Water-Soluble Organic Nitrogen in High Mountain Snow Samples from Central Japan

We measured water-soluble organic nitrogen (WSON) in snow pit samples, which were collected at the Murodo-Daira snowfield near the summit of Mt. Tateyama, central Japan in 2008, 2009, and 2011. The concentrations of WSON ranged from 12.8 to 96.7 ng g, which were significantly lower than those reported in continental wet deposition samples from the Asian continent. WSON may be largely diluted in the snow samples during snowing processes over the high mountains. We found that WSON significantly correlated with nss-Ca and water-soluble organic carbon (WSOC). Concentrations of WSON are likely controlled by the intensity of Asian dust events. Contributions of WSON to water soluble total nitrogen (WSTN) in snow pit sequence was found to be 15 ± 10%, which is lower than those (63–91%) of reference dust materials collected in China. Mass concentration ratio of WSOC/WSON was on average 23.7, which is significantly higher than the C/N weight ratio (5.6) calculated from the Redfield ratio. This result suggests that WSOC is largely produced by secondary photochemical oxidation of anthropogenic volatile organic compounds during the transport from East Asia to the high mountain areas in Japan. On the other hand, WSON may be contributed from Asian dusts from arid areas in China whereas water-soluble inorganic nitrogen may be derived from pollution sources.


INTRODUCTION
The annual global emissions of water-soluble inorganic nitrogen (WSIN) and water-soluble organic nitrogen (WSON) in particulate phase were estimated to be 34 and 3.9 Tg-N, respectively (Neff et al., 2002).WSON accounts for 10-40% of the water-soluble total nitrogen (WSTN) on a global scale (Cornell, 2011).Cape et al. (2011) reported that WSON concentrations in wet deposition are increasing recently; relatively high concentrations of WSON in wet deposition are observed in China due to the enhanced anthropogenic sources (Zhang et al., 2008).Increased emissions of nitrogen species to the air not only control the nitrogen saturation in terrestrial and aquatic ecosystems (e.g., Aber et al., 1989;Cornell et al., 1995), but also affect human health by decreasing the quality of air and water (e.g., Harrison and Yin, 2000;Zhang and Anastasio, 2001).Global nitrogen cycle should also be influenced by enhanced human activity.Watersoluble components including WSON in the atmosphere contribute to the formation of cloud condensation nuclei (CCN) and ice nuclei (IN) and thus have positive or negative radiative forcing, affecting global climate change (e.g., Kanakidou et al., 2005).
WSON is composed of various chemical species including amines, amino acids, urea, and nitrophenols (e.g., Cape et al., 2011).The WSON species can be emitted from marine organisms (Miyazaki et al., 2011), combustion system (Nojima et al., 1983), fertilizer (Ham and Tamiya, 2006), forest ecosystem (Miyazaki et al., 2014), and soil dust (Wang et al., 2013).It is known that removal process and/or chemical loss of water-soluble inorganic nitrogen (WSIN) (NO 3 -and NH 4 + ) during long-range transport largely depend on the atmospheric processing time (e.g., Cornell, 2011) and meteorological conditions such as precipitation and fog (Sasakawa et al., 2002).However, the long-range transport of WSON and its fate in the atmosphere have been poorly understood.
Mt. Tateyama is located in central Japan, facing to the Sea of Japan (Fig. 1).Murodo-Daira locates near the summit of Mt.Tateyama and is covered with thick snow (> 6 m) in winter to spring.In our previous study, low molecular weight (LMW) monocarboxylic acids in the Murodo-Daira snowpit sequences have been reported (Kawamura et al., 2012).We found that higher concentrations of LMW monocarboxylic acids in the snow pit sequences are associated with the presence of Asian dust layers (Kawamura et al., 2012;Mochizuki et al., 2015).However, quantitative analysis of WSON has never been conducted in the snow pit sequences from the Japanese high mountain site.In this study, we measured WSON in 22 snowpack samples collected from the Murodo-Daira snowfield site near the summit of Mt. Tateyama, central Japan in 2008, 2009, and 2011.Here, we report WSON concentrations in the snow pit sequences and discuss long-range atmospheric transport of WSON from East Asia to the Japanese Islands during the season of Asian dust events.

MATERIALS AND METHODS
Snow accumulation during one winter season in 2008, 2009, and 2011 were 6.6 m, 6.0 m, and 6.5 m, respectively in the Murodo-Daira site (36.58ºN, 137.36ºE; elevation, 2450 m) near Mt.Tateyama (elevation, 3015 m), central Japan.Snowpack samples were collected in 2008 (n = 7), 2009 (5), and 2011 (10) from the Murodo-Daira site (Table 1).Because several dirty layers due to the occurrence of Asian dusts were recognized by visual observation, we have sampled the snow pit samples with dust layers to understand relationship between WSON and Asian outflow during the Asian dust season.After the collection of snow samples from the different layers of pit sequences, snow samples were placed in a pre-cleaned glass jar (8 L) and then mercuric chloride (HgCl 2 ) was added immediately to the samples to prevent microbial activity.The sample jar was sealed with a Teflon-lined screw cap, transported to the laboratory in Sapporo, and stored in the dark refrigerator room at 4°C prior to analysis.Kattner (1999) reported that the poisoning with HgCl 2 can make it possible to store inorganic nitrogen for a long period.Similarly, degradation of water-soluble organic nitrogen and water soluble organic carbon can be minimized by poisoning with HgCl 2 and storing the samples in cool and dark conditions.
After removing the particles from the melt snow sample using a disk filter (0.22 µm, Millipore Millex-GV, Merck, USA), water sample was analyzed for water-soluble organic carbon (WSOC) and water-soluble total nitrogen (WSTN) using a total organic carbon (TOC)/total nitrogen (TN) analyzer (Model TOC-Vcsh, Shimadzu).Before the WSOC and WSTN analysis, the extracts were treated with 1.2 M HCl and purged with a pure air to remove inorganic carbon and volatile organic compounds.We calculated WSON by subtracting WSIN (NH 4 + -N and NO 3 --N measured by ion chromatography) from WSTN (WSON = WSTN -WSIN) (Miyazaki et al., 2011).The uncertainties based on the combined analytical errors in the measurements of WSIN and WSTN were within 18% (Miyazaki et al., 2011).The concentrations of selected ionic species (NO 3 -, NH 4 + , and Ca 2+ ) in 2008, 2009, and 2011 are used from previous publications (Kawamura et al., 2012;Mochizuki et al., 2015).Concentrations of non-sea salt Ca 2+ in melt snow sample is calculated by the following equation: where [nss-Ca 2+ ] is concentrations of non-sea-salt (nss) Ca 2+ and [Ca 2+ ] and [Na + ] are concentrations of total Ca 2+ and Na + , respectively (Berg and Winchester, 1978;Duce et al., 2003).
To evaluate the WSTN (WSIN and WSON) and WSOC of reference dust materials (Kosa); Chinese loess (CJ-1), Tengger desert (CJ-2), and Gobi desert (Gobi) provided by National Institute for Environmental Studies (Nishikawa et al., 2000(Nishikawa et al., , 2013)), were analyzed for WSIN, WSTN, and WSOC.A dust sample (0.5 g) was extracted with 50 mL ultrapure water and analyzed by the method described above.Granular snow.Asian dust events over Imizu, Toyama prefecture, Japan near Mt.Tateyama during December to March in each year were recorded by the lidar observations (http://www-lidar.nies.go.jp/Toyama/).This result is supported by the air mass back trajectories at height of 3000 m (Figs.2(a)-2(c)).
We calculated seven-day air mass back trajectories at height of 3000 m above sea level using the Meteorological Data Explorer (METEX) provided by the National Institute for Environmental Studies, Tsukuba, Japan (http://db.cger.nies.go.jp/metex/index.html).

Air Mass Back Trajectory
The seven day air mass back trajectories at the Murodo-Daira site during the snowfall period (November-April) showed that most of the air masses passed through the Asian continent including China, Mongolia, and Korea and the Sea of Japan (Figs. 2(a)-2(c)), suggesting that snow pit samples are affected by the outflows from East Asia.In particular, the air masses associated with dust layers in 2008, 2009, and 2011 were mainly delivered from Taklamakan Desert, Gobi Desert, and Loess Plateau, which are characterized by heavy Asian dust events (see bold lines in Figs.2(a)-2(c) and Table 1).In addition, air mass heights over the Asian continent and Sea of Japan were between 2500 m and 6000 m.The monthly average of precipitation recorded in Urumqi (China), Beijing (China), and Seoul (Korea) were less than 20 mm, 6.0 mm, and 33 mm, respectively (Japan Meteorological Agency website: http://www.data.jma.go.jp/gmd/cpd/monit or/mainstn/obslist.php).Such low precipitations are important for the intensified uplift of dusts and subsequent long-range atmospheric transport from arid regions.

Water Soluble Organic Nitrogen
Fig. 3(a) presents concentrations of WSTN and WSON in the snowpack samples from the Murodo-Daira site at Mt. Tateyama, Japan in 2008, 2009, and 2011.The WSTN and WSON concentrations across all snow pack samples in 2008, 2009, and 2011 ranged from 94.7 to 1760 ng g -1 (425 ± 455 ng g -1 ) and 12.8 to 96.7 ng g -1 (35.6 ± 22.5 ng g -1 ), respectively.The average concentration of WSON in 2008 (44.4 ng g -1 ) and 2009 (48.2 ng g -1 ) are higher than that in 2011 (24.3 ng g -1 ).Overall, WSON concentrations in the snow samples are significantly lower than those reported in rainwater samples from multiple sites (180 ng g -1 ) in the world (Cornell, 2011) and from Shengsi Island, China in March (ca.420 ng g -1 ) (Zhang et al., 2011).However, WSON levels reported in the Murodo-Daira site are similar to those of the ice floe samples from the Fram Strait near the Arctic Ocean (ca.50 ng g -1 ) (Thomas et al., 1995), suggesting that WSON concentrations in the Japanese high mountain snow samples are near the background level on the Earth.Interestingly, the WSON levels in the Murodo-Daira site are very low, in spite that this site is affected by the outflows of air masses from the Asian continent (Figs.2(a)-2(c)).WSON is emitted from marine biota (e.g., Miyazaki et al., 2011), but biological activity is low during winter.Our results suggest that WSON is less supplied from the Sea of Japan through long-range transport by the winter Asian Monsoon.The details of sources, decomposition, and transport of WSON will be discussed in later section.

Sources and Long-Range Transport of WSON
Several types of nitrogen compounds contribute to WSON.For example, amino acids are emitted not only from marine biological sources such as bacteria and degraded algal proteins but also from terrestrial sources such as plant debris and livestock farming.Organic nitrates and nitrophenols can be produced by the reaction of anthropogenic and biogenic hydrocarbons with nitrate radicals.Amines are derived from biogenic sources.These potential sources have been discussed in Cape et al. (2011).In addition, urea is emitted not only from primary sources such as microbial modification of agricultural fertilizers (Cape et al., 2011) but also from secondary formation via atmospheric reactions (Fu et al., 2014).Unfortunately, we are not able to identify these WSON components in this study.
Relatively high abundances of WSON were observed in snow layers with dusts such as snow samples Nos. 3 and 10.We found a positive correlation (r = 0.88) between WSON and nss-Ca 2+ , an Asian dust tracer, using all the data points (Fig. 4(a)).This result suggests that Asian dust is an important controlling factor of WSON concentrations in snowpack samples.Co-variation was reported for nss-Ca 2+ and LMW monocarboxylic acids at the Mt.Tateyama snow site (Kawamura et al., 2012;Mochizuki et al., 2015).On the other hand, relatively weak correlation was found between WSON and WSIN concentrations (r = 0.46) (Fig. 4(b)).In contrast, there is no correlation between WSIN and nss-Ca 2+ (r = 0.22, not shown as a figure).We suggest that Asian dust can control the long-range transport of WSON and the production process in the atmosphere and that adsorption processes on the dust particles are similar between WSON and LMW monocarboxylic acids but these processes are different between WSON and WSIN.
Fig. 5 plots WSON concentrations as a function of WSOC concentrations.WSON shows a good correlation with WSOC (r = 0.81).It is known that WSOC is derived from biomass burning and secondary production via the photooxidation of gaseous hydrocarbons and other non-polar organic precursors (Snyder et al., 2009;Matsumoto et al., 2014).Our result indicates that WSON is biomass burning and secondarily produced by photochemical nitrification of organic gasses.
In contrast, WSON/WSTN ratio in aerosol samples from Okinawa Island, Japan, an outflow region of Asian dust, was about 14% (Kunwar and Kawamura, 2014).We found that WSON does not comprise of major fraction of WSTN in the Japanese high mountain snow samples.
In conclusion, WSON in the snowpack samples has less significant influence from the Asian continent over the Japanese Islands due to the photochemical decomposition of WSON during long-range transport.In the future, the potential chemical aging of WSON and its degradation rate in the atmosphere should be investigated.

CONCLUSIONS
Snowpack samples were collected in the Murodo-Daira snowfield site near the summit of Mt.Tateyama, central Japan in 2008, 2009, and 2011, and analyzed for the measurement of WSON.The concentrations of WSON in the high mountain snow samples ranged from 12.8 to 96.7 ng g -1 , which are near the background level in the ice floe samples from the Fram Strait near the Arctic Ocean.On average, the average contribution of WSON to WSTN is 15 ± 10%.WSON is not a major component of WSTN in the high mountain snow samples.We found that concentrations of WSON significantly correlate with nss-Ca 2+ (Asian dust tracer) and WSOC (secondary formation).Our study suggests that WSON is mainly derived from long-range transport of Asian dust associated with Chinese air pollutants over the Japanese Islands during winter.In addition, the ratios of

Fig. 1 .
Fig. 1.Location of the snow pit sampling site near Mt.Tateyama, central Japan.
Seven-day air mass back trajectories at a level of 3000 m a.s.l.during main snow fall period (January-March) in (a) 2008, (b) 2009, and (c) 2011.Bold lines show the trajectories associated with dust layers as observed by a lidar.