ISSN 00014338, Izvestiya, Atmospheric and Oceanic Physics, 2015, Vol. 51, No. 4, pp. 423–430. © Pleiades Publishing, Ltd., 2015. Original Russian Text © E.F. Mikhailov, S.Yu. Mironova, M.V. Makarova, S.S. Vlasenko, T.I. Ryshkevich, A.V. Panov, M.O. Andreae, 2015, published in Izvestiya AN. Fizika Atmosfery i Okeana, 2015, Vol. 51, No. 4, pp. 484–492.

Studying Seasonal Variations in Carbonaceous Aerosol Particles in the Atmosphere over Central Siberia E. F. Mikhailova, S. Yu. Mironovaa, M. V. Makarovaa, S. S. Vlasenkoa, T. I. Ryshkevicha, A. V. Panovb, and M. O. Andreaec a

St. Petersburg State University, ul. Ul’yanovskaya 1, Petrodvorets, 198504 Russia email: [email protected] b Sukachev Institute of Forest, Siberian Branch, Russian Academy of Sciences, Akademgorodok 50, Krasnoyarsk, 660036 Russia c Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, D55020 Germany Received April 8, 2014; in final form, June 4, 2014

Abstract—The results of 2year (2010–2012) measurements of the concentrations of organic carbon (OC) and elemental carbon (EC), which were taken at the Zotino Tall Tower Observatory (ZOTTO) Siberian back ground station (61° N, 89° E), are given. Despite the fact that this station is located far from populated areas and industrial zones, the concentrations of OC and EC in the atmosphere over boreal forests in central Siberia significantly exceed their background values. In winter and fall, high concentrations of atmospheric carbon aceous aerosol particles are caused by the longrange transport (~1000 km) of air masses that accumulate pol lutants from large cities located in both southern and southwestern regions of Siberia. In spring and summer, the pollution level is also high due to regional forest fires and agricultural burning in the steppe zone of west ern Siberia in the Russian–Kazakh border region. Background concentrations of carbonaceous aerosol par ticles were observed within relatively short time intervals whose total duration was no more than 20% of the entire observation period. In summer, variations in the background concentrations of OC closely correlated with air temperature, which implies that the biogenic sources of organicparticle formation are dominating. Keywords: atmospheric composition, boreal forests, carbonaceous aerosol, thermooptical method DOI: 10.1134/S000143381504009X

longrange transport of aerosol emissions from fires and large cities.

1. INTRODUCTION Exchange processes between the forest ecosystem of Siberia and the atmosphere are the important fac tors of climate changes in Eurasia. Among them are carbon emissions in the form of greenhouse gases (СО2 and СН4) and organic substances in both gas eous and aerosol forms [1, 2]. The international Zotino Tall Tower Observatory (ZOTTO) station (central Siberia, 61° N, 89° E) was set up in 2006 in order to continuously monitor atmo spheric composition and analyze the state of forest ecosystems. A tower 300 m in height was mounted at the ZOTTO station, making it possible to study a rela tively homogeneous atmospheric region with a large coverage area of ~106 km2 [3]. The city of Krasnoyarsk (with a population of about 950000) and the village of Zotino (with a population of about 500) are the near est populated areas located at distances of 600 km and 25 km from the ZOTTO station, respectively. The remoteness of this station from local sources of anthropogenic pollution provides unique conditions for studying the composition of background aerosols over boreal forests and makes it possible to record the

Studying the composition and microphysical prop erties of background aerosols is important for con structing models that describe natural climate changes. Such models are necessary to obtain more precise estimates of the level of climate change caused by anthropogenic activities [4–6]. In addition, bio genic aerosols over boreal forests noticeably contribute to global radiative forcing (from –0.03 to –1.1 W/m2), and their local effect may reach values from –5 to ⎯14 W/m2 [7]. Forest fires and the biogenic activity of coniferous trees and forest floors are the main sources of carbon aceous aerosols emitted into the atmosphere over boreal forests. Data obtained from a chemical analysis of smokeaerosol samples imply that elemental carbon (EC), organic carbon (OC), and sulfates are the main components of aerosol particles. The molecular com position of the organic fraction of aerosol particles is still little understood. It is only known that the prod ucts of the thermal decomposition of plant cellulose and lignin make the largest contribution to the organic mass. Among them are sugar anhydrides (levoglu

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cosan, galactosan, and mannosan), phenol com pounds (salicylic, vanillic, and syringic acids), fatty acids, alcohols, esters, aldehydes and ketones [8–10]. During the summer, due to emissions from biogenic sources, the atmosphere is additionally supplied with biological aerosols (fungal spores, plant pollen, and bacteria) and lowvolatile cyclic hydrocarbons (iso prene, terpenes, and aromatic hydrocarbons). The lat ter form secondary organic aerosols as a result of pho tochemicaloxidation reactions [11]. The aging pro cesses—coagulation, adsorption, and heterogeneous reactions—are accompanied by the aggregation of particles and changes in their chemical composition [12]. As a result, climatically active submicron frac tions (0.1–1.0 µm) of organicsalt particles are formed. Particles of such sizes absorb and scatter radi ation in both visible and infrared spectral regions (direct aerosol forcing) [13], and they are also active cloud condensation nuclei (indirect aerosol forcing) [14]. Similarly to salt fractions, the organic compo nent scatters radiation, while elemental carbon absorbs radiation. The ratio between these quantities determines the total effect of the carbonaceous frac tion of particles on the radiation balance [9, 15]. For the Siberian region, the optical properties of black carbon aerosol were measured with both station ary and mobile instruments; the results of these mea surements are given in [16–18]. Data obtained from these measurements showed that forest fires strongly affect the single scattering albedo and the absolute value of the volume scattering coefficient. The chemical composition of aerosols in the Sibe rian region has analytically been studied since the 1990s. However, such measurements are episodic and the list of determined substances is limited mainly to inorganic ions and heavy metals [19]. The exception is [20], where results of the 12year (2001–2012) moni toring of both seasonal and annual trends of the OC and EC contents were analyzed. However, these data describe trends characteristic of urbanized areas, because this monitoring station is located at a distance of 30 km from the city of Novosibirsk in the village of Klyuchi. Our work presents the first results of contin uous OC and EC measurements taken at the ZOTTO background station from April 2010 to May 2012. These data supplement earlier results obtained from the monitoring of the particulate composition and optical properties of aerosols [21, 22]. 2. METHODS FOR AEROSOL SAMPLING AND CHEMICAL ANALYSIS Aerosol samples were taken at a height of 302 m. The conditions of sampling and transport losses of aerosol particles are described in [23]. The concentra tion of aerosol particles in the air was measured by dividing the mass of precipitated particles by the vol ume of the air pumped through the filter (with a rate of 19 L/min). Samples were taken using quartz filters

(Tissuquartz TM 2500 QAT) with a diameter of 47 mm. These filters were preliminarily baked in a muffle fur nace at a temperature of 800°С for 12 h. Two filters located one after the other were used for every sam pling. The mass of aerosol was determined as the dif ference between precipitated masses on the first and second filters [24]. Each filter was weighed 3–5 times on a XP6 balance (Mettler Toledo) with a sensitivity of 0.6 µg. The statistical scatter in determining the mass of aerosol amounted to 0.04 mg. From April 2010 to June 2012, 118 filter samples were taken and analyzed. The mean time for taking one sample amounted to about 5 days. Some samples were rejected mainly due to increased air humidity (>90%) in the sampling sys tem. The contents of OC, EC, and total carbon (TC) (TC = OC + EC) in the samples was determined using a commercial thermooptical analyzer (US Sunset Laboratory Inc.). The error in measuring both OC and EC concentrations includes a constant portion (detec tion limit) amounting to 0.2 µg/cm2 and a variable portion (random error) that does not exceed 6% of the mass of carbon material on the filter. The method of analyzing filter samples is described in [25]. In addition to carbon, the organic matter (OM) of particles involves oxygen, hydrogen, and nitrogen. These elements are not determined by the thermo optical analyzer, therefore, a correction coefficient is used to convert the mass of OC into the mass of OM. This coefficient varies from 1.2 to 2.4, depending on the source of emission and the oxidation level of organic matter during aerosol transport [26]. A weightedaverage correction coefficient of 1.8 was used in our work. As a result, the total carbonaceous fraction mass (TCM) was calculated from the formula: TCM = OM + EC = 1.8OC + EC. The multiplier 1.8 was earlier used in determining OM at both the SMEAR II and Kpuszta European background sta tions [27, 28]. This value provided the best agreement in the weight balance between the gravimetric mass of particles and the total mass of components measured using independent methods. 3. RESULTS AND DISCUSSION Figure 1 gives the results of processing filter aerosol samples taken from April 2010 to May 2012. It is easily seen that the contents of OC and EC in samples for fall and winter are significantly higher than for summer. Seasonal variations in the concentrations of carbon aceous aerosol have repeatedly been noted before. Their high values in fall and winter are mainly associ ated with an increase in the consumption of coal and liquid hydrocarbons during the heating season and with a decrease in the height of the inversion layer. Seasonal average pollution sources in Siberia for the 2006–2011 period have recently been analyzed in [22]. On the basis of the STILT model [26], the authors of [22] (using carbon monoxide as an exam

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Fig. 1. Seasonal variations in the concentrations of (1) EC, (2) OC, and (3) total aerosol mass.

ple) showed that, in both winter and summer seasons, pollution arrived in the ZOTTO station region from large cities located in southern and southwestern Sibe ria. In summer, in addition to emissions from anthro pogenic sources from southern directions, emissions from local forest fires in central Siberia were also observed. A comparison between results given in [22] and our measurement data obtained from April 2010 to May 2012 showed that the main sources of OC and carbonaceous aerosol are identical. Therefore, in our work we will emphasize some episodes that are respon sible for both extremely high and low concentrations of the carbonaceous fraction of aerosol. It follows from Fig. 1 that high concentrations of EC and OC are revealed in aerosol samples taken in the second half of April 2010 and 2011. Moreover, on April 28, 2010, the maximum (over the entire observa tion period) concentrations of OC and EC amounted to 18.9 and 1.5 µg/m3, respectively. Figure 2a shows six 5day backward trajectories of airmass transport (HYSPLIT model [30]), which were constructed with an interval of 2 h. These trajectories are numbered in such a way that the first one arrives at the ZOTTO sta tion at the very beginning of measurements (~14:00) and the last (sixth) arrives at the end of the measure ment period (at midnight). These trajectories charac terize the region within which air masses were located for 5 days before their arrival at the station. The dashed lines show two regions with strong agricultural burning (black points denote fire pixels detected by the MODIS instruments [31]). One region covers the Tyumen, Omsk, and Novosibirsk oblasts and Altai krai, and the other region covers the middle belt of European Russia. It is seen that, on April 28, 2010, IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS

from 19:00 (fifth and sixth trajectories), air masses enriched with combustion products from intense agri cultural burning started to arrive at the station. It is necessary to note that the first, second, and third tra jectories (Fig. 2a) initially passed over the region of intense fires, which is located in the middle belt of Russia. Thus, the longrange airmass transport observed in the period under consideration (Fig. 2b) resulted in strong air pollution by combustion prod ucts from the two regions of spring agricultural burn ing, which was manifested in record high carbon aceousaerosol concentrations in the samples of April 28, 2010. Against the background of relatively high EC con centrations observed over the entire observation period, there are short intervals (June 26–August 1, 2010 and June 30–August 31, 2011) (Fig. 1) within which the content of EC did not exceed 0.02 µg/m3 and the content of OC varied from 0.4 to 2.2 µg/m3. An analysis of meteorological conditions and back ward trajectories showed that, during these intervals, either longrange advective transport was not observed at all, or air masses arrived at the observation site from the Arctic. As an example, Fig. 3a shows the trajecto ries of motion of air masses that arrived at the ZOTTO station within the period June 15–26, 2010. Within this period, the concentrations of OC and EC were fairly low (0.97 and 0.02 µg/m3, respectively). It fol lows from Fig. 3a that only 3 (1, 2, and 3) out of 11 tra jectories passed over densely populated regions (Fig. 3a shows the highway network) and air masses moving along these three trajectories could accumu late anthropogenic emissions (Fig. 3b). Data obtained with the MODIS satellite instrument showed that no intense forest fires were observed along the path of air Vol. 51

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masses from the Arctic: during this time, all combus tion sources were located southeast of the station in the region of Lake Baikal and along the main high ways. Low anthropogenic signals imply that, within the indicated time intervals, the organic fraction of aero sol was formed mainly due to biogenic sources. In boreal forests, the main sources of biogenic particles are isoprene (С5H8) and monoterpenes (С10Р16) emit ted by coniferous trees [32]. When these molecules are oxidized by hydroxyl radical (OH), ozone (О3), and nitrogen trioxide radical (NO3), they form lowvola tile highmolecularweight substances which (in the presence of nucleation cores) can turn into aerosols [33]. Emissions of organicparticle precursors depend on air temperature and, consequently, the concentra tions of formed biogenic (secondary) aerosols are bound to increase with increased air temperature [34]. This effect is supported by our measurement data. Fig

ure 4 shows the dependence of the concentration of OC on daily mean air temperature for July 2010 and July–August 2011. It is seen that, within the tempera ture range under consideration, the concentration of aerosol OC almost linearly related to air temperature (correlation coefficient r = 0.90). Combustion processes are sources of both OC and EC emissions. The ratio between them depends on the regime of combustion and the nature of combustible material. As a rule, during the combustion of coal in boilers or petroleum products in gas engines, the degree of conversion of hydrocarbons is higher than that during the combustion of wood or dry grass [10]. Therefore, the OC/EC ratio may serve as an indicator of a pollution source. Figure 5 shows the regression relationships between the contents of OC and EC. For the summer period, dates with low EC concentrations were eliminated when OC in aerosols was formed due to biogenic sources. A close correlation between OC

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Fig. 3. (a) Backward trajectories of the motion of air masses and (b) the height of their transport within the period June 15–26, 2010. The circles correspond to daily intervals. The trajectories are constructed with an interval of 24 h. The trajectories (num bered 1, 2, and 3) pass over large populated areas in southern Siberia. The highway network is shown.

and EC during winter (r = 0.85) and summer (r = 0.93) implies that both components entered the atmo sphere from the same combustion sources, and a sig nificant seasonal difference between the regression coefficients (OC/EC = 6.5 ± 0.5 for winter and 20.7 ± 1.4 for summer) suggests that the nature of the sources of carbonaceousaerosol emissions is different. Low relative contents of OC in aerosol samples in winter are indicative of anthropogenic pollution sources, and high contents in summer are indicative of the domi nant contribution of emissions from forest fires [35]. This conclusion is supported by the results of a trajec tory analysis. High OC and EC concentrations IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS

observed within the periods August 21–September 6, 2010, and June 3–22, 2011, (Fig. 1) were caused by forest fires localized in central Siberia and Transbaika lia. During the winter periods December 2–6, 2010, January 3–8, 2011, and January 6–7, 2012, air masses transported pollutants from urbanized areas in south ern Siberia. During the periods December 16–20, 2010, and January 1–3, 2011, both northwesterly and northeasterly winds transported pollutants from gas production areas in Novyi Urengoi and partially from the neighboring village of Zotino. The table gives the seasonal concentrations of aero sols and carbonaceous components from April 2010 to Vol. 51

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Fig. 4. Dependence of the content of biogenic OC on daily mean air temperature in summer: (1) experimental data and (2) linear regression with the correlation coefficient r = 0.90.

Fig. 5. Correlation between EC and OC for (1) winter and (2) summer: linear regressions for (3) winter (r = 0.85, OC/EC = 6.5) and (4) summer (r = 0.90, OC/EC = 20.7).

May 2012. An approximately 40fold range of varia tions in the concentrations of OC and EC (mean max/min value over the components), which is observed for all seasons, and high mean values of EC/TC confirm that the influence of anthropogenic sources and fires on the composition of aerosols is strong. As a result, the contribution of carbonaceous material to the aerosol mass (TCM/PM) throughout the whole year was significant with a yearly mean value of 49%. Minimum seasonal concentrations were recorded mainly within periods of dominating Arctic air masses passing over underpopulated regions or dur ing periods without advection.

In order to have an idea of the air quality over the ZOTTO station during background periods, it is rea sonable to compare the measurement data on the con tent of anthropogenic EC with similar data obtained at other background stations. For such a comparison, the SMEAR II station (Finland, 61° N, 24° E), which is also located in the zone of boreal forests, is of great interest [36]. At this station, in August 2006, when the atmosphere was free from anthropogenic emissions, the concentration of EC varied within 101–191 ng/m3 with a modal value of 164 ng/m3. At the Kpuszta sta tion (Hungary, 46° N, 19° E) located further south and surrounded by coniferous forests, the background

Maximum (max), minimum (min), and mean seasonal concentrations (µg/m3) of carbonaceous aerosol components from April 2010 to May 2012 Spring Compo nents PM OC EC TC OM TCM

Summer

max

min mean max

54.4 18.8 1.47 20.3 34.0 35.4

3.14 0.50 0.017 0.52 0.91 0.92

12.5 3.09 0.22 3.31 5.56 5.78

min

Fall mean max

76.2 0.77 12.4 36.3 13.8 0.29 3.45 8.17 0.67 <0.001* 0.13 0.64 14.3 0.50 3.58 8.49 24.8 1.38 6.21 14.7 25.3 1.38 6.34 22.5

Winter

All seasons

min mean max

min mean max

1.82 0.49 0.022 0.56 0.89 0.96

2.09 0.86 0.028 0.96 1.37 1.47

10.8 2.31 0.15 2.47 4.17 4.32

22.9 8.71 0.91 9.62 13.9 14.9

7.1 46.7

27 89

min

mean

10.7 54.4 0.77 11.1 2.89 18.8 0.46 2.94 0.35 1.47 <0.001 0.21 3.24 20.3 0.48 3.15 4.63 34.0 0.82 5.14 4.97 35.4 0.84 5.35

Ratio, % EC/TC 8.8 TCM/PM 84

3.3 17

6.5 38

5.4 95

<0.1 24

2.7 63

19.4 3.4 72 13.8

4.5 23

10.7 49

19.4 <0.1 95 13.8

6.7 49

PM (particle mass) corresponds to the total mass concentration of aerosols (weightanalysis data), OC (organic carbon), EC (elemental carbon), OM (organic mass), TC (total carbon), and TCM (total carbonaceousfraction mass) concentration. * The content of EC was estimated by dividing the detection limit by the volume of pumped air. IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS

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content of EC varied from 124 to 192 ng/m3 with a modal value of 143 ng/m3 from midMay to June 2006 [37]. For comparison, at the ZOTTO station, in August 2010 and 2011, the range of EC variations was 0–131 ng/m3 with a most probable value of 63 ng/m3; i.e., the content of anthropogenic carbon at the Euro pean Hyytiälä and Kpuszta stations is, on average, 2.4 times higher than at the Siberian station. It should be noted that, despite the fact that the ZOTTO station is located far from large cities, the total duration of time intervals with background concentrations amounts to no more than 20% of the entire observa tion period. CONCLUSIONS The composition of the carbonaceous fraction of aerosol samples taken at the ZOTTO station from April 2010 to May 2012 was studied using a thermo optical method. Seasonally mean contributions of car bonic component to the mass of aerosol particles (TCM/PM) varied within 38–63%. Measurement data show that, despite the fact that the ZOTTO sta tion is located far from populated areas and industrial zones, the contents of OC and EC in the atmosphere over boreal forests in central Siberia significantly exceed their background values. During the cold seasons, the main sources of pollu tion are large cities located in the industrial zones in southern and southwestern Siberia, which combust coal and hydrocarbons for heating. Within the spring– summer period, the pollution level is high due to regional forest fires and agricultural burning in the steppe zone of Siberia. The difference between seasonal pollution sources is clearly pronounced in the ratio between OC and EC. An especially marked difference is noted between the winter and summer periods, for which the mean OC/EC ratios differ by more than a factor of three. Their background concentrations were observed within relatively short time intervals, the total duration of which amounted to no more than 20% of the entire observation period. During the summer, variations in the concentra tion of OC closely correlated with those in air temper ature, which implies that biogenic sources of its for mation are dominating. During this period, the EC content was 2.4 times lower than that for the European background stations. ACKNOWLEDGMENTS The authors are grateful to A.A. Tsukanov, N.V. Timokhina, S.V. Titov, and N.V. Sidenko from the Sukachev Institute of Forest for aerosol sampling at the ZOTTO station. This work was supported by the Russian Founda tion for Basic Research (project nos. 120500620a), StPSU project no. 11.38.650.2013, the StPSU IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS

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Translated by B. Dribinskaya

IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS

Vol. 51

No. 4

2015