45, 532 and 781 nm (Table 1), respectively, using MAC total ¼ B meas ðλþ= ðec mass þ OC mass Þ abs where, B meas abs (λ) is the absorption coefficient measured by PASS-3, EC mass and OC mass are the mass concentration of elemental carbon (EC) and organic carbon (OC) given by DRI 21A. The value of MAC total is calculated by the slope of the BC mass concentration (µg/m 3 ) B abc,532 (Mm -1 ) B sca,532 (Mm -1 ) 3 24 18 12 6 12 9 6 3 8 6 4 2 SSA 532 A abs A sca 1..95.9.85.8 2. 1.8 1.6 1.4 1.2 2.5 2. 1.5 1..5. 6 12 18 24 6 12 18 24 6 12 18 24 Time (hr) Time (hr) Time (hr) RH (%) Wind speed (m/sec) Vis (km) 1 8 6 4 2 4.5 3.6 2.7 1.8.9. 1 5 Fig. 4 Diurnal variations of B sca, B abs, A sca, A abs, SSA, Vis, wind speed, RH and mass concentration of black carbon (BC). The error bars represent standard deviation (S.D.).
4 JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 2 RH 2% 4% 6% 8% a Fitted curve with RH<7% b c Visibility (km) 15 1 5 Vis=2.43/B Vis=2.17/B sca ext R 2 =.72 R 2 =.71.4.8 1.2 1.6.3.6.9 1.2 1.5.5.1.15.2 B ext (km -1 ) B sca (km -1 ) B abs (km -1 ) Fig. 5 Visibility as a function of dry extinction coefficient (B ext ), B sca and B abs at 532 nm from 14 to 28 November, 212. fitting curve in Fig. 8a. The light absorption from uncoated EC (B est abs ) was acquired from: B est abs ¼ MAC denuded EC meas mass where, MAC denuded for denuded soot is the published MAC values (1.6 m 2 /g at 45 nm, 7.32 m 2 /g at 532 nm, and 4.24 m 2 /g at 781 nm). The mass absorption efficiencies of brown carbon (MAC BrC ) determined by (B meas abs B est abs )/OC mass were 1.8 ±.4,.8 ±.2 and.7 ±.1 at three wavelengths. The average absorption enhancement factor for coating was about 1.6 during the sampling period. The value is slightly lower than that observed in a field study (Cross et al. 21). Additionally, the contributions of EC and EC coating were 38% and 64% of B abs,45 (B abs at 45 nm) and 44% and 75% of B abs,532 (B abs at 532 nm). Therefore, OC accounted for 36% of B abs,45 and 26% of B abs,532. Light absorption is enhanced by brown carbon and coatings on EC cores. 2.3.3. Particle size and mass concentration To investigate the influence of aerosol size distribution on the aerosol scattering and absorption, B sca and B abs were plotted vs. f ext (RH) B ext,532 nm (Mm -1 ) 12 8 4 3 2 1 3 38 47 55 63 72 8 RH (%) Dry Ambient 14 16 18 2 22 24 26 28 Date Fig. 6 Time series of B ep (ambient), B ext (dry) and f ext (RH) with relative humidity (RH) shown by color from 14 to 28 November, 212. B ep (ambient) and B ext (dry) refer to the extinction coefficients at ambient and RH < 4% conditions, and f ext (RH) is the hygroscopic growth factor of aerosol extinction. b a the number concentration of PM 2.5 with diameter above 1 nm (N 1 ), as shown in Fig. 9a. The effective average cross-sections of the PM 2.5 (N 1 ) during sampling period are 4.4 1 14 m 1 for scattering and.57 1 14 m 1 for absorption. The relationship between B sca, B abs and fine particle (PM 1 ) can be seen in Fig. 9b. The slope of the linear fitting curve (multiplied by four) can represent the average geometric scattering and absorption efficiencies (Q sca and Q abs ) (Garland et al. 28). The Q sca and Q abs are 1.4 and.13, respectively, which are lower than the results of Guangzhou (Garland et al. 28). Assuming the aerosol density of 1.7 g/cm 3,PM 2.5 mass concentration can be calculated from the size distributions. Fig. 9c shows the B sca and B abs plotted versus PM 2.5 mass concentration. Mass scattering and absorption efficiencies (the slope of the fitting line) are 2.72 and.3 m 2 /g, respectively. All the correlation coefficients are higher than.6. 3. Conclusions During an intensive field campaign from 14 to 28 November 212, the aerosol optical properties, chemical composition and size distribution were investigated in the Northern suburb of Nanjing in YRD of China. The hourly average dry B sca and B abs (at 532 nm) f ext (RH) 8 6 4 2 Curve fitting 2 4 6 8 Relative humility (%) Fig. 7 The relationship between f ext (RH) and RH.
JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 41 Table 1 Total mass absorption efficiencies (MAC total ), brown carbon MAC (MAC BrC ), absorption enhancement factor (f) and the contribution to total absorption. λ (nm) MAC total MAC BrC f EC EC + coating OC 45 3.6 ±.3 1.8 ±.4.38.64.36 532 2.1 ±.2.8 ±.2 1.6.44.75.26 781 1.4 ±.1.7 ±.1 λ: the three wavelengths used in PASS-3; EC: elemental carbon; OC: organic carbon. were 375.7 ± 29.5 and 41.6 ± 18.7 Mm 1, respectively. High B sca and B abs resulted in the decrease of visibility (4.3 ± 2.3 km). The relative low visibility showed that haze frequently occurred in the autumn of Nanjing. Single scattering albedo (SSA) at 532 nm was.9 ±.3, with the value of 1.58 ±.21 for A sca (45/781) and 1.31 ±.4 for A abs (45/781). The higher A sca indicated that smaller size particles were dominant. Aging of aerosol led to an increase in A abs of aerosols. Diurnal variation of optical parameters with multiple peaks was strongly associated with meteorological factors such as wind speed and RH. Values of B sca were higher in the morning and evening. BC emitted from combustion processes resulted in higher B abs, with two peaks at 7: and 22:. The emitted fresh soot in the morning led to a decrease in A abs. SSA showed a valley at 7: due to the light absorption enhancement. The MLD with lower wind speed should be responsible for higher B sca, B abs and lower visibility in the morning. The optical properties of aerosols were affected by many factors. Visibility was inversely related to B ext, B sca and B abs with RH below 7%. The hygroscopic growth factor f ext (RH) was calculated, with an average value of 1.8 ± 1.2. A two-parameter function was appropriate to fit the relationship between f ext (RH) and RH. Total aerosol mass absorption efficiencies MAC total 45 nm 532 nm 781 nm 2 a b 15 c 15 1 (Mm -1 ) B meas abs 1 (Mm -1 ) B meas B est abs abs - 5 5 2 4 6 2 4 6 8 1 1 2 3 4 OC+EC (µg/m 3 ) (Mm -1 ) OC (µg/m 3 ) B est abs Fig. 8 The contribution of organic carbon (OC) and elemental carbon (EC) to light absorption. B meas abs (λ) is the measured absorption coefficient by PASS-3, and B est abs is the light absorption from uncoated EC. B abs and B sca (Mm -1 ) 18 12 6 a B abs y=.57x-2.96 R 2 =.62 B abs y=.32x-3.29 R 2 =.72 B abs y=.3x R 2 =.89 B sca y=.44x+28.5 R 2 =.67 B sca y=.26x+16.6 R 2 =.81 B sca y= 2.72x R 2 =.95 Linear fitting curve Linear fitting curve Linear fitting curve b c 5 1 15 2 2 4 6 2 4 6 N 1 ( 1 3 cm -3 ) PM 1 surface concentration ( 1 3 µm 2 /cm 3 ) PM 2.5 mass (µg/m 3 ) Fig. 9 Correlation between absorption and scattering coefficients and the number concentration of PM 2.5 with diameter above 1 nm (N 1 ), surface and mass concentration. PM 1 and PM 2.5 represent the particles with the diameter less than 1 and 2.5 μm, respectively.
42 JOURNAL OF ENVIRONMENTAL SCIENCES 4 (216) 35 43 were 3.6 ±.3, 2.1 ±.2 and 1.4 ±.1, while MAC BrC were 1.8 ±.4,.8 ±.2 and.7 ±.1 at 45, 532 and 781 nm. The contributions of EC, EC coating and OC to absorption were 44%, 75% and 26%. Light absorption was enhanced by brown carbon and coatings of EC cores. The B sca and B abs increased with increasing N 1,PM 1 surface concentration and PM 2.5 mass concentration. The effective average cross sections of PM 2.5 were 4.4 1 14 m 1 for scattering and.57 1 14 m 1 for absorption. The average geometric scattering and absorption efficiencies (Q sca and Q abs ) were 1.4 and.13. Mass scattering and absorption efficiencies were 2.72 and.3 m 2 /g. 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