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Simultaneous measurement of the water-soluble organic nitrogen in the gas phase and aerosols at a forested site in Japan
Atmospheric Environment 200 (2019) 312–318
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Simultaneous measurement of the water-soluble organic nitrogen in the gas phase and aerosols at a forested site in Japan
T
Kiyoshi Matsumotoa,∗, Yuuya Watanabeb, Ken Horiuchib, Takashi Nakanoc a
Division of Life and Environmental Sciences, University of Yamanashi, 4-4-37, Takeda, Kofu, 400-8510, Japan Faculty of Life and Environmental Sciences, University of Yamanashi, 4-4-37, Takeda, Kofu, 400-8510, Japan c Mt. Fuji Research Institute, Yamanashi Prefectural Government, 5597-1, Kenmarubi, Kamiyoshida, Fujiyoshida, 403-0005, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Water-soluble organic nitrogen Nitrogen compound Aerosol Gas phase Secondary organic aerosol Forest
A simultaneous observation of the water-soluble organic nitrogen (WSON) in the gaseous and particulate phases was conducted at a forested site in Japan for two years to clarify the generation mechanism and possible sources of the gaseous and particulate WSON. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, ranging from the concentration below the detection limit (DL) to 0.302 μg m−3. A large portion (about 73%) of the particulate WSON was distributed in the fine-mode range (d < 2.0 μm). The concentration of the gaseous WSON was 0.041 μg m−3 on average, ranging from the concentration below the DL to 0.195 μg m−3. About 37% of the atmospheric WSON was distributed in the gas phase, and about 59% of the gaseous WSON was basic. The coarse-mode (d = 2.0–10 μm) WSON showed an unclear seasonal trend, but increased in the summer. Its concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. Suspended organic materials from primary sources, such as plant debris, would be a possible source for the coarse-mode WSON. The WSON in the fine-mode range showed a clear seasonality with enhanced concentrations from the spring to summer. The fine-mode WSON concentration showed a strong correlation with the acidity of the fine particles and the concentration of the gaseous basic WSON, suggesting the secondary production of the WSON in the acid fine particles from the gaseous basic WSON. The basic WSON in the gas phase showed a similar seasonal trend with the gaseous ammonia and strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species from terrestrial vegetation and/or soil. The seasonal trend of the gaseous acid WSON was unclear, and its concentration did not show significant correlations with any parameters. The possible source and chemical forms of the gaseous acid WSON cannot be deduced at the present time.
1. Introduction Water-soluble organic nitrogen (WSON) has been reported to account for a significant fraction of particulate nitrogen (Cape et al. (2011), and references therein). Its deposition on the earth's surface has been considered as a significant input pathway of bioavailable nitrogen to the terrestrial and marine ecosystems (Spokes et al., 2000; Russell et al., 2003; Qi et al., 2013; Matsumoto et al., 2014, 2018). In addition, organic nitrogen species in the aerosols have also been paid attention in the recent studies of climate change and public health, because they would act as ice nuclei that affect the global climate and cause adverse health effects due to their allergenicity (Gruijthuijsen et al., 2006; Pummer et al., 2015). Various potential sources for the particulate WSON including both primary and secondary processes have been suggested in previous ∗
studies, but the sources have not yet been sufficiently clarified. The generation of the WSON in the fine particles from the gaseous WSON via gas-to-particle conversion processes has been suggested in recent studies (Matsumoto et al., 2017, 2018), although the primary sources, such as biomass burning (Mace et al., 2003; Zamora et al., 2011), bioparticles (Matsumoto et al., 2014, 2018), and dust particles (Nakamura et al., 2006; Lesworth et al., 2010), have also been considered as significant sources of the particulate WSON. A simultaneous measurement of the gaseous and particulate WSON would be favorable to clarify the source and generation mechanism of the particulate WSON, but have been rarely reported to date. The total amount of the WSON in the gas phase itself has rarely been measured in previous investigations although the measurements of individual gaseous organic nitrogen species, such as alkylamines and alkylnitrates, have been conducted (Cape et al. (2011); Ge et al. (2011), and references therein). Only a few
Corresponding author. E-mail address: [email protected] (K. Matsumoto).
https://doi.org/10.1016/j.atmosenv.2018.12.011 Received 21 August 2018; Received in revised form 3 December 2018; Accepted 5 December 2018 Available online 19 December 2018 1352-2310/ © 2018 Elsevier Ltd. All rights reserved.
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pioneering studies reported the measurements of the WSON in the gas phase (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2017). They demonstrated that about 33–67% of the WSON in the atmosphere is distributed in the gas phase, suggesting that the gaseous WSON occupies a large fraction of the atmospheric WSON. To date, the generation mechanism of the particulate WSON from its precursor gases has not been verified by the simultaneous measurement of the WSON in the gas phase and aerosols. The possible sources of the precursor gases have also not been discussed. The present study conducted the simultaneous measurement of the WSON in the gas phase and aerosols for two years, and discussed the generation mechanism and possible sources of the particulate and gaseous WSON.
HNO3, SO2, and NH3, respectively. The DTN in the extract was measured using the TOC/TN analyzer, and the difference between the concentrations of the DTN and IN is considered to be the gaseous WSON. The WSON from the Na2CO3/glycerol and H3PO4/glycerol-impregnated filter is defined as the gaseous acid and basic WSON, respectively, in this study. Measurements of the total carbon (TC) and elemental carbon (EC) in the aerosols were conducted by a thermal oxidation technique (Kaneyasu and Murayama, 2000). Two pieces were cut from the filter collected aerosol sample. The amount of carbon on one piece was determined using a CHN elemental analyzer (MT-6; Yanaco Analytical Industry Co., Ltd.), which can be considered to be the total carbon (TC). Another piece was heated at 340 °C in flowing air for 20 min, then the carbon on the piece was determined by the same technique as the TC, which can be considered as elemental carbon (EC). The difference in the TC and EC is defined as organic carbon (OC) in this study. Although the overestimation of the EC and underestimation of the OC can be expected in this method due to the pyrolysis of some parts of the OC, our previous experiments demonstrated that these overestimations and underestimations were small especially for the samples with small amounts of carbon (Matsumoto et al., 2017).
2. Experiment 2.1. Sample collection Sample collections were conducted at a forest area located at the northern foot of Mt. Fuji in Japan. The sampling site (35.5ºN, 138.8ºE) was constructed at an open space in the forest and surrounded by red pines with tree heights of about 10–15 m. A downtown area with a population of about 66,000 was about 5 km away from the site. The air sampler was installed about 2 m above the ground surface. The distance between the sampler and the nearest trees was about 13 m. Two size-fractionated aerosol samples with diameters of 2.0–10 μm (coarse-mode) and < 2.0 μm (fine-mode) were collected on quartz fiber filters (QR100; Toyo Roshi Kaisha, Ltd.) using an NILU filter holder with two-stage aluminum alloy impactors (NL Series; Tokyo Dylec Corp.). Before the sample collections, the quartz fiber filters were heated at 850 °C for 4 h. Behind the collection filter of the fine-mode aerosol, a 2% Na2CO3/1% glycerol-impregnated cellulose filter (5A; Toyo Roshi Kaisha, Ltd.) and a 2% H3PO4/1% glycerol-impregnated cellulose filter (5A; Toyo Roshi Kaisha, Ltd.) were placed in this order to collect the acid and basic gases, respectively. Sample collections were conducted at the flow rate of 20.0 L min−1. The air samples were collected from 22 January 2016 to 10 January 2018. The sampling time of one sample was about 14 days. During the sample collections, operational blank tests were performed at given intervals. All of the filter samples were stored below −20 °C until chemically analyzed.
2.3. Quality assurance and quality control in the analysis The averaged field operational blank values of the DTN, ion species, WSOC, TC, and EC (n = 24 for DTN, 31 for ionic species and WSOC, and 17 for TC and EC) were subtracted from the measurements of the collected samples. The detection limit (DL) of these measured species was defined as follows: DL = mb +3 σb, where mb and σb indicate the mean and standard deviation of the blank measurements of each constituent, respectively. Because the WSON and OC were calculated using the measurements of four nitrogen species and two carbon species, respectively, the propagation of the errors in the measurements of each species determined the DL of the WSON and OC. The DL of the WSON was defined as follows: DL = (σaDTN2 + σaNO22 + σaNO32 + σaNH42 + σbDTN2 + σbNO22 + σbNO32 + σbNH42)0.5,
2.2. Sample analysis
where σaDTN, σaNO2, σaNO3, and σaNH4 denote the standard deviations of the repetitive analyses of the DTN, NO2−-N, NO3−-N, and NH4+-N, respectively, in the sample extracts and σbDTN, σbNO2, σbNO3, and σbNH4 denote the standard deviations of the blank concentrations of the DTN, NO2−-N, NO3−-N, and NH4+-N, respectively. In addition, the intercomparability of the TOC/TN analyzer and ion chromatograph was carried out with 5–10 repetitive analyses of the 1, 3, and 5 ppmN NH4NO3 solutions, to examine the systematic error in the WSON analysis. The averaged ratio of the IN from the ion chromatograph to the DTN from the TOC/TN analyzer was 1.013 with the standard deviation of 0.021 (Matsumoto et al., 2014). In this study, therefore, the sample with a higher IN/DTN ratio than 0.950 that was obtained by subtracting three times the standard deviation (0.021) from the mean (1.013) was also considered to be below the DL for the WSON. The DL of the OC was defined as follows:
One-fourth of the filter collected aerosol sample was ultrasonically extracted with 20 ml of ultrapure water (specific resistivity > 18 MΩ cm) in a glass vial. The water extract was filtered using a PTFE membrane filter (13HP045AN; Toyo Roshi Kaisha, Ltd.). The dissolved total organic carbon (DTOC) and total nitrogen (DTN) in the extract were measured using a total organic carbon (TOC)/total nitrogen (TN) analyzer (Model TOC-Vcsh/TNM-1, Shimadzu) that can detect the DTOC with an NDIR detector and the DTN with an ozone chemiluminescence detection after a thermo-catalytic oxidation process. The DTOC and DTN are considered to be the water-soluble organic carbon (WSOC) and water-soluble total nitrogen (WSTN), respectively, in this study. The ion species (Cl−, NO2−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, and Ca2+) in the extract were measured using ion chromatographs (DX-120; Dionex Corp. and ICS-1100; Thermo Fisher Scientific Inc.). The concentrations of the total inorganic nitrogen (IN) were calculated from the sum of the NO2−-N, NO3−-N and NH4+-N concentrations, and the difference between the concentrations of the DTN and IN is considered to be the WSON. The whole of the filter collected acid or basic gases was ultrasonically extracted in 40 ml of ultrapure water in a glass vial, and the extract was filtered using the PTFE membrane filter. Cl−, NO2−, NO3−, SO42−, and NH4+ in the extract were measured using the ion chromatographs to determine the concentrations of the gaseous HCl, HNO2,
DL = (σaTC2 + σaEC2 + σbTC2 + σbEC2)0.5, where σaTC and σaEC denote the standard deviation for the repetitive analyses of the TC and EC in the collected samples, respectively, and σbTC and σbEC denote the standard deviation of the blank measurements of the TC and EC, respectively. The concentrations below the DLs were considered to be zero in the following statistical analyses and discussion. About 31%, 21%, 43%, and 13% of the WSON measurements from the coarse-mode, fine-mode, 313
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basic and acid gas samples, respectively, were below the DL in this study. 3. Results and discussion 3.1. WSON in the aerosols Measurements of the nitrogen species in the aerosols are summarized in Table 1. Matsumoto and Yamato (2016) demonstrated that some parts of the particulate WSON collected on the aerosol collection filter would volatilize during the sample collection and detect as the gaseous WSON by the downstream gas collection filter. They estimated that approximately 32.5% of the WSON in the fine particles was lost. They also found that these artifacts did not show any seasonal trends and temperature dependence. These artifacts would also be expected in the measurements of the particulate and gaseous inorganic nitrogen species. Because the magnitude of these artifacts would be largely influenced by the meteorological condition and particle chemical composition, literature values of the magnitude cannot be applied to our measurements. Therefore, the artifacts in the measurements of the nitrogen species were not corrected in the following discussion. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, which is within previous measurements from the rural or forested sites (Mace et al., 2003; González Benítez et al., 2010; Miyazaki et al., 2014; Matsumoto et al., 2014). The ratio of the WSON to WSTN was about 13% on average in the aerosols, which is within our previous measurements at an urban site of about 40 km from the present sampling site (Matsumoto et al., 2014, 2018) and is a lower tail of the reported ratios from many previous studies (Shi et al. (2010); Cape et al. (2011), and references therein). Higher ratios of the WSON/WSTN of about 25% on average were found in the coarse-mode range compared with those in the fine-mode range of about 11% on average. This would be due to the contribution of the primary sources, such as plant debris, on the coarse-mode aerosols as will be discussed later. A large portion (about 73% on average) of the WSON was distributed in the fine-mode range, which has also been already published in many previous reports (Matsumoto et al. (2017), and references therein). The seasonal averaged concentrations of the nitrogen species in the aerosols are summarized in Fig. 1. In the coarse-mode range, nitrate was the most dominant nitrogen compound showing the highest
Fig. 1. Seasonal averaged concentrations of nitrogen species in the coarse-mode (upper panel) and fine-mode (lower panel) particles. Only for NH4+-N in the fine-mode, refer to the right axis.
concentration in the spring due to the uptake of the gaseous nitric acid by Asian dust particles. The coarse-mode WSON showed the highest concentration in the summer and lowest in the autumn, but its seasonal trend was unclear. Correlations of the coarse-mode and fine-mode WSON with other particulate species, gaseous WSON, gaseous pollutants (NOx and Ox), and meteorological factors are summarized in Tables 2 and 3, respectively. The data of the NOx and Ox concentrations were from the hourly data at the Yoshida ambient air pollution monitoring station, and those of the meteorological factors were from the hourly data at the Kawaguchiko weather station. Both stations are located in the downtown area about 5 km away from our sampling site. Table 2 Correlations of the coarse-mode WSON with other coarse-mode species, gaseous WSON, gaseous pollutants, and meteorological factors. The number of samples was 52. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Table 1 Measurements of nitrogen species in the aerosols. The unit is μgN m−3 except for the WSON/WSTN. The number of samples was 52, but only the coarse-mode WSON/WSTN was 43. mean
median
Coarse-mode WSTN 0.078 0.074 NO2−-N 0.000 b.d.a − NO3 -N 0.054 0.047 NH4+-N 0.012 0.010 WSON 0.019 0.015 WSON/WSTN 0.251 0.257 Fine-mode WSTN 0.447 0.449 NO2−-N 0.001 b.d.a − NO3 -N 0.017 0.009 NH4+-N 0.380 0.372 WSON 0.051 0.036 WSON/WSTN 0.105 0.078 Total (Coarse-mode + Fine-mode) WSTN 0.526 0.532 NO2−-N 0.001 b.d.a − NO3 -N 0.071 0.066 NH4+-N 0.392 0.379 WSON 0.070 0.058 WSON/WSTN 0.127 0.118 a
standard deviation
maximum
minimum
Coarse-mode species NO2− NO3− NH4+ nss-SO42Na+ nss-K+ nss-Ca2+ [nss-SO42-] + [NO3−] – [NH4+] WSOC OC EC Gaseous WSON Basic WSON Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
a
0.051 0.001 0.037 0.013 0.020 0.182
0.184 0.008 0.183 0.057 0.084 0.734
b.d. b.d.a 0.011 b.d.a b.d.a 0.000
0.169 0.003 0.021 0.146 0.056 0.099
0.823 0.012 0.118 0.787 0.269 0.385
0.136 b.d.a b.d.a 0.133 b.d.a 0.000
0.191 0.003 0.046 0.150 0.059 0.088
0.888 0.012 0.211 0.797 0.302 0.361
0.136 b.d.a 0.014 0.133 b.d.a 0.000
Below the detection limit. 314
r
Significance
−0.14 −0.06 0.03 0.04 −0.05 0.05 −0.14 −0.05 0.32 0.18 0.25
– – – – – – – – * – –
−0.16 0.05
– –
0.03 −0.16
– –
0.13 0.10 −0.12 −0.13
– – – –
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explained by the primary emission from biomass burning and the secondary production from precursor gases (e.g., Ho et al., 2015; Yu et al., 2017; Xu et al., 2017). In the present study, however, the influence of biomass burning on the WSON measurements was not found due to few occurrences of biomass burning around the sampling site, which resulted in the fact that only the secondary production significantly contributed to the fine-mode WSON.
Table 3 Correlations of the fine-mode WSON with other fine-mode species, gaseous WSON, gaseous pollutants, and meteorological factors. The number of samples was 52. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Fine-mode species NO2− NO3− NH4+ nss-SO42Na+ nss-K+ nss-Ca2+ [nss-SO42-] + [NO3−] – [NH4+] WSOC OC EC Gaseous WSON Basic WSON Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
−0.22 −0.05 0.20 0.30 −0.09 0.07 0.26 0.44 0.30 0.22 0.23
– – – * – – – *** * – –
0.44 0.27
*** *
−0.13 0.18
– –
0.14 0.08 −0.08 −0.10
– – – –
3.2. WSON in the gas phase Table 4 summarizes the measurements of the nitrogen species in the gas phase. As already mentioned, the WSON in the gas phase has rarely been measured in the previous investigations. A few studies reported the measurements of the WSON in the gas phase with concentrations that ranged within 0.022–0.253 μg m−3 (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2016, 2017). The concentration of the gaseous WSON in this study was 0.041 μg m−3 on average, which is lower than those at the urban and rural sites (González Benítez et al., 2010; Matsumoto and Yamato, 2016, 2017), similar to that at the coastal site (Zamora et al., 2011), and higher at the remote oceanic site (Zamora et al., 2011). The WSON concentrations in the gas phase were detected to be at a comparable level with those in the particulate phase. About 37% of the atmospheric WSON was distributed in the gas phase on average. As already discussed, the collection of the gaseous WSON after the collection of the aerosols on a filter can lead to underestimation of the WSON in the aerosols and overestimation of the WSON in the gas phase due to volatilization loss of the WSON in the aerosols collected on the filter (Matsumoto and Yamato, 2016). Therefore, it is possible that the distribution of the atmospheric WSON into the gas phase was overestimated in this study. Compared to previous measurements from a similar collection technique of the particulate and gaseous WSON with this study (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2016), the partitioning of the atmospheric WSON into the gas phase is somewhat lower in this study. In the gaseous WSON, basic compounds were dominant; about 59% of the gaseous WSON was basic, which is a similar result from our previous investigation (Matsumoto and Yamato, 2017). The gaseous WSON occupied only about 9% of the total nitrogen in the measured gaseous nitrogen species (WSON, NH3-N, HNO2-N, and HNO3-N) on average. The seasonal averaged concentrations of the nitrogen species in the gas phase are summarized in Fig. 2. The most dominant species was ammonia with the highest concentration in the summer due to enhancement of its emission at high temperatures. The second most dominant species was nitric acid that increased in the spring and summer due to enhanced photochemical production. The basic WSON in the gas phase showed a similar seasonal trend with ammonia, suggesting that the similar process with the emission of ammonia would be a possible source for the gaseous basic WSON. On the other hand, the
As shown in Table 2, the coarse-mode WSON concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. The correlations of the coarsemode WSON concentration with the gaseous species and meteorological factors were not significant. The contribution of the suspended organic materials from mature leaves, leaf litters, and their decomposed matters to the coarse-mode WSOC has been reported in past studies (Pashynska et al., 2002; Puxbaum and Tenze-Kunit, 2003; Medeiros et al., 2006; Tominaga et al., 2011). The present results could suggest that the suspended organic materials from the primary sources, such as plant debris, would be an effective source for the coarse-mode WSON, although the correlation of the WSON with nss-K+ was not statistically significant in this study. In the fine-mode range, on the other hand, ammonium was the most dominant nitrogen compound with higher concentrations in the warm season from the spring to summer. Fine-mode ammonium was associated with sulfate that increased in the warm season due to the photochemical production. The WSON in the fine-mode range was also enhanced in the warm season. As shown in Table 3, the fine-mode WSON concentration showed a strong correlation with the difference in the equivalent concentrations between the sum of nss-SO42- + NO3− and NH4+ ([nss-SO42-] + [NO3−] – [NH4+]) among the measured species in the fine aerosols, suggesting the secondary production of the WSON in the acid particles as discussed in our previous measurements at the urban and maritime sites (Matsumoto et al., 2017, 2018). Correlations of the fine-mode WSON concentration with the gaseous WSON concentrations were also significant, especially strong with the gaseous basic WSON, which supports the secondary production of the fine-mode WSON from the gaseous WSON. Enhanced concentrations of the particulate sulfate due to enhanced photochemical reactions in the warm season would cause higher acidity of the fine particles, which leaded to higher concentrations of the fine-mode WSON during this season. Indeed, the correlation of the WSON with the nss-SO42- was significant in the fine-mode range as shown in Table 2. The correlation of the WSON with the WSOC was also significant in the fine-mode range, which can be attributed to the fact that a large portion of the WSOC is also derived from the photochemical processes (Miyazaki et al., 2006, 2007). Recent studies reported that a large portion of the particulate WSON can be
Table 4 Measurements of nitrogen species in the gas phase. The unit is μgN m−3. The number of samples was 53.
HNO2-N HNO3-N NH3-N Acid WSON Basic WSON Total (acid + basic) WSON a
315
mean
median standard deviation
maximum minimum
0.025 0.093 0.274 0.017 0.024 0.041
0.022 0.087 0.223 0.012 0.012 0.028
0.057 0.172 0.703 0.060 0.195 0.195
Below the detection limit.
0.012 0.038 0.176 0.015 0.036 0.039
0.005 0,027 0.040 b.d.a b.d.a b.d.a
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weight aliphatic amines. In past studies, methylamine, dimethylamine, trimethylamine, diethylamine, and triethylamine have been often detected in the gas phase (Schade and Crutzen, 1995; Gibb et al., 1999; Vandenboer et al., 2011; Ge et al., 2011; You et al., 2014). The concentrations of these aliphatic amines are largely dependent on local sources, but the nitrogen concentration ratios of these aliphatic amines with ammonia have been estimated to be within about 0.001–0.16 in the ambient gas phase (Schade and Crutzen, 1995; Gibb et al., 1999; Vandenboer et al., 2011; You et al., 2014). In the present study, the ratio of the average nitrogen concentration of the basic WSON to that of ammonia was 0.088, suggesting the possibility that a significant portion of the gaseous basic WSON fraction can be explained by these aliphatic amines. In past studies, the emissions from the terrestrial vegetation and soil have been pointed out as possible natural sources of these amines and ammonia (Schulten and Schnitzer (1998); Ge et al. (2011); Sintermann and Neftel (2015), and references therein). Their emissions from these sources would be enhanced in the warm season. On the other hand, the acid WSON concentrations in the gas phase did not show significant correlations with any parameters. Possible sources and chemical forms of the gaseous acid WSON cannot be deduced at the present time.
Fig. 2. Seasonal averaged concentrations of nitrogen species in the gas phase. Only for NH3-N, refer to the right axis.
Table 5 Correlations of the gaseous basic WSON with other measured gaseous species, gaseous pollutants, and meteorological factors. The number of samples was 53. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Measured gaseous species HCl HNO2 HNO3 SO2 NH3 Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
−0.03 0.32 0.29 −0.22 0.34 −0.03
– * * – * –
−0.25 −0.03
– –
0.42 0.32 −0.09 0.08
*** * – –
3.3. Partitioning of the WSON between the gaseous and particulate phases As already discussed, the coarse-mode WSON can be considered to be derived from primary emission sources and not from the gaseous WSON. On the other hand, the fine-mode WSON would be mainly derived from the gaseous WSON through the gas-to-particle conversion reaction although it is possible that some parts of the fine-mode WSON were derived from primary sources. Fig. 3 shows the seasonal trends of the partitioning of the WSON between the gas phase and fine-mode particles and the total concentrations of the gaseous and fine-mode WSON. The seasonal trend of the total concentrations peaked in the summer. This can be attributed to increases in the fine-mode and gaseous basic WSON in the summer as already described, which would be caused by enhancement of the emission of the gaseous basic WSON due to higher temperatures and subsequent generation of the fine-mode WSON via gas-to-particle conversion reactions. On the other hand, the seasonality of the partitioning was unclear, but the partitioning shifted toward the gas phase in the winter and summer. In the summer, the gaseous basic WSON emissions would be enhanced due to the high temperature, which shifted the partitioning toward the gas phase. In the winter, on the other hand, the emission of the gaseous basic WSON
seasonal trend of the gaseous acid WSON was unclear, although it showed higher concentrations from the winter to spring. Correlations of the basic and acid gaseous WSON with other measured gaseous species, NOx, Ox, and meteorological factors are summarized in Tables 5 and 6, respectively. The basic WSON concentrations in the gas phase were strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species as with the ammonia. The most possible compounds in the gaseous basic WSON fraction would be low-molecular Table 6 Correlations of the gaseous acid WSON with other measured gaseous species, gaseous pollutants, and meteorological factors. The number of samples was 53. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Measured gaseous species HCl HNO2 HNO3 SO2 NH3 Basic WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
0.01 −0.18 −0.07 0.20 −0.09 −0.03
– – – – – –
−0.03 0.22
– –
−0.13 −0.24 0.25 0.01
– – – –
Fig. 3. Seasonal averaged total concentrations of the gaseous and fine-mode WSON (upper panel) and concentration ratios of the WSON between the gas phase and fine-mode particles (lower panel). 316
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would decrease, which leaded to the lower concentrations of the gaseous basic WSON and fine-mode WSON, but the gaseous acid WSON existed at relatively higher concentrations that shifted the partitioning toward the gas phase.
Acknowledgement We would like to acknowledge R. Ueno, M. Ishikawa, and K. Sakata for their kind support of the sample collections and chemical analyses. This study was financially supported by JSPS KAKENHI (Grant Number 16K05620).
4. Summary
Appendix A. Supplementary data
The present study conducted the simultaneous measurement of the WSON in the gaseous and particulate phases at a forested site for two years, and suggested the generation mechanism and possible sources of the WSON in both phases. To date, the generation mechanism of the particulate WSON from its precursor gases has not been verified by the simultaneous measurement of the WSON in the gas phase and aerosols. The possible sources of the precursor gases have also not been discussed. The summary of the present study is as follows. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, ranging from the concentration below the DL to 0.302 μg m−3. The ratio of the WSON to WSTN in the aerosols was about 13%. Higher ratios of the WSON/WSTN of about 25% were found in the coarse-mode range compared to those in the fine-mode range of about 11%. About 73% of the particulate WSON was distributed in the fine-mode range. The coarse-mode WSON showed an unclear seasonal trend, but increased in the summer. The coarse-mode WSON concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. Suspended organic materials from primary sources, such as plant debris, would be a possible source of the coarse-mode WSON. The WSON in the fine-mode range showed a clear seasonality with enhanced concentrations in the warm season from the spring to summer. The fine-mode WSON concentration showed a strong correlation with the difference in the equivalent concentrations between the sum of nss-SO42- + NO3− and NH4+ ([nssSO42-] + [NO3−] – [NH4+]) among the measured species in the fine aerosols, suggesting the secondary production of the WSON in the acid particles. The correlation of the fine-mode WSON concentration with the gaseous basic WSON concentration was also strong, which suggests the secondary production of the WSON in the acid fine particles from the gaseous basic WSON. Enhanced concentrations of the particulate sulfate due to enhanced photochemical reactions in the warm season would cause higher concentrations of the fine-mode WSON during this season. The concentration of the gaseous WSON in this study was 0.041 μg m−3 on average, ranging from the concentration below the DL to 0.195 μg m−3. About 37% of the atmospheric WSON was distributed in the gas phase, and about 59% of the gaseous WSON was basic. The gaseous WSON occupied only about 9% of the total nitrogen in the measured gaseous nitrogen species. The basic WSON in the gas phase showed a similar seasonal trend with ammonia and strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species from terrestrial vegetation and/or soil. The ratio of the nitrogen concentration of the basic WSON to that of ammonia suggests the possibility that a significant portion of the gaseous basic WSON can be explained by aliphatic amines, such as methylamine, dimethylamine, trimethylamine, diethylamine, and triethylamine. On the other hand, the seasonal trend of the gaseous acid WSON was unclear, but showed higher concentrations from the winter to spring. The gaseous acid WSON concentrations did not show significant correlations with any parameters. Possible sources and chemical forms of the gaseous acid WSON cannot be deduced at the present time. The seasonal trend of the total concentrations of the gaseous and fine-mode WSON peaked in the summer, probably due to the enhanced emission of the gaseous basic WSON from their sources in the summer. However, the seasonality of the partitioning of the WSON between the gas phase and fine particles was unclear.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.atmosenv.2018.12.011. References Cape, J.N., Cornell, S.E., Jickells, T.D., Nemitz, E., 2011. Organic nitrogen in the atmosphere - where does it come from? A review of sources and methods. Atmos. Res. 102, 30–48. Ge, X., Wexler, A., Clegg, S., 2011. Atmospheric amines - Part I. A review. Atmos. Environ. 45, 524–546. Gibb, S.W., Mantoura, R.F.C., Liss, P.S., 1999. Ocean-atmosphere exchange and atmospheric speciation of ammonia and methylamines in the region of the NW Arabian Sea. Global Biogeochem. Cycles 13, 161–177. González Benítez, J.M., Cape, J.N., Heal, M.R., 2010. Gaseous and particulate watersoluble organic and inorganic nitrogen in rural air in southern Scotland. Atmos. Environ. 44, 1506–1514. Gruijthuijsen, Y.K., Grieshuber, I., Stöcklinger, A., Tischler, U., Fehrenbach, T., Weller, M.G., Vogel, L., Vieths, S., Pöschl, U., Duschl, A., 2006. Nitration enhances the allergenic potential of proteins. Int. Arch. Allergy Immunol. 141, 265–275. Ho, K.F., Ho, S.S.H., Huang, R.J., Liu, S.X., Cao, J.J., Zhang, T., Chuang, H.C., Chan, C.S., Hu, D., Tian, L., 2015. Characteristics of water‐soluble organic nitrogen in fine particulate matter in the continental area of China. Atmos. Environ. 106, 252–261. Kaneyasu, N., Murayama, S., 2000. High concentrations of black carbon over middle latitudes in the North Pacific Ocean. J. Geophys. Res. 105, 19881–19890. Lesworth, T., Baker, A., Jickells, T., 2010. Aerosol organic nitrogen over the remote Atlantic Ocean. Atmos. Environ. 44, 1887–1893. Mace, K., Artaxo, P., Duce, R., 2003. Water-soluble organic nitrogen in Amazon Basin aerosols during the dry (biomass burning) and wet seasons. J. Geophys. Res. 108, 4512. https://doi.org/10.1029/2003JD003557. Matsumoto, K., Yamamoto, Y., Kobayashi, H., Kaneyasu, N., Nakano, T., 2014. Watersoluble organic nitrogen in the ambient aerosols and its contribution to the dry deposition of fixed nitrogen species in Japan. Atmos. Environ. 95, 334–343. Matsumoto, K., Yamato, K., 2016. Uncertainties in the measurements of water-soluble organic nitrogen in the aerosol. Atmos. Environ. 144, 220–225. Matsumoto, K., Yamato, K., 2017. Water-soluble organic nitrogen in the gas phase measured by the denuder-filter pack method. Tellus B 69, 1306916. https://doi.org/ 10.1080/16000889.2017.1306916. Matsumoto, K., Yamamoto, Y., Nishizawa, K., Kaneyasu, N., Irino, T., Yoshikawa-Inoue, H., 2017. Origin of the water-soluble organic nitrogen in the maritime aerosol. Atmos. Environ. 167, 97–103. Matsumoto, K., Takusagawa, F., Suzuki, H., Horiuchi, K., 2018. Water-soluble organic nitrogen in the aerosols and rainwater at an urban site in Japan: implications for the nitrogen composition in the atmospheric deposition. Atmos. Environ. 191, 267–272. Medeiros, P.M., Conte, M.H., Weber, J.C., Simoneit, B.R.T., 2006. Sugars as source indicators of biogenic organic carbon in aerosols collected above the Howland Experimental Forest, Maine. Atmos. Environ. 40, 1694–1705. Miyazaki, Y., Kondo, Y., Takegawa, N., Komazaki, Y., Fukuda, M., Kawamura, K., Mochida, M., Okuzawa, K., Weber, R.J., 2006. Time-resolved measurements of watersoluble organic carbon in Tokyo. J. Geophys. Res. 111, D23206. https://doi.org/10. 1029/2006JD007125. Miyazaki, Y., Kondo, Y., Han, S., Koike, M., Kodama, D., Komazaki, Y., Tanimoto, H., Matsueda, H., 2007. Chemical characteristics of water-soluble organic carbon in the Asian outflow. J. Geophys. Res. 112, D22S30. https://doi.org/10.1029/ 2007JD009116. Miyazaki, Y., Fu, P.Q., Ono, K., Tachibana, E., Kawamura, K., 2014. Seasonal Cycles of water-soluble organic nitrogen aerosols in a deciduous broadleaf forest in northern Japan. J. Geophys. Res. 119, 1440–1454. https://doi.org/10.1029/2013JD020713. Nakamura, T., Ogawa, H., Maripi, D., Uematsu, M., 2006. Contribution of water soluble organic nitrogen to total nitrogen in marine aerosols over the East China Sea and western North Pacific. Atmos. Environ. 40, 7259–7264. Pashynska, V., Vermeylen, R., Vas, G., Maenhaut, W., Claeys, M., 2002. Development of a gas chromatographic/ion trap mass spectrometric method for the determination of levoglucosan and saccharidic compounds in atmospheric aerosols. Application to urban aerosols. J. Mass Spectrom. 37, 1249–1257. Pummer, B.G., Budke, C., Augustin-Bauditz, S., Niedermeier, D., Felgitsch, L., Kampf, C.J., Huber, R.G., Liedl, K.R., Loerting, T., Moschen, T., Schauperl, M., Tollinger, M., Morris, C.E., Wex, H., Grothe, H., Pöschl, U., Koop, T., Fröhlich-Nowoisky, J., 2015. Ice nucleation by water-soluble macromolecules. Atmos. Chem. Phys. 15, 4077–4091. Puxbaum, H., Tenze-Kunit, M., 2003. Size distribution and seasonal variation of atmospheric cellulose. Atmos. Environ. 37, 3693–3699. Qi, J.H., Shi, J.H., Gao, H.W., Sun, Z., 2013. Atmospheric dry and wet deposition of
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alkyl amines in continental particulate matter and their online detection in the gas and particle phase. Atmos. Chem. Phys. 11, 4319–4332. Xu, W., Sun, Y., Wang, Q., Du, W., Zhao, J., Ge, X., Han, T., Zhang, Y., Zhou, W., Li, J., Fu, P., Wang, Z., Worsnop, D.R., 2017. Seasonal characterization of organic nitrogen in atmospheric aerosols using high resolution aerosol mass spectrometry in Beijing, China. ACS Earth Space Chem. 1, 673–682. You, Y., Kanawade, V.P., de Gouw, J.A., Guenther, A.B., Madronich, S., Sierra-Hernandez, M.R., Lawler, M., Smith, J.N., Takahama, S., Ruggeri, G., Koss, A., Olson, K., Baumann, K., Weber, R.J., Nenes, A., Guo, H., Edgerton, E.S., Porcelli, L., Brune, W.H., Goldstein, A.H., Lee, S.-H., 2014. Atmospheric amines and ammonia measured with a chemical ionization mass spectrometer (CIMS). Atmos. Chem. Phys. 14, 12181–12194. Yu, X., Yu, Q., Zhu, M., Tang, M., Li, S., Yang, W., Zhang, Y., Deng, W., Li, G., Yu, Y., Huang, Z., Song, W., Ding, X., Hu, Q., Li, J., Bi, X., Wang, X., 2017. Water soluble organic nitrogen (WSON) in ambient fine particles over a megacity in South China: spatiotemporal variations and source apportionment. J. Geophys. Res. 122, 13045–13060. Zamora, L.M., Prospero, J.M., Hansell, D.A., 2011. Organic nitrogen in aerosols and precipitation at Barbados and Miami: implications regarding sources, transport and deposition to the western subtropical North Atlantic. J. Geophys. Res. 116. https:// doi.org/10.1029/2011JD015660.
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Simultaneous measurement of the water-soluble organic nitrogen in the gas phase and aerosols at a forested site in Japan
T
Kiyoshi Matsumotoa,∗, Yuuya Watanabeb, Ken Horiuchib, Takashi Nakanoc a
Division of Life and Environmental Sciences, University of Yamanashi, 4-4-37, Takeda, Kofu, 400-8510, Japan Faculty of Life and Environmental Sciences, University of Yamanashi, 4-4-37, Takeda, Kofu, 400-8510, Japan c Mt. Fuji Research Institute, Yamanashi Prefectural Government, 5597-1, Kenmarubi, Kamiyoshida, Fujiyoshida, 403-0005, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Water-soluble organic nitrogen Nitrogen compound Aerosol Gas phase Secondary organic aerosol Forest
A simultaneous observation of the water-soluble organic nitrogen (WSON) in the gaseous and particulate phases was conducted at a forested site in Japan for two years to clarify the generation mechanism and possible sources of the gaseous and particulate WSON. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, ranging from the concentration below the detection limit (DL) to 0.302 μg m−3. A large portion (about 73%) of the particulate WSON was distributed in the fine-mode range (d < 2.0 μm). The concentration of the gaseous WSON was 0.041 μg m−3 on average, ranging from the concentration below the DL to 0.195 μg m−3. About 37% of the atmospheric WSON was distributed in the gas phase, and about 59% of the gaseous WSON was basic. The coarse-mode (d = 2.0–10 μm) WSON showed an unclear seasonal trend, but increased in the summer. Its concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. Suspended organic materials from primary sources, such as plant debris, would be a possible source for the coarse-mode WSON. The WSON in the fine-mode range showed a clear seasonality with enhanced concentrations from the spring to summer. The fine-mode WSON concentration showed a strong correlation with the acidity of the fine particles and the concentration of the gaseous basic WSON, suggesting the secondary production of the WSON in the acid fine particles from the gaseous basic WSON. The basic WSON in the gas phase showed a similar seasonal trend with the gaseous ammonia and strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species from terrestrial vegetation and/or soil. The seasonal trend of the gaseous acid WSON was unclear, and its concentration did not show significant correlations with any parameters. The possible source and chemical forms of the gaseous acid WSON cannot be deduced at the present time.
1. Introduction Water-soluble organic nitrogen (WSON) has been reported to account for a significant fraction of particulate nitrogen (Cape et al. (2011), and references therein). Its deposition on the earth's surface has been considered as a significant input pathway of bioavailable nitrogen to the terrestrial and marine ecosystems (Spokes et al., 2000; Russell et al., 2003; Qi et al., 2013; Matsumoto et al., 2014, 2018). In addition, organic nitrogen species in the aerosols have also been paid attention in the recent studies of climate change and public health, because they would act as ice nuclei that affect the global climate and cause adverse health effects due to their allergenicity (Gruijthuijsen et al., 2006; Pummer et al., 2015). Various potential sources for the particulate WSON including both primary and secondary processes have been suggested in previous ∗
studies, but the sources have not yet been sufficiently clarified. The generation of the WSON in the fine particles from the gaseous WSON via gas-to-particle conversion processes has been suggested in recent studies (Matsumoto et al., 2017, 2018), although the primary sources, such as biomass burning (Mace et al., 2003; Zamora et al., 2011), bioparticles (Matsumoto et al., 2014, 2018), and dust particles (Nakamura et al., 2006; Lesworth et al., 2010), have also been considered as significant sources of the particulate WSON. A simultaneous measurement of the gaseous and particulate WSON would be favorable to clarify the source and generation mechanism of the particulate WSON, but have been rarely reported to date. The total amount of the WSON in the gas phase itself has rarely been measured in previous investigations although the measurements of individual gaseous organic nitrogen species, such as alkylamines and alkylnitrates, have been conducted (Cape et al. (2011); Ge et al. (2011), and references therein). Only a few
Corresponding author. E-mail address: [email protected] (K. Matsumoto).
https://doi.org/10.1016/j.atmosenv.2018.12.011 Received 21 August 2018; Received in revised form 3 December 2018; Accepted 5 December 2018 Available online 19 December 2018 1352-2310/ © 2018 Elsevier Ltd. All rights reserved.
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pioneering studies reported the measurements of the WSON in the gas phase (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2017). They demonstrated that about 33–67% of the WSON in the atmosphere is distributed in the gas phase, suggesting that the gaseous WSON occupies a large fraction of the atmospheric WSON. To date, the generation mechanism of the particulate WSON from its precursor gases has not been verified by the simultaneous measurement of the WSON in the gas phase and aerosols. The possible sources of the precursor gases have also not been discussed. The present study conducted the simultaneous measurement of the WSON in the gas phase and aerosols for two years, and discussed the generation mechanism and possible sources of the particulate and gaseous WSON.
HNO3, SO2, and NH3, respectively. The DTN in the extract was measured using the TOC/TN analyzer, and the difference between the concentrations of the DTN and IN is considered to be the gaseous WSON. The WSON from the Na2CO3/glycerol and H3PO4/glycerol-impregnated filter is defined as the gaseous acid and basic WSON, respectively, in this study. Measurements of the total carbon (TC) and elemental carbon (EC) in the aerosols were conducted by a thermal oxidation technique (Kaneyasu and Murayama, 2000). Two pieces were cut from the filter collected aerosol sample. The amount of carbon on one piece was determined using a CHN elemental analyzer (MT-6; Yanaco Analytical Industry Co., Ltd.), which can be considered to be the total carbon (TC). Another piece was heated at 340 °C in flowing air for 20 min, then the carbon on the piece was determined by the same technique as the TC, which can be considered as elemental carbon (EC). The difference in the TC and EC is defined as organic carbon (OC) in this study. Although the overestimation of the EC and underestimation of the OC can be expected in this method due to the pyrolysis of some parts of the OC, our previous experiments demonstrated that these overestimations and underestimations were small especially for the samples with small amounts of carbon (Matsumoto et al., 2017).
2. Experiment 2.1. Sample collection Sample collections were conducted at a forest area located at the northern foot of Mt. Fuji in Japan. The sampling site (35.5ºN, 138.8ºE) was constructed at an open space in the forest and surrounded by red pines with tree heights of about 10–15 m. A downtown area with a population of about 66,000 was about 5 km away from the site. The air sampler was installed about 2 m above the ground surface. The distance between the sampler and the nearest trees was about 13 m. Two size-fractionated aerosol samples with diameters of 2.0–10 μm (coarse-mode) and < 2.0 μm (fine-mode) were collected on quartz fiber filters (QR100; Toyo Roshi Kaisha, Ltd.) using an NILU filter holder with two-stage aluminum alloy impactors (NL Series; Tokyo Dylec Corp.). Before the sample collections, the quartz fiber filters were heated at 850 °C for 4 h. Behind the collection filter of the fine-mode aerosol, a 2% Na2CO3/1% glycerol-impregnated cellulose filter (5A; Toyo Roshi Kaisha, Ltd.) and a 2% H3PO4/1% glycerol-impregnated cellulose filter (5A; Toyo Roshi Kaisha, Ltd.) were placed in this order to collect the acid and basic gases, respectively. Sample collections were conducted at the flow rate of 20.0 L min−1. The air samples were collected from 22 January 2016 to 10 January 2018. The sampling time of one sample was about 14 days. During the sample collections, operational blank tests were performed at given intervals. All of the filter samples were stored below −20 °C until chemically analyzed.
2.3. Quality assurance and quality control in the analysis The averaged field operational blank values of the DTN, ion species, WSOC, TC, and EC (n = 24 for DTN, 31 for ionic species and WSOC, and 17 for TC and EC) were subtracted from the measurements of the collected samples. The detection limit (DL) of these measured species was defined as follows: DL = mb +3 σb, where mb and σb indicate the mean and standard deviation of the blank measurements of each constituent, respectively. Because the WSON and OC were calculated using the measurements of four nitrogen species and two carbon species, respectively, the propagation of the errors in the measurements of each species determined the DL of the WSON and OC. The DL of the WSON was defined as follows: DL = (σaDTN2 + σaNO22 + σaNO32 + σaNH42 + σbDTN2 + σbNO22 + σbNO32 + σbNH42)0.5,
2.2. Sample analysis
where σaDTN, σaNO2, σaNO3, and σaNH4 denote the standard deviations of the repetitive analyses of the DTN, NO2−-N, NO3−-N, and NH4+-N, respectively, in the sample extracts and σbDTN, σbNO2, σbNO3, and σbNH4 denote the standard deviations of the blank concentrations of the DTN, NO2−-N, NO3−-N, and NH4+-N, respectively. In addition, the intercomparability of the TOC/TN analyzer and ion chromatograph was carried out with 5–10 repetitive analyses of the 1, 3, and 5 ppmN NH4NO3 solutions, to examine the systematic error in the WSON analysis. The averaged ratio of the IN from the ion chromatograph to the DTN from the TOC/TN analyzer was 1.013 with the standard deviation of 0.021 (Matsumoto et al., 2014). In this study, therefore, the sample with a higher IN/DTN ratio than 0.950 that was obtained by subtracting three times the standard deviation (0.021) from the mean (1.013) was also considered to be below the DL for the WSON. The DL of the OC was defined as follows:
One-fourth of the filter collected aerosol sample was ultrasonically extracted with 20 ml of ultrapure water (specific resistivity > 18 MΩ cm) in a glass vial. The water extract was filtered using a PTFE membrane filter (13HP045AN; Toyo Roshi Kaisha, Ltd.). The dissolved total organic carbon (DTOC) and total nitrogen (DTN) in the extract were measured using a total organic carbon (TOC)/total nitrogen (TN) analyzer (Model TOC-Vcsh/TNM-1, Shimadzu) that can detect the DTOC with an NDIR detector and the DTN with an ozone chemiluminescence detection after a thermo-catalytic oxidation process. The DTOC and DTN are considered to be the water-soluble organic carbon (WSOC) and water-soluble total nitrogen (WSTN), respectively, in this study. The ion species (Cl−, NO2−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, and Ca2+) in the extract were measured using ion chromatographs (DX-120; Dionex Corp. and ICS-1100; Thermo Fisher Scientific Inc.). The concentrations of the total inorganic nitrogen (IN) were calculated from the sum of the NO2−-N, NO3−-N and NH4+-N concentrations, and the difference between the concentrations of the DTN and IN is considered to be the WSON. The whole of the filter collected acid or basic gases was ultrasonically extracted in 40 ml of ultrapure water in a glass vial, and the extract was filtered using the PTFE membrane filter. Cl−, NO2−, NO3−, SO42−, and NH4+ in the extract were measured using the ion chromatographs to determine the concentrations of the gaseous HCl, HNO2,
DL = (σaTC2 + σaEC2 + σbTC2 + σbEC2)0.5, where σaTC and σaEC denote the standard deviation for the repetitive analyses of the TC and EC in the collected samples, respectively, and σbTC and σbEC denote the standard deviation of the blank measurements of the TC and EC, respectively. The concentrations below the DLs were considered to be zero in the following statistical analyses and discussion. About 31%, 21%, 43%, and 13% of the WSON measurements from the coarse-mode, fine-mode, 313
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basic and acid gas samples, respectively, were below the DL in this study. 3. Results and discussion 3.1. WSON in the aerosols Measurements of the nitrogen species in the aerosols are summarized in Table 1. Matsumoto and Yamato (2016) demonstrated that some parts of the particulate WSON collected on the aerosol collection filter would volatilize during the sample collection and detect as the gaseous WSON by the downstream gas collection filter. They estimated that approximately 32.5% of the WSON in the fine particles was lost. They also found that these artifacts did not show any seasonal trends and temperature dependence. These artifacts would also be expected in the measurements of the particulate and gaseous inorganic nitrogen species. Because the magnitude of these artifacts would be largely influenced by the meteorological condition and particle chemical composition, literature values of the magnitude cannot be applied to our measurements. Therefore, the artifacts in the measurements of the nitrogen species were not corrected in the following discussion. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, which is within previous measurements from the rural or forested sites (Mace et al., 2003; González Benítez et al., 2010; Miyazaki et al., 2014; Matsumoto et al., 2014). The ratio of the WSON to WSTN was about 13% on average in the aerosols, which is within our previous measurements at an urban site of about 40 km from the present sampling site (Matsumoto et al., 2014, 2018) and is a lower tail of the reported ratios from many previous studies (Shi et al. (2010); Cape et al. (2011), and references therein). Higher ratios of the WSON/WSTN of about 25% on average were found in the coarse-mode range compared with those in the fine-mode range of about 11% on average. This would be due to the contribution of the primary sources, such as plant debris, on the coarse-mode aerosols as will be discussed later. A large portion (about 73% on average) of the WSON was distributed in the fine-mode range, which has also been already published in many previous reports (Matsumoto et al. (2017), and references therein). The seasonal averaged concentrations of the nitrogen species in the aerosols are summarized in Fig. 1. In the coarse-mode range, nitrate was the most dominant nitrogen compound showing the highest
Fig. 1. Seasonal averaged concentrations of nitrogen species in the coarse-mode (upper panel) and fine-mode (lower panel) particles. Only for NH4+-N in the fine-mode, refer to the right axis.
concentration in the spring due to the uptake of the gaseous nitric acid by Asian dust particles. The coarse-mode WSON showed the highest concentration in the summer and lowest in the autumn, but its seasonal trend was unclear. Correlations of the coarse-mode and fine-mode WSON with other particulate species, gaseous WSON, gaseous pollutants (NOx and Ox), and meteorological factors are summarized in Tables 2 and 3, respectively. The data of the NOx and Ox concentrations were from the hourly data at the Yoshida ambient air pollution monitoring station, and those of the meteorological factors were from the hourly data at the Kawaguchiko weather station. Both stations are located in the downtown area about 5 km away from our sampling site. Table 2 Correlations of the coarse-mode WSON with other coarse-mode species, gaseous WSON, gaseous pollutants, and meteorological factors. The number of samples was 52. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Table 1 Measurements of nitrogen species in the aerosols. The unit is μgN m−3 except for the WSON/WSTN. The number of samples was 52, but only the coarse-mode WSON/WSTN was 43. mean
median
Coarse-mode WSTN 0.078 0.074 NO2−-N 0.000 b.d.a − NO3 -N 0.054 0.047 NH4+-N 0.012 0.010 WSON 0.019 0.015 WSON/WSTN 0.251 0.257 Fine-mode WSTN 0.447 0.449 NO2−-N 0.001 b.d.a − NO3 -N 0.017 0.009 NH4+-N 0.380 0.372 WSON 0.051 0.036 WSON/WSTN 0.105 0.078 Total (Coarse-mode + Fine-mode) WSTN 0.526 0.532 NO2−-N 0.001 b.d.a − NO3 -N 0.071 0.066 NH4+-N 0.392 0.379 WSON 0.070 0.058 WSON/WSTN 0.127 0.118 a
standard deviation
maximum
minimum
Coarse-mode species NO2− NO3− NH4+ nss-SO42Na+ nss-K+ nss-Ca2+ [nss-SO42-] + [NO3−] – [NH4+] WSOC OC EC Gaseous WSON Basic WSON Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
a
0.051 0.001 0.037 0.013 0.020 0.182
0.184 0.008 0.183 0.057 0.084 0.734
b.d. b.d.a 0.011 b.d.a b.d.a 0.000
0.169 0.003 0.021 0.146 0.056 0.099
0.823 0.012 0.118 0.787 0.269 0.385
0.136 b.d.a b.d.a 0.133 b.d.a 0.000
0.191 0.003 0.046 0.150 0.059 0.088
0.888 0.012 0.211 0.797 0.302 0.361
0.136 b.d.a 0.014 0.133 b.d.a 0.000
Below the detection limit. 314
r
Significance
−0.14 −0.06 0.03 0.04 −0.05 0.05 −0.14 −0.05 0.32 0.18 0.25
– – – – – – – – * – –
−0.16 0.05
– –
0.03 −0.16
– –
0.13 0.10 −0.12 −0.13
– – – –
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explained by the primary emission from biomass burning and the secondary production from precursor gases (e.g., Ho et al., 2015; Yu et al., 2017; Xu et al., 2017). In the present study, however, the influence of biomass burning on the WSON measurements was not found due to few occurrences of biomass burning around the sampling site, which resulted in the fact that only the secondary production significantly contributed to the fine-mode WSON.
Table 3 Correlations of the fine-mode WSON with other fine-mode species, gaseous WSON, gaseous pollutants, and meteorological factors. The number of samples was 52. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Fine-mode species NO2− NO3− NH4+ nss-SO42Na+ nss-K+ nss-Ca2+ [nss-SO42-] + [NO3−] – [NH4+] WSOC OC EC Gaseous WSON Basic WSON Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
−0.22 −0.05 0.20 0.30 −0.09 0.07 0.26 0.44 0.30 0.22 0.23
– – – * – – – *** * – –
0.44 0.27
*** *
−0.13 0.18
– –
0.14 0.08 −0.08 −0.10
– – – –
3.2. WSON in the gas phase Table 4 summarizes the measurements of the nitrogen species in the gas phase. As already mentioned, the WSON in the gas phase has rarely been measured in the previous investigations. A few studies reported the measurements of the WSON in the gas phase with concentrations that ranged within 0.022–0.253 μg m−3 (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2016, 2017). The concentration of the gaseous WSON in this study was 0.041 μg m−3 on average, which is lower than those at the urban and rural sites (González Benítez et al., 2010; Matsumoto and Yamato, 2016, 2017), similar to that at the coastal site (Zamora et al., 2011), and higher at the remote oceanic site (Zamora et al., 2011). The WSON concentrations in the gas phase were detected to be at a comparable level with those in the particulate phase. About 37% of the atmospheric WSON was distributed in the gas phase on average. As already discussed, the collection of the gaseous WSON after the collection of the aerosols on a filter can lead to underestimation of the WSON in the aerosols and overestimation of the WSON in the gas phase due to volatilization loss of the WSON in the aerosols collected on the filter (Matsumoto and Yamato, 2016). Therefore, it is possible that the distribution of the atmospheric WSON into the gas phase was overestimated in this study. Compared to previous measurements from a similar collection technique of the particulate and gaseous WSON with this study (González Benítez et al., 2010; Zamora et al., 2011; Matsumoto and Yamato, 2016), the partitioning of the atmospheric WSON into the gas phase is somewhat lower in this study. In the gaseous WSON, basic compounds were dominant; about 59% of the gaseous WSON was basic, which is a similar result from our previous investigation (Matsumoto and Yamato, 2017). The gaseous WSON occupied only about 9% of the total nitrogen in the measured gaseous nitrogen species (WSON, NH3-N, HNO2-N, and HNO3-N) on average. The seasonal averaged concentrations of the nitrogen species in the gas phase are summarized in Fig. 2. The most dominant species was ammonia with the highest concentration in the summer due to enhancement of its emission at high temperatures. The second most dominant species was nitric acid that increased in the spring and summer due to enhanced photochemical production. The basic WSON in the gas phase showed a similar seasonal trend with ammonia, suggesting that the similar process with the emission of ammonia would be a possible source for the gaseous basic WSON. On the other hand, the
As shown in Table 2, the coarse-mode WSON concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. The correlations of the coarsemode WSON concentration with the gaseous species and meteorological factors were not significant. The contribution of the suspended organic materials from mature leaves, leaf litters, and their decomposed matters to the coarse-mode WSOC has been reported in past studies (Pashynska et al., 2002; Puxbaum and Tenze-Kunit, 2003; Medeiros et al., 2006; Tominaga et al., 2011). The present results could suggest that the suspended organic materials from the primary sources, such as plant debris, would be an effective source for the coarse-mode WSON, although the correlation of the WSON with nss-K+ was not statistically significant in this study. In the fine-mode range, on the other hand, ammonium was the most dominant nitrogen compound with higher concentrations in the warm season from the spring to summer. Fine-mode ammonium was associated with sulfate that increased in the warm season due to the photochemical production. The WSON in the fine-mode range was also enhanced in the warm season. As shown in Table 3, the fine-mode WSON concentration showed a strong correlation with the difference in the equivalent concentrations between the sum of nss-SO42- + NO3− and NH4+ ([nss-SO42-] + [NO3−] – [NH4+]) among the measured species in the fine aerosols, suggesting the secondary production of the WSON in the acid particles as discussed in our previous measurements at the urban and maritime sites (Matsumoto et al., 2017, 2018). Correlations of the fine-mode WSON concentration with the gaseous WSON concentrations were also significant, especially strong with the gaseous basic WSON, which supports the secondary production of the fine-mode WSON from the gaseous WSON. Enhanced concentrations of the particulate sulfate due to enhanced photochemical reactions in the warm season would cause higher acidity of the fine particles, which leaded to higher concentrations of the fine-mode WSON during this season. Indeed, the correlation of the WSON with the nss-SO42- was significant in the fine-mode range as shown in Table 2. The correlation of the WSON with the WSOC was also significant in the fine-mode range, which can be attributed to the fact that a large portion of the WSOC is also derived from the photochemical processes (Miyazaki et al., 2006, 2007). Recent studies reported that a large portion of the particulate WSON can be
Table 4 Measurements of nitrogen species in the gas phase. The unit is μgN m−3. The number of samples was 53.
HNO2-N HNO3-N NH3-N Acid WSON Basic WSON Total (acid + basic) WSON a
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mean
median standard deviation
maximum minimum
0.025 0.093 0.274 0.017 0.024 0.041
0.022 0.087 0.223 0.012 0.012 0.028
0.057 0.172 0.703 0.060 0.195 0.195
Below the detection limit.
0.012 0.038 0.176 0.015 0.036 0.039
0.005 0,027 0.040 b.d.a b.d.a b.d.a
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weight aliphatic amines. In past studies, methylamine, dimethylamine, trimethylamine, diethylamine, and triethylamine have been often detected in the gas phase (Schade and Crutzen, 1995; Gibb et al., 1999; Vandenboer et al., 2011; Ge et al., 2011; You et al., 2014). The concentrations of these aliphatic amines are largely dependent on local sources, but the nitrogen concentration ratios of these aliphatic amines with ammonia have been estimated to be within about 0.001–0.16 in the ambient gas phase (Schade and Crutzen, 1995; Gibb et al., 1999; Vandenboer et al., 2011; You et al., 2014). In the present study, the ratio of the average nitrogen concentration of the basic WSON to that of ammonia was 0.088, suggesting the possibility that a significant portion of the gaseous basic WSON fraction can be explained by these aliphatic amines. In past studies, the emissions from the terrestrial vegetation and soil have been pointed out as possible natural sources of these amines and ammonia (Schulten and Schnitzer (1998); Ge et al. (2011); Sintermann and Neftel (2015), and references therein). Their emissions from these sources would be enhanced in the warm season. On the other hand, the acid WSON concentrations in the gas phase did not show significant correlations with any parameters. Possible sources and chemical forms of the gaseous acid WSON cannot be deduced at the present time.
Fig. 2. Seasonal averaged concentrations of nitrogen species in the gas phase. Only for NH3-N, refer to the right axis.
Table 5 Correlations of the gaseous basic WSON with other measured gaseous species, gaseous pollutants, and meteorological factors. The number of samples was 53. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Measured gaseous species HCl HNO2 HNO3 SO2 NH3 Acid WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
−0.03 0.32 0.29 −0.22 0.34 −0.03
– * * – * –
−0.25 −0.03
– –
0.42 0.32 −0.09 0.08
*** * – –
3.3. Partitioning of the WSON between the gaseous and particulate phases As already discussed, the coarse-mode WSON can be considered to be derived from primary emission sources and not from the gaseous WSON. On the other hand, the fine-mode WSON would be mainly derived from the gaseous WSON through the gas-to-particle conversion reaction although it is possible that some parts of the fine-mode WSON were derived from primary sources. Fig. 3 shows the seasonal trends of the partitioning of the WSON between the gas phase and fine-mode particles and the total concentrations of the gaseous and fine-mode WSON. The seasonal trend of the total concentrations peaked in the summer. This can be attributed to increases in the fine-mode and gaseous basic WSON in the summer as already described, which would be caused by enhancement of the emission of the gaseous basic WSON due to higher temperatures and subsequent generation of the fine-mode WSON via gas-to-particle conversion reactions. On the other hand, the seasonality of the partitioning was unclear, but the partitioning shifted toward the gas phase in the winter and summer. In the summer, the gaseous basic WSON emissions would be enhanced due to the high temperature, which shifted the partitioning toward the gas phase. In the winter, on the other hand, the emission of the gaseous basic WSON
seasonal trend of the gaseous acid WSON was unclear, although it showed higher concentrations from the winter to spring. Correlations of the basic and acid gaseous WSON with other measured gaseous species, NOx, Ox, and meteorological factors are summarized in Tables 5 and 6, respectively. The basic WSON concentrations in the gas phase were strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species as with the ammonia. The most possible compounds in the gaseous basic WSON fraction would be low-molecular Table 6 Correlations of the gaseous acid WSON with other measured gaseous species, gaseous pollutants, and meteorological factors. The number of samples was 53. Significance is as follows; ***: p < 0.005, **: p < 0.01, *: p < 0.05, and -: p > 0.05.
Measured gaseous species HCl HNO2 HNO3 SO2 NH3 Basic WSON Gaseous pollutants NOx Ox Meteorological factors Temperature Relative humidity Wind speed Rainfall amount
r
Significance
0.01 −0.18 −0.07 0.20 −0.09 −0.03
– – – – – –
−0.03 0.22
– –
−0.13 −0.24 0.25 0.01
– – – –
Fig. 3. Seasonal averaged total concentrations of the gaseous and fine-mode WSON (upper panel) and concentration ratios of the WSON between the gas phase and fine-mode particles (lower panel). 316
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would decrease, which leaded to the lower concentrations of the gaseous basic WSON and fine-mode WSON, but the gaseous acid WSON existed at relatively higher concentrations that shifted the partitioning toward the gas phase.
Acknowledgement We would like to acknowledge R. Ueno, M. Ishikawa, and K. Sakata for their kind support of the sample collections and chemical analyses. This study was financially supported by JSPS KAKENHI (Grant Number 16K05620).
4. Summary
Appendix A. Supplementary data
The present study conducted the simultaneous measurement of the WSON in the gaseous and particulate phases at a forested site for two years, and suggested the generation mechanism and possible sources of the WSON in both phases. To date, the generation mechanism of the particulate WSON from its precursor gases has not been verified by the simultaneous measurement of the WSON in the gas phase and aerosols. The possible sources of the precursor gases have also not been discussed. The summary of the present study is as follows. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, ranging from the concentration below the DL to 0.302 μg m−3. The ratio of the WSON to WSTN in the aerosols was about 13%. Higher ratios of the WSON/WSTN of about 25% were found in the coarse-mode range compared to those in the fine-mode range of about 11%. About 73% of the particulate WSON was distributed in the fine-mode range. The coarse-mode WSON showed an unclear seasonal trend, but increased in the summer. The coarse-mode WSON concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. Suspended organic materials from primary sources, such as plant debris, would be a possible source of the coarse-mode WSON. The WSON in the fine-mode range showed a clear seasonality with enhanced concentrations in the warm season from the spring to summer. The fine-mode WSON concentration showed a strong correlation with the difference in the equivalent concentrations between the sum of nss-SO42- + NO3− and NH4+ ([nssSO42-] + [NO3−] – [NH4+]) among the measured species in the fine aerosols, suggesting the secondary production of the WSON in the acid particles. The correlation of the fine-mode WSON concentration with the gaseous basic WSON concentration was also strong, which suggests the secondary production of the WSON in the acid fine particles from the gaseous basic WSON. Enhanced concentrations of the particulate sulfate due to enhanced photochemical reactions in the warm season would cause higher concentrations of the fine-mode WSON during this season. The concentration of the gaseous WSON in this study was 0.041 μg m−3 on average, ranging from the concentration below the DL to 0.195 μg m−3. About 37% of the atmospheric WSON was distributed in the gas phase, and about 59% of the gaseous WSON was basic. The gaseous WSON occupied only about 9% of the total nitrogen in the measured gaseous nitrogen species. The basic WSON in the gas phase showed a similar seasonal trend with ammonia and strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species from terrestrial vegetation and/or soil. The ratio of the nitrogen concentration of the basic WSON to that of ammonia suggests the possibility that a significant portion of the gaseous basic WSON can be explained by aliphatic amines, such as methylamine, dimethylamine, trimethylamine, diethylamine, and triethylamine. On the other hand, the seasonal trend of the gaseous acid WSON was unclear, but showed higher concentrations from the winter to spring. The gaseous acid WSON concentrations did not show significant correlations with any parameters. Possible sources and chemical forms of the gaseous acid WSON cannot be deduced at the present time. The seasonal trend of the total concentrations of the gaseous and fine-mode WSON peaked in the summer, probably due to the enhanced emission of the gaseous basic WSON from their sources in the summer. However, the seasonality of the partitioning of the WSON between the gas phase and fine particles was unclear.
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