Environ. Sci. Technol. 2010, 44, 2348–2353
Single-Particle Characterization of Summertime Arctic Aerosols Collected at Ny-Ålesund, Svalbard H O N G G E N G , †,‡ J I Y E O N R Y U , † HAE-JIN JUNG,† HYEOK CHUNG,§ K A N G - H O A H N , § A N D C H U L - U N R O * ,† Department of Chemistry, Inha University, YonghyunDong, NamGu, 402-751, Incheon, Korea, Research Center of Environmental Science and Engineering, Shanxi University, Taiyuan, 030006, China, and Department of Mechanical Engineering, Hanyang University, Sangnok-gu, 425-791, Ansan, Korea
Received October 27, 2009. Revised manuscript received January 20, 2010. Accepted February 14, 2010.
Single-particle characterization of summertime Arctic aerosols is useful to understand the impact of air pollutants on the polar atmosphere. In the present study, a quantitative single particle analytical technique, low-Z particle electron probe X-ray microanalysis, was used to characterize 8100 individual particles overall in 16 sets of aerosol samples collected at NyÅlesund, Svalbard, Norway on 25-31 July, 2007. Based on theirX-rayspectralandsecondaryelectronimagedataofindividual particles, 13 particle types were identified, in which particles of marine origin were the most abundant, followed by carbonaceous and mineral dust particles. A number of aged (reacted) sea salt (and mixture) particles produced by the atmospheric reaction of genuine sea-salts, especially with NOx or HNO3, were significantly encountered in almost all the aerosol samples. They greatly outnumbered genuine sea salt particles, implying that the summertime Arctic atmosphere, generally regarded as a clean background environment, is disturbed by anthropogenic air pollutants. The main sources of airborne NOx (or HNO3) are probably ship emissions around the Arctic Ocean, industry emission from northern Europe and northwestern Siberia, and renoxification of NO3within or on the melting snow/ice surface.
1. Introduction The atmosphere of the Arctic region is characterized by its cold temperature, stable stratification of the boundary layer, and unusual light conditions. The Arctic atmosphere is regarded as a unique natural laboratory to understand the occurrence, nature, origin, and transport of atmospheric aerosols and their effect on global climate change (1–4). At various Arctic locations such as the Svalbard Islands (Norway), Alert (Canada), Barrow (Alaska), and coastal northern Greenland, it has been observed that atmospheric aerosols were often disturbed by the well-known Arctic haze, a winter/ spring Arctic pollution phenomenon (2, 5), suggesting that the atmosphere of the Arctic region could be polluted by * Corresponding author phone: +82 32 860 7676; fax: +82 32 867 5604; e-mail:
[email protected]. † Inha University. ‡ Shanxi University. § Hanyang University. 2348
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anthropogenic air pollutants in the winter and spring. The summertime Arctic atmosphere has been generally regarded as a clean background environment with low aerosol burden and pollution levels. However, some studies have reported that the summertime Arctic atmosphere was unexpectedly influenced by airborne pollutants, leading to high aerosol loading and pollution events (5–8). For example, in July 2004, about 5.8 million hectare of boreal forest was burned in North America. The fire emitted a biomass burning pollution plume that reached Barrow, Alert, Greenland, and Ny-Ålesund (Svalbard), leading to elevated light-absorbing aerosols and a seasonal anomaly of CO concentrations (7, 8). Nevertheless, our understanding of the chemical composition of summertime Arctic aerosols is still in its infancy (3). Single-particle analytical techniques are useful for obtaining detailed information on chemical species, morphologies, and mixing state of individual particles (9–11). Thus, single-particle characterization of summertime Arctic atmospheric particles is expected to increase our knowledge on what the characteristics of their chemical compositions are and whether they are disturbed by anthropogenic SO2 and NOx in the summer. To characterize the summertime ambient Arctic aerosol particles, a quantitative single-particle analytical technique called low-Z particle electron probe X-ray microanalysis (low-Z particle EPMA) was applied to examine aerosol samples collected on July 25-31, 2007 at a Korean scientific base located at Ny-Ålesund, Svalbard. Low-Z particle EPMA is capable of determining chemical composition and morphology of individual atmospheric particles, and provides satisfactory analytical results without complicated sample preparation (9–11). Using low-Z particle EPMA, environmentally important atmospheric particles such as mineral dust, sulfates, nitrates, and carbonaceous species can be reliably analyzed. Even the formation of nitrate and sulfate on sea-salts and mineral dust particles can be clearly elucidated (12). The objective of the present study is to characterize summertime Arctic aerosol particles by using the low-Z particle EPMA technique and to investigate the impact of anthropogenic air pollutants on the chemical components of summertime Arctic aerosol particles.
2. Materials and Methods 2.1. Sampling. Overall, 16 sets of aerosol samples were collected on July 25-31, 2007 at a Korean polar research station located at Ny-Ålesund (78°55′N, 11°56′E), Svalbard, Norway (see Supporting Information (SI) Figure S1). Svalbard is an archipelago in the Arctic Ocean, including three populated islands: Spitsbergen, Bear Island, and Hopen. NyÅlesund is located at the remote northern settlement of Spitsbergen, where the sun is above the horizon for 24 h a day from late April to late August. During the sampling period, the local temperature was 5-12 °C, relative humidity 57-82%, air pressure 1003-1010 mbar, and average wind speed 1-10 m s-1 (SI Table S1). Particles were sampled on Al foils using a seven-stage May cascade impactor (13). The May impactor, at a 20 L min-1 sampling flow, has aerodynamic cut-offs of 16, 8, 4, 2, 1, 0.5, and 0.25 µm for stages 1-7, respectively. Sampling durations for each stage were adjusted to avoid the collection of agglomerated particles. PM2-4, PM1-2, and PM0.5-1 particles on stages 4, 5, and 6, respectively, for all samples as well as PM8-16 and PM4-8 particles on stages 2 and 3, respectively, for samples collected on July 26, 27, and 30, 2007, were examined. Six-day (144 h) backward air mass trajectories were obtained using the hybrid lagrangian single-particle inte10.1021/es903268j
2010 American Chemical Society
Published on Web 03/03/2010
FIGURE 1. Typical secondary electron images of aerosol particles collected at Ny-Ålesund, Svalbard. Droplets rich in C, N, O, and S are abbreviated as “(C, N, O, S)”, which are likely the mixture of elemental/organic carbon and H2SO4/NH4HSO4/(NH4)2SO4. “Aluminosilicate*” is for aluminosilicate particles containing Fe, for example, particles no. 9, no. 11, and no. 35. grated trajectory (HYSPLIT) model from the NOAA Air Resources Laboratory’s web server (http://www.arl.noaa.gov/ ready/hysplit4.html), as shown in SI Figure S2. The back trajectories showed diverse origins for the air masses at different altitudes (100, 500, and 1000 m), especially for the air masses on July 29-31. On July 25-28, 2007, air masses at heights of 100, 500, and 1000 m above the sampling site originated from and stayed on the Arctic Ocean (SI Figure S2a-c); and then, on July 29 and 30, air masses at the height of 500 and 1000 m passed over the Barents Sea (SI Figure S2d and e); in the last period of the sampling (around July 31), air masses that originated from the Arctic Ocean and passed over the Barents Sea, northern Europe, and northwestern Siberia were predominant (SI Figure S2f). 2.2. Measurement and Analytical Techniques. Individual particles were measured by a JEOL JSM-6390 SEM equipped with an Oxford Link SATW ultrathin window energydispersive X-ray (EDX) detector. X-ray spectra were obtained using INCA software (Oxford). The resolution of the detector was 133 eV for Mn-KR X-ray. A 10 kV of accelerating voltage and 0.5 nA of beam current were used. Overall, about 8100 individual particles (∼150 individual particles for each stage sample) were manually investigated to obtain information on their size, secondary electron image (SEI), chemical composition, and mixing state. The net X-ray intensities of the elements were obtained by nonlinear least-squares fitting, using the AXIL program (14). The elemental concentrations of individual particles were determined from their X-ray intensities by using a Monte
Carlo calculation combined with reverse successive approximations (15, 16). The quantification procedure provided results with an accuracy of within 12% relative deviations between the calculated and nominal elemental concentrations for various standard particles (17). The “expert system” program, which can rapidly perform chemical speciation, was used to determine the formula concentrations and group distributions (18). The basic classification rule for the measured particles is given elsewhere (9, 17).
3. Results and Discussion 3.1. Particle Types and Their Relative Abundances. Based on the X-ray spectra and SEI data of all the analyzed individual particles, 13 particle types were identified. Their characteristics and relative abundances in different stage samples are described below. 3.1.1. Sea-Salt Aerosols. Sea-salt aerosols (SSA) are ubiquitous in the marine boundary layer (MBL), and can significantly impact particulate matter concentrations in coastal regions (19). The genuine (or fresh) sea-salt particles, which did not experience atmospheric chemical reactions after being emitted into the air, look bright on SEI and are cubic in shape (e.g., particles no. 16 and no. 17 in Figure 1). Their X-ray spectra show that Na and Cl signals are predominant, with the atomic concentration ratio of [Na]: [Cl] ≈ 1:1 (SI Figure S3a). Often, a small peak of oxygen is observed, which likely comes from the NaOH shell, an alkaline hygroscopic coating around the NaCl (20). Occasionally, VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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MgCO3 and MgSO4, which might originate from the evaporation of sea salts, were observed with less than 0.6% of relative abundance. In the atmosphere, the reacted (or aged) sea-salts containing NaNO3 and/or Na2SO4 are produced when genuine sea-salts react with nitrogen and sulfur oxides species, which results in chlorine loss, sometimes without remaining any chloride if the reaction is complete (21, 22). Hence, SSA often acts as a sink for anthropogenic gases such as nitrogen and sulfur oxides species. Herein, the reacted sea-salts were classified into three types based on their X-ray spectral data. The first type is of those containing nitrates such as Na(Cl, NO3) and (Na, Mg)(Cl, NO3), which are the reaction products of genuine sea-salts and NOx or HNO3, for example, particles no. 6, no. 10, no. 15, no. 18, no. 21, no. 29, no. 31, no. 40, and no. 43 in Figure 1 and SI Figure S3b. Particles internally mixed with NaCl, MgCl2, NaNO3, and Mg(NO3)2 are denoted as (Na, Mg)(Cl, NO3). The second type is of those containing methanesulfonate (CH3SO3-) and nonsea-salt sulfate (nss-SO42-) such as Na2SO4 and (Na, Mg)SO4 (e.g., particle no. 46 in Figure 1d), which resulted from the reaction of genuine sea-salts with SO2/H2SO4 or methylsulfonic acid (MSA)sa product of dimethylsulfide (DMS) (22). The third type is of those containing both NO3and CH3SO3-/nss- SO42-, for example, particles no. 23, no. 24, and no. 38 in Figure 1 and SI Figure S3c. Sometimes, the reacted sea-salts are mixed with mineral dust particles, which were classified into the “reacted sea-salt and mixture” group, for example, particles no. 19, no. 26, no. 51 in Figure 1 and SI Figure S3d. The majority of reacted sea-salt (and mixture) particles are nitrate-containing (SI Table S2). Their round or semispherical shape on SEIs (Figure 1b) indicates that they were liquid droplets at the time of collection. The presence of water in SSA greatly enhances the reactivity of NaCl with gases such as HNO3, ClONO2, and N2O5, and the formation of hygroscopic nitrates in turn additionally lowers the deliquescence and efflorescence points of secondary SSA particles, making them remain as liquid droplets in the air (21, 23). Water evaporated from the reacted sea-salt particles after their collection, and crystalline NaCl particles were fractionally crystallized out, as illustrated in Figure 1b. Organic species present in the reacted sea-salt particles, inferred from significant carbon signals in their X-ray spectra (SI Figure S3), possibly originate from humic or humic-like substances in marine environment (11). 3.1.2. Mineral Dust Particles. The representative types of mineral dust particles include aluminosilicate (AlSi-containing), quartz (SiO2), calcite (CaCO3)/dolomite (CaMg(CO3)2), gypsum (CaSO4 · 2H2O), Fe-rich (mostly iron oxides), and rutile (TiO2). They appear irregular and bright on their SEIs (Figure 1). The obtained atomic concentration ratios for SiO2, CaCO3, gypsum, and TiO2 particles are close to their stoichiometry, that is, [Si]:[O] ≈ 1:2, [Ca]:[C]:[O] ≈ 1:1:3, [Ca]: [S]:[O] ≈ 1:1:6, and [Ti]:[O] ≈ 1:2, respectively. For aluminosilicate particles, the atomic concentration ratios of O, Si, and Al are liable to vary as a wide variety of cations such as Na+, K+, Ca2+, Mg2+, and Fe2+ in them can affect the stoichiometry of O, Si, and Al (24). Fe-rich particles look angular on their SEIs and usually contain more than 20 at.% Fe (e.g., particle no. 37 in Figure 1), often with minor amounts of C, Si, and Al. Fe-rich particles as well as aluminosilicatecontaining Fe2+/Fe3+ can play an important role in enhancing biological activity in the Arctic Ocean, as they can act as a nutrient for biota in marine environments with their depositions (25). Reacted mineral dust particles are classified mainly as “reacted CaCO3/CaMg(CO3)2” and “aluminosilicate + (N, S)” types in which (N, S) notation represents compounds containing either nitrates, sulfates, or both. They are produced 2350
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when aluminosilicate (especially Ca2+-containing) and CaCO3/ CaMg(CO3)2 particles react with airborne sulfur and nitrogen oxides in the presence of moisture or with “secondary acids” such as H2SO4, HNO3, and HCl (12, 26, 27). They can be formed either through the reaction of aluminosilicate and CaCO3/CaMg(CO3)2 particles with H2SO4 and/or HNO3, or from adsorption of NH4NO3 or (NH4)2SO4/NH4HSO4 on particle surface (27), for example, particles no. 2, no. 8, and no. 39 in Figure 1 (SI Figure S4 for typical X-ray spectra). 3.1.3. Carbonaceous Particles. Carbonaceous particles, generated mainly from incomplete combustion of fossil fuels and biomass, are ubiquitous in the tropospheric atmosphere and have strong radiative-forcing effects (28). They are generally differentiated into two groups: elemental carbon (EC, termed “carbon-rich particle” in this article) and organic carbon (OC). The sum of C and O contents (sometimes including N) is more than 90 at.% for both EC and OC. Carbonrich (EC) particles are defined when the carbon content is 3-fold greater than the oxygen content and almost no other elements are present (9, 11). The morphologies of carbonrich particles include a fractal-like chain structure (e.g., soot aggregates), separate spherules (e.g., tar balls), and an irregular-shaped structure (e.g., chars) (28). Soot agglomerates, which are abundant in urban atmosphere, were not observed in the summertime Arctic samples, possibly due to the absence of combustion sources of diesel oil or coal in Ny-Ålesund. A few tar balls (SI Figure S5a) and chars were observed, which have irregular or spherical morphology, quite different from that of soot aggregates. They can be generated by gas-to-particle conversion within smoke plumes and are particularly abundant in slightly aged biomass smoke (29). The OC particles were categorized into four different types based on their morphological and X-ray spectral data. The first type is of primary or water-insoluble secondary organic particles, which look bright and angular on their SEI and have high levels of C, N, and O. It has been termed (C, N, O)-rich particles or nitrogen-containing organic compounds (NOC) by other researchers (30), and may be generated mostly by biomass burning. The second type is of biogenic particles (e.g., pollen, spore, algae, bacteria, and plant or insect fragments), a special type of organic particles. They have a “biogenic fingerprint”, for example, the presence of N, K, P, S and/or Cl together with C and O (e.g., particles no. 47 and no. 48 in Figure 1 and SI Figure S5b) (11, 24). Most of the biogenic particles are likely from the seawater of the Arctic Ocean or the surrounding seas, as the sampling site is near the sea. The third type is of liquid droplet particles (probably containing water-soluble organic carbon (WSOC)), which look dark and round and have high levels of C, N, and O, often together with a trace amount of Na and Mg elements indicating their marine origin (SI Figure S5c). These (C, N, O)-rich droplets are possibly composed of organic matter and NH4NO3. Oxidation of volatile organic compounds, humic substances or “humic-like substances (HULIS)” in the marine environment, and crude oils spilled on the sea surface from ship tankers are perhaps contributory to the organic matter (24). The fourth type of OC particles is of droplet particles rich in C, N, O, and S, represented as (C, N, O, S)-rich droplets, which were also significantly observed in the summertime aerosol samples collected at Tokchok Island, Korea (11). They look dark on their SEIs (e.g., many particles in Figure 1d) and are likely internally mixed with (NH4)2SO4/NH4HSO4 and organic matter (SI Figure S5d). The (C, N, O, S)-rich droplets were frequently encountered in samples collected at 22:04-8: 04, 7/25-26/2007 and 20:40-8:40, 7/30-31/2007, particularly in PM1-2 and PM0.5-1 fractions, and their average atomic concentrations of C, N, O, and S are 26.2 ((7.4) %, 14.1 ((5.5) %, 53.0 ((10.3) %, and 3.3 ((1.2) %, respectively, often with minor contents of Na and Mg (on average, 0.7 at.% and 2.1
FIGURE 2. Relative abundances of various particle types in summertime Arctic samples.
at.%, respectively) (SI Table S3). Ambient sulfate is often neutralized by ammonia (mainly from animal waste, fertilizer application, soil and industrial emissions, and direct emissions from vegetation and oceans) and its most common form is (NH4)2SO4. However, if ammonia is scarce, sulfate can remain in more acidic forms such as NH4HSO4 or H2SO4 (31). Ammonia is usually scarce in the Arctic region being without major ammonia sources (5), so that the (C, N, O, S)-rich droplet particles might include more acidic NH4HSO4 (or H2SO4) rather than (NH4)2SO4. There are three possible sources of SO2 and SO42- in the Arctic region: the first is from ship emissions around the Arctic Ocean (25, 32); the second is likely from industrial emissions in western Siberia (Russia) and northern Europe when air masses passed over these regions (SI Figure S2); the third is from DMS in seawater, as high biological activity in ocean margins results in a significant production of DMS, which can be oxidized to form various sulfur containing products, such as MSA, SO2, nss-SO42- (22, 33). The organic matter in the (C, N, O, S)-rich droplet particles is likely similar to that in the (C, N, O)-rich droplets. Perhaps, some products of DMS are involved (22). 3.2. Relative Abundances of Various Types of Particles Observed in Summertime Arctic Samples. Figure 2 shows the relative abundances of major particle types observed in the samples. Sea-salt particles, including both genuine and aged (reacted) species, are the most frequently encountered, with average relative abundances of 55.9, 59.2, and 54.9% for PM2-4, PM1-2, and PM0.5-1 fractions, respectively. The air mass back trajectories at the heights of 100, 500, and 1000 m show that the air masses for nearly all the samples originated from the Arctic Ocean and stayed over it for a long time, implying the strong marine influence on the samples (SI Figure S2). The genuine sea-salt particles are encountered with low
frequency, except in PM2-4, PM1-2, and PM0.5-1 fractions of samples collected at 22:20-19:45, 7/26-27/2007. Relative humidity was low when the samples with abundant genuine sea-salts were collected (SI Table S1), probably limiting heterogeneous atmospheric reactions in sea-salts (34). The reacted sea-salt (and mixture) particles are frequently encountered in samples collected on July 26, 28, 29, and 31 (with relative abundances of 40∼88%), and greatly outnumber the genuine ones. The nitrate-containing reacted sea-salts are significantly encountered in all the samples and their relative abundances vary depending strongly on the air masses. Friedman et al. (35) also reported the similar finding. The reason the reacted sea-salt (and mixture) particles, especially nitrate-containing species (SI Table S2), are so abundant will be discussed in Section 3.3. Among mineral dust particles, aluminosilicate particles are the most frequently encountered, followed by CaCO3/ (Ca,Mg)CO3, SiO2, CaSO4, and Fe-rich particles. The sums of relative abundances of mineral dust particles (including reacted ones) are 35.1, 30.9, and 24.3% on average for PM2-4, PM1-2, and PM0.5-1 fractions, respectively, with peak abundances for samples collected at 9:10-20:00, 7/30/2007. From the observations that mineral dust particles are abundantly observed in big size fractions (PM8-16 and PM4-8) and the nonreacted (genuine) mineral dust particles greatly outnumber the reacted ones (only 5.3, 6.4, and 7.0% on average for PM4-2, PM2-1, and PM1-0.5 fractions, respectively), it is concluded that many of the mineral dust particles should be of local origin, mostly from snow-free mountains during summertime (5). The possible reasons for the predominance of the genuine mineral dust particles over the reacted ones are (i) anthropogenic emissions are not strong in the remote Arctic region so that mineral dust particles had no chance to react with air pollutants; (ii) the reactivity of mineral dust VOL. 44, NO. 7, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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particles with NOx and SO2 is much lower than that of seasalts (34). Like the reacted sea-salt particles, reacted mineral dust particles containing nitrates are predominant over those containing sulfates (SI Table S4), indicating that NOx or HNO3 contributed to the formation of reacted mineral dust particles more than SO2 or H2SO4 did. Organic carbon (OC) particles are primarily composed of liquid droplets rich in (C, N, O) and (C, N, O, S). They greatly outnumber carbon-rich ones whose relative abundances are 0.5, 0.3, and 0.8% on average for PM4-2, PM2-1, and PM1-0.5 fractions, respectively (SI Table S5). It implies that EC particles, which are produced from smoke plumes of forest fires or from oil combustion by ships around the periphery of the Arctic Ocean (25), make much less influence on the summertime Arctic atmosphere than the mixture of organic matter and secondary anthropogenic pollutants such as NH4NO3 and NH4HSO4/(NH4)2SO4 do. The relative abundances of (C, N, O, S)-rich droplets are greatly higher than those of (C, N, O)-rich droplets (SI Table S5). On average, (C, N, O)-rich droplets are encountered with relative abundances of 1.9, 1.7, and 3.4% in PM4-2, PM2-1, and PM1-0.5 fractions, respectively; whereas (C, N, O, S)-rich droplets are significantly encountered in PM4-2, PM2-1, and PM1-0.5 fractions for almost all the samples, peaking at 50∼60% of relative abundances for PM4-2 fraction of samples collected at 20:40-8:40, 7/30-31/2007. Possible reasons for this observation are (i) NH4HSO4/(NH4)2SO4 was easier to be formed in the summer than NH4NO3 and/or (ii) the air masses that originated from continental European countries and passed over the south Arctic Ocean and Barents Sea carried more NH3 and SO2. It has been reported that air masses from western Siberia can be contaminated by anthropogenic pollutants such as SO2 from smelting factory in Norilsk (36). And nss-sulfate concentrations in precipitations at NyÅlesund were reported to be high when air masses arriving at Ny-Ålesund had passed over Scandinavia and western Siberia regions (37). 3.3. Possible Formation Mechanisms for Reacted SeaSalt Particles over the Arctic Region. As stated above, seasalt particles were abundant in our summertime Arctic aerosol samples and the reacted sea-salt (and mixture) particles outnumbered the genuine ones. In addition, the abundances of nitrate-containing sea-salt particles greatly outweighed those of the CH3SO3-/nss-SO42--containing ones for all the samples except the sample collected on July 27, 2007 (SI Table S2). Why were such a large number of nitratecontaining sea-salt particles formed and where did NOx come from in the summertime Arctic region? The possible explanations are described as follows: (1) Sunlight or solar radiation provides favorable conditions for the enhancement of the reaction of NOx (or HNO3) and sea-salt aerosols. Gard et al. (38) observed that active heterogeneous chemical reaction of sea-salt particles and NOx (or HNO3) primarily occurred during the daytime. It was also reported that sea-salt particles were almost unprocessed during the nighttime, while they were completely converted mostly into NaNO3 particles during the daytime (21). At Ny-Ålesund, the all-day-long daytime during the sampling period should enhance photochemical activities, consequently resulting in the production of nitric acid that readily reacts with sea-salt particles. (2) Levels of NOx and some precursors of secondary aerosols are increasing in the Arctic Ocean because of the rapid increase of shipping traffic in the region (25, 32). It was reported that the high level of nitrate was observed over and downwind of shipping lanes, as the relatively low deposition rate of high level of NOx species emitted from traveling ships provided ample time for rate-limited mass transfer for the displacement of chloride (19). In Russian, Canadian, and U.S. Arctic regions, shipping activities such as the running 2352
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of icebreakers, transportation of crude oil, and deliveries of general cargo and petroleum products are extensive. Air pollutant emissions from shipping traffic and fuel usage along the main navigation routes in the Arctic Ocean should have an impact on chemical compositions of the Arctic aerosols in the marine boundary layer (25, 32). (3) Anthropogenic air pollutants such as nitrogen and sulfur oxides from industrial emissions in northern Europe and western Siberia can be transported to Ny-Ålesund during the summer (1, 36). (4) Renoxification (i.e., reduction of NO3- to NOx and HONO) may be one of the most important mechanisms (39, 40). During the continuous daytime in the Arctic region, photochemically induced reactions are active. Photochemical reactions occurring within the snowpack or on the snow/ice surface where NOx and NO3- were accumulated or enriched in the winter and spring (particularly when the Arctic haze occurred) can produce high level of NOx from nitrate (41). Under solar radiation, the deposited NO3- on the Arctic snowpack or ice can be reactivated or reduced to nitrite (NO2-) or to NO2 and hydroxyl radicals (OH), which may eventually lead to reemission of nitrogenous compounds to the atmosphere (42), resulting in elevated reacted (aged) seasalt particles by the reaction of NOx or HNO3 with sea salts. The nitrogen cycling rates in the Arctic were reported to increase as shrub growth is stimulated by increasing summer temperature and nitrogen supply (43). Also, the acceleration of summer warming in the Arctic region can result in the advancing of snowmelt and the declining of summer sea ice. (5) The decomposition of peroxyacetyl nitrate (PAN) in the Arctic atmosphere during the polar night period also contributes to the formation of NO2 (44). The thermal decomposition of PAN to NO2 in the Arctic troposphere (e.g., at Svalbard) occurs mostly in the summer, although the amounts of PAN and NO2 from PAN are small (40).
Acknowledgments This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (ROA-2007-000-20030-0) and by the fund of Integrated Research on the Composition of Polar Atmosphere and Climate Change (COMPAC)′ (PE09030 of Korea Polar Research Institute).
Supporting Information Available Additional details are provided in five tables and five figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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