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Characterization of Natural and Affected Environments
Humic-like substances #HULIS# in aerosols of central Tibetan Plateau (Nam Co, 4730 m asl): Abundance, light absorption properties and sources Guangming Wu, Xin Wan, Shaopeng Gao, Pingqing Fu, Yongguang Yin, Gang Li, Guoshuai Zhang, Shichang Kang, Kirpa Ram, and Zhiyuan Cong Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01251 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018
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Humic-like substances （ HULIS） ） in aerosols of central
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Tibetan Plateau (Nam Co, 4730 m asl): Abundance, light
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absorption properties and sources
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Guangming Wu1,7, Xin Wan1,7, Shaopeng Gao1, Pingqing Fu2, Yongguang
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Yin3, Gang Li5, Guoshuai Zhang1, Shichang Kang4,6, Kirpa Ram8,
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Zhiyuan Cong1,6*
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
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5 6 7 8
Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China Institute of Arid Meteorology, China Meteorological Administration, Lanzhou 730020, China Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi 221005, India
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ABSTRACT
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Humic-like substances (HULIS) are major components of light-absorbing brown
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carbon that play an important role in Earth’s radiative balance. However, their
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concentration, optical properties and sources are least understood over Tibetan Plateau
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(TP). In this study, the analysis of total suspended particulate (TSP) samples from
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central of TP (i.e., Nam Co) reveal that atmospheric HULIS are more abundant in
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summer than that in winter without obvious diurnal variations. The light absorption
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ability of HULIS in winter is 2 to 3 times higher than that in summer. In winter,
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HULIS are mainly derived from biomass burning emissions in South Asia by
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long-range transport. In contrast, the oxidation of anthropogenic and biogenic
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precursors from northeast part of India and southeast of TP are major sources of
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HULIS in summer.
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Keywords: HULIS, Aerosol, Tibetan Plateau, Optical properties, Sources
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INTRODUCTION
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Carbonaceous aerosols are ubiquitous in the atmosphere, which play an
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important role in regional and global climate system by direct and indirect effect on
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Earth’s energy balance.1, 2 Recent studies have highlighted that, in addition to the
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well-known black carbon (BC), certain type of organic aerosols also exert
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considerable light absorption capability.3-6 These kind of organic aerosols are referred
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to as brown carbon (BrC), which exhibit abruptly increased absorption from visible to
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ultraviolet wavelength due to their strong wavelength dependence.3 Chung, et al.
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estimated that the global direct radiative forcing of carbonaceous aerosol was about
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0.65 Wm-2, approximately 20% of which could be attributed to the absorption by BrC.
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The chemical composition and optical property of BrC are strongly dependent on
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emission, transport and formation processes. However, physico-chemical and optical
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properties of BrC are poorly understood and demand a systematic study, particularly
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over the pristine and climatic sensitive region of Tibetan Plateau (TP).
7
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Atmospheric humic-like substances (HULIS) are class of macromolecular
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organic compounds prevalent in diverse environmental media (e.g., aerosol and rain
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water), which can be easily isolated and recognized as important components of
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BrC.8-10 They can be emitted directly during the process of biomass burning.11
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Biopolymers like cellulose and lignin could thermally breakdown to (semi-) volatile,
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low molecular weight substances, which could further condense to higher molecular
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weight HULIS through gas-phase reactions.9 HULIS could also be produced by
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complex secondary reactions (e.g., heterogeneous and photosensitized reactions) from
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anthropogenic or biogenic volatile organic compounds (VOCs).8, 12
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The Tibetan Plateau (TP) is the highest and the most extensive plateau in the
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world with average the elevation higher than 4000 m asl (above sea level) and the
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land area approximately 2.5 × 106 km2.13 Due to high altitude and thin air, the solar
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radiation is relative strong over TP with annual mean value of 205 W m-2, which is
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larger than those over most areas on the earth.14 The TP holds more than 105 km2
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mountain glaciers, which are sensitive to the darkening of snow surface by deposition
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of carbonaceous and dust aerosols15, 16 and direct heating in atmosphere.17 During the
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outbreak period of atmospheric brown cloud over South Asia, the air pollutants could
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be easily transported to the central of TP (i.e., Nam Co) within a few days,18 which
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may have considerable impact on the air conditions over the TP. Several field
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campaigns and modeling studies have been carried out about the probable sources,
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transport and climate effect of carbonaceous aerosols over TP.16, 19, 20 However, most
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of these studies mainly focused on BC and total OC (organic carbon) with less
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attention on BrC aerosols. Previous studies have reported the ratio of OC/BC can
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reach up to 34.8 at Nam Co19 and 17.7 at Lulang21, highlighting the potential
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importance of BrC aerosols over TP. Nevertheless, the scientific knowledge on
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abundance as well as optical properties of BrC is extremely limited over the TP, which
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hinder the comprehensive understanding of the role of carbonaceous aerosols in
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regional climatic system.
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In this work, we investigated the diurnal variations of concentrations and optical
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properties of isolated BrC (i.e., HULIS) at the central part of TP during winter and
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summer. Our goals are to understand their sources and formation process in the
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pristine environment of TP and to provide a basic dataset for optimization of regional
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climate modeling.
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EXPERIMENTAL SECTION
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Research site and aerosol sampling. The total suspended particle (TSP)
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samples were collected at Nam Co station (30°46.44′N, 90°59.31′E, 4730 m asl,
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Figure S1), which was located at central part of TP. The atmospheric conditions at
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Nam Co represent a pristine environment with low annual aerosol optical depth (AOD
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value of 0.029 at 550 nm).22 The geographic characteristics at Nam Co region include
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mountains, glaciers, lakes, rivers and grassland, and thus can be considered as a
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representative of the diversity of TP. The meteorological parameters during the
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sampling period are shown in Figure S2. The aerosol samples were collected from
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December 21, 2014 to February 2, 2015 and August 3 to September 7, 2015 on a
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day/night pattern by high-volume aerosol sampler (TH-1000 CH, Wuhan Tianhong
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Instrument Co., Ltd, China). Sixty TSP samples in total were collected and stored
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frozen in prebaked glass jars (150 mL) until further analysis.23 More detailed ACS Paragon Plus Environment
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information can be found in Supporting Information.
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Isolation and measurement of HULIS. The method used to isolate HULIS
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from aerosol was based on the HLB solid phase extraction protocol, which has
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excellent reproducibility and high recovery yield.24 Briefly, an aliquot of 30 cm2
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filters was extracted with 10 ml ultrapure water (Millipore, USA) for 30 min under
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ultrasonication and the extract was purified with syringe-driven filter (MillexGV
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PVDF, 0.22 µm; Millipore, Ireland). Then the extract was acidified with HCl to pH of
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2 for the subsequent isolation. Oasis HLB (30 µm, 60 mg/cartridge, Waters, USA)
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cartridge was pre-conditioned with methanol and ultrapure water before loading the
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prepared extracts. The cartridge was subsequently rinsed with ultrapure water to wash
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away the possible residues and drying in inert (100% N2) environment. Finally,
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HULIS were eluted from the cartridge with 5 ml methanol, evaporated to dryness
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under gentle stream of nitrogen and re-dissolved into ultrapure water for the
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measurement of light absorption property and concentration. During that process, the
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flow rate was kept below 1 ml min-1. The recovery of this method was tested with
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SRFA (Suwannee River Fulvic Acid, International Humic Substances Society), which
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were recognized as standard substances of HULIS.9, 25 Our results indicated that the
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extraction efficiency of HLB for HULIS was better than 85%.
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The concentration of HULIS was measured by total carbon analyzer (TOC-L,
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Shimadzu, Japan) with standard deviation of reproducibility test less than 3%. The
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light absorption spectra of HULIS were measured by UV-Vis Spectrophotometer
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(UV-3600, Shimadzu, Japan) at low-speed mode between 200 to 800 nm in a 1-cm
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path length quartz cuvette. During the measurement, ultrapure water was introduced
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as blank sample for the data correction. The light absorbance (A) of HULIS can be
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directly obtained by weighting the intensities of incident and transmitted light signal.
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The light absorption capability, also known as mass absorption efficiency (MAE, unit:
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m2 g-1), was estimated in equation (1), which represents the light absorbance per unit
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mass of HULIS.26, 27
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= −
×
× ×
× ln 10
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where Vw (ml) is the volume of extraction, V (m3) refers to the volume of air sampled
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through the filter during the sampling, L (cm) is the path length of light, and C (µg m-3)
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corresponds to the concentration of HULIS. To eliminate the possible errors due to the
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baseline drift, light absorbance was referenced to the absorption at the wavelength of
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700 nm (the average value between 695 and 705 nm).
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Absorption Ångström exponent (AAE) is important parameter to characterize the
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wavelength dependence of light absorption capability of HULIS. It can be derived
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from the MAE values between wavelength λ1 and λ2:
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AAE = ln / ln
(2)
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OC, EC, WSOC and organic tracer measurement. The OC and EC
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concentrations were measured by carbon analyzer (DRI model 2001, Desert Research
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Institute, USA) follow the well-established IMPROVE-A thermal/optical reflectance
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protocol28 with analytical uncertainty less than 5%. The WSOC was analyzed with
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total carbon analyzer after the extraction by ultrapure water. The organic tracers (i.e.,
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levoglucosan, malic acid, pimelic acid, 2-methylglyceric acid, 2-methyltetrols,
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β-caryophyllinic acid and 2, 3-dihydroxy-4-oxopentanoic acid (DHOPA)) were
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measured by chromatography/mass spectrometry (GC/MS) after derivatization with
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99% N, O-bis-trifluoroacetamide (containing 1% trimethylsilyl chloride)/ pyridine
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(2:1; v/v) solution. The relative standard deviation in duplicate analysis for these
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organic tracers was lower than 15%.28 More detail informations concerning these
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processes can be seen in the supporting materials.
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RESULTS AND DISCUSSION
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Abundances and variations of carbonaceous species. The concentrations of
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OC, EC, WSOC and HULIS at Nam Co as well as their ratios are summarized in
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Table 1. The average concentrations of OC in winter and summer are 1.94 ± 1.58 µg C
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m-3 and 0.85 ± 0.66 µg C m-3, respectively, and the corresponding abundances of EC
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are 0.19 ± 0.31 µg C m-3 and 0.07 ± 0.11 µg C m-3, respectively. The mass
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concentrations of OC and EC exhibit distinct changes with winter concentrations
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almost a factor of 2 higher than those in summer (Figure 1). Similar seasonal variation
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has also been reported by previous studies at Lhasa29 and the south edge of TP (Mt.
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Everest).30 However, relatively higher OC/EC ratios (14.1 ± 8.2 in winter and 13.8 ±
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5.5 in summer) emphasize that OC is the dominant composition of carbonaceous
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aerosol at Nam Co.
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The average abundance of WSOC during winter and summer are 0.35 ± 0.12 µg
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C m-3 and 0.41 ± 0.19 µg C m-3, respectively, contributing to about 50% of OC in
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summer and 18% in winter. HULIS have been reported as the dominant components
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of WSOC worldwide and can make up to almost 72% of the latter.31 In this study, the
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concentrations of atmospheric HULIS are 0.15 ± 0.11 µg C m-3 and 0.22 ± 0.09 µg C
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m-3 in winter and summer, respectively (Table 1 and Figure 1). They are nearly 1 to 10
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- fold lower than those in aerosols from European and Oceania cities, 10 to 30 times
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lower than those from east of China and Japan, and 100 times lower than that from the
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Amazon forest (Table S1). This indicates Nam Co can be taken as a regional
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background site with relatively low absolute concentrations of HULIS. Nevertheless,
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the HULIS contributes to about 40-60% of WSOC and 12-31% of OC at Nam Co,
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which makes it an important component of WSOC and OC.
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Table 1. The average abundances of HULIS and other carbonaceous aerosols at Nam Co, TP. Winter
Summer
Daytime
Nighttime
Average
Daytime
Nighttime
Average
OC (µg C m-3)
1.69±1.29
2.22±1.85
1.94±1.58
0.97±0.87
0.75±0.35
0.85±0.66
EC (µg C m-3)
0.18±0.21
0.21±0.40
0.19±0.31
0.07±0.13
0.07±0.09
0.07±0.11
WSOC (µg C m-3)
0.38±0.13
0.32±0.11
0.35±0.12
0.40±0.18
0.41±0.21
0.41±0.19
HULIS (µg C m-3)
0.15±0.11
0.15±0.12
0.15±0.11
0.22±0.08
0.23±0.09
0.22±0.09
OC/EC
12.9±8.0
15.9±8.3
14.1±8.2
10.6±5.0
23.7±30.9
WSOC/OC
0.32±0.21
0.23±0.14
0.28±0.18
0.50±0.19
0.57±0.20
0.55±0.20
HULIS/WSOC
0.37±0.21
0.41±0.21
0.38±0.21
0.58±0.15
0.60±0.14
0.59±0.15
HULIS/OC
0.10±0.09
0.10±0.10
0.12±0.14
0.29±0.13
0.33±0.11
0.31±0.12
25.0±21.7
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Figure 1. The concentrations (seasonal and diurnal) of OC, EC, HULIS and WSOC in
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the aerosols collected at Nam Co, TP.
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Figure 2. The ratios of OC/EC, WSOC/OC, HULIS/WSOC and HULIS/OC in the
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aerosols from Nam Co, TP.
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From Figure S2, different meteorological conditions were found between
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daytime and nighttime at Nam Co. However, the abundances of atmospheric HULIS
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are 0.15 ± 0.11 µg C m-3 (daytime) and 0.15 ± 0.12 µg C m-3 (nighttime) in winter,
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0.22 ± 0.08 µg C m-3 (daytime) and 0.23 ± 0.09 µg C m-3 (nighttime) in summer
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(Table 1) with weak diurnal variations. Similar variation pattern also existed in other
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carbonaceous species (i.e., OC, EC and WSOC) as well as the contributions of HULIS
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to WSOC and OC (Figure 2). This indicates that they may not formed locally but
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derived from the long-range transport from other regions or produced during the
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transport processes. This is quite possible as Nam Co region represents a pristine
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environment and local emissions of VOCs are negligible.
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Optical properties of HULIS. The optical properties of HULIS in Nam Co
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aerosol are presented in Table 2. The values of MAE365 in winter daytime (nighttime)
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and summer daytime (nighttime) are 0.55 ± 0.27 m2 g-1 (0.73 ± 0.48 m2 g-1) and 0.23
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± 0.10 m2 g-1 (0.28 ± 0.14 m2 g-1), respectively. The MAE365 value exhibits strong
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seasonal variation and is 2 to 3 times higher in winter than that in summer (Table 2,
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Figure S3), which indicates stronger light absorption capability of HULIS in winter.
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This may due to different sources and formation processes. For example, Fan, et al. 32
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found that atmospheric HULIS from biomass burning emissions have higher light
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absorption capability than that from photochemical reactions at the Pearl River Delta,
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South China. Therefore, the higher MAE values in winter may be correlated with
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biomass burning emissions. Similar seasonality has been reported at the Himalaya by
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Kirillova, et al.
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monsoon. Comparing with the optical properties of HULIS from published literatures,
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the MAE365 values in Nam Co winter were similar to those (MAE365, 0.62 - 0.76 m2
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g-1) in the biomass burning influenced Amazon basin.11 In addition, MAE365 values in
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our summer samples are comparable to those (MAE355, 0.32 ± 0.04 m2 g-1) from an
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Central European background site, k-puszta (Hungary), where substantial amount of
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VOCs are emitted from forests and grasslands.34 Comparing with other well-known
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light-absorbing substances in aerosol, the MAE365 of HULIS at Nam Co are 18
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(winter) and 45 (summer) times lower than that of freshly emitted BC (MAE550=7.5
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, which the MAE365 values of BrCws in winter higher than that in
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m2 g-1, AAE=1).35
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Table 2. The light-absorbing properties of HULIS in the atmosphere at Nam Co, TP. Winter
HULIS
Summer
Daytime
Nighttime
Average
Daytime
Nighttime
Average
MAE365 (m2 g-1)
0.55±0.27
0.73±0.48
0.64±0.40
0.23±0.10
0.28±0.14
0.25±0.12
AAE(300-400 )
7.14±2.27
7.60±2.77
7.37±2.54
9.35±2.47
9.02±2.71
9.28±2.59
MAE250 (m2 g-1)
4.22±1.87
4.78±2.06
4.49±1.98
1.96±0.50
2.08±0.76
2.02±0.66
E250/E365
8.67±5.53
8.09±4.75
8.39±5.17
10.1±4.6
9.29±5.75
9.77±5.27
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The parameter AAE(300-400) indicates absorption Ångström exponent calculated in
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the wavelengths from 300 to 400 nm because the strong light absorption of HULIS
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occurs in this range.8, 36 The averaged AAE(300-400) value of HULIS in winter daytime
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(nighttime) and in summer daytime (nighttime) are 7.14 ± 2.27 (7.60 ± 2.77) and 9.35
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± 2.47 (9.02 ± 2.71), respectively (Table 2). This suggest that the light absorption by
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HULIS in Nam Co aerosols, especially absorption Ångström exponent, is more
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variable compared to BC, whose AAE value is typically around one over ultraviolet to
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visible wavelength range for freshly emitted from biomass burning.8, 37 This highlights
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the importance of HULIS in solar absorption, especially over the ultraviolet
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wavelength range. Furthermore, this may also has far-reaching impact on the
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tropospheric photochemistries in atmosphere like the formation of ozone and other
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radicals (e.g., OH and HO2).3, 11, 38 Moreover, these values are slightly larger than the
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reported AAE (6.4, daytime and 6.8, nighttime) from the biomass burning aerosols,11
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and even larger than the AAE values (5.87 ± 0.49) at the European forest-covered
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site,34 pointing out the more wavelength dependent of HULIS’s light absorption
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properties in the Nam Co aerosols. In addition, the wavelength dependence of HULIS
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at Nam Co region also presents a clear seasonal variation with the AAE(300-400) values
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higher in summer compared to that in winter (Table 2). This trend is reverse to the
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seasonal variation of MAE365, which have stronger light absorption capability in
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winter (Table 2). Namely, HULIS in summer exhibit higher AAE value while weaker
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light absorption capability (i.e., MAE) than those in winter, which may have strong
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implication of their seasonal variations in sources or the chemical reaction processes.
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It has been documented that secondary organic aerosols tend to have stronger
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wavelength dependence with less absorption than primary organic aerosols in the long
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wavelength.39 Recent studies like Ajtai, et al.
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applied the AAE values of different aerosols to the source apportionment studies.
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However, due to the limited data support, this study cannot give precise explanation
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about the relationship between AAE value and chemical compositions. Moreover,
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according to the research by Dinar, et al.25, different extraction protocols used in the
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isolation of HULIS may have considerable impact on its chemical compositions.
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Therefore, the influence by extraction procedures on the chemical compositions of
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HULIS are also needed consideration in future research.
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, Utry, et al.
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and Pinter, et al.
42
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Source attribution of HULIS. In winter, HULIS have moderate correlations
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with OC (R2=0.43, P
