Spatiotemporal variations of monocarboxylic acids in snow layers along a transect from Zhongshan Station to Dome A, eastern Antarctica

Atmospheric Research 158–159 (2015) 79–87

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Spatiotemporal variations of monocarboxylic acids in snow layers along a transect from Zhongshan Station to Dome A, eastern Antarctica Chuanjin Li a,⁎, Cunde Xiao a,b,⁎⁎, Guitao Shi c, Minghu Ding a,b, Shichang Kang a,d, Lulu Zhang e, Shugui Hou f, Bo Sun c, Dahe Qin a, Jiawen Ren a a The State Key Laboratory of the Cryospheric Sciences, the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b Climate System Institute, Chinese Academy of Meteorological Sciences, Beijing 100081, China c Polar Research Institute of China, Shanghai 200136, China d Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China e Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China f School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 28 September 2014 Received in revised form 12 February 2015 Accepted 13 February 2015 Available online 21 February 2015 Keywords: Monocarboxylic acids Spatiotemporal variations Human activities Eastern Antarctica

a b s t r a c t The spatiotemporal distributions of formate and acetate in snow layers along a transect from Zhongshan Station to Dome A are presented here. The mean concentrations of mono-carboxylic acids in summer surface snow layers were 2.93 ± 1.72 ng g−1 and 10.07 ± 5.87 ng g−1 for formate and acetate, respectively. In the snow pit samples, the concentrations varied between 0.47 ± 0.14 ng g−1 and 3.12 ± 4.24 ng g−1 for formate and between 5.31 ± 1.55 ng g−1 and 13.29 ± 4.64 ng g−1 for acetate. Spatially, the concentrations of both acids featured negative trends with increasing elevation and distance inland for the initial 600 km of the transect, which implies that marine sources from the coastal oceans dominate the acid supply. Different distribution styles of the acids in the interior section (600–1248 km) suggest that different source region and transporting mechanism may be responsible for the acid deposition in the interior regions. Seasonal variations in the amounts of acid in a coastal snow pit (29-A) indicate higher values in the summer and lower amounts in the winter. An enlarged source region and intensified production and transport mechanisms were primarily responsible for the higher values in summer. Longer records from the interior snow pits (29-L and 29-M) indicate elevated values in the 1970s and lower values in the 1980s and early 1990s. The increases in the mono-carboxylic acids since 1999 in snow pit 29-L and since 2005 in snow pit 29-M were temporally coincident with Chinese expedition activities in the area, suggesting that human activities were responsible for the increases in the acid load during recent decades. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. ⁎⁎ Correspondence to: C. Xiao, The State Key Laboratory of the Cryospheric Sciences, the Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail addresses: [email protected] (C. Li), [email protected] (C. Xiao).

http://dx.doi.org/10.1016/j.atmosres.2015.02.008 0169-8095/© 2015 Elsevier B.V. All rights reserved.

Formate and acetate are the chief carboxylic compounds in the troposphere, and in addition to sulfuric and nitric acids, they play a prominent role in the aqueous and gaseous phases of atmospheric acidity (De Angelis et al., 2012; Khare et al., 1999; Talbot et al., 1988; Shannigrahi et al., 2014). Although they are weak acids, formate and acetate contribute significantly to the free acidity and ionic balance of precipitation and

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the chemical reactions in the atmosphere, particularly in remote areas (Li et al., 2003; Talbot et al., 1988; Belosi et al., 2014; Wu et al., 2015). Although there is still little knowledge of the sources of carboxylic acids, their sources have been identified in general. In mid-latitude continental regions, other than photochemical reactive vehicular emission, possible sources of formate and acetate are direct emissions from vegetation (Andreae et al., 1988; Talbot et al., 1988), burning biomass (Andreae et al., 1988; Lefer et al., 1994; Talbot et al., 1988), photochemical reactions of unsaturated hydrocarbons (Madronich and Calvert, 1990; Su et al., 1979), and anthropogenic pollution due to the burning of fossil fuels, wood and garbage (Hartstein and Forshey, 1974; Kawamura et al., 1985; Madronich and Calvert, 1990; Talbot et al., 1988). In tropical continental areas, direct emissions from vehicles, ants, soil, vegetation, and burning biomass are important source of these species (Khare et al., 1999). The likely sources in marine locations are photochemical reactions, biogenic emissions, and long-range transport from continental sites (Khare et al., 1999). During the past few decades, researchers have paid much attention to the sources of carboxylic acids, their temporal variations and the associated driving factors (Khare et al., 1999). Polar ice sheets and alpine glaciers provide information in a unique way regarding the past variations in carboxylic acids in response to climatic and environmental changes (Legrand et al., 2003, 2004; Li et al., 2003). In 1987, Saigne et al. (1987) first identified the potential of studying the formate and acetate in the Dome C ice core, Antarctica. In 1995, Legrand and De Angelis (1995) studied the formate records in a 307-m-long ice core recovered in Adelie Land (D10) and found that formate concentrations there during the Holocene are one order of magnitude lower than in Greenland and that formate concentrations in ice from the Pleistocene are lower than those from the Holocene by a factor of nearly five. A long record of formate and acetate concentrations in the EDC and EDML ice cores from Antarctica showed that, except for the sporadic arrival of diluted continental plumes during glacial extremes, the primary source of the acetate deposited over the EDC does not seem to have changed significantly over the past 300 kyr and is related to marine biogenic activity (De Angelis et al., 2012). In low latitudes, records of carboxylic acids obtained from alpine ice cores have provided additional information regarding remote continental sites in the Northern Hemispheres. Based on high-resolution records of formate and acetate from a high-elevation glacier in the French Alps (Col du, 4250 m above sea level), it was concluded that, in addition to convective transport from the boundary layer, the secondary production of both carboxylic acids in the troposphere may contribute to the carboxylic acid budget in the midtroposphere over Europe (Legrand et al., 2003). However, human activity is only a minor contributor to the carboxylic acids budget, even for populated areas (Legrand et al., 2003; Paulot et al., 2011). The ice core from Glacier 1 at the head of the Urumqi River, Tianshan, western China (43°06′N, 86°49′E, 4040 m above sea level), indicated that the two carboxylic acids co-varied during the past decades and exhibited periods of high concentrations from the early 1960s to the middle 1970s and from the early 1980s to the middle 1990s. Additionally, local/regional anthropogenic pollution may influence the deposition of carboxylic acids (Li et al., 2003).

Although much effort has been expended studying the records of the two carboxylic acids in snow and ice, the spatial coverage has not been sufficient to study their global distribution patterns and influencing factors. Little attention has been paid to the air-snow exchange, which is an essential part of the interpretation of ice core profiles (De Angelis et al., 2012). In this work, we present detailed formate and acetate data collected from snow layers along a transect from Zhongshan station to Dome A, which is the highest point on the eastern Antarctic ice sheet. The transect was 1,248 km long, and the elevation varied from sea level to 4,100 m. Thirteen snow pits were excavated to obtain samples. The distribution pattern of the carboxylic acids at various elevations, their temporal variations during the previous few decades, and the factors that influence these patterns were investigated. 2. Sampling and analysis procedures The field expedition of the 29th Chinese National Antarctic Research Expedition (CHINARE) started at Zhongshan Station (69°37′31″S, 76°37′22″E) in December 2012 and ended at the Chinese interior research site, Kunlun Station, which is located on the highest point in eastern Antarctica, Dome A (80°25′01S, 77°06′58E) (Ding et al., 2011; Xiao et al., 2008) in January 2013 (Fig. 1). The route was 1,248 km long, and the elevation rose from sea level at Zhongshan Station to 4,093 m at Dome A. Detailed geographic information regarding the route and the snow accumulation are listed in Table 1 and have been presented by others (Ding et al., 2011; Hou et al., 2007; Xiao et al., 2008). In total, 13 snow pits were dug as the expedition moved inland, and 585 snow or firn samples were collected from the pits. Detailed information regarding the snow pit sites is shown in Table 1. The accumulation rates were measured along the return traverse based on the heights of stakes placed during the outbound traverse. The density of the surface snow, the snow temperature at15-cm depth, and the air temperatures 50 cm above the surface were measured simultaneously. In total, 66 groups of data were obtained during the traverse. Moreover, 181 surface snow samples were collected at regular intervals along the route; 66 of these were selected for trace metal analysis, and the other 115 samples were analyzed for major ions. All the surface snow sampling was executed at the site 20 m up wind from the route. All of the sampling containers were 250-ml Nalgene LDPE wide-mouth bottles, which were thoroughly cleaned following strict cleaning procedures (Hong et al., 2000; Liu et al., 2011) at the State Key Laboratory of the Cryospheric Sciences (SKLCS) in Lanzhou and double sealed in acid-washed LDPE bags. All of the sampling tools, such as the polypropylene scraper and knives, were also pre-cleaned before embarking on the field expedition. After digging the snow pits, another 20-cm profile was further peeled off using clean scrapers. All of the samples were collected by pushing bottles into the wall of the snow pits, and two samplers were involved following the “Clean Hands-Dirty Hands” protocol described in our previous work (Zhang et al., 2012). Details regarding the cleaning and sample-handling protocols can be found in the relevant references (Huang et al., 2012; Loewen et al., 2007; Zhang et al., 2012). The surface snow samples were collected by vertically pushing the sample bottles into the snow surface to a depth of at least 5 cm. Afterward, the samples were

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Fig. 1. Sampling sites along the transect from Zhongshan Station to Dome A. Thirteen snow pit samples were collected during the inland journey, and detailed geographic and sampling information can be found in Table 1. Blue circles and red points were the locations where the surface snow samples were collected on the return trip and used for mercury and major ions analysis, respectively. Antarctic map with the locations of South Pole, Dome A, Vostok, and Dome C are also shown.

double sealed in clean bags, kept in clean, dark boxes and transported back to SKLCS in a frozen state (b−10 °C) and stored in a dark refrigerator (−18 °C) until further processing. To avoid potential contamination from the plastic containers as the samples melted, an approximately 100 ml aliquot of the snow sample (equivalent to ~40 ml of liquid) was transferred to pre-cleaned glass bottles before analyzing the sample for carboxylic acids. The samples were analyzed for ions immediately after melting in a clean room (1000 class for the room and 100 class for the operating chamber). Cations were analyzed using a Dionex ISC 3000 ion chromatograph equipped with an Ion Pac CS12A column, 20 mM MSA (methanesulfonic acid) eluent, and a CSRS suppresser. The anions were analyzed using a Dionex ISC3000 ion chromatograph equipped with an Ion Pac AS11-HC column, 25 mM KOH eluent, and an ASRS suppresser. The detection limits for formate and acetate, which were defined as 3 times the standard deviation of the baseline noise, were approximately 0.06 ng/g for HCOO− and 0.24 ng/g for CH3COO−. The analytical precision and accuracy of these concentrations was better than 10%. Three sampling bottles were used as field blank and they were brought to the field together with other bottles. In the lab, the empty bottles were filled with deionized water and analyzed following the same procedure as other samples, and the concentrations of the two acids were both lower than the detection limits, suggesting that the contamination introduced during the sampling procedure may be negligible.

Dating of the three selected snow pits (29-A, 29-L, and 29-M) was performed by a combination of seasonal and absolute markers. For seasonal snow-layer identification (relative dating), we used a visual comparison between δ18O and seasonal variations in chemical profiles (Na+, MSA, nssSO2− 4 ). Absolute age dating was performed using the volcanic marker from the Pinatubo eruption of 1991, which is indicated by a prominent rise in nssSO2− in snow layers (snow pits 29-L and 29-M). The 4 age dating indicated that there were three summer peaks in the snow exposed in snow pit 29-A, 37 years (1976–2012) of accumulation in snow pit 29-L, and approximately 40 years (1973–2012) of accumulation in snow pit 29-M. 3. Results and discussions 3.1. Spatial distribution of formate and acetate The mean concentrations of formate and acetate in the surface snow samples for the entire transect were 2.93 ± 1.72 ng/g and 10.07 ± 5.87 ng/g, respectively. The mean concentrations in the snow pit samples varied from 0.47 ± 0.14 ng/g to 3.12 ± 4.24 ng/g for formate and from 5.31 ± 1.55 ng/g to 13.29 ± 4.64 ng/g for acetate (Fig. 2). In the Dome A region, the two monocarboxylic acids in the snow pits displayed concentrations similar to those collected from the Dome C snow pack during the 1997–1998 field season (Table 2) (De Angelis et al., 2012). The similar or lower concentrations in our samples

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Table 1 Geographic and sampling information together with the mean formate and acetate concentrations in the 13 snow pits along the transect from Zhongshan Station to Dome A. Sampling site 29-A 29-B 29-C 29-D 29-E 29-F 29-G 29-H 29-I 29-J 29-K 29-L 29-M a b c

Location 69°42′39.5″S 76°28′43″E 70°30′11.9″S 76°49′36.5″E 71°11′12.7″S 77°21′52.2″E 71°58′42.8″S 77°56′45.3″E 72°51′27.2″S 77°22′31.9″E 73°26′7.1″S 76°59′20.7″E 73°54′52.7″S 76°59′15.6″E 75°27′0.7″S 76°53′42.6″E 76°20′45.2″S 77°02′14.4″E 77°12′2.7″S 76°58′3.7″E 77°59′51.1″S 77°06′42.2″E 79°05′57.1″S 76°59′42.9″E 80°22′00″S 77°21′11″E

Elevation (m)

Distance from start (km)

Depth (cm)

Accumulation a (kg m−2 yr−1)

Nb

Formate c (ng/g)

Acetate c (ng/g)

Sampling date

832

40

200

268.0

8

1.62 ± 1.07

9.90 ± 2.19

Dec/16/2012

1597

130

200

138.5

20

2.89 ± 2.53

13.29 ± 4.64

Dec/17/2012

2074

210

150

172.0

20

1.25 ± 0.53

12.12 ± 2.29

Dec/18/2012

2351

300

200

121.6

20

3.12 ± 4.24

10.13 ± 8.81

Dec/20/2012

2514

400

200

94.6

20

2.36 ± 1.87

10.90 ± 3.20

Dec/21/2012

2551

466

200

120.6

20

2.29 ± 2.23

8.39 ± 5.33

Dec/22/2012

2631

520

300

54.7

60

0.88 ± 0.42

5.82 ± 2.58

Dec/24/2012

2799

690

100

29.7

10

0.47 ± 0.14

7.38 ± 2.71

Dec/26/2012

2830

792

200

62.2

20

0.83 ± 0.34

8.00 ± 2.30

Dec/28/2012

2967

886

200

88.0

80

0.61 ± 0.39

11.79 ± 3.17

Dec/29/2012

3169

976

200

33.3

80

0.84 ± 0.41

5.81 ± 2.41

Dec/30/2012

3757

1100

250

25.4

100

0.95 ± 0.39

5.31 ± 1.55

Jan/02/2013

4093

1248

300

23.5

100

1.13 ± 0.40

6.46 ± 2.01

Jan/05/2013

The accumulation rate was the mean value observed in each snow pit divided by the age in years. N represents the total number of the samples collected from each snow pit. Concentrations of the two monocarboxylic acids are shown as the mean value ± standard deviation.

indicated to us that little contamination was introduced during our sampling and analyzing procedures (Table 2). Comparisons of the concentrations with other Antarctic sites indicated that our formate concentrations were within the ranges of Holocene samples collected elsewhere. However, our acetate concentrations were higher than those shown in Table 2. Greenland and low-latitude areas typically display higher monocarboxylic acid concentrations due to their proximity to human activity and natural sources (Legrand et al., 2003; Udisti et al., 1998; Vimeux et al., 2008), and the observed concentrations from these areas were higher than our results, particularly for formate (Savarino and Legrand, 1998). The spatial distribution pattern of the two monocarboxylic acids in surface snow and snow pit samples varies between sections of the transect route. The initial section (0–600 km) exhibited consistency between the concentrations in the surface and snow pit samples; both showed general decreasing trends with increasing distance inland (Fig. 2). Two mechanisms may account for this consistency. First, monocarboxylic acids are primarily produced and transported inland during the summer season (Legrand et al., 2004). Second, little evaporation of the deposited carboxylic acids may have occurred in this section due to the relatively higher accumulation rates (Ding et al., 2011). A slight positive correlation (0.49, P b 0.01) between the two monocarboxylic acids was identified in the surface snow samples, which implies that these compounds originated from the same sources and underwent similar transport and deposition. The formate displayed positive correlation with sea salt ions (Na+, 0.55, P b 0.01), which implies that a coastal marine source was primarily responsible

for the deposition of carboxylic acids, and they were transported inland together with sea salt aerosols (Legrand et al., 2004). However, the influences from wind currents, surface topography, dust, and other post-depositional effects cannot be ignored here because the decreasing trend was ambiguous in some certain regions (Fig. 2) (Ma et al., 2010). Two significant differences were apparent between the coastal and inland section. First, lower deposition rates and mean concentrations of the two acids were observed in the inland snow pits than in the coastal area, particularly with regard to formate. Second, the mean concentrations of both monocarboxylic acids in the snow pits were lower than the summer surface snow samples (Fig. 2). Post-depositional loss from the snow pack may be the primary reason for the differences: once deposited on the snow surface, the monocarboxylic acids can re-evaporate into the atmosphere, thereby leading to lower concentrations in the snow pits (De Angelis and Legrand, 1995; Dibb et al., 1994; Dibb and Arsenault, 2002). However, the post-depositional effect does not comprehensively explain the large differences in acid concentrations in different sections with similar accumulations (Fig. 2). Moreover, the good correlation between the carboxylic acids and sea salt ions in the surface samples disappeared in the interior section (Table 3). Intensified evaporation of the volatile acids in interior region with lower accumulation may be one reason (De Angelis and Legrand, 1995; Dibb et al., 1994; Dibb and Arsenault, 2002). The acids instead displayed good correlation with nssSO2− 4 , which primarily originates from biological activity in lower-latitude oceans, anthropogenic emissions or crustal erosion (Becagli et al., 2004; Legrand and Mayewski,

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Fig. 2. Spatial distribution of the formate and acetate in surface snow and snow pits along the transect route from Zhongshan station to Dome A and the geophysical parameters. (a) Formate concentrations in surface snow (open circles) and snow pits (solid squares) together with the flux (red solid circle); (b) acetate concentrations in surface snow (open circles) and snow pits (solid squares) together with the flux (red solid circles); (c) accumulation rates measured using the stakes during the previous three years (2010–2012) (gray vertical bars) and from the snow pits (solid squares), with elevations (gray circles) for reference.

snow surface (De Angelis et al., 2012). Therefore, we speculate that different sources and transport mechanisms may be responsible for the acid deposition in the interior section (Wang et al., 2013; Xiao et al., 2004, 2012). The deposition of

1997). Particle-borne species such as sulfate are incorporated into Antarctic interior surface snow by wet and dry deposition, especially during summer season (Harder et al., 2000), and this is also helpful for the monocarboxylic acids deposition on the

Table 2 Comparison of the formate and acetate concentrations in snow pits along the transect route from Zhongshan Station to Dome A and ice layers of other remote sitesa. Sites

Location and altitude

Accumulation Formate (ng g−1)b (g cm−2 yr−1)

Acetate (ng g−1)b

Reference

Dome Summit (Greenland) San Valentin (Patagonia) Col du Dome (French Alps) D10 Northern Victoria Land Berkner Island Talos Dome DML 05 DC Vostok Lambert glacier

72° 36′N, 38° 25′W, 3250 m

23

10.7 (1)

9.3 (1.4)

46°35′S, 73°19′W, 3747 m

__

6.5 (5.5)

5.1 (0.5)

Legrand and de Angelis (1995) Savarino and Legrand (1998) Vimeux et al. (2008)

45°28′N, 9°11′E, 4250 m

10 ~ 30

100

27

Legrand et al. (2003)

66°36′S,138°11′ E, 270 m 73°06′S,165°28′ E, 2960 m 78°19′S,46°20′ W, 869 m 72°49′S,159°11′ E, 2315 m 73°10′S, 124° 10′ E, 3240 m 74°40′S, 124° 10′ E, 3240 m 78°28′S, 106° 48′ E, 3490 m 69°43′S,76°29′E to 80°22′ S,77°21′E 832 m to 4093 m

19 (6) __ 13 7.5 6.2 3.2 2.2 2.4 ~ 26.8

1.4 (0.7) 16.89 (15.88) 0.1 (0.08) 0.3 (0.33) 0.55 (0.24) 0.22 (0.14) 0.16 (0.07) 0.47 (0.14) ~3.12 (4.24)

__ 17.39 (12.97) 0.11 (0.09) 1.2 (1.6) 0.86 (0.67) 0.32 (0.24) 0.16 (0.16) 5.31 (1.55) ~13.29 (4.64)

Legrand and de Angelis (1995) Udisti et al. (1998) De Angelis et al. (2012) De Angelis et al. (2012) De Angelis et al. (2012) De Angelis et al. (2012) De Angelis et al. (2012) This study

a b

A portion of the data is from de Angelis. The numbers in brackets represent one standard deviation.

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Table 3 Correlation matrix of the concentrations of formate and acetate and other elements in surface snow samples and their geophysical parameters. 40–600 km

Formate Acetate Na+ NH+ 4 MSA nssSO2− 4 Distance Elevation Accumulation

600–1248 km

Formate

Acetate

Formate

Acetate

– 0.49⁎⁎ 0.55⁎⁎ −0.05 −0.19 −0.13 −0.02 0.00 0.04

0.49⁎⁎ – 0.15 0.17 −0.13 −0.11 −0.20 −0.20 0.07

– 0.85⁎⁎ −0.11 0.12 0.08 0.30⁎ 0.55⁎⁎ 0.63⁎⁎ −0.20

0.85⁎⁎ – −0.24 0.25⁎ −0.01 0.25⁎ 0.44⁎⁎ 0.54⁎⁎ −0.19

⁎⁎ Correlation at P = 0.01. ⁎ Correlation at P = 0.05.

monocarboxylic acids in the interior may occur via stratospheric transport, whereas lower atmospheric transport may dominate the coastal section. The significantly positive correlations between the acid concentrations and the distance inland in the interior section (600–1248 km) may indicate that the acids in the interior area originate from lower latitudes (MassonDelmotte et al., 2008). 3.2. Temporal variations in formate and acetate One coastal (29-A, Fig. 3) and two interior snow pits (29-L and 29-M, Fig. 4) were selected for the study of temporal variations in the two monocarboxylic acids. Clear seasonal variations in the amounts of formate and acetate were observed in snow pit 29-A. We detected high acid concentrations in the summer and the opposite in the winter. The concentration of acetate (10.61 ± 1.98 ng/g) in summer was higher than the winter (9.19 ± 2.43 ng/g). Snow pit 29-A was located near the coast, and marine influences were the primary factor affecting the monocarboxylic acids there (Legrand et al., 2004). The extent of sea ice decreases quickly in the summer (November to February), and the large areas of open ocean can

contribute significantly to alkene emissions from the ocean (Legrand et al., 2004; Li et al., 2014). Alkene production (primarily ethene, propene, butene, and pentene) is related to the release of dissolved organic material by algae that is then photochemically converted to alkenes (Ratte et al., 1993, 1998). Moreover, during the summer, short-wavelength solar radiation is at a maximum; thus, the enhanced oceanic photodegradation of dissolved organic matter and the photochemical production of alkenes may be other factors affecting the monocarboxylic acid loading in snow (Legrand et al., 2004). The strong northern wind from the ocean during the summer may be another important factor in the greater deposition of formate and acetate during the summer (Li et al., 2014; Ma et al., 2010). However, there are different opinions regarding whether the higher penguin populations during the summer near the sampling site affect the monocarboxylic acid load (Legrand et al., 1998, 2004). In snow pit 29-A, the influence of penguins cannot be excluded because of the co-existence of ammonium and monocarboxylic acids in the summer snow layers of 2011 and 2012 (Legrand et al., 1998). The deposition of formate and acetate on Dome A during the past four decades was studied in two depositional profiles in snow pits 29-L and 29-M (Fig. 4). In both snow pits, the two monocarboxylic acids exhibited similar variations, which implies that the same sources, transport, and depositional mechanisms were responsible for their records. In the snow dating to the 1970s in snow pit 29-M, both of the monocarboxylic acids displayed significant increasing trends, and two peak values corresponding to the end of 1970s were identified. However, sudden decreases in the levels of the acids at the beginning of the 1980s were observed. After 1980s, there was a consistent increase in the concentrations during the following decade. The mean ratio between formate and acetate in both snow pits during the two decades are 0.18 ± 0.05 for 29-M and 0.18 ± 0.07 for 29-L. According to Talbot et al. (1988) founding that air masses that were influenced chiefly by anthropogenic emissions had ratios lower than one, while those influenced mainly by natural sources, vegetation emissions in particular, had the ratios larger than one. Moreover, according to temporal

Fig. 3. Vertical profile of the formate (a) and acetate (b) concentrations in snow pit 29-A, together with nss-SO2− (c) and oxygen isotopes (d). 4

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research on mercury deposition in Dome A region, they show similar variation trend with the acids. Hence, we speculate that major influence on monocarboxylic acids were from anthropogenic activities in low latitudes via long-range transport. The two acids displayed slight decreasing trends during the 1990s, reached another low value in 2004, and then increased markedly to the present. According to statistical data collected by CHINARE regarding the population and vehicles in the Dome A region, the increase in the levels of the two acids were temporally coincident with anthropogenic activity in the region. During the 15th CHINARE in 1998–1999, the expedition team only reached site DT401 (near snow pit 29-L), which is located 120 km from Dome A, and thus it is conceivable that no significant increase in the acid levels occurred at the end of the 1990s in snow pit 29-M. In comparison with 29-M, the acid records in snow pit 29-L displayed lower amplitude variations, which may be associated with the stronger post-depositional effect at site DT401 (Ma et al., 2010; Ren et al., 2010). The temporal variations in the levels of the two monocarboxylic acids were similar in the 29-M records, i.e., a slight increase during the 1970s and low values during the 1980s. However, the 29-L record displayed earlier concentration increases at the end of 1990s than did the 29-M record. As mentioned earlier, the first arrival at DT401 was during the 1998–1999 season, and the simultaneous increase in the two acids may be associated with the expedition activities at that time. Another difference between the 29-M and 29-L records was the consistent increasing trend since 2004/2005 in the 29-M record in contrast to the sustained plateau in concentrations since 1998/1999 in the 29-L record. After the 15th CHINARE, the next expedition merely passed by site DT401 to reach

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Dome A. During the last four expeditions, more scientific and logistic works has been performed at Dome A, therefore likely resulting in more emissions from vehicle exhaust (Fig. 4). Moreover, since 1999, the supplementary deposition of the two monocarboxylic acids (original concentration minus background values) featured a mean ratio of 0.6 ± 0.4 (mass/mass) in snow pit 29-L and a slightly smaller ratio (0.53 ± 0.48) in the post-2005 record of snow pit 29-M. Vehicle emissions usually contain formate and acetate at a ratio of approximately 0.5 (Talbot et al., 1988); the similar results in the Dome A region during the previous decade may be further proof of the influence of human activity on monocarboxylic acid deposition in Dome A snow. The slightly greater standard deviation may indicate that other natural influences were also important in the acid deposition. 4. Conclusions The spatiotemporal distribution of formate and acetate in a transect from Zhongshan Station to Dome A was presented. Spatially, both acids exhibit negative trends with increasing elevation and distance inland for the initial 600 km of the transect, which implies that marine sources from coastal oceans primarily dominate the acid supply. However, the interior section (600–1248 km of the transect) differs with regard to the distribution patterns of the acids; a different source region and different transporting mechanisms were responsible for acid deposition in the interior regions. Seasonal variations in the monocarboxylic acids in the coastal region resulted in greater deposition in the summer and less deposition in the winter. The enlarged area of open sea water

Fig. 4. Temporal variations in the anomalies of formate (red open circles with the red solid line for 10-point smoothed data) and acetate (solid black points with the black solid line for 10-point smoothed data) concentrations in snow pit 29-L and 29-M. The numbers of participants and vehicles used in each CHINARE since 1999 are also shown.

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region during the summer was responsible for the intensified production of alkenes, which are an important precursor for the formation of monocarboxylic acids. Moreover, the intensified photochemical reactions and meridional transport from the ocean were also responsible for the higher acid values in the summer snow layers. Long records from the interior snow pits (29-L and 29-M) displayed higher values in the 1970s and subsequent decreases in the 1980s and early 1990s. The increases in the monocarboxylic acids since 1999 in snow pit 29-L and since 2005 in snow pit 29-M snow pits were temporally coincident with the expedition activities of Chinese researchers (CHINARE) in this area, which indicates that human activity has been responsible for the increase in the acid load during recent decades. Acknowledgments The authors thank all of the members who participated in the 2012–2013 Chinese National Antarctic Research Expedition field campaigns for sample collection. In addition, special thanks are given to Ms. Xiaoxiang Wang, Xiaoqing Cui, and Yuman Zhu from the Cold and Arid Regions Environmental and Engineering Research Institute for completing portions of the analysis. This work was financially supported by the Innovative Research Group, the National Natural Science Foundation of China (501100001809) (41121001), the National Basic Research Program of China (973 Program, 2013CBA01804), the State Key Laboratory of Cryospheric Sciences, the National Natural Science Foundation of China (Grant Nos. 41201069 and 41476164), the State Oceanic Administration of People's Republic of China Project on Climate in Polar Regions (Grant Nos. CHINARE 2014-04-04 and CHINARE 2014-02-02), and the Foundation for Excellent Youth Scholars of CAREERI, CAS. References Andreae, M.O., et al., 1988. Formic and acetic acid over the central Amazon region. J. Geophys. Res. 93 (D2), 1616–1624. Becagli, S., et al., 2004. Chemical characterization of the last 250 years of snow deposition at Talos Dome (East Antarctica). Int. J. Environ. Anal. Chem. 84 (6–7), 523–536. Belosi, F., Santachiara, G., Prodi, F., 2014. Ice-forming nuclei in Antarctica: new and past measurements. Atmos. Res. 145–146, 105–111. De Angelis, M., Legrand, M., 1995. Preliminary investigations of post depositional effects on HCl, HNO3, and organic acids in polar firn layers. In: Delmas, R.J. (Ed.), Ice Core Studies of Global Biogeochemical Cycles, NATO ASI Series I, Vol. 30. Proceedings of the NATO Advanced Research Workshop held in Annecy, France, March 26–31, 1993. Springer, Berlin, pp. 361–381. De Angelis, M., et al., 2012. Long-term trends of mono-carboxylic acids in Antarctica: comparison of changes in sources and transport processes at the two EPICA deep drilling sites. Tellus B 64 (17331), 1–21. Dibb, J.E., Arsenault, M., 2002. Shouldn't snowpacks be sources of monocarboxylic acids? Atmos. Environ. 36, 2513–2522. Dibb, J.E., et al., 1994. Soluble acidic species in air and snow at Summit, Greenland. Geophys. Res. Lett. 21, 1627–1630. Ding, M., et al., 2011. Spatial variability of surface mass balance along a traverse route from Zhongshan station to Dome A, Antarctica. J. Glaciol. 57 (204), 658–666. Harder, Susan, Warren, Stephen G., Charlson, Robert J., 2000. Sulfate in air and snow at the South Pole: implications for transport and deposition at sites with low snow accumulation. J. Geophys. Res. 105 (D18), 22,825–22,832. Hartstein, A.M., Forshey, D.R., 1974. Coal mine combustion products: neoprenes, polyvinyl chloride compositions, urethane foam, and wood. US Dept. of the Interior, Washington, D. C. Hong, S., Lluberas, A., Rodriguez, F., 2000. A clean protocol for determining ultralow heavy metal concentrations: its application to the analysis of Pb, Cd, Cu, Zn and Mn in Antarctic snow. Korean J. Polar Res. 11, 35–47.

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