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Size-segregated sugar composition of transported dust aerosols from Middle-East over Delhi during March 2012
Atmospheric Research 189 (2017) 24–32
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Size-segregated sugar composition of transported dust aerosols from Middle-East over Delhi during March 2012 S. Kumar a,b, S.G. Aggarwal a,b,⁎, P.Q. Fu c, M. Kang c, B. Sarangi a,b, D. Sinha d, R.K. Kotnala a,b a
Environmental Sciences and Biomedical Metrology Division, CSIR-National Physical Laboratory, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory Campus, New Delhi 110012, India Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China d Government Nagarjun Post Graduate Science College, Raipur 492010, India b c
a r t i c l e
i n f o
Article history: Received 3 September 2016 Received in revised form 15 January 2017 Accepted 19 January 2017 Available online 21 January 2017 Keywords: Aerosols Size-segregated Sugars Trehalose Tracers
a b s t r a c t During March 20–22, 2012 Delhi experienced a massive dust-storm which originated in Middle-East. Size segregated sampling of these dust aerosols was performed using a nine staged Andersen sampler (5 sets of samples were collected including before dust-storm (BDS)), dust-storm day 1 to 3 (DS1 to DS3) and after dust storm (ADS). Sugars (mono and disaccharides, sugar-alcohols and anhydro-sugars) were determined using GC–MS technique. It was observed that on the onset of dust-storm, total suspended particulate matter (TSPM, sum of all stages) concentration in DS1 sample increased by N2.5 folds compared to that of BDS samples. Interestingly, fine particulate matter (sum of stages with cutoff size b2.1 μm) loading in DS1 also increased by N 2.5 folds as compared to that of BDS samples. Sugars analyzed in DS1 coarse mode (sum of stages with cutoff size N2.1 μm) samples showed a considerable increase (~1.7–2.8 folds) compared to that of other samples. It was further observed that mono-saccharides, disaccharides and sugar-alcohols concentrations were enhanced in giant (N 9.0 μm) particles in DS1 samples as compared to other samples. On the other hand, anhydro-sugars comprised ~13–27% of sugars in coarse mode particles and were mostly found in fine mode constituting ~66–85% of sugars in all the sample types. Trehalose showed an enhanced (~2–4 folds) concentration in DS1 aerosol samples in both coarse (62.80 ng/m3) and fine (8.57 ng/m3) mode. This increase in Trehalose content in both coarse and fine mode suggests their origin to the transported desert dust and supports their candidature as an organic tracer for desert dust entrainments. Further, levoglucosan to mannosan (L/M) ratios which have been used to predict the type of biomass burning influences on aerosols are found to be size dependent in these samples. These ratios are higher for fine mode particles, hence should be used with caution while interpreting the sources using this tool. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Deserts are the major sources of mineral dust aerosols which can be transported over long distances (Fu et al., 2012; Park and Cho, 2013; Wang et al., 2009; Wang et al., 2013; Yamaguchi et al., 2012) to different parts of the globe. Dust-storms are known to significantly impact on the chemical composition and mass-size distribution of particles, thereby affecting the aerosol physical and microphysical properties (Agarwal et al., 2010; Fu et al., 2012; Wang et al., 2009; Wang et al., 2013). Most importantly, such events have been reported to increase the number of pathogenic micro-organisms (Nourmoradi et al., 2015; Yamaguchi et al., 2012) and metals including criteria pollutants like Ni (Kumar et al., 2016) which might impact adversely on the air quality and human ⁎ Corresponding author at: Environmental Sciences and Biomedical Metrology Division, CSIR-National Physical Laboratory, New Delhi 110012, India. E-mail address: [email protected] (S.G. Aggarwal).
http://dx.doi.org/10.1016/j.atmosres.2017.01.012 0169-8095/© 2017 Elsevier B.V. All rights reserved.
health (Chiu et al., 2008; Nourmoradi et al., 2015; Singh and Naseema Beegum, 2013; Yamaguchi et al., 2012). Out of the several known dust source regions which include Saharan, Middle-East, and Central Asian deserts of Mongolia and China (Prospero et al., 2002), North-India is impacted mostly by dust-storms originating from neighbouring Thar desert or occasionally from Middle-East deserts (Kang et al., 2016; Kumar et al., 2016; Ram et al., 2016; Singh and Naseema Beegum, 2013). Several studies related to effects of duststorm on radiative forcing have been carried out in and around New Delhi (Ram et al., 2016; Singh and Naseema Beegum, 2013). However, studies with the focus on the chemical compositions of these dust aerosols are very scarce, especially from this region (Kang et al., 2016; Kumar et al., 2016). Further, globally the studies on size segregation and organic compositions of dust aerosols are also very limited (Agarwal et al., 2010; Fu et al., 2012; Wang et al., 2013). Size segregated studies of organic composition are useful for studying the health effects and sources of aerosols (Agarwal et al., 2010; Fu et
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al., 2012; Simoneit et al., 1999; Simoneit et al., 2004; Urban et al., 2014; Wang et al., 2013). Sugars are an important component of water-soluble organic carbon (WSOC) fraction of aerosols (Agarwal et al., 2010; Fu et al., 2012; Fu et al., 2016; Simoneit et al., 2004, 1999). It is found to be ubiquitous and have been reported to be originating from several sources (Agarwal et al., 2010; Aggarwal and Kawamura, 2009; Fu et al., 2012; Fu et al., 2016; Simoneit et al., 1999; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013; Yttri et al., 2007; Yttri et al., 2011). Primary saccharides such as fructose, glucose, sucrose, and trehalose in aerosols are found primarily from sources like developing leaves, phloem of plants as well as suspended soil dusts (affected by biomass burnings) and also form a major fraction of soil organic matter (Yttri et al., 2007; Yttri et al., 2011). Sugar-alcohols (arabitol, mannitol and inositol) and trehalose have been attributed to the biogenic emissions from fungal spores (Fu et al., 2016; Simoneit et al., 2004; Wang et al., 2009). Anhydro-sugars, i.e. levoglucosan, mannosan and galactosan are specifically related to the biomass burnings as they are formed by pyrolysis of cellulose, lignin and starch found in abundance in biomass (Fine et al., 2002; Nolte et al., 2001; Schauer et al., 2001; Simoneit et al., 1999). Presence of sugars and their variable concentrations have often been interpreted as tracers for different sources (Agarwal et al., 2010; Aggarwal and Kawamura, 2009; Fu et al., 2012; Fu et al., 2016; Scaramboni et al., 2015; Simoneit et al., 1999; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013; Yttri et al., 2007; Yttri et al., 2011). For example, levoglucosan is the most widely used tracer for the biomass burning in aerosols (Fine et al., 2002; Nolte et al., 2001; Schauer et al., 2001; Simoneit et al., 1999). Ratio of levoglucosan to mannosan has been further used to interpret the type of biomass burning tracers (Cheng et al., 2013; Engling et al., 2014; Urban et al., 2014). Similarly, the presence of sugar-alcohols has been related with biogenic emissions. Total sugar concentration has been also suggested to be a tracer for biomass burnings (Scaramboni et al., 2015). The high abundances of these saccharides in the samples suggest a significant contribution of re-suspended soil organic matter from agricultural activities to the tropospheric aerosols (Rogge et al., 2007; Simoneit et al., 2004; Yttri et al., 2011). In this study, we present the size segregated sugar composition in urban aerosols collected during an intense dust-storm originating in Middle-East and crossed over New Delhi on March 20–22, 2012 (Fig. 1). This dust-storm has been reported to impact significantly on regional climate. Some studies on physical properties of these dust aerosols have been reported recently (Singh and Naseema Beegum, 2013).
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Also a study has been undertaken to determine selected metal composition of aerosols collected during this dust-storm (Kumar et al., 2016). Here in this study, we characterize the size-segregated sugar in dust-storm affected urban and urban background aerosols. As per our best of knowledge, this is the first study done from Indian region to assess the size-segregated sugar compositions in the transported dust aerosols during a severe dust-storm which originated in Middle-East desert. In this study, the compositions of mono-saccharides, disaccharides, sugar alcohols and anhydro-sugars are discussed. Our results revealed that these sugars (especially trehalose) can be used as organic tracers for transported dust from desert and arid regions. 2. Materials and methods 2.1. Sampling and analysis Delhi encountered dust-storm on March 20, 2012 and was under the envelope of dust for nearly two days (Fig. 1). Size segregated aerosol sampling was performed using Andersen impactor sampler having eight stages with cutoff diameters of N 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.7, 0.4 μm and a backup filter (i.e. with nine size bins: N 9.0, 9.0–5.8, 5.8– 4.7, 4.7–3.3, 3.3–2.1, 2.1–1.1, 1.1–0.7, 0.7–0.4 and b 0.4 μm). Pre-baked (at 450 °C, at least for 6 h) quartz filters were used for sampling and the sampler was operated at an average flow rate of 28.3 l/min on the rooftop of National Physical Laboratory (NPL) at ~ 15 m above ground level (agl). NPL site is located in central Delhi, a representative urban site (discussed in details in Kumar et al., 2015, Fig. 1). Cascade impactor samplers have an inherent problem of particle resuspension but it can be minimized if (1) overloading is avoided and (2) the impaction surface is having materials which can capture or hold the impacting particles appropriately (Dunbar et al., 2005). Since we use the sampler at the flow-rate of 28.3 lpm (low-volume) and quartz filters, we assume a negligible resuspension. Also during the dust-event days when the chances of overloading are high, we have reduced the sampling time from usual 72 h to 24 h and thus assume only a little effect of the resuspension. Briefly, this site is located in central Delhi, which is about half a km away from the busy road, and is surrounded by institutional and residential areas, and protected ridge vegetation (Google imagery of the sampling site is shown in Fig. 1). In total five set of aerosol samples were collected. 72 h aerosol samplings were performed on March 06, 2012 and April 03, 2012 representing before dust-storm (BDS) period and after dust-storm (ADS) period, respectively. During dust-storm
Fig. 1. In the left panel, three days backward trajectories of the air parcel (using NOAA HYSPLIT backward trajectory at 1000 m altitude on reanalysis data) reaching over New Delhi on the five sampling days are shown. On March 20, 2012 the trajectory can be seen to be passing over the Arabian sea which suggests that they might have originated from the Middle-East. Published literature for this period clearly shows the satellite image of dust originating from the Middle-East and crossing over New Delhi (Kumar et al., 2016). Right upper panel shows the Google imagery of Delhi state and the lower right panel shows the sampling site image.
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(DS), ~ 24 h aerosol samplings (three sets) were performed starting from March 20 to March 23, 2012 (hereafter, these three sets of samples are referred as DS1, DS2 and DS3 samples, respectively). Andersen sampler was started in the evening of March 20, 2016 and run for a period of about twenty three and half hours and the filters were replaced. Sampling was resumed after the filter change and similarly conducted for two more days to collect ~24 h aerosol samples. Sum of mass concentrations found in all size-bins is referred as total suspended particulate matter (TSPM) in text here after. Blank filter samples were also collected by loading the filter in sampler without sucking air and removed immediately. After sampling, each filter was packed in separate glass bottle and stored in a refrigerator at ~4 °C. 2.2. Extraction of Sugars and analysis Sugars were extracted from filter samples by taking a piece of filter in a glass bottle and adding 10 ml of dichloromethane/methanol (2:1, v/v) solution and ultrasonicating this for 10 min. This process was repeated thrice for complete extraction. The solvent extracts were filtered through quartz wool packed in a Pasteur pipette, and then concentrated using a rotary evaporator, and then air dried with pure nitrogen gas. The extracts were reacted with 50 μl of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) (contains 1% trimethylsilyl chloride and 10 μl of pyridine) at 70 °C for 3 h. After being reacted, the derivatives were added with 40 μl of n-hexane containing 1.43 ng/μl of the internal standard (C13 n-alkane) prior to GC–MS analysis (Aggarwal and Kawamura, 2009; Fu et al., 2012). Gas Chromatography – Mass Spectroscopy (GC–MS) analysis were performed on Agilent model 7890 GC coupled to Agilent model 5975C MSD. Sugar separation by GC was achieved on a DB-5 m s fused silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness) with a GC oven temperature program: temperature was increased from 50 °C (2 min) to 120 °C at a rate of 30 °C/min and further it was ramped to 300 °C at a rate of 6 °C/min with a final isotherm hold at 300 °C for 16 min. The GC injector was maintained at 280 °C. The mass spectrometer was operated on Electron Ionization (EI) mode at 70 eV and scanned over the range of 50–650 Da. GC–MS response factors were determined with authentic standards. Fragment ions of sugar compounds at m/z = 217 and 204 were used for quantification. Field blank filters were treated as real samples for quality assurance. Also quartz filters spiked with known concentrations of sugar standards were analyzed for their recoveries and some real samples were analyzed in duplicate for the calculation of repeatability. Recoveries of sugars were generally better than 80% (individual sugar recoveries: glucose N 90%; levoglucosan N90%; trehalose 90%; sucrose N85%; arabitol N80%; fructose.80%; mannitol N 80%; inositol N70%). Relative standard deviation of the concentrations based on duplicate analyses was generally b10%. The results showed negligible background concentration for the targeted species in filter blanks. Sum of sugars determined in all sizebins is referred as total concentration of sugar.
Guanzhong Basin, central China) and Mt. Tai (North China Plain, east China), which was originated from Gobi desert, respectively. In general, dust-storms have been observed to increase particle mass loadings in the coarse mode (Agarwal et al., 2010; Wang et al., 2013). In this study, percentage contribution of PM mass in coarse (PM N 2.1 μm) mode is 68.5, 70.8, 66.5, 59.5 and 64.8% of TSPM on BDS, DS1, DS2, DS3 and ADS sampling days, respectively. Also, particulate concentration is observed to be maximum in DS1 samples (255 μg/m3) in coarse mode. Mass concentration of particles in fine mode (PM b 2.1 μm) is observed to be 117, 308, 324, 400 and 149 μg/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively, Fig. 2A and B. Particulate concentration in DS1 samples in fine mode is ~ 2.5 times higher than that observed in BDS samples and it increased further in DS2 and DS3 samples. This further increase in fine particulate concentrations in DS2 and DS3 samples is probably due to the continuous addition of fine mode particles from urban anthropogenic sources in the accumulation mode. Comparatively slower removal rate of fine particles than that of coarse particles probably resulted in fine PM loadings to increase in DS2 and DS3 samples. Further it was also observed that all the samples show the characteristic bimodal distribution for urban aerosols suggesting that urban contribution was still considerable. We observed a characteristic fine mode peak in all samples in 1.1–2.1 μm size range which is generally characteristic of combustion emissions (Bounanno et al., 2009). Another peak is observed in 5.8–9.0 μm range which signifies the contribution of suspended dusts. Bounanno et al. (2009) showed the lognormal mass-size distribution of various sources including urban, traffic and vegetation burning and they have mentioned that all these distributions are bimodal. Further they reported that the magnitude of particle mass is high in fine mode particles emitted from traffic and vegetation burning sources and lower in urban sources. In this study, we found that the peaks in fine and coarse modes are comparable in magnitude, possibly suggesting that these urban aerosols are affected by biomass burnings, traffic emissions as well as also the dust resuspension (accounts for higher concentration of coarse mode particles). Considering the mass-size distribution of PM in giant mode (PM N 9.0 μm), it was observed that the percentage contribution of giant particles increased considerably in DS1 samples (20.33%, 214.1 μg/m3) compared to BDS (14.19%, 52.9 μg/m3), DS2 (18.85%, 182.4 μg/m3), DS3 (11.90%, 117.6 μg/m3) and ADS (13.50%, 57.0 μg/ m3) samples as expected. This increase was also observed in ultra-fine (PM b 0.4 μm) particles (i.e. 0.89% (3.31 μg/m3), 6.07% (64.00 μg/m3), 6.02% (58.26 μg/m3), 8.35% (82.53 μg/m3) and 1.96% (8.27 μg/m3)in BDS, DS1, DS2, DS3 and ADS samples, respectively), suggesting that dust-storms also increase the ultra-fine particles which is likely to become more prominent in the days to follow as observed in DS2 and DS3 samples. This increase in ultra-fine particles may be due to the possibly enhanced lifetime of these particles compared to coarse particles in the atmosphere (where the wind speed during DS1, DS2 and DS3 was recorded to be 14 ± 3.5, 17 ± 1.4 and 16 ± 3.5 km/h, respectively (Kang et al., 2016)), and also possibly because of the promotion of new particle formation and growth due to the polluted dust (Nie et al., 2014; Sarangi et al., 2015).
3. Results and discussion 3.2. Total sugar distribution during the study period 3.1. High particle mass loading observed during the dust-storm Sum of particle mass concentration of all the stages and the backup filter provides total suspended particulate matter (TSPM) mass concentration. This is observed to be 1053, 968 and 988 μg/m3 in DS1, DS2 and DS3 samples, respectively at New Delhi sampling site. Similar episodic high mass concentration values (1031 and 10,702 μg/m3) have been reported for the Delhi and Bikaner (a city in Thar desert) aerosols affected by dust-storm arising from Thar desert in May 2000, respectively (Yadav and Rajamani, 2006). Similarly, Wang et al. (2013) observed high PM10 mass concentration values, i.e. 506 ± 331 and 1343 ± 450 μg/m3 during a heavy dust storm over Mt. Hua (located in
Total saccharides have been suggested to be a suitable alternative tracer for a variety of biomass burning sources in aerosols because of their presence in the form of polysaccharides in plants (Scaramboni et al., 2015) which account for about 75% of the dry weight of plants. However, studies also suggested that enhanced sugar concentrations in aerosols may be also due to soil resuspension (Rogge et al., 2007). There are also biogenic sources which may increase the total sugar concentration in aerosols (Graham et al., 2003; Elbert et al., 2007; Jia et al., 2010; Jia and Fraser, 2011; Jia and Fraser, 2011; Medeiros et al., 2006; Yttri et al., 2007; Yttri et al., 2011). Scaramboni et al. (2015) further suggested that total sugars could be used to evaluate the influence of wood fires
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Fig. 2. Mass distribution of the aerosol particles collected on March 06, 2012 (before dust storm, BDS), March 20–22, 2012 (dust storm days, DS1, DS2 and DS3) and April 03, 2012 (after dust storm, ADS). (A) Mass size distribution of aerosols. (B) Lognormal mass size distribution of aerosols.
in urban areas, and could be a better tracer than potassium in coastal regions, where the aerosols often contain high levels of sea-salt potassium. Size segregated sugar concentrations have been reported to provide better insights to the sources of aerosols (Agarwal et al., 2010; Wang et al., 2013). In this study, concentrations of total sugar (including sugar-alcohols and anhydro-sugars in all the size-bins) were determined to be 355, 440, 185, 192 and 184 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively. Concentration of total sugars in DS1 samples was found to be the maximum but the comparative increase with respect to BDS samples is merely ~ 1.2 times. However, while considering the total sugar concentration in DS1 samples compared to ADS samples, this increase is ~2.4 folds which is comparable to the increase in TSPM mass concentration (~2.5 folds). On the contrary, this increase of total sugar concentration in DS1 samples in coarse mode (PM N 2.1 μm) is 2.8 and 1.6 folds to that of BDS and ADS samples while in fine mode it is 0.6 and 1.4 folds, respectively. Concentrations of total sugar reported in this study are comparable to the concentrations that have been reported for the Chennai aerosols (Chennai is an Indian city) (Fu et al., 2010) except for DS1 and BDS samples. Seasonal variation of the concentrations of total sugars was not observed in summer (144 ng/m3) and winter (143 ng/m3) aerosols from a natural forest area sampling site (Fu et al., 2010), whereas large seasonal variation of levoglucosan (forming a significant part of total sugars in urban aerosols) has been reported for Delhi region in PM2.5 ranging from 210 to 5258 ng/m3, maximum being observed in winter and minimum in summer (Chowdhury et al., 2007). Size-segregated total sugar concentration is shown in Fig. 3A and B. Concentration of total sugars in size range N9.0 μm was observed to
be ~ 2.4–5.4 folds higher in DS1 samples compared to other samples. This increase may be largely attributed as a consequence of the duststorm. Contribution of sugars determined in giant particles to coarse mode total sugar concentration was 25.41%, 50.60%, 36.04%, 55.55% and 26.55% in BDS, DS1, DS2, DS3 and ADS samples, respectively. This shows an increase in total sugar concentrations in giant particles during dust-storm. In DS1 samples, PM concentrations were observed to increase in each size bin but on the other hand, total sugars showed increase in DS1 samples only compared to ADS samples. Total sugar concentration in the DS1 samples was lower than that observed for BDS samples, which is possibly due to the high influence of biomass burnings on BDS samples (a point to be discussed in the following section).
3.3. Anhydro-sugar distribution in aerosol samples Biomass burning is known to contribute significantly to aerosols (Aggarwal and Kawamura, 2009; Chowdhury et al., 2007; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013). Biomass is rich in oligosaccharides and polysaccharides (cellulose, lignin and starch) which on combustion produce significant amount of monosaccharides, disaccharides, sugar-alcohols and anhydro-sugars in addition to the other simpler molecules. Anhydro-sugars are formed by pyrolysis of carbohydrates such as cellulose and starch. Hence, anhydro-sugars (levoglucosan, mannosan and galactosan) have been used as a tracer for biomass combustions (Simoneit et al., 2004). Biomass burning is observed in the form of open burning of agricultural residues, grassland and forest fires, and residential combustion of biomass for cooking
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Fig. 3. Mass distribution total sugar in aerosol samples. (A) Mass size distribution of total sugars in aerosol samples. (B) Lognormal mass size distribution of total sugars in aerosol samples.
and heating purposes. This practice is prevalent in most of the developing countries including India. Levoglucosan is the most abundant anhydro-sugar which is found in biomass burning aerosols and it is an established organic tracer. Apart from this, it is also one of the most abundant organic species found in aerosols, which are influenced by anthropogenic activities (Fu et al., 2010). In this study, total concentration (sum of all size-bins) of levoglucosan, are found to be 141.9, 86.7, 47.3, 62.6 and 63.7 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively. These values are similar to average levoglucosan concentrations (111 ng/m3, (range 50.7–213 ng/m3), 69 ± 46 ng/m3 (29–196 ng/m3)) reported in PM10 aerosol collected in summer from Chennai (Fu et al., 2010) and Mumbai (Aggarwal et al., 2013), other metropolitan cities in India, respectively. However, the average levoglucosan concentration found in this study 80.45 ± 37.12 ng/m3 is lower than the values (410 ng/m3 (range 150– 570 ng/m3)) reported in Delhi for PM10 aerosols collected in early March 2010 (Perrino et al., 2011). Levoglucosan is observed to dominate the fine mode sugar composition (Fig. 4). Levoglucosan constitutes ~57 to 74% of the total sugars in fine mode (Fig. 4), as compared to other anhydro-sugars (mannosan and galactosan) which are found to constitute ~ 6–7% and ~ 3–5%, respectively. Concentration of mannosan (18.0, 12.1, 7.4, 9.4 and 8.4 ng/m3, respectively) and galactosan (12.8, 7.9, 3.7, 5.0 and 5.1 ng/m3, respectively) in TSPM are much lower compared to that of levoglucosan. Further in this study, average levoglucosan concentration in fine particles is determined to be 42.96 ± 31.89 ng/m3, and is lower than that of the values observed in summer (June and July 2001) PM2.5 aerosols at Kolkata (75 ± 15 ng/m3), Delhi at the same NPL
sampling site (210 ± 40 ng/m3) and Chandigarh (140 ± 30 ng/m3), respectively (Chowdhury et al., 2007). Also average levoglucosan values found in this study are much smaller than that of the average concentrations observed during spring, autumn and winter of Delhi (1026.6, 1773.6 and 5258 ng/m3, respectively), Kolkata (336.4, 474.4 and 5491.9 ng/m3, respectively) (Chowdhury et al., 2007). Chowdhury et al. (2007) suggested a profound influence of seasonal anthropogenic activities on the concentration of levoglucosan on urban aerosols which is less in summers and more in winters. Knowledge of other anhydro-sugars in addition to levoglucosan has been used for qualitative assessment of different specific biomass sources (Engling et al., 2014; Cheng et al., 2013). Engling et al. (2014)
Fig. 4. Levogucosan to mannosan (L/M) ratio on different sampling days and their variation in different size-bins.
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suggest that smoke particles derived from softwood combustion are characterized by low levoglucosan to mannosan (L/M) ratios of 3 to 5, while hard wood smoke L/M ratios ranged from 15 to 25 and those for rice straw are even higher (~40). Similarly, Cheng et al. (2013) used L/ M ratios in combination of levoglucosan to potassium ratio to identify the type of biomass/biofuel burned (softwood, hardwood, coal, etc.). Schmidl et al. (2008) have reported L/M ratios ranged from 5.2–6.0 for the leaf (pear, walnut and birch trees) combustion aerosols whereas a ratio of 9 ± 5 has been reported for biomass burnings (sugarcane harvesting season) affected aerosols (Urban et al., 2014). Cheng et al. (2013) reported that L/M ratios in the Beijing PM2.5 aerosols and the averaged ratios were reported to be 25.01 ± 13.20, 12.65 ± 3.38 and 9.49 ± 1.63 during the summer biomass burning episode, typical summer and winter period, respectively. High ratio of L/M was attributed to the wheat straw burning during crop residue burning period. In the present study, L/M ratio in TSPM samples was calculated to be 7.9, 7.1, 6.4, 6.6 and 7.6 in BDS, DS1, DS2, DS3 and ADS samples, suggesting biomass burnings influence on aerosols which may be from a combination of leaves, softwood and hardwood. Further, L/M ratio has been found to be size specific. Levoglucosan to mannosan ratio was found to be in the range of 6.39–7.87 (average 7.12 ± 0.62), 3.88–5.77 (4.95 ± 0.70) and 8.81–10.25 (9.72 ± 0.60) for TSPM, coarse and fine particles, respectively. Fig. 4 shows the variation of L/M ratio in different size bins of Andersen sampler on different sampling days. Generally, L/M ratio in this study is observed to be higher in fine mode particles which gradually decrease for coarse particles (Fig. 4). Also we observe small variation in these ratios on different sampling days suggesting that there is little change in these ratios even when major episodic events like dust-storms impact on urban aerosols. Levoglucosan to mannosan ratio is dependent on aerosol size, hence it is important to be cautious while interpreting results based on this ratio. Average ratio of L/M may be lower for PM10 as found in this study because of lower ratios in coarse fraction. Also the DS1 samples are representative of urban aerosol mixing with transported dust aerosols, these ratios represent the combined effect of the two. In the present study, the highest levoglucosan value (141.9 ng/m3) was observed in the BDS samples suggesting greater influence of biomass burnings. This is one of the reasons that we observed high concentration of total sugars in the BDS samples compared to DS1 samples. Chowdhury et al. (2007) have also reported much higher value of levoglucosan (1026 ng/m3) in spring samples of Delhi. Declining values in other samples suggest the dilution effect as well as decreasing influence of biomass burning in rest of the samples. 3.4. Sugar-alcohols and other sugars Sugar alcohols, especially arabitol and mannitol constitute an important fraction of the dry weight of fungi, and in particular mannitol can contribute between 20 and 50% of the mycelium dry weight (Vélëz et al., 2007). Although other natural sources of these two polyols (lower plants, bacteria, insects and algae) (Graham et al., 2003; Vélëz et al., 2007; Zhang et al., 2010) are also known to exist, arabitol and mannitol in airborne particulate matter can be associated to fungal spores as biomarkers (Carvalho et al., 2003). Arabitol and mannitol have been also reported to be enhanced in biomass burning aerosols (Giri et al., 2013; Nirmalkar et al., 2015; Yang et al., 2012). Further, Burshtein et al. (2011) suggested that ergosterol is a better marker for fungi and mannitol and arabitol may not be specific biomarkers especially during spring and autumn which was attributed to high levels because of vegetation during spring blossoms and autumn decomposing. Di Filippo et al. (2013) also suggested that ergosterol was the only reliable marker that may be used as biomarkers for fungi in urban as well as suburban regions of Rome. Sugar-alcohols (arabitol, mannitol and inositol) determined in this study show no specific trend except that inositol was the least in each sample (Fig. 5). Concentration of arabitol in BDS, DS1, DS2, DS3 and
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ADS samples is 9.3, 44.7, 15.4, 17.1 and 15.6 ng/m3, whereas that of mannitol is 16.3, 34.1, 10.6, 13.8 and 16.8 ng/m3, respectively. It is evident that dust-storm lead to an increase in sugar-alcohols as DS1 samples show the maximum concentration. Further it is observed that mannitol dominates in BDS and ADS samples and in the rest of the samples arabitol leads. Concentration of inositol was 4.5, 5.7, 1.8, 3.0 and 2.2 ng/m3 in the respective samples. Fig. 5 shows a similarity in the mass-size distribution patterns of arabitol and mannitol which are observed to be unimodal in the coarse range (5.8–9.0 μm size bins except in DS1 samples where it is peaked in N9.0 μm size). These distributions clearly suggest that these compounds are primarily result from the resuspension of soil materials and biogenic sources like fungal spores, which contribute to these chemicals in coarse mode particles. Ambient concentrations of arabitol and mannitol in PM2.5 observed in Chengdu (China) were extremely high, ranging from 5.2 to 79.6 ng/m3 and from 20.2 to 121.8 ng/m3 with average concentrations of 21.5 ± 16.6 and 43.9 ± 19.3 ng/m3, respectively, which have been found to statistically correlate with levoglucosan and potassium concentrations (Yang et al., 2012). Hence, they concluded that abundances of fugal tracers as well as biomass tracers are a result of the co-located sources (Yang et al., 2012). In the present study, BDS samples show significant amount of levoglucosan (141.9 ng/m3 in BDS samples compared to 86.7 ng/m3 in DS1 samples) which decreases further in other samples, whereas the concentration of arabitol and mannitol increase significantly (44.7 and 34.0 ng/m3 compared to 9.3 and 16.3 ng/m3 in BDS samples) in DS1 samples. An increase in the concentration of arabitol and mannitol possibly suggests that dust-storm can significantly be enriched in micro-organisms (Vélëz et al., 2007). Other studies have also reported that duststorms lead to an increase in the number of micro-organisms (Nourmoradi et al., 2015; Yamaguchi et al., 2012) Mono-saccharides and disaccharides were also determined in these samples. Mass-size distributions of these compounds are shown in Figs. 6 and 7. Dust-storm affected samples (DS1) show the maximum concentration of these sugars. Glucose, fructose, sucrose and trehalose can originate from numerous microorganisms, plants, and animals. However, these compounds have also been proposed as tracers for soil biota (Simoneit et al., 2004; Rogge et al., 2007; Wang et al., 2009). The high abundances of these saccharides in the samples have been suggested as a significant contribution of resuspended soil organic matter from agricultural activities in East Asian countries to the tropospheric aerosols. Total concentration of mono-saccharides, i.e., fructose is found to be 21.6, 27.8, 13.6, 13.1 and 11.1 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, whereas glucose concentration is 16.6, 47.3, 24.3, 25.2 and 23.5 ng/m3, respectively. Sucrose concentration in these samples is 83.4, 102.4, 41.9, 27.2 and 28.8 ng/m3, whereas that of trehalose is 31.0, 71.37, 19.2, 15.6 and 9.1 ng/m3, respectively. Concentration of these sugars in coarse and fine mode is shown in Table 1. It is observed that among the disaccharides, increase in trehalose concentration in DS1 samples is more prominent than sucrose. Trehalose and sucrose have similar sources, but their higher content in DS1 samples further suggests that these aerosols are affected by dust-storms arising from arid region (a point to be discussed in next section). 3.5. Trehalose as a tracer for dust aerosols Trehalose is a nonreducing disaccharide of glucose. It is generally found in high concentrations (as much as 20% of dry weight) in many organisms capable of surviving in complete dehydration (anhydrobiotic organisms) (Crowe et al., 1984; Ilhan et al., 2015). Special ability of trehalose to stabilize dry membranes in anhydrobiotic organisms provides them the ability to cope up with thermo-, freeze- and desiccation stresses (Crowe et al., 1984; Ilhan et al., 2015). These organisms include spores of certain fungi, macrocysts of the slime mold dictyostelium, dry active baker's yeast, brine shrimp cysts (dry gastrulae of Artemia salina), and the dry larvae and adults of several species of nematodes.
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Fig. 5. Mass-size distribution of sugar-alcohols.
These organisms can persist in the anhydrobiotic state for many years. Survival of dehydration by some of these organisms is correlated with synthesis of trehalose during dehydration or its degradation following rehydration (Crowe et al., 1984; Ilhan et al., 2015). Trehalose has been also reported to be found in plants including resurrection plants, Arabidopsis thaliana, Capparis ovata (caper), rice and tobacco which are well adapted to drought and high temperature stress in arid and semi-arid regions of the Mediterranean. Water replacement and glass formation mechanisms of this unusual sugar are thought to be responsible for providing tolerance to abiotic and biotic stresses in several plants (Fernandez et al., 2010). Further, dust-storms have been reported to increase the air borne microorganisms (Nourmoradi et al., 2015; Griffin, 2007). Nourmoradi et al. (2015) reported ~1.5 folds increase in bacteria and ~ 3.8 folds of
fungi during a dust-storm period in Sanandaj, Iran. They reported that the predominant species of bacteria was Bacillus spp. (56.2% of total bacteria) and on the other hand, the major fungi was Mycosporium spp. (28.6% of total fungi) during the occurrence of a dust-storm. Griffin (2007) and references therein present several cases where significant increase in bacterial and fungal concentrations have been observed during dust-storms. This suggests that during dust-storm microorganism related chemicals are expected to increase. Similar observations have been also made previously and researchers have proposed trehalose as a likely tracer for the transported aerosols from arid regions (Fu et al., 2012; Wang et al., 2013). Here in this study, we report the size-segregated trehalose concentration in urban aerosols including aerosols collected during the duststorm period. Total concentration of trehalose (i.e., found in TSPM) is
Fig. 6. Mass-size distribution of glucose and fructose.
Fig. 7. Mass-size distribution of sucrose and trehalose.
S. Kumar et al. / Atmospheric Research 189 (2017) 24–32
31
Table 1 Mass concentration of sugars in the samples. Particulate matter (PM) concentrations are reported in μg/m3 and that of sugars in ng/m3. 06-03-2012
PM Levoglucosan Galactosan Mannosan Fructose Glucose Sucrose Trehalose Arabitol Inositol Mannitol Total sugar
20-03-2012
21-03-2012
22-03-2012
03-04-2012
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
255 44.40 6.08 7.70 19.43 13.77 75.24 28.87 8.09 3.32 10.30 217.21
117 97.51 6.71 10.34 2.13 2.79 8.20 2.14 1.20 1.19 5.96 138.17
746 36.47 4.45 7.20 25.98 44.52 96.14 62.80 43.70 4.81 32.97 359.05
308 50.25 3.41 4.93 1.78 2.80 6.22 8.57 1.01 0.86 1.09 80.91
643 21.37 2.11 4.46 11.08 19.95 37.40 17.99 14.36 1.29 9.52 139.54
324 25.93 1.57 2.94 2.48 4.37 4.50 1.16 1.04 0.52 1.06 45.58
588 20.82 2.96 5.36 11.94 23.24 23.98 14.36 16.61 2.51 13.46 135.24
400 41.82 2.02 4.08 1.14 1.98 3.17 1.26 0.46 0.45 0.31 56.69
274 22.46 2.79 4.28 9.94 21.49 27.55 8.58 14.86 1.70 14.15 127.79
149 41.24 2.30 4.15 1.19 2.01 1.24 0.48 0.77 0.51 2.67 56.57
31.0, 71.4, 19.2, 15.6 and 9.1 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples and that of sucrose is 83.4, 102.4, 41.9, 27.2 and 28.8 ng/m3, respectively. An increase (~2 folds) in trehalose concentration observed in the DS1 samples is probably due the dust enriched in biogenic materials arising from the deserts. Significant increase in glucose concentration was also observed, which is ~ 2.9 folds higher than that observed in BDS samples. Glucose concentration is observed to be 16.6, 47.3, 24.3, 25.2 and 23.5 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, whereas that of fructose is 21.6, 27.8, 13.6, 13.1 and 11.1 ng/m3, respectively. Wang et al. (2013) determined sugars in PM10 and size-resolved particles at Mt. Hua and Mt. Tai in central and east China, respectively during the spring of 2009 including a massive dust storm. Concentrations of disaccharides like sucrose and trehalose were reported to increase sharply in coarse particles during dust-storm. Authors suggested the elevated concentration (10–30 times) of trehalose in these dust-storm samples compared to non-dust samples as a tracer for transported dust from Gobi desert regions. Further, studying the ratios of levoglucosan to trehalose (L/T), a biomass burning emission product to that of biogenic emission product reveals that during the dust-storm, the ratio decreases. It was observed that L/T ratio varied between 2.47 and 7.03 in rest of the samples except in DS1 samples where it was the least 1.22. Similar variations were also observed for sucrose to trehalose (S/T) ratios which ranged between 1.43 and 3.18, the smallest value was observed in DS1 samples. Contrary to higher levoglucosan concentration in fine mode, trehalose concentration is low and sometimes below detection limit in finer size bins.
sugars suggests that anhydro-sugars are mostly confined to the fine mode particles. During dust-storm episode, concentration of anhydrosugars (levoglucosan, mannosan and galactosan) is found to be decreased, which is probably because of the dilution effect. However, sugar-alcohols and other sugars increased in DS1 samples probably due to their abundance in transported dust. Also the percentage composition of these respective sugars in giant particles was enhanced in DS1 samples as compared to other samples. Levoglucosan to mannosan (L/M) ratio is often used to predict the type of biomass burning influences on the aerosols. In this study, we observed that this ratio is size dependent. Ratios are lower in larger sized particles. Hence caution should be taken while interpreting these results. Also, the effect of dust storm episode was found to be negligible on these ratios. Increase in trehalose concentration was most significant in DS1 samples. It shows an enhanced (~2–4 folds) concentration in DS1 aerosol samples in both coarse and fine mode. Trehalose is found in high concentrations in the micro-organisms which have the ability to cope up harsh conditions like thermal stress, desiccation, etc. and also in the xerophytic plants. Hence, their enhanced concentrations suggest aerosols originating from such regions, and here in this case, it is desert dust aerosols. Further, levoglucosan to trehalose (L/T) ratio decrease (~ 1) in DS1 compared to that of other samples, which ranged 2.47–7.03. Hence, lowered L/T ratio and enhanced trehalose concentration in aerosol samples may represent the influence of transported desert dust aerosols. This study supports the candidature of trehalose as an organic tracer for desert dust entrainments.
4. Conclusions Aerosol size-segregated samples were collected during a massive dust-storm episode (March 20–22, 2012) observed in New Delhi, which was originated in Middle-East. First time we reported the sizesegregated sugar content of this dust-storm affected urban aerosols together with before dust-storm and after dust-storm aerosols collected in New Delhi. During the dust-storm episode, a significant increase (~2–3 times) in total suspended particulate matter (TSPM) mass concentration was observed in dust-storm samples (day 1, i.e. DS1) compared to that of before dust-storm (BDS), following days (DS2 and DS3) and after duststorm (ADS) aerosols. Size-segregated mass distribution of particles in DS1 samples showed an increase in all the size bins suggesting that the transported aerosols also affected significantly to the fine mode (b2.1 μm). Further, particle mass concentration in DS1 samples in giant particles (N9.0 μm) increased several folds compared to other samples, which suggest that transported dust is enriched in coarse mode particles. In dust-storm samples (DS1, DS2 and DS3), concentrations of total sugars (mono-saccharides, disaccharides, sugar-alcohols and anhydrosugars) were observed to be enhanced. Size-segregated study of total
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Ambient aerosol concentrations of sugars and sugar-alcohols at four different sites in Norway. Atmos. Chem. Phys. 7, 4267–4279. Yttri, K.E., Simpson, D., Nøjgaard, J.K., Kristensen, K., Genberg, J., Stenström, K., Swietlicki, E., Hillamo, R., Aurela, M., Bauer, H., 2011. Source apportionment of the summer time carbonaceous aerosol at Nordic rural background sites. Atmos. Chem. Phys. 11, 13339–13357. Zhang, T., Engling, G., Chan, C.-Y., Zhang, Y.-N., Zhang, Z.-S., Lin, M., Sang, X.-F., Li, Y.D., Li, Y.-S., 2010. Contribution of fungal spores to particulate matter in a tropical rainforest. Environ. Res. Lett. 5.
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Size-segregated sugar composition of transported dust aerosols from Middle-East over Delhi during March 2012 S. Kumar a,b, S.G. Aggarwal a,b,⁎, P.Q. Fu c, M. Kang c, B. Sarangi a,b, D. Sinha d, R.K. Kotnala a,b a
Environmental Sciences and Biomedical Metrology Division, CSIR-National Physical Laboratory, New Delhi 110012, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory Campus, New Delhi 110012, India Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China d Government Nagarjun Post Graduate Science College, Raipur 492010, India b c
a r t i c l e
i n f o
Article history: Received 3 September 2016 Received in revised form 15 January 2017 Accepted 19 January 2017 Available online 21 January 2017 Keywords: Aerosols Size-segregated Sugars Trehalose Tracers
a b s t r a c t During March 20–22, 2012 Delhi experienced a massive dust-storm which originated in Middle-East. Size segregated sampling of these dust aerosols was performed using a nine staged Andersen sampler (5 sets of samples were collected including before dust-storm (BDS)), dust-storm day 1 to 3 (DS1 to DS3) and after dust storm (ADS). Sugars (mono and disaccharides, sugar-alcohols and anhydro-sugars) were determined using GC–MS technique. It was observed that on the onset of dust-storm, total suspended particulate matter (TSPM, sum of all stages) concentration in DS1 sample increased by N2.5 folds compared to that of BDS samples. Interestingly, fine particulate matter (sum of stages with cutoff size b2.1 μm) loading in DS1 also increased by N 2.5 folds as compared to that of BDS samples. Sugars analyzed in DS1 coarse mode (sum of stages with cutoff size N2.1 μm) samples showed a considerable increase (~1.7–2.8 folds) compared to that of other samples. It was further observed that mono-saccharides, disaccharides and sugar-alcohols concentrations were enhanced in giant (N 9.0 μm) particles in DS1 samples as compared to other samples. On the other hand, anhydro-sugars comprised ~13–27% of sugars in coarse mode particles and were mostly found in fine mode constituting ~66–85% of sugars in all the sample types. Trehalose showed an enhanced (~2–4 folds) concentration in DS1 aerosol samples in both coarse (62.80 ng/m3) and fine (8.57 ng/m3) mode. This increase in Trehalose content in both coarse and fine mode suggests their origin to the transported desert dust and supports their candidature as an organic tracer for desert dust entrainments. Further, levoglucosan to mannosan (L/M) ratios which have been used to predict the type of biomass burning influences on aerosols are found to be size dependent in these samples. These ratios are higher for fine mode particles, hence should be used with caution while interpreting the sources using this tool. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Deserts are the major sources of mineral dust aerosols which can be transported over long distances (Fu et al., 2012; Park and Cho, 2013; Wang et al., 2009; Wang et al., 2013; Yamaguchi et al., 2012) to different parts of the globe. Dust-storms are known to significantly impact on the chemical composition and mass-size distribution of particles, thereby affecting the aerosol physical and microphysical properties (Agarwal et al., 2010; Fu et al., 2012; Wang et al., 2009; Wang et al., 2013). Most importantly, such events have been reported to increase the number of pathogenic micro-organisms (Nourmoradi et al., 2015; Yamaguchi et al., 2012) and metals including criteria pollutants like Ni (Kumar et al., 2016) which might impact adversely on the air quality and human ⁎ Corresponding author at: Environmental Sciences and Biomedical Metrology Division, CSIR-National Physical Laboratory, New Delhi 110012, India. E-mail address: [email protected] (S.G. Aggarwal).
http://dx.doi.org/10.1016/j.atmosres.2017.01.012 0169-8095/© 2017 Elsevier B.V. All rights reserved.
health (Chiu et al., 2008; Nourmoradi et al., 2015; Singh and Naseema Beegum, 2013; Yamaguchi et al., 2012). Out of the several known dust source regions which include Saharan, Middle-East, and Central Asian deserts of Mongolia and China (Prospero et al., 2002), North-India is impacted mostly by dust-storms originating from neighbouring Thar desert or occasionally from Middle-East deserts (Kang et al., 2016; Kumar et al., 2016; Ram et al., 2016; Singh and Naseema Beegum, 2013). Several studies related to effects of duststorm on radiative forcing have been carried out in and around New Delhi (Ram et al., 2016; Singh and Naseema Beegum, 2013). However, studies with the focus on the chemical compositions of these dust aerosols are very scarce, especially from this region (Kang et al., 2016; Kumar et al., 2016). Further, globally the studies on size segregation and organic compositions of dust aerosols are also very limited (Agarwal et al., 2010; Fu et al., 2012; Wang et al., 2013). Size segregated studies of organic composition are useful for studying the health effects and sources of aerosols (Agarwal et al., 2010; Fu et
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al., 2012; Simoneit et al., 1999; Simoneit et al., 2004; Urban et al., 2014; Wang et al., 2013). Sugars are an important component of water-soluble organic carbon (WSOC) fraction of aerosols (Agarwal et al., 2010; Fu et al., 2012; Fu et al., 2016; Simoneit et al., 2004, 1999). It is found to be ubiquitous and have been reported to be originating from several sources (Agarwal et al., 2010; Aggarwal and Kawamura, 2009; Fu et al., 2012; Fu et al., 2016; Simoneit et al., 1999; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013; Yttri et al., 2007; Yttri et al., 2011). Primary saccharides such as fructose, glucose, sucrose, and trehalose in aerosols are found primarily from sources like developing leaves, phloem of plants as well as suspended soil dusts (affected by biomass burnings) and also form a major fraction of soil organic matter (Yttri et al., 2007; Yttri et al., 2011). Sugar-alcohols (arabitol, mannitol and inositol) and trehalose have been attributed to the biogenic emissions from fungal spores (Fu et al., 2016; Simoneit et al., 2004; Wang et al., 2009). Anhydro-sugars, i.e. levoglucosan, mannosan and galactosan are specifically related to the biomass burnings as they are formed by pyrolysis of cellulose, lignin and starch found in abundance in biomass (Fine et al., 2002; Nolte et al., 2001; Schauer et al., 2001; Simoneit et al., 1999). Presence of sugars and their variable concentrations have often been interpreted as tracers for different sources (Agarwal et al., 2010; Aggarwal and Kawamura, 2009; Fu et al., 2012; Fu et al., 2016; Scaramboni et al., 2015; Simoneit et al., 1999; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013; Yttri et al., 2007; Yttri et al., 2011). For example, levoglucosan is the most widely used tracer for the biomass burning in aerosols (Fine et al., 2002; Nolte et al., 2001; Schauer et al., 2001; Simoneit et al., 1999). Ratio of levoglucosan to mannosan has been further used to interpret the type of biomass burning tracers (Cheng et al., 2013; Engling et al., 2014; Urban et al., 2014). Similarly, the presence of sugar-alcohols has been related with biogenic emissions. Total sugar concentration has been also suggested to be a tracer for biomass burnings (Scaramboni et al., 2015). The high abundances of these saccharides in the samples suggest a significant contribution of re-suspended soil organic matter from agricultural activities to the tropospheric aerosols (Rogge et al., 2007; Simoneit et al., 2004; Yttri et al., 2011). In this study, we present the size segregated sugar composition in urban aerosols collected during an intense dust-storm originating in Middle-East and crossed over New Delhi on March 20–22, 2012 (Fig. 1). This dust-storm has been reported to impact significantly on regional climate. Some studies on physical properties of these dust aerosols have been reported recently (Singh and Naseema Beegum, 2013).
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Also a study has been undertaken to determine selected metal composition of aerosols collected during this dust-storm (Kumar et al., 2016). Here in this study, we characterize the size-segregated sugar in dust-storm affected urban and urban background aerosols. As per our best of knowledge, this is the first study done from Indian region to assess the size-segregated sugar compositions in the transported dust aerosols during a severe dust-storm which originated in Middle-East desert. In this study, the compositions of mono-saccharides, disaccharides, sugar alcohols and anhydro-sugars are discussed. Our results revealed that these sugars (especially trehalose) can be used as organic tracers for transported dust from desert and arid regions. 2. Materials and methods 2.1. Sampling and analysis Delhi encountered dust-storm on March 20, 2012 and was under the envelope of dust for nearly two days (Fig. 1). Size segregated aerosol sampling was performed using Andersen impactor sampler having eight stages with cutoff diameters of N 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.7, 0.4 μm and a backup filter (i.e. with nine size bins: N 9.0, 9.0–5.8, 5.8– 4.7, 4.7–3.3, 3.3–2.1, 2.1–1.1, 1.1–0.7, 0.7–0.4 and b 0.4 μm). Pre-baked (at 450 °C, at least for 6 h) quartz filters were used for sampling and the sampler was operated at an average flow rate of 28.3 l/min on the rooftop of National Physical Laboratory (NPL) at ~ 15 m above ground level (agl). NPL site is located in central Delhi, a representative urban site (discussed in details in Kumar et al., 2015, Fig. 1). Cascade impactor samplers have an inherent problem of particle resuspension but it can be minimized if (1) overloading is avoided and (2) the impaction surface is having materials which can capture or hold the impacting particles appropriately (Dunbar et al., 2005). Since we use the sampler at the flow-rate of 28.3 lpm (low-volume) and quartz filters, we assume a negligible resuspension. Also during the dust-event days when the chances of overloading are high, we have reduced the sampling time from usual 72 h to 24 h and thus assume only a little effect of the resuspension. Briefly, this site is located in central Delhi, which is about half a km away from the busy road, and is surrounded by institutional and residential areas, and protected ridge vegetation (Google imagery of the sampling site is shown in Fig. 1). In total five set of aerosol samples were collected. 72 h aerosol samplings were performed on March 06, 2012 and April 03, 2012 representing before dust-storm (BDS) period and after dust-storm (ADS) period, respectively. During dust-storm
Fig. 1. In the left panel, three days backward trajectories of the air parcel (using NOAA HYSPLIT backward trajectory at 1000 m altitude on reanalysis data) reaching over New Delhi on the five sampling days are shown. On March 20, 2012 the trajectory can be seen to be passing over the Arabian sea which suggests that they might have originated from the Middle-East. Published literature for this period clearly shows the satellite image of dust originating from the Middle-East and crossing over New Delhi (Kumar et al., 2016). Right upper panel shows the Google imagery of Delhi state and the lower right panel shows the sampling site image.
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(DS), ~ 24 h aerosol samplings (three sets) were performed starting from March 20 to March 23, 2012 (hereafter, these three sets of samples are referred as DS1, DS2 and DS3 samples, respectively). Andersen sampler was started in the evening of March 20, 2016 and run for a period of about twenty three and half hours and the filters were replaced. Sampling was resumed after the filter change and similarly conducted for two more days to collect ~24 h aerosol samples. Sum of mass concentrations found in all size-bins is referred as total suspended particulate matter (TSPM) in text here after. Blank filter samples were also collected by loading the filter in sampler without sucking air and removed immediately. After sampling, each filter was packed in separate glass bottle and stored in a refrigerator at ~4 °C. 2.2. Extraction of Sugars and analysis Sugars were extracted from filter samples by taking a piece of filter in a glass bottle and adding 10 ml of dichloromethane/methanol (2:1, v/v) solution and ultrasonicating this for 10 min. This process was repeated thrice for complete extraction. The solvent extracts were filtered through quartz wool packed in a Pasteur pipette, and then concentrated using a rotary evaporator, and then air dried with pure nitrogen gas. The extracts were reacted with 50 μl of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) (contains 1% trimethylsilyl chloride and 10 μl of pyridine) at 70 °C for 3 h. After being reacted, the derivatives were added with 40 μl of n-hexane containing 1.43 ng/μl of the internal standard (C13 n-alkane) prior to GC–MS analysis (Aggarwal and Kawamura, 2009; Fu et al., 2012). Gas Chromatography – Mass Spectroscopy (GC–MS) analysis were performed on Agilent model 7890 GC coupled to Agilent model 5975C MSD. Sugar separation by GC was achieved on a DB-5 m s fused silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness) with a GC oven temperature program: temperature was increased from 50 °C (2 min) to 120 °C at a rate of 30 °C/min and further it was ramped to 300 °C at a rate of 6 °C/min with a final isotherm hold at 300 °C for 16 min. The GC injector was maintained at 280 °C. The mass spectrometer was operated on Electron Ionization (EI) mode at 70 eV and scanned over the range of 50–650 Da. GC–MS response factors were determined with authentic standards. Fragment ions of sugar compounds at m/z = 217 and 204 were used for quantification. Field blank filters were treated as real samples for quality assurance. Also quartz filters spiked with known concentrations of sugar standards were analyzed for their recoveries and some real samples were analyzed in duplicate for the calculation of repeatability. Recoveries of sugars were generally better than 80% (individual sugar recoveries: glucose N 90%; levoglucosan N90%; trehalose 90%; sucrose N85%; arabitol N80%; fructose.80%; mannitol N 80%; inositol N70%). Relative standard deviation of the concentrations based on duplicate analyses was generally b10%. The results showed negligible background concentration for the targeted species in filter blanks. Sum of sugars determined in all sizebins is referred as total concentration of sugar.
Guanzhong Basin, central China) and Mt. Tai (North China Plain, east China), which was originated from Gobi desert, respectively. In general, dust-storms have been observed to increase particle mass loadings in the coarse mode (Agarwal et al., 2010; Wang et al., 2013). In this study, percentage contribution of PM mass in coarse (PM N 2.1 μm) mode is 68.5, 70.8, 66.5, 59.5 and 64.8% of TSPM on BDS, DS1, DS2, DS3 and ADS sampling days, respectively. Also, particulate concentration is observed to be maximum in DS1 samples (255 μg/m3) in coarse mode. Mass concentration of particles in fine mode (PM b 2.1 μm) is observed to be 117, 308, 324, 400 and 149 μg/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively, Fig. 2A and B. Particulate concentration in DS1 samples in fine mode is ~ 2.5 times higher than that observed in BDS samples and it increased further in DS2 and DS3 samples. This further increase in fine particulate concentrations in DS2 and DS3 samples is probably due to the continuous addition of fine mode particles from urban anthropogenic sources in the accumulation mode. Comparatively slower removal rate of fine particles than that of coarse particles probably resulted in fine PM loadings to increase in DS2 and DS3 samples. Further it was also observed that all the samples show the characteristic bimodal distribution for urban aerosols suggesting that urban contribution was still considerable. We observed a characteristic fine mode peak in all samples in 1.1–2.1 μm size range which is generally characteristic of combustion emissions (Bounanno et al., 2009). Another peak is observed in 5.8–9.0 μm range which signifies the contribution of suspended dusts. Bounanno et al. (2009) showed the lognormal mass-size distribution of various sources including urban, traffic and vegetation burning and they have mentioned that all these distributions are bimodal. Further they reported that the magnitude of particle mass is high in fine mode particles emitted from traffic and vegetation burning sources and lower in urban sources. In this study, we found that the peaks in fine and coarse modes are comparable in magnitude, possibly suggesting that these urban aerosols are affected by biomass burnings, traffic emissions as well as also the dust resuspension (accounts for higher concentration of coarse mode particles). Considering the mass-size distribution of PM in giant mode (PM N 9.0 μm), it was observed that the percentage contribution of giant particles increased considerably in DS1 samples (20.33%, 214.1 μg/m3) compared to BDS (14.19%, 52.9 μg/m3), DS2 (18.85%, 182.4 μg/m3), DS3 (11.90%, 117.6 μg/m3) and ADS (13.50%, 57.0 μg/ m3) samples as expected. This increase was also observed in ultra-fine (PM b 0.4 μm) particles (i.e. 0.89% (3.31 μg/m3), 6.07% (64.00 μg/m3), 6.02% (58.26 μg/m3), 8.35% (82.53 μg/m3) and 1.96% (8.27 μg/m3)in BDS, DS1, DS2, DS3 and ADS samples, respectively), suggesting that dust-storms also increase the ultra-fine particles which is likely to become more prominent in the days to follow as observed in DS2 and DS3 samples. This increase in ultra-fine particles may be due to the possibly enhanced lifetime of these particles compared to coarse particles in the atmosphere (where the wind speed during DS1, DS2 and DS3 was recorded to be 14 ± 3.5, 17 ± 1.4 and 16 ± 3.5 km/h, respectively (Kang et al., 2016)), and also possibly because of the promotion of new particle formation and growth due to the polluted dust (Nie et al., 2014; Sarangi et al., 2015).
3. Results and discussion 3.2. Total sugar distribution during the study period 3.1. High particle mass loading observed during the dust-storm Sum of particle mass concentration of all the stages and the backup filter provides total suspended particulate matter (TSPM) mass concentration. This is observed to be 1053, 968 and 988 μg/m3 in DS1, DS2 and DS3 samples, respectively at New Delhi sampling site. Similar episodic high mass concentration values (1031 and 10,702 μg/m3) have been reported for the Delhi and Bikaner (a city in Thar desert) aerosols affected by dust-storm arising from Thar desert in May 2000, respectively (Yadav and Rajamani, 2006). Similarly, Wang et al. (2013) observed high PM10 mass concentration values, i.e. 506 ± 331 and 1343 ± 450 μg/m3 during a heavy dust storm over Mt. Hua (located in
Total saccharides have been suggested to be a suitable alternative tracer for a variety of biomass burning sources in aerosols because of their presence in the form of polysaccharides in plants (Scaramboni et al., 2015) which account for about 75% of the dry weight of plants. However, studies also suggested that enhanced sugar concentrations in aerosols may be also due to soil resuspension (Rogge et al., 2007). There are also biogenic sources which may increase the total sugar concentration in aerosols (Graham et al., 2003; Elbert et al., 2007; Jia et al., 2010; Jia and Fraser, 2011; Jia and Fraser, 2011; Medeiros et al., 2006; Yttri et al., 2007; Yttri et al., 2011). Scaramboni et al. (2015) further suggested that total sugars could be used to evaluate the influence of wood fires
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Fig. 2. Mass distribution of the aerosol particles collected on March 06, 2012 (before dust storm, BDS), March 20–22, 2012 (dust storm days, DS1, DS2 and DS3) and April 03, 2012 (after dust storm, ADS). (A) Mass size distribution of aerosols. (B) Lognormal mass size distribution of aerosols.
in urban areas, and could be a better tracer than potassium in coastal regions, where the aerosols often contain high levels of sea-salt potassium. Size segregated sugar concentrations have been reported to provide better insights to the sources of aerosols (Agarwal et al., 2010; Wang et al., 2013). In this study, concentrations of total sugar (including sugar-alcohols and anhydro-sugars in all the size-bins) were determined to be 355, 440, 185, 192 and 184 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively. Concentration of total sugars in DS1 samples was found to be the maximum but the comparative increase with respect to BDS samples is merely ~ 1.2 times. However, while considering the total sugar concentration in DS1 samples compared to ADS samples, this increase is ~2.4 folds which is comparable to the increase in TSPM mass concentration (~2.5 folds). On the contrary, this increase of total sugar concentration in DS1 samples in coarse mode (PM N 2.1 μm) is 2.8 and 1.6 folds to that of BDS and ADS samples while in fine mode it is 0.6 and 1.4 folds, respectively. Concentrations of total sugar reported in this study are comparable to the concentrations that have been reported for the Chennai aerosols (Chennai is an Indian city) (Fu et al., 2010) except for DS1 and BDS samples. Seasonal variation of the concentrations of total sugars was not observed in summer (144 ng/m3) and winter (143 ng/m3) aerosols from a natural forest area sampling site (Fu et al., 2010), whereas large seasonal variation of levoglucosan (forming a significant part of total sugars in urban aerosols) has been reported for Delhi region in PM2.5 ranging from 210 to 5258 ng/m3, maximum being observed in winter and minimum in summer (Chowdhury et al., 2007). Size-segregated total sugar concentration is shown in Fig. 3A and B. Concentration of total sugars in size range N9.0 μm was observed to
be ~ 2.4–5.4 folds higher in DS1 samples compared to other samples. This increase may be largely attributed as a consequence of the duststorm. Contribution of sugars determined in giant particles to coarse mode total sugar concentration was 25.41%, 50.60%, 36.04%, 55.55% and 26.55% in BDS, DS1, DS2, DS3 and ADS samples, respectively. This shows an increase in total sugar concentrations in giant particles during dust-storm. In DS1 samples, PM concentrations were observed to increase in each size bin but on the other hand, total sugars showed increase in DS1 samples only compared to ADS samples. Total sugar concentration in the DS1 samples was lower than that observed for BDS samples, which is possibly due to the high influence of biomass burnings on BDS samples (a point to be discussed in the following section).
3.3. Anhydro-sugar distribution in aerosol samples Biomass burning is known to contribute significantly to aerosols (Aggarwal and Kawamura, 2009; Chowdhury et al., 2007; Simoneit et al., 2004; Wang et al., 2009; Wang et al., 2013). Biomass is rich in oligosaccharides and polysaccharides (cellulose, lignin and starch) which on combustion produce significant amount of monosaccharides, disaccharides, sugar-alcohols and anhydro-sugars in addition to the other simpler molecules. Anhydro-sugars are formed by pyrolysis of carbohydrates such as cellulose and starch. Hence, anhydro-sugars (levoglucosan, mannosan and galactosan) have been used as a tracer for biomass combustions (Simoneit et al., 2004). Biomass burning is observed in the form of open burning of agricultural residues, grassland and forest fires, and residential combustion of biomass for cooking
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Fig. 3. Mass distribution total sugar in aerosol samples. (A) Mass size distribution of total sugars in aerosol samples. (B) Lognormal mass size distribution of total sugars in aerosol samples.
and heating purposes. This practice is prevalent in most of the developing countries including India. Levoglucosan is the most abundant anhydro-sugar which is found in biomass burning aerosols and it is an established organic tracer. Apart from this, it is also one of the most abundant organic species found in aerosols, which are influenced by anthropogenic activities (Fu et al., 2010). In this study, total concentration (sum of all size-bins) of levoglucosan, are found to be 141.9, 86.7, 47.3, 62.6 and 63.7 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, respectively. These values are similar to average levoglucosan concentrations (111 ng/m3, (range 50.7–213 ng/m3), 69 ± 46 ng/m3 (29–196 ng/m3)) reported in PM10 aerosol collected in summer from Chennai (Fu et al., 2010) and Mumbai (Aggarwal et al., 2013), other metropolitan cities in India, respectively. However, the average levoglucosan concentration found in this study 80.45 ± 37.12 ng/m3 is lower than the values (410 ng/m3 (range 150– 570 ng/m3)) reported in Delhi for PM10 aerosols collected in early March 2010 (Perrino et al., 2011). Levoglucosan is observed to dominate the fine mode sugar composition (Fig. 4). Levoglucosan constitutes ~57 to 74% of the total sugars in fine mode (Fig. 4), as compared to other anhydro-sugars (mannosan and galactosan) which are found to constitute ~ 6–7% and ~ 3–5%, respectively. Concentration of mannosan (18.0, 12.1, 7.4, 9.4 and 8.4 ng/m3, respectively) and galactosan (12.8, 7.9, 3.7, 5.0 and 5.1 ng/m3, respectively) in TSPM are much lower compared to that of levoglucosan. Further in this study, average levoglucosan concentration in fine particles is determined to be 42.96 ± 31.89 ng/m3, and is lower than that of the values observed in summer (June and July 2001) PM2.5 aerosols at Kolkata (75 ± 15 ng/m3), Delhi at the same NPL
sampling site (210 ± 40 ng/m3) and Chandigarh (140 ± 30 ng/m3), respectively (Chowdhury et al., 2007). Also average levoglucosan values found in this study are much smaller than that of the average concentrations observed during spring, autumn and winter of Delhi (1026.6, 1773.6 and 5258 ng/m3, respectively), Kolkata (336.4, 474.4 and 5491.9 ng/m3, respectively) (Chowdhury et al., 2007). Chowdhury et al. (2007) suggested a profound influence of seasonal anthropogenic activities on the concentration of levoglucosan on urban aerosols which is less in summers and more in winters. Knowledge of other anhydro-sugars in addition to levoglucosan has been used for qualitative assessment of different specific biomass sources (Engling et al., 2014; Cheng et al., 2013). Engling et al. (2014)
Fig. 4. Levogucosan to mannosan (L/M) ratio on different sampling days and their variation in different size-bins.
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suggest that smoke particles derived from softwood combustion are characterized by low levoglucosan to mannosan (L/M) ratios of 3 to 5, while hard wood smoke L/M ratios ranged from 15 to 25 and those for rice straw are even higher (~40). Similarly, Cheng et al. (2013) used L/ M ratios in combination of levoglucosan to potassium ratio to identify the type of biomass/biofuel burned (softwood, hardwood, coal, etc.). Schmidl et al. (2008) have reported L/M ratios ranged from 5.2–6.0 for the leaf (pear, walnut and birch trees) combustion aerosols whereas a ratio of 9 ± 5 has been reported for biomass burnings (sugarcane harvesting season) affected aerosols (Urban et al., 2014). Cheng et al. (2013) reported that L/M ratios in the Beijing PM2.5 aerosols and the averaged ratios were reported to be 25.01 ± 13.20, 12.65 ± 3.38 and 9.49 ± 1.63 during the summer biomass burning episode, typical summer and winter period, respectively. High ratio of L/M was attributed to the wheat straw burning during crop residue burning period. In the present study, L/M ratio in TSPM samples was calculated to be 7.9, 7.1, 6.4, 6.6 and 7.6 in BDS, DS1, DS2, DS3 and ADS samples, suggesting biomass burnings influence on aerosols which may be from a combination of leaves, softwood and hardwood. Further, L/M ratio has been found to be size specific. Levoglucosan to mannosan ratio was found to be in the range of 6.39–7.87 (average 7.12 ± 0.62), 3.88–5.77 (4.95 ± 0.70) and 8.81–10.25 (9.72 ± 0.60) for TSPM, coarse and fine particles, respectively. Fig. 4 shows the variation of L/M ratio in different size bins of Andersen sampler on different sampling days. Generally, L/M ratio in this study is observed to be higher in fine mode particles which gradually decrease for coarse particles (Fig. 4). Also we observe small variation in these ratios on different sampling days suggesting that there is little change in these ratios even when major episodic events like dust-storms impact on urban aerosols. Levoglucosan to mannosan ratio is dependent on aerosol size, hence it is important to be cautious while interpreting results based on this ratio. Average ratio of L/M may be lower for PM10 as found in this study because of lower ratios in coarse fraction. Also the DS1 samples are representative of urban aerosol mixing with transported dust aerosols, these ratios represent the combined effect of the two. In the present study, the highest levoglucosan value (141.9 ng/m3) was observed in the BDS samples suggesting greater influence of biomass burnings. This is one of the reasons that we observed high concentration of total sugars in the BDS samples compared to DS1 samples. Chowdhury et al. (2007) have also reported much higher value of levoglucosan (1026 ng/m3) in spring samples of Delhi. Declining values in other samples suggest the dilution effect as well as decreasing influence of biomass burning in rest of the samples. 3.4. Sugar-alcohols and other sugars Sugar alcohols, especially arabitol and mannitol constitute an important fraction of the dry weight of fungi, and in particular mannitol can contribute between 20 and 50% of the mycelium dry weight (Vélëz et al., 2007). Although other natural sources of these two polyols (lower plants, bacteria, insects and algae) (Graham et al., 2003; Vélëz et al., 2007; Zhang et al., 2010) are also known to exist, arabitol and mannitol in airborne particulate matter can be associated to fungal spores as biomarkers (Carvalho et al., 2003). Arabitol and mannitol have been also reported to be enhanced in biomass burning aerosols (Giri et al., 2013; Nirmalkar et al., 2015; Yang et al., 2012). Further, Burshtein et al. (2011) suggested that ergosterol is a better marker for fungi and mannitol and arabitol may not be specific biomarkers especially during spring and autumn which was attributed to high levels because of vegetation during spring blossoms and autumn decomposing. Di Filippo et al. (2013) also suggested that ergosterol was the only reliable marker that may be used as biomarkers for fungi in urban as well as suburban regions of Rome. Sugar-alcohols (arabitol, mannitol and inositol) determined in this study show no specific trend except that inositol was the least in each sample (Fig. 5). Concentration of arabitol in BDS, DS1, DS2, DS3 and
29
ADS samples is 9.3, 44.7, 15.4, 17.1 and 15.6 ng/m3, whereas that of mannitol is 16.3, 34.1, 10.6, 13.8 and 16.8 ng/m3, respectively. It is evident that dust-storm lead to an increase in sugar-alcohols as DS1 samples show the maximum concentration. Further it is observed that mannitol dominates in BDS and ADS samples and in the rest of the samples arabitol leads. Concentration of inositol was 4.5, 5.7, 1.8, 3.0 and 2.2 ng/m3 in the respective samples. Fig. 5 shows a similarity in the mass-size distribution patterns of arabitol and mannitol which are observed to be unimodal in the coarse range (5.8–9.0 μm size bins except in DS1 samples where it is peaked in N9.0 μm size). These distributions clearly suggest that these compounds are primarily result from the resuspension of soil materials and biogenic sources like fungal spores, which contribute to these chemicals in coarse mode particles. Ambient concentrations of arabitol and mannitol in PM2.5 observed in Chengdu (China) were extremely high, ranging from 5.2 to 79.6 ng/m3 and from 20.2 to 121.8 ng/m3 with average concentrations of 21.5 ± 16.6 and 43.9 ± 19.3 ng/m3, respectively, which have been found to statistically correlate with levoglucosan and potassium concentrations (Yang et al., 2012). Hence, they concluded that abundances of fugal tracers as well as biomass tracers are a result of the co-located sources (Yang et al., 2012). In the present study, BDS samples show significant amount of levoglucosan (141.9 ng/m3 in BDS samples compared to 86.7 ng/m3 in DS1 samples) which decreases further in other samples, whereas the concentration of arabitol and mannitol increase significantly (44.7 and 34.0 ng/m3 compared to 9.3 and 16.3 ng/m3 in BDS samples) in DS1 samples. An increase in the concentration of arabitol and mannitol possibly suggests that dust-storm can significantly be enriched in micro-organisms (Vélëz et al., 2007). Other studies have also reported that duststorms lead to an increase in the number of micro-organisms (Nourmoradi et al., 2015; Yamaguchi et al., 2012) Mono-saccharides and disaccharides were also determined in these samples. Mass-size distributions of these compounds are shown in Figs. 6 and 7. Dust-storm affected samples (DS1) show the maximum concentration of these sugars. Glucose, fructose, sucrose and trehalose can originate from numerous microorganisms, plants, and animals. However, these compounds have also been proposed as tracers for soil biota (Simoneit et al., 2004; Rogge et al., 2007; Wang et al., 2009). The high abundances of these saccharides in the samples have been suggested as a significant contribution of resuspended soil organic matter from agricultural activities in East Asian countries to the tropospheric aerosols. Total concentration of mono-saccharides, i.e., fructose is found to be 21.6, 27.8, 13.6, 13.1 and 11.1 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, whereas glucose concentration is 16.6, 47.3, 24.3, 25.2 and 23.5 ng/m3, respectively. Sucrose concentration in these samples is 83.4, 102.4, 41.9, 27.2 and 28.8 ng/m3, whereas that of trehalose is 31.0, 71.37, 19.2, 15.6 and 9.1 ng/m3, respectively. Concentration of these sugars in coarse and fine mode is shown in Table 1. It is observed that among the disaccharides, increase in trehalose concentration in DS1 samples is more prominent than sucrose. Trehalose and sucrose have similar sources, but their higher content in DS1 samples further suggests that these aerosols are affected by dust-storms arising from arid region (a point to be discussed in next section). 3.5. Trehalose as a tracer for dust aerosols Trehalose is a nonreducing disaccharide of glucose. It is generally found in high concentrations (as much as 20% of dry weight) in many organisms capable of surviving in complete dehydration (anhydrobiotic organisms) (Crowe et al., 1984; Ilhan et al., 2015). Special ability of trehalose to stabilize dry membranes in anhydrobiotic organisms provides them the ability to cope up with thermo-, freeze- and desiccation stresses (Crowe et al., 1984; Ilhan et al., 2015). These organisms include spores of certain fungi, macrocysts of the slime mold dictyostelium, dry active baker's yeast, brine shrimp cysts (dry gastrulae of Artemia salina), and the dry larvae and adults of several species of nematodes.
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Fig. 5. Mass-size distribution of sugar-alcohols.
These organisms can persist in the anhydrobiotic state for many years. Survival of dehydration by some of these organisms is correlated with synthesis of trehalose during dehydration or its degradation following rehydration (Crowe et al., 1984; Ilhan et al., 2015). Trehalose has been also reported to be found in plants including resurrection plants, Arabidopsis thaliana, Capparis ovata (caper), rice and tobacco which are well adapted to drought and high temperature stress in arid and semi-arid regions of the Mediterranean. Water replacement and glass formation mechanisms of this unusual sugar are thought to be responsible for providing tolerance to abiotic and biotic stresses in several plants (Fernandez et al., 2010). Further, dust-storms have been reported to increase the air borne microorganisms (Nourmoradi et al., 2015; Griffin, 2007). Nourmoradi et al. (2015) reported ~1.5 folds increase in bacteria and ~ 3.8 folds of
fungi during a dust-storm period in Sanandaj, Iran. They reported that the predominant species of bacteria was Bacillus spp. (56.2% of total bacteria) and on the other hand, the major fungi was Mycosporium spp. (28.6% of total fungi) during the occurrence of a dust-storm. Griffin (2007) and references therein present several cases where significant increase in bacterial and fungal concentrations have been observed during dust-storms. This suggests that during dust-storm microorganism related chemicals are expected to increase. Similar observations have been also made previously and researchers have proposed trehalose as a likely tracer for the transported aerosols from arid regions (Fu et al., 2012; Wang et al., 2013). Here in this study, we report the size-segregated trehalose concentration in urban aerosols including aerosols collected during the duststorm period. Total concentration of trehalose (i.e., found in TSPM) is
Fig. 6. Mass-size distribution of glucose and fructose.
Fig. 7. Mass-size distribution of sucrose and trehalose.
S. Kumar et al. / Atmospheric Research 189 (2017) 24–32
31
Table 1 Mass concentration of sugars in the samples. Particulate matter (PM) concentrations are reported in μg/m3 and that of sugars in ng/m3. 06-03-2012
PM Levoglucosan Galactosan Mannosan Fructose Glucose Sucrose Trehalose Arabitol Inositol Mannitol Total sugar
20-03-2012
21-03-2012
22-03-2012
03-04-2012
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
N2.1
b2.1
255 44.40 6.08 7.70 19.43 13.77 75.24 28.87 8.09 3.32 10.30 217.21
117 97.51 6.71 10.34 2.13 2.79 8.20 2.14 1.20 1.19 5.96 138.17
746 36.47 4.45 7.20 25.98 44.52 96.14 62.80 43.70 4.81 32.97 359.05
308 50.25 3.41 4.93 1.78 2.80 6.22 8.57 1.01 0.86 1.09 80.91
643 21.37 2.11 4.46 11.08 19.95 37.40 17.99 14.36 1.29 9.52 139.54
324 25.93 1.57 2.94 2.48 4.37 4.50 1.16 1.04 0.52 1.06 45.58
588 20.82 2.96 5.36 11.94 23.24 23.98 14.36 16.61 2.51 13.46 135.24
400 41.82 2.02 4.08 1.14 1.98 3.17 1.26 0.46 0.45 0.31 56.69
274 22.46 2.79 4.28 9.94 21.49 27.55 8.58 14.86 1.70 14.15 127.79
149 41.24 2.30 4.15 1.19 2.01 1.24 0.48 0.77 0.51 2.67 56.57
31.0, 71.4, 19.2, 15.6 and 9.1 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples and that of sucrose is 83.4, 102.4, 41.9, 27.2 and 28.8 ng/m3, respectively. An increase (~2 folds) in trehalose concentration observed in the DS1 samples is probably due the dust enriched in biogenic materials arising from the deserts. Significant increase in glucose concentration was also observed, which is ~ 2.9 folds higher than that observed in BDS samples. Glucose concentration is observed to be 16.6, 47.3, 24.3, 25.2 and 23.5 ng/m3 in BDS, DS1, DS2, DS3 and ADS samples, whereas that of fructose is 21.6, 27.8, 13.6, 13.1 and 11.1 ng/m3, respectively. Wang et al. (2013) determined sugars in PM10 and size-resolved particles at Mt. Hua and Mt. Tai in central and east China, respectively during the spring of 2009 including a massive dust storm. Concentrations of disaccharides like sucrose and trehalose were reported to increase sharply in coarse particles during dust-storm. Authors suggested the elevated concentration (10–30 times) of trehalose in these dust-storm samples compared to non-dust samples as a tracer for transported dust from Gobi desert regions. Further, studying the ratios of levoglucosan to trehalose (L/T), a biomass burning emission product to that of biogenic emission product reveals that during the dust-storm, the ratio decreases. It was observed that L/T ratio varied between 2.47 and 7.03 in rest of the samples except in DS1 samples where it was the least 1.22. Similar variations were also observed for sucrose to trehalose (S/T) ratios which ranged between 1.43 and 3.18, the smallest value was observed in DS1 samples. Contrary to higher levoglucosan concentration in fine mode, trehalose concentration is low and sometimes below detection limit in finer size bins.
sugars suggests that anhydro-sugars are mostly confined to the fine mode particles. During dust-storm episode, concentration of anhydrosugars (levoglucosan, mannosan and galactosan) is found to be decreased, which is probably because of the dilution effect. However, sugar-alcohols and other sugars increased in DS1 samples probably due to their abundance in transported dust. Also the percentage composition of these respective sugars in giant particles was enhanced in DS1 samples as compared to other samples. Levoglucosan to mannosan (L/M) ratio is often used to predict the type of biomass burning influences on the aerosols. In this study, we observed that this ratio is size dependent. Ratios are lower in larger sized particles. Hence caution should be taken while interpreting these results. Also, the effect of dust storm episode was found to be negligible on these ratios. Increase in trehalose concentration was most significant in DS1 samples. It shows an enhanced (~2–4 folds) concentration in DS1 aerosol samples in both coarse and fine mode. Trehalose is found in high concentrations in the micro-organisms which have the ability to cope up harsh conditions like thermal stress, desiccation, etc. and also in the xerophytic plants. Hence, their enhanced concentrations suggest aerosols originating from such regions, and here in this case, it is desert dust aerosols. Further, levoglucosan to trehalose (L/T) ratio decrease (~ 1) in DS1 compared to that of other samples, which ranged 2.47–7.03. Hence, lowered L/T ratio and enhanced trehalose concentration in aerosol samples may represent the influence of transported desert dust aerosols. This study supports the candidature of trehalose as an organic tracer for desert dust entrainments.
4. Conclusions Aerosol size-segregated samples were collected during a massive dust-storm episode (March 20–22, 2012) observed in New Delhi, which was originated in Middle-East. First time we reported the sizesegregated sugar content of this dust-storm affected urban aerosols together with before dust-storm and after dust-storm aerosols collected in New Delhi. During the dust-storm episode, a significant increase (~2–3 times) in total suspended particulate matter (TSPM) mass concentration was observed in dust-storm samples (day 1, i.e. DS1) compared to that of before dust-storm (BDS), following days (DS2 and DS3) and after duststorm (ADS) aerosols. Size-segregated mass distribution of particles in DS1 samples showed an increase in all the size bins suggesting that the transported aerosols also affected significantly to the fine mode (b2.1 μm). Further, particle mass concentration in DS1 samples in giant particles (N9.0 μm) increased several folds compared to other samples, which suggest that transported dust is enriched in coarse mode particles. In dust-storm samples (DS1, DS2 and DS3), concentrations of total sugars (mono-saccharides, disaccharides, sugar-alcohols and anhydrosugars) were observed to be enhanced. Size-segregated study of total
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