Isotopic components and source analysis of inorganic nitrogen in coastal aerosols of the Yellow Sea

The coastal atmospheric environment is one of the most complex environments on earth. It is shaped by terrestrial, marine, and atmospheric processes and acts as an external nutrient source for coastal waters. At present, there are few observations of inorganic nitrogen isotopes of China coastal aerosols, let alone the Yellow Sea. In this study, a weekly collection of total suspended particulate aerosols was conducted on the Qianliyan Island in 2018 for the measurements of inorganic nitrogen species (NO₃⁻ and NH₄⁺) and their isotopic ratios (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, and δ¹⁵N-NH₄⁺). At the Qianliyan Island, the average NO₃⁻ and NH₄⁺ concentrations were 2.49 ± 2.12 and 3.33 ± 2.68 μg·m⁻³, respectively; the average δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, and δ¹⁵N-NH₄⁺ were 2.4‰ ± 5.7‰, 78.7‰ ± 8.0‰, and −2.6‰ ± 6.3‰, respectively. The major nitrate formation pathways were •OH oxidation and N₂O₅ hydrolysis paths, and the dominant sources of inorganic nitrogen aerosols were coal combustion (29% ± 7%), marine (19% ± 15%), and fertilizer (16% ± 13%). Aerosol δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ were obviously higher in winter and lower in summer; conversely, aerosol δ¹⁵N-NH₄⁺ was slightly higher in summer and slightly lower in winter. The difference in nitrogen sources was considered to be the best explanation for the aerosol δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ differences between summer and winter, of which coal combustion contributed the most. The seasonal difference in nitrate formation paths was considered to be the best explanation for the difference of Qianliyan aerosol nitrate δ¹⁸O-NO₃⁻ between summer and winter. Aerosol inorganic nitrogen deposition flux was estimated to be 3.4 nmol N·m⁻²·s⁻¹, which induced less than 1% to marine primary production, and aerosol inorganic nitrogen deposition, compared with N₂ fixation, contributed some 80% of δ¹⁵N-NO₃⁻ depression of the summer Yellow Sea thermocline.

Introduction

Atmospheric nitrogen deposition is widely regarded as one of the important sources of nutrients in the marine system (Duce et al., 2008; Galloway et al., 2008). Aerosol has direct or indirect effects on climate, environment, and human health (Jickells et al., 2005; Pöschl, 2005; Das and Jayaraman, 2012; Xu and Penner, 2012; Seinfeld and Pandis, 2016). The rapid population growth has been egregiously negatively impacting on the coastal landscape and atmospheric environment in many parts of the world (Arteaga et al., 2019; Aswini and Hegde, 2021; Mohamed et al., 2021; Refulio-Coronado et al., 2021; Wang et al., 2021). For example, human activities have caused a large number of nutrients to be transported to the coastal waters, resulting in a surge in the concentration of inorganic nitrogen compared with that of phosphorus and the disharmony of nitrogen, phosphorus, and silicon in the coastal waters of China (Zhang et al., 2007b; Galloway, 2013; Qi et al., 2020). The weak exchange capacity of seawater on the continental shelf of China exacerbates the eutrophication of nearshore waters of China, resulting in the frequent occurrence of harmful algal blooms (Xin et al., 2019; Wang et al., 2021).

Anthropogenic/terrestrial dissolved inorganic nitrogen (DIN) sources commonly contain industrial and civil sources (coal combustion), agricultural sources (fertilizer, animal waste, and biomass burning), and transportation sources (road dust/soil and vehicle emission). As the distance increases from the shoreline, the nutrient concentrations in atmospheric total suspended particulate (TSP) aerosols decrease exponentially (Morin et al., 2009; Seok et al., 2021) and the anthropogenic/terrestrial contributions to seawater decrease. Therefore, the influence of anthropogenic/terrestrial DIN in aerosol on marginal sea is much greater than that on open sea. In addition to anthropogenic/terrestrial sources, the significance of a seawater-derived (named “marine-sourced”) DIN, which is related to biological activities and acted on the atmosphere through the sea–air interface, has recently been recognized (Altieri et al., 2021; Dobashi et al., 2022). Surface seawater is considered to be the source of gaseous organic nitrogen and ammonia in the marine atmosphere (Altieri et al., 2013; Bertram et al., 2018; Altieri et al., 2021; Yu and Li, 2021). The marine emission of organic alkyl nitrates is converted into atmospheric NO_x and eventually nitrate (Burger et al., 2022). As marine atmospheric environment is acidic, ammonium is formed right after ammonia is discharged (Altieri et al., 2014). Therefore, it is necessary to consider both anthropogenic/terrestrial and marine sources for DIN in marginal-sea aerosols.

As the main species of aerosol DIN, nitrate and ammonium concentrations only provide information on DIN composition and deposition flux. The isotopic compositions of atmospheric inorganic nitrogen species are imperative to acquire further information, which goes far beyond the judgment based on the continental inorganic nitrogen species concentrations being usually higher than those from marine. From the limited studies, the average δ¹⁵N-NO₃⁻ values are more positive in the Pacific Ocean than that in the Atlantic Ocean, and the average aerosol δ¹⁸O-NO₃ ⁻ values (70‰–85‰) are similar in the Atlantic Ocean and the Pacific Ocean (Morin et al., 2009; Kundu et al., 2010; Kawashima and Kurahashi, 2011; Gobel et al., 2013; Savarino et al., 2013; Kamezaki et al., 2019; Carter et al., 2021; Joyce et al., 2022), though the regularity is not shown for aerosol nitrate concentrations in the Atlantic Ocean when comparing with that in the Pacific Ocean, because sometimes nitrate concentration in the coastal area of Japan (Kawashima and Kurahashi, 2011) is even lower than that in the open Atlantic Ocean (28°S–52°N) (Morin et al., 2009). From even limited studies, it concludes that both aerosol ammonium concentrations and δ¹⁵N-NH₄⁺ values were relatively higher in the Pacific Ocean than that in the Atlantic Ocean, whether in the coastal or open-sea atmosphere (Kundu et al., 2010; Kawashima and Kurahashi, 2011; Lin et al., 2016; Kamezaki et al., 2019; Joyce et al., 2022). In the open Atlantic Ocean (40°S–50°N), aerosol δ¹⁵N-NH₄⁺ can be as low as −12‰ (Lin et al., 2016); in the marginal sea of the Western Pacific Ocean, aerosol δ¹⁵N-NH₄⁺ can reach 38‰ (Kawashima and Kurahashi, 2011).

Along the marginal seas of China, the nitrate aerosol concentration decreases from the Bohai Sea to the South China Sea, with the aerosol nitrate concentrations of 6.0 ± 3.4 and 6.6 ± 2.7 μg·m⁻³, respectively, in the Bohai Sea, and the boundary between the Bohai Sea and the Yellow Sea were (Zong et al., 2017; Zong et al., 2022) and as low as 0.5 ± 0.2 μg·m⁻³ in the South China Sea (Xiao et al., 2015). The variation of aerosol δ¹⁵N-NO₃⁻ is opposite to that of δ¹⁸O-NO₃⁻. From the Bohai Sea, the boundary between the Bohai Sea and the Yellow Sea to the South China Sea, annual average δ¹⁵N-NO₃⁻ values were 7.8‰ ± 5.0‰, 8.3‰ ± 6.2‰, and 1.5‰ ± 1.6‰, respectively, and annual average δ¹⁸O-NO₃⁻ values were 72.6‰ ± 13.5‰, 76.9‰ ± 11.8‰, and 83.2‰ ± 10.6‰, respectively. So far, there is no report on aerosol nitrogen isotope of ammonium over the Chinese marginal seas.

On these basis of DIN isotopic values and the developed methods, studies on stable nitrogen and oxygen isotopes have been used for source tracing and formation pathways (Walters and Michalski, 2015; Walters and Michalski, 2016; Elliott et al., 2019; Walters et al., 2019). In order to control coastal air pollution from the source, researches in coastal areas of China focused on source tracing. Coal combustion was the main source of aerosol nitrate in the South China Sea in cold months (October to January), whereas biomass burning, animal manure, and soil emission were the main sources of aerosol nitrate in the South China Sea in warm months (May to August) by comparison with the source values (Xiao et al., 2015). Coal combustion was also the main source of annual aerosol nitrate at the junction of the Yellow Sea and the Bohai Sea (36% ± 7%; Zong et al., 2017) and in the Bohai Sea (46% ± 16%; Zong et al., 2022). The less important contributors to aerosol nitrate were biomass burning, vehicle emission, and soil emission. As coal-fire combustion has higher δ¹⁵N-NO_x relative to biomass burning and fertilized soil emission (Elliott et al., 2019), the annual low proportion of coal combustion contribution may be one of the reasons for the lower aerosol δ¹⁸O-NO₃⁻ in the South China Sea when compared with the Yellow Sea and the Bohai Sea. However, these studies exclude marine source in the source analysis, though marine source has its significance as previously mentioned. Tianjin, a coastal city east to the Bohai Sea, has an estimated marine source of ~4% in PM_2.5 (Dong et al., 2022). Usually, the contribution of marine source to TSP aerosols increases with the increase of aerosol particle size (Yeatman et al., 2001), making it reasonable to consider marine source for Qianliyan TSP aerosols. As for formation pathways of nitrate, estimation has been done that •OH oxidation and N₂O₅ hydrolysis paths contribute 41%–42% and 28%–41%, respectively, of total aerosol nitrate at a global scale (Alexander et al., 2020). Nitrate production in the winter equatorial Pacific atmosphere was dominated by •OH oxidation, and the portion ranged from 58% to 63% (Carter et al., 2021), which is comparable to the portion of •OH path for aerosol nitrate in summer Bohai Sea (Zong et al., 2022). However, the portion of •OH path for aerosol nitrate declined by 27% in the winter Bohai Sea (Zong et al., 2022). Therefore, it is necessary to discern the seasonal variation of nitrate formation path of Qianliyan.

In the view that dry deposition fluxes of inorganic nitrogen in atmospheric aerosols were well reported (Qi et al., 2020) but limited studies have reported the isotopic composition of nitrate and/or ammonium in coastal aerosols over the Chinese marginal seas, we report the aerosol nitrate and ammonium isotopes in the Yellow Sea. In order to avoid the degradation of coastal environment caused by excessive inorganic nitrogen deposition, we trace the sources of inorganic nitrogen in the Yellow Sea aerosol in consideration of marine source. In addition, we estimate the contribution of aerosol DIN deposition to marine primary production and the constrain of aerosol DIN isotopes on new nitrogen inputs to thermocline.

Materials and methods

Aerosol sampling

Aerosols are a mixing of terrigenous/anthropogenic and marine sources at the Qianliyan Island along the Yellow Sea coast of China, including the continental aerosols from China, Korea, and Japan, and marine aerosols from the North-West Pacific Ocean on the downwind of East Asian mass transport path controlled by the East Asian Summer and Winter Monsoon (Chang et al., 2021).

Aerosol samples were weekly collected (24 h per each time) by a high-volume sampler (Laoying 2031 typed intelligent large flow TSP sampler, Qingdao Laoying Environmental Technology Co., Ltd) at a flow rate of 1.05 m⁻³·min⁻¹ with Whatman 41 filter at Qianliyan Island (36.27°N, 121.38°E) of the western Yellow Sea throughout 2018 and were subsequently frozen (−20°C) until they were determined. Aerosol sampling did carefully avoid precipitations because of the aerosol removal by precipitations (Yeatman et al., 2001).

Before and after sampling, the filters were weighed on electronic balance with the readability up to 0.01 mg (Sartorius CPA224S analytical lab balance, Sartorius, Göttingen, Germany) immediately after being in a desiccator (25°C, 10% relative humidity [RH]) to a constant weight. Constant weight refers to the weight difference of the filter dried twice in succession being less than 0.01 mg for a blank filter and 0.2 mg for a sampled filter. The second and subsequent weighing were carried out after further drying for 1 h under same conditions. Generally, drying for more than 24 h ensures constant filter weight.

Ultrasound extraction

Sampled filters were divided into 16 equal parts. One central fixed position portion of each sample was ultrasonically extracted for 1 h using Milli-Q water (18.2 MΩ·cm), and extractions were filtered by a disposable filter (0.45 μm).

Analytical method and quality control

Inorganic nitrogen concentrations

NO₃⁻, NO₂^−,and NH₄⁺ concentrations were measured by the nutrient autoanalyzer (QuAAtro39-SFA, SEAL Analytical Group) with detection limits of 0.02, 0.01, and 0.02 μmol·L⁻¹, respectively, and all precisions (RSD) were less than 2%. Among all samples, NO₂⁻ accounted for 0–0.1% of the sum of NO₃⁻ and NO₂⁻, with an average of 0.03% ± 0.03%. Hence, the results of NO₂⁻ were not reported and were not deducted in raw results of NO₃⁻ channel in this study. Since the proportions of NO₂⁻ in the sum of NO₃⁻ and NO₂⁻ were less than 1%, NO₂⁻ influence can be ignored when determining NO₃⁻ isotopes (Casciotti and McIlvin, 2007).

Inorganic nitrogen isotopes

Additionally, nitrogen isotopic composition of nitrate was measured by the biochemical method based on the conversion of NO₃⁻ to N₂O by denitrifying bacteria (Pseudomonas aureofaciens, ATC: 13958) (Sigman et al., 2001), and nitrogen isotopic composition of ammonium was measured by the chemical method based on the conversion of NH₄⁺ to NO₂⁻ and then to N₂O, which was then analyzed by isotope ratio mass spectrometry (IsoPrime 100, Elementar Analysensysteme GmbH) (Zhang et al., 2007a). The isotope ratios were calibrated against standard reference materials IAEA-N3 (δ¹⁵N: +4.7‰, δ¹⁸O: +25.6‰), USGS-34 (δ¹⁵N: −1.8‰, δ¹⁸O: −27.9‰), and USGS-35 (δ¹⁵N: −1.8‰, δ¹⁸O: +57.5‰) (Böhlke et al., 2003) and laboratory mixed standard spike (δ¹⁵N: +14.3‰, δ¹⁸O: −23.0‰) for nitrate nitrogen and oxygen isotopes and standard reference materials IAEA-N1 (δ¹⁵N: +0.4‰), USGS-25 (δ¹⁵N: −30.4‰), and USGS-26 (δ¹⁵N: +53.7‰) for ammonium nitrogen isotopes (Böhlke et al., 1993). Correction to nitrate δ¹⁵N for δ¹⁷O contribution needed to be made because of the joint contribution of ¹⁴N¹⁴N¹⁷O and ¹⁴N¹⁵N¹⁶O to the peak at mass 45. The correction followed the steps of Hastings et al. (2003) and a better adjustment of δ¹⁷O calculation by δ¹⁷O = 0.86 × δ¹⁸O + 0.05 (Michalski et al., 2004). The standard solutions and blank samples were measured every 10 samples to make quality control. The deep water from the Mariana Trench (8,000 m in depth, δ¹⁵N-NO₃⁻: 5.2‰ ± 0.1‰, δ¹⁸O-NO₃⁻: 1.9‰ ± 0.2‰) and IAEA-N1 (δ¹⁵N-NH₄⁺: 0.43‰ ± 0.22‰) were measured in batches to assure the data quality. Based on replicate analyses of the standards and the unknowns, the average precisions of nitrogen and oxygen isotope delta values are ±0.2‰ and ±0.5‰, respectively, or better.

Data processing

Back trajectories

The online HYSPLIT trajectory model (https://www.ready.noaa.gov/) is used for the air-mass back trajectory of 72 h—72 h is in consideration of the annual mean lifetime of nitrate and ammonium in the troposphere against dry deposition to the earth’s surface (Asman et al., 1998; Alexander et al., 2020). This model determines the height of the mixed (boundary) layer from the meteorological data set. In order to better capture the source characteristics of air-mass, the mid-boundary layer height (one half of the mixed layer height) that reflects the characteristics of the average boundary layer flow field is selected as the starting height, and four start times, which are the start time of each sample (T), T + 6 h, T + 12 h, and T + 18 h, for each sample are set.

A three-endmember model for nitrate formation

A three-endmember model (Walters and Michalski, 2016) is induced to clarify the nitrate formation paths. The model includes three formation paths, namely, •OH oxidation (NO₂ +·OH → HNO₃), N₂O₅ hydrolysis (N₂O₅ + H₂O → 2HNO₃), and hydrocarbon paths (NO₃ + R [DMS/hydrocarbons] → HNO₃). NO_x reacts with different oxidants to produce NO₃⁻. During the day, •OH is the main oxidant because of the strong solar radiation (Lelieveld et al., 2004); during the night, O₃ oxidation dominates and reacts with NO₂ to produce N₂O₅/NO₃ (Walters and Michalski, 2015). Based on N and O isotopic mass-balance and the assumptions that atmospheric isotopic equilibrium is achieved between NO and NO₂ during the daytime, and NO₂, NO₃ and N₂O₅ during the nighttime, aerosol nitrates formed by •OH oxidation, N₂O₅ hydrolysis and hydrocarbon paths yield distinctive δ¹⁵N-δ¹⁸O (Walters and Michalski, 2016). The calculation formula of three-endmember values of δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ is written in Text S1, and the method for calculating the portions of nitrate formation paths (the Bayesian mixing model simmr) is introduced in the next section.

Bayesian mixing model

The simmr, a Bayesian stable isotope mixing model implemented in R, is applicable to the generic situation where data include N measurements on J isotopes with K (K > J + 1) sources. Similarly, simmr can be applied to quantify the respective contributions of three nitrate formation path on data set of two isotopes from seasonal samples. Based on the results of potential sources/nitrate formation paths identification, the contribution of each endmember is described as

$\begin{array}{ll}{\text{X}}_{\text{ij}}={\sigma }_{\text{k}=1}^{\text{K}}{\text{F}}_{\text{k}}\left({\text{S}}_{\text{jk}}+{\text{c}}_{\text{jk}}\right)+{\text{ϵ}}_{\text{ij}}& (1)\end{array}$

$\begin{array}{ll}{\text{S}}_{\text{jk}}\sim \text{N}\left({\text{μ}}_{\text{jk}},{\text{ω}}_{\text{jk}}^2\right)& (2)\end{array}$

$\begin{array}{ll}{\text{c}}_{\text{jk}}\sim \text{N}\left({\text{λ}}_{\text{jk}},{\text{τ}}_{\text{jk}}^2\right)& (3)\end{array}$

$\begin{array}{ll}{\text{ϵ}}_{\text{ij}}\sim \text{N}\left(0,{ }_{\text{σj}}^2\right)& (4)\end{array}$

where X_ij is the isotopic value j (1, 2, 3, ⋯, J) of the mixture i (1, 2, 3, ⋯, N), F_k is the source/path k proportion derived from simmr, S_jk is the source value k (1, 2, 3, ⋯, K) on isotope j, c_jk is the fractionation factor for isotope j on source/path k, and ϵ_ij is the residual error representing the additional unquantified variation among individual mixtures; S_jk is normally distributed with mean μ_jk and standard deviation ω_jk c_jk is normally distributed with mean λ_jk and standard deviation τ_jk and ϵ_ij is normally distributed with mean 0 and standard deviation σ_j . The detail of simmr refers to Parnell et al. (2010). Because of the homology of inorganic nitrogen species (Dong et al., 2022) and distinct source ranges of δ¹⁵N-NH₄⁺–δ¹⁵N-NO₃⁻ from field source sampling (Table S1), it provides a solution to source apportionment of inorganic nitrogen in aerosols. As reported, biomass burning, coal combustion, vehicle emission, and soil emission were main sources for terrigenous aerosol nitrate (Xiao et al., 2015; Zong et al., 2017) and fertilizer, animal waste, vehicle emission, and coal combustion for terrigenous aerosol ammonium (Pan et al., 2016; Pan et al., 2020). Furthermore, marine impact cannot be ignored for Qianliyan aerosols due to marine–atmosphere exchange (Savoie and Prospero, 1982; O’Dowd, 2002). The nitrogen isotope compositions of expected sources are presented in Table S1, and the nitrogen and oxygen isotope compositions of three nitrate formation paths are depicted in Figure S1.

Results

Air-mass source characteristic

The computed back trajectories (n = 45) were categorized into two clusters (continental and maritime) that mainly based on the origins of air-masses and trajectories pathways (Figure S2). The continental cluster (n = 39) consisted three subclusters, which were the eastern (northeast to southeast, n = 2), the southern (southeast to west, n = 1), and the northern (west to northeast, n = 36) continental origins. The northern continental subcluster included the most backward trajectories that originated from the Siberian plateau of Russia via Mongolia and inland of the North China. The eastern continental subcluster included the most backward trajectories that originated from developed countries Japan and South Korea. The sole southern continental subcluster carried near-ground pollution from south China, which is well known as one of economically developed regions in China. The maritime cluster (n = 6) comprised two subclusters, which were the open-sea (n = 2) and marginal sea (n = 4) origins. The open-sea cluster referred to back trajectories from remote Pacific Ocean, which barely went through human activity zone, whereas the marginal-sea cluster referred to back trajectories mostly from the Yellow Sea and the sole one from Sea of East/Japan. On average, the TSP concentration of the maritime cluster (81 ± 59 μg·m⁻³) was lower than the continental cluster (171 ± 53 μg·m⁻³). Specifically, TSP concentrations of the eastern continental, southern continental, northern continental, open-sea, and marginal-sea subclusters were 159 ± 35, 198, 171 ± 54, 47 ± 13, and 98 ± 68 μg·m⁻³, respectively. This delineated the decent trend of atmospheric TSP concentrations from land to ocean; therefore, three continental subclusters were integrated as one in the following results and discussions.

NO₃⁻ and NH₄⁺ concentrations

The NO₃⁻ concentrations ranged from 0.45 to 13.19 μg·m⁻³, with a numerical average of 2.49 ± 2.12 μg·m⁻³, and the NH₄⁺ concentrations ranged from 0.41 to 16.47 μg·m⁻³, with a numerical average of 3.33 ± 2.68 μg·m⁻³. In addition, NO₃⁻ concentration positively and significantly correlated with NH₄⁺ concentration (Table S2), which was expressed as ${\text{c}}_{{\text{NO}}_3^{-}}$ = 0.7047 × ${\text{ c}}_{{\text{NH}}_4^{+}}$ + 0.1421 (R² = 0.793). Classified by the air-mass sources, NO₃⁻ and NH₄⁺ concentration ranges were respectively 0.45–13.19 and 0.69–16.47 μg·m⁻³ for the continental cluster, 0.56–0.57 and 0.41–0.83 μg·m⁻³ for the open-sea cluster, and 0.83–2.69 and 1.12–5.81 μg·m⁻³ for the marginal-sea cluster. Different-orientation NO₃⁻ and NH₄⁺ concentrations were not significantly different (NO₃⁻: Kruskal–Wallis test, H = 5.7, p > 0.05; NH₄⁺: Kruskal–Wallis test, H = 5.1, p > 0.05). Classified by seasons, average NO₃⁻ and NH₄⁺ concentrations were the lowest in summer, which is consistent with the lowest concentration of TSP in summer (Figure S3). Specifically, average NO₃⁻ concentration was 3.2 ± 3.1 μg·m⁻³ in spring, 1.3 ± 0.7 μg·m⁻³ in summer, 3.4 ± 1.7 μg·m⁻³ in autumn, and 1.9 ± 1.3 μg·m⁻³ in winter, whereas average NH₄⁺ concentration was 4.4 ± 3.8 μg·m⁻³ in spring, 2.3 ± 1.8 μg·m⁻³ in summer, 3.9 ± 2.4 μg·m⁻³ in autumn, and 2.6 ± 1.4 μg·m⁻³ in winter at the Qianliyan Island (Figure S3). Seasonal NO₃⁻ concentrations were significantly different (Kruskal–Wallis test, H = 11.1, p < 0.05), whereas seasonal NH₄⁺ concentrations were not significantly different (Kruskal–Wallis test, H = 5.3, p > 0.05).

δ¹⁵N values of NO₃⁻ and NH₄⁺

To sum up, δ¹⁵N-NO₃⁻ in Qianliyan aerosols ranged from −5.5‰ to 14.1‰, with a numerical average of 2.4‰ ± 5.7‰, and δ¹⁵N-NH₄⁺ ranged from −17.1‰ to 12.1‰, with a numerical average of −2.6‰ ± 6.3‰ (Figure 1). Classified by the air-mass sources, the arithmetic means of δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ were 1.8‰ ± 1.1 ‰ and −9.6‰ ± 2.6‰ for the open-sea cluster, −2.0‰ ± 2.5‰ and −3.6‰ ± 2.6‰ for the marginal-sea cluster, and 2.9‰ ± 5.8‰ and −2.2‰ ± 6.5‰ for the continental cluster at the Qianliyan Island (Figure 2). Different-orientation δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values were not significantly different (δ¹⁵N-NO₃⁻: Kruskal–Wallis test, H = 3.2, p > 0.05; δ¹⁵N-NH₄⁺: Kruskal–Wallis test, H = 3.5, p > 0.05). Classified by seasons, the numerical average of δ¹⁵N-NO₃⁻ was 1.3‰ ± 4.5‰ in spring, −2.2‰ ± 2.6‰ in summer, 0.3‰ ± 3.8‰ in autumn, and 9.2‰ ± 3.8‰ in winter, whereas the numerical average of δ¹⁵N-NH₄⁺ was −0.6‰ ± 6.9‰ in spring, −0.8 ± 7.7‰ in summer, −4.2 ± 5.5 ‰ in autumn and was −5.1 ± 3.9‰ in winter at the Qianliyan Island (Figure S3). Seasonal δ¹⁵N-NO₃⁻ values were significantly different (Kruskal–Wallis test, H = 24.0, p < 0.05), whereas seasonal δ¹⁵N-NH₄⁺ values were not significantly different (Kruskal–Wallis test, H = 4.2, p > 0.05).

FIGURE 1

Figure 1 Aerosol nitrate and ammonium isotopic compositions plotted against aerosol nitrate and ammonium concentrations with samples coded by seasons at the Qianliyan Island in 2018.

FIGURE 2

Figure 2 Aerosol nitrate and ammonium isotopic compositions plotted against aerosol nitrate and ammonium concentrations with samples coded based on the air-mass back trajectories at the Qianliyan Island in 2018.

δ¹⁸O values of NO₃⁻

To sum up, δ¹⁸O-NO₃⁻ in Qianliyan aerosols ranged from 63.3‰ to 98.0 ‰, with a numerical average of 78.7‰ ± 8.0‰ (Figure 1). Classified by the air-mass sources, the arithmetic mean of δ¹⁸O-NO₃⁻ was 64.1‰ ± 1.1‰ for the open-sea cluster, 74.9‰ ± 3.7‰ for the marginal-sea cluster, and 81.4‰ ± 6.9‰ for the continental cluster at the Qianliyan Island (Figure 2). Different-orientation δ¹⁸O-NO₃⁻ showed significant difference (Kruskal–Wallis test, H = 7.6, p < 0.05). Classified by seasons, the numerical average of δ¹⁸O-NO₃⁻ was 78.6‰ ± 4.7‰ in spring, 70.5‰ ± 5.4 ‰ in summer, 80.3‰ ± 6.1 ‰ in autumn, and 84.4‰ ± 8.8 ‰ in winter at the Qianliyan Island (Figure S3). Seasonal δ¹⁸O-NO₃⁻ showed significant difference (Kruskal–Wallis test, H = 17.6, p < 0.05).

Nitrate formation path

Inferred from a three-endmember model, the results showed that •OH oxidation and N₂O₅ hydrolysis paths were two main formation pathways for aerosol nitrate formation (Figure 3). The average contribution of the •OH oxidation path was high in summer (74.9 ± 4.7%) and low in winter (41.2 ± 7.0%); N₂O₅ hydrolysis path was low in summer (12.6 ± 5.9%) and high in winter (53.8 ± 7.6%), indicating a transition of the leading pathway of nitrate formation from the •OH oxidation in summer to the O₃ oxidation in winter because N₂O₅ was produced by O₃, which reacts with NO₂.

FIGURE 3

Figure 3 Contribution ratios of •OH oxidation, N₂O₅ hydrolysis and hydrocarbon paths for aerosol nitrate in different seasons at the Qianliyan Island.

Inorganic nitrogen source apportionment

Annually, the dominant sources of inorganic nitrogen were coal combustion (29% ± 7%), marine (19% ± 15%), and fertilizer (16% ± 13%) (Figure 4). The highest proportion of coal burning in winter, the highest portion of marine source in summer, and the lowest proportion of fertilizer in winter were the three main characteristics of the seasonal sources of inorganic nitrogen isotopes. The primary source of Qianliyan inorganic nitrogen aerosols was coal combustion in winter (55% ± 6%), which acted in concert with the heating period. On the contrary, fertilizer accounted for the smallest proportion in winter (9% ± 8%), which acted in concert with the agricultural activity inactive period. Marine contribution was slightly raised to 25% ± 20% in summer but diminished to 10% ± 7% in winter, which is consistent with the dominant wind direction controlled by the East Asian monsoon.

FIGURE 4

Figure 4 Source apportion via bi-nitrogen isotopes of aerosol nitrate and ammonium at the Qianliyan Island.

Discussions

Interpretations for seasonal variations of aerosol inorganic nitrogen isotopes

Seasonal δ¹⁵N-NO₃⁻ variation

The seasonal characteristic of Qianliyan aerosol nitrate δ¹⁵N-NO₃⁻ is that its value was significantly higher in winter and lower in summer, which was common in aerosols over Northwest Pacific marginal seas near Qianliyan (Figure 5) and the other seas (Savarino et al., 2013; Li et al., 2021). Additionally, it was also observed that the patterns of δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ are higher in winter and lower in summer in aerosols over land (Zhao et al., 2020; Zhu et al., 2021; Dong et al., 2022; Luo et al., 2022). For example, average δ¹⁵N-NO₃⁻ values of aerosols in the Beijing–Tianjin–Hebei region with serious air pollution were 10.8‰ ± 3.6‰ in winter and −0.4‰ ± 2.8‰ in summer (Dong et al., 2022). These observational results indicate that there might exist unified reasons for the nitrate nitrogen isotope seasonality difference, regardless of the values of NO₃⁻ concentration and δ¹⁵N-NO₃⁻ in different sites and the difference values of δ¹⁵N-NO₃⁻ in summer and winter being different.

FIGURE 5

Figure 5 The seasonal and annual average ( ± SD) of nitrate concentrations (μg·m⁻³) and nitrogen and oxygen isotopic compositions (‰) of nitrate over the marginal seas of China and the East/Japan sea. Data source: the Bohai Sea (Zong et al., 2022), Beihuangcheng (Zong et al., 2017), Jeju (Kundu et al., 2010), Akita (Kawashima, 2019), Uljin (Kim et al., 2019), and Yongxing (Xiao et al., 2015).

At the Qianliyan Island, the difference of average nitrate δ¹⁵N-NO₃⁻ in aerosols between summer and winter was 11.4‰. Nitrate δ¹⁵N-NO₃⁻ in Qianliyan aerosol exhibited negative correlations with temperature, RH, and sunshine hour (Table S2), showing a significant sensitivity to temperature, RH, and sunshine hour. Based on current cognition, aerosol nitrate δ¹⁵N-NO₃⁻ are affected by source δ¹⁵N-NO_x (Elliott et al., 2007; Elliott et al., 2019) and ¹⁵N isotope fractionation during the conversion of NO_x oxidation to nitrate, of which ¹⁵N isotope fractionation coefficients during nitrate transformation were related to temperature and formation pathway (Freyer, 1991; Morin et al., 2009; Walters and Michalski, 2015; Walters and Michalski, 2016; Li et al., 2019 and references therein). Because both NO_x sources (e.g., fertilizer related to agricultural activities and coal combustion related to winter heating) and ¹⁵N isotope fractionations during the conversion of NO_x oxidation to nitrate had seasonality, it needs further distinguishment of the effects of temperature, nitrate formation path, and NO_x source.

The nitrogen isotope fractionation of nitrate caused by the temperature difference of Qianliyan between summer and winter was 3.9‰ at large based on the isotope fractionation theory (Walters and Michalski, 2015), which is far less than the difference of average δ¹⁵N-NO₃⁻ of Qianliyan aerosols between summer and winter. The nitrogen isotope fractionation of nitrate caused by the nitrate formation path variation of Qianliyan between summer and winter was 12.0‰ at large using the average portions of three nitrate formation paths that inferred from a three-endmember model with the fixed δ¹⁵N-NO_x and temperature. However, it was reported that the seasonality of δ¹⁵N-NO₃⁻ was largely explained by source change rather than isotopic fractionation driven by NO_x photochemical equilibrium at Shenyang, a Chinese megacity (Li et al., 2019).

The δ¹⁵N-NO₃⁻ difference caused by the NO_x sources of Qianliyan between summer and winter was 11.1‰ using the average source δ¹⁵N-NO₃⁻ and the average portions of each source via simmr. Among all the sources, the proportions of coal and marine varied greatly in summer and winter. Compared with summer, the proportion of coal increased by 45%, and that of sea source decreased by 15% in winter, which multiplied with their source δ¹⁵N-NO₃⁻ were 8.7‰ and 0.3‰, respectively. According to the Winter Clean Heating Plan in Northern China (2017–2021) (https://www.gov.cn/xinwen/2017-12/20/5248855/files/7ed7d7cda8984ae39a4e9620a4660c7f.pdf), the annual consumption of coal for heating during winter in the Northern China was approximately equivalent to 4 × 10⁸ t of standard coal, and according to the China Energy Stastical Yearbook (2019) (www.stats.gov.cn/tjsj/ndsj/2019/indexch.htm), the national coal consumption was 27.38 × 10⁸ t of standard coal in 2018. Thus, in winter, coal combustion was the primary source of the North China nitrate aerosols (59% ± 14%; Fan et al., 2020) and the primary source of the Bohai Sea nitrate aerosols (57% ± 11%; Zong et al., 2022). This meant that coal burning preferred to have the greatest impact on δ¹⁵N-NO₃⁻ difference between summer and winter.

At other sites over the marginal seas of the Northwest Pacific, the minimum of the difference of average nitrate δ¹⁵N-NO₃⁻ in aerosols between summer and winter was 0.9‰ at the South China Sea (Yongxing), and the maximum was 13.3 ‰ at the junction of the Yellow Sea and the Bohai Sea (Figure 5). The observed average temperature in East Asia was 18.9°C in summer and −7.5°C in winter (Jiang et al., 2005), which the temperature-dependent isotope fractionation seemed enough for small δ¹⁵N-NO₃⁻ difference between winter and summer in the South China Sea aerosol though it was also necessary to exclude the impact of changes in sources and formation pathways. If the difference of average nitrate δ¹⁵N-NO₃⁻ in aerosols between summer and winter exceeded 3.1‰, the changes in sources and formation pathways existed. However, in view of the information that we have at this moment, it is still difficult to identify the influences of the source and formation path.

Seasonal δ¹⁸O-NO₃⁻ variation

The seasonal characteristic of Qianliyan aerosol nitrate δ¹⁸O-NO₃⁻ is that its value was significantly higher in winter and lower in summer, which was common in aerosols over Northwest Pacific marginal seas near Qianliyan (Figure 5) and the other seas (Savarino et al., 2013; Li et al., 2021). Additionally, it was also observed that the patterns of δ¹⁸O-NO₃⁻ are higher in winter and lower in summer in aerosols over land (Zhao et al., 2020; Zhu et al., 2021; Dong et al., 2022; Luo et al., 2022). For example, average δ¹⁸O-NO₃⁻ of aerosols in an industrial base of Northeast China, Chang Chun, was 70.5‰ ± 10.0‰ in winter and 58.7‰ ± 4.5‰ in summer (Zhao et al., 2020). These observational results indicate that there might exist consistent reasons for the nitrate oxygen isotope seasonality difference, regardless of the values of NO₃⁻ concentration and δ¹⁸O-NO₃⁻ in different sites and the difference values of δ¹⁸O-NO₃⁻ in summer and winter being different.

At the Qianliyan Island, the difference of average nitrate δ¹⁸O-NO₃⁻ in aerosols between summer and winter was 13.9‰. Nitrate δ¹⁸O-NO₃⁻ in Qianliyan aerosol exhibited negative correlations with temperature, RH, and sunshine hour (Table S2), showing a significant sensitivity to temperature, RH, and sunshine hour. Based on current cognition, aerosol nitrate δ¹⁸O-NO₃⁻ are affected by source δ¹⁸O-NO_x, δ¹⁸O-H₂O, and ¹⁸O isotope fractionation during the conversion of NO_x oxidation to nitrate, of which ¹⁸O isotope fractionation coefficients during nitrate transformation were related to temperature and formation pathway (Walters and Michalski, 2016 and references therein). Because both NO_x sources, δ¹⁸O-H₂O and ¹⁸O isotope fractionations during the conversion of NO_x oxidation to nitrate, had seasonality, it needs further distinguishment.

The oxygen isotope fractionation of nitrate caused by the temperature difference of Qianliyan between summer and winter was 2.5‰ at large based on the isotope fractionation theory (Walters and Michalski, 2016), which is far less than the difference of average δ¹⁸O-NO₃⁻ of Qianliyan aerosols between summer and winter. The oxygen isotope fractionation of nitrate caused by the nitrate formation path variation of Qianliyan between summer and winter was 17.1‰ at large using the average portions of three nitrate formation paths that inferred from a three-endmember model with the fixed δ¹⁸O-NO_x, δ¹⁸O-H₂O, and temperature. Generally, seasonal effect on δ¹⁸O-H₂O is that δ¹⁸O-H₂O is positively correlated with the temperature, which varies from about 0‰ per °C at the equator and about 0.5‰ per °C at the high latitudes (http://www-naweb.iaea.org/napc/ih/documents/global_cycle/vol%20II/cht_ii_04.pdf). At the Qianliyan Island, a midlatitude region, Δδ¹⁸O-H₂O predicts to be 0.25‰ per °C, and the calculated δ¹⁸O-NO₃⁻ difference that was caused by δ¹⁸O-H₂O between summer and winter was 2.9‰ at large, because at most one third of the oxygen atoms in nitrate come from the oxygen atoms in water. As for source δ¹⁸O-NO_x values, few studies have measured the nitrogen isotope of NO_x and the oxygen isotope at the same time (Felix and Elliott, 2014; Walters et al., 2018), because NO_x is emitted to the atmosphere primarily as NO, and it will be oxidized to NO₂ by oxidants, of which O₃ oxidation accounts for 84%–85% of global total oxidation paths and HO₂/RO₂ oxidation accounts for 14%–15% (Alexander et al., 2020). Reported δ¹⁸O-O₃ (95‰–130‰) is much larger than δ¹⁸O-RO₂ (~23‰) (Lee, 2019 and references therein). Thus, the enrichment of the O-atoms in terminal positions of O₃ is believed to be transferred to various NO_x species and to nitrate during oxidation by O₃, which means that the characteristics of source δ¹⁸O-NO_x will be replaced (Machalski et al., 2014). All in all, the formation path of nitrate has the greatest influence on aerosol δ¹⁸O-NO₃⁻ of Qianliyan.

At other sites over the marginal seas of the Northwest Pacific, the difference of average nitrate δ¹⁸O-NO₃⁻ in aerosols between summer and winter exceeded that of Qianliyan (Figure 5). The oxygen isotope fractionation of nitrate caused by the temperature difference of East Asia between summer and winter was 2.0‰ at large. Meanwhile, the influence of the δ¹⁸O-H₂O variation caused by temperature on δ¹⁸O-NO₃⁻ difference is 0–4.4 ‰. These two factors were too small to explain the δ¹⁸O-NO₃⁻ difference between summer and winter of 18.0‰–26.5‰ at fixed stations. Because source δ¹⁸O-NO₃⁻ will be eliminated by oxidants, it is believed that the formation path of nitrate has the greatest influence on aerosol δ¹⁸O-NO₃⁻. The seasonal distribution of •OH concentration is higher in summer and lower in winter (Bahm and Khalil, 2004; Zong et al., 2017). On the contrary, O₃ concentration is high in summer and low in winter (Chen et al., 2022); The average contribution of the •OH pathway to nitrate aerosol in summer is expected to decrease in winter. For example, Zong et al. (2022) reported that the average contribution of the •OH pathway to nitrate aerosol was 61.8% ± 18.3% in summer and was 36.3% ± 20.6% in winter over the Bohai Sea. Zhao et al. (2020) reported that the estimated average contribution of the •OH pathway for fine nitrate aerosols in northeast China accounted for a relatively high proportion in all seasons, with the annual average contribution of 61.0% ± 18.8%, especially 79.4% ± 6.1% in summer. Xiao et al. (2020) reported that the estimated average contribution of •OH oxidation, N₂O₅ hydrolysis, and hydrocarbon paths for fine nitrate aerosols in autumn and early winter in southeast China were 37.1% ± 33.4%, 60.3% ± 32.2%, and 2.6% ± 2.7%, respectively. In winter, the equator has the longest sunshine duration relative to the Northern Hemisphere, thus aerosol nitrate production in the equatorial Pacific atmosphere is dominated by •OH oxidation (60%) in winter (Carter et al., 2021). However, Kamezaki et al. (2019) reported that the estimated average contribution of •OH oxidation, N₂O₅ hydrolysis, and hydrocarbon paths for fine nitrate aerosols over Pacific Ocean (40°S–65°N) were 76% ± 12%, 5% ± 6%, and 18% ± 7%, respectively, without obvious difference in summer and winter. They explained that •OH oxidation paths dominating in winter was because of the warm-period sampling. If the difference of average nitrate δ¹⁸O-NO₃⁻ in aerosols between summer and winter exceeded 4.4%, the formation path of nitrate accounts over source and temperature.

Seasonal δ¹⁵N-NH₄⁺ variation

Unlike δ¹⁵N-NO₃⁻, Qianliyan aerosol ammonium δ¹⁵N-NH₄⁺ had no obvious significant correlation with meteorological parameters (Table S2). Although the seasonal and different-orientation δ¹⁵N-NH₄⁺ of Qianliyan aerosols were not significantly different, average δ¹⁵N-NH₄⁺ of Qianliyan aerosols was slightly higher in summer and lower in winter like other coastal sites, Baengnyeong, Jeju, and Akita (Figure 6). Additionally, it was also observed that the patterns of δ¹⁵N-NH₄⁺ are higher in summer and lower in winter in aerosols over land (Park et al., 2018; Wu et al., 2019b; Dong et al., 2022). The average δ¹⁵N-NH₄⁺ difference in aerosols between summer and winter was low to 0.3‰ in Seoul, Korea (Park et al., 2018), and up to 36.6‰ in Tianjin, China (Dong et al., 2022). These observational results indicate that there might exist accordant reasons for the ammonium isotope seasonality difference, regardless of the values of NH₄⁺ concentration and δ¹⁵N-NH₄⁺ in different sites and the differences of δ¹⁵N-NH₄⁺ in summer and winter being different.

FIGURE 6

Figure 6 The seasonal and annual average (± SD) ammonium concentrations (μg·m⁻³) and nitrogen isotopic compositions (‰) of ammonium over the Yellow sea and the East/Japan sea. Data source: Baengnyeong (Park et al., 2018), Jeju (Kundu et al., 2010), and Akita (Kawashima, 2019).

Because N is conserved during the nonoxidation reaction of gaseous NH₃ to particulate NH₄⁺ in the atmosphere, to a large extent, δ¹⁵N-NH₄⁺ reflects the sources of NH₃ (Elliott et al., 2019). For example, summer heavier δ¹⁵N-NH₄⁺ in Akita aerosol was explained by animal wastes (Kawashima and Kurahashi, 2011); summer heavier δ¹⁵N-NH₄⁺ in Jeju aerosols was partly explained by biomass burning with enriched ¹⁵N from China-originated air-masses (Kundu et al., 2010). In addition, aerosol ammonium δ¹⁵N-NH₄⁺ is affected by ¹⁵N isotope fractionation from NH₃ to NH₄⁺. The negative kinetic isotope fraction (−28‰; Pan et al., 2016) occurs when gaseous NH₃ unidirectionally reacts with gaseous H₂SO₄, whereas the positive equilibrium isotope fraction occurs when gaseous NH₃ reversibly reacts with gaseous HCl or HNO₃ or H₂O under rich ammonia environment (NH₃/H₂SO₄ molar ratio > 2) between atmospheric NH₄⁺ and gaseous NH₃ (Walters et al., 2019 and references therein). Since sufficient observations showed that δ¹⁵N values were sorted as δ¹⁵N-NH₄⁺ (solid, s), δ¹⁵N-NH₄⁺ (liquid, l), δ¹⁵N-NH₃ (gas, g) from big to small (Savard et al., 2017; Ti et al., 2018; Zheng et al., 2018; Kawashima, 2019), the negative kinetic isotope effect on aerosol NH₄⁺ must be washed out as equilibrium conditions are established between gaseous NH₃ and aerosol NH₄⁺. The equilibrium fractionation factors can be theoretically estimated as ${ }^{{15}_{{ϵ}_{{\text{NH}}_{4(aq)}^{+}/{\text{NH}}_{\text{s}(\text{g})}}}}=14058/\text{T}(\text{K})-12.20$ and ${ }^{{15}_{{ϵ}_{N{H}_{4(s)}^{+}/N{H}_{s(g)}}}}=12522/T(K)-11.31$ (Walters et al., 2019), and theoretical calculations are confirmed as calculated values at 25°C(${ }^{{15}_{{ϵ}_{N{H}_{4(aq)}^{+}/N{H}_{s(g)}}}}:35‰$; ${ }^{{15}_{{ϵ}_{N{H}_{4(s)}^{+}/N{H}_{s(g)}}}}:30‰$) are largely identical to the chamber experiment result ${ }^{{15}_{{ϵ}_{N{H}_{4(aerosol)}^{+}/N{H}_{s(g)}}}}:33‰$, Heaton et al., 1997). The sources of NH₃ and ¹⁵N isotope fractionation during the conversion of NH₃ oxidation to ammonium had seasonality, and both factors may lead to aerosol δ¹⁵N-NH₄⁺ difference between summer and winter.

At the Qianliyan Island, the difference of average ammonium δ¹⁵N-NH₄⁺ in Qianliyan aerosol between summer and winter was 4.3‰. Temperature and the NH₃ concentration level affect the chemical processes of NH₃ to NH₄⁺. However, the correlations between ammonium δ¹⁵N-NH₄⁺ in Qianliyan aerosols and ambient temperature, RH, and sunshine hour were not significant (Table S2), which implied that the reason for the trend cannot be explained by temperature alone. For ¹⁵N isotope fractionation from NH₃ to NH₄⁺, the effect of RH can be negligible (Kawashima and Ono, 2019), and the influence of temperature was opposite to the seasonal trend of δ¹⁵N-NH₄⁺ of Qianliyan aerosols. Hence, it can be inferred that the seasonal variation of NH₃ sources had the greatest impact on the seasonal variation of ammonium δ¹⁵N-NH₄⁺ in Qianliyan aerosols.

The seasonal impress of δ¹⁵N-NH₃ source had been recognized with a laboratory study pointing out that δ¹⁵N-NH₃ from agricultural activities might increase with the increase of temperature and ammonia loss (Schulz et al., 2001). However, it was difficult to be directly proved neither by NH₃ source apportion after the adjustment of field ¹⁵N fractionation coefficients of the chemical transformation from gas NH₃ to aerosol NH₄⁺ due to the lack of NH₃ concentrations and δ¹⁵N-NH₃ values nor the emission and deposition list of NH₃ sources in this study. If ¹⁵N isotope fractionation between gaseous NH₃ and aerosol NH₄⁺ and the fraction of NH₃ that converted to NH₄⁺ are known, the δ¹⁵N-NH₄⁺ for a certain source can be determined (Pan et al., 2016). Or observe the δ¹⁵N-NH₄⁺ for a certain source directly. At the Qianliyan Island, δ¹⁵N-NH₄⁺ of open-sea aerosols (−11.4‰ and −7.8‰) was apparently close to marine-source δ¹⁵N-NH₄⁺ (−10.2‰ to −2.2‰; David Felix et al., 2013). Thus, observed δ¹⁵N-NH₄⁺ values of various sources (Table S1) were used for source apportion in the absence of gas NH₃ concentrations and δ¹⁵N-NH₃ values. The δ¹⁵N-NH₄⁺ difference caused by the sources of Qianliyan between summer and winter was 5.4‰ using the average source δ¹⁵N-NH₄⁺ and the average portions of each source via simmr. The largest seasonal source change of Qianliyan was reflected in the increased proportion of coal combustion in winter than that in summer. The enhanced fossil fuel source of ammonium aerosols of the South China (Wu et al., 2019b) and the dominant source of fossil fuel in winter ammonium aerosols in the North China (Pan et al., 2016) were found in the urban atmosphere. As one of the most abundant and affordable fossil fuels worldwide (64% in China in 2018, http://www.stats.gov.cn/tjsj/ndsj/2019/indexch.htm), it was reasonable to consider coal as the main source of ammonium in Qianliyan aerosols under the control of the East Asian winter monsoon. In addition, there were seasonal differences in fertilizer application because Chinese agricultural activities significantly decreased in winter due to the farming system, which was mainly dominated by one crop a year in the north (https://www.caas.cn/en/agriculture/agriculture_in_china/), and seasonal differences in marine contribution as the reversion of the dominant wind direction controlled by the East Asian monsoon. However, these two sources led to δ¹⁵N-NH₄⁺ difference of less than 1‰ using the average source δ¹⁵N-NH₄⁺ and the average portions of each source via simmr.

At other sites over the marginal seas of the Northwest Pacific, the difference of average ammonium δ¹⁵N-NH₄⁺ in aerosols between summer and winter exceeded that of Qianliyan (Figure 6), which the greater difference deduced the more pronounced seasonal change of NH₃/NH₄⁺ sources. More research on seasonal source changes is needed to support this result. In addition, as a missing piece of all types of the isotope fraction, the transport isotope fraction of nitrogen species, which assumed it default to none in this study, has not been well studied so far and needs to be studied further.

Impacts of coastal aerosol inorganic nitrogen to the marginal sea

The concentration of nutrients in TSPs in the atmosphere decreased exponentially with the increase of the distance from the nearest land (Seok et al., 2021). Therefore, priority was give to the impact of coastal aerosol DIN on the Yellow Sea in this study.

Contribution to primary production

Aerosol DIN deposition flux is estimated by F_Dry=C_Dry×v_d , of which C_Dry  is the nutrient concentration and v_d is the nutrient deposition rate. The deposition rate of NO₃⁻ and NH₄⁺ are 1.15 and 0.6 cm·s⁻¹, respectively (Zhang et al., 2011 and references therein). At the Qianliyan Island, the deposition fluxes of aerosol nitrate and ammonium were 2.0 and 1.4 nmol N·m⁻²·s⁻¹, respectively. The average dry deposition fluxes of nitrate and ammonium in atmospheric aerosols over the marginal seas and Northwest Pacific were 0.5 ± 0.3 and 0.7 ± 0.3 nmol N·m⁻²·s⁻¹, respectively (Qi et al., 2020). Since the high fluxes of nitrate and ammonium were located in the Bohai Sea, the Yellow Sea, and the East China Sea, which were three to six times higher than those in the Northwest Pacific Ocean, our estimation of the deposition fluxes of nitrate and ammonium aerosol in the Yellow Sea were comparable to their results though a relatively low v_d for nitrate was used in the study of Qi et al. (2020).

The primary production was 0.3–0.8 mmol C·m⁻³·h⁻¹ on the Yellow Sea surface (Zhang et al., 2012). Also, the new primary production brought by aerosol DIN was overestimated to be 3.3 μmol C·m⁻³·h⁻¹ based on the Redfield ratio (C:N = 106:16). Thus, aerosol DIN deposition contributes less than 1% to primary production.

Impact on new nitrogen inputs to thermocline

The nutrient discharges from the atmosphere to the Yellow Sea were small compared with those from rivers, and aerosol DIN deposition flux was small compared with precipitation deposition flux (Liu et al., 2003). Since aerosol DIN dissolves quickly in seawater before deposits downward, aerosol DIN deposition rate was 0.7 μmol N·m⁻³·h⁻¹ above the thermocline (~20 m, Wu et al., 2019a), which was an order of magnitude higher than the reported average N₂ fixation rate of 72 nmol N·m⁻³·h⁻¹ on the Yellow Sea surface (Zhang et al., 2012).

The stratification of the Yellow Sea is obvious in summer, whereas the water mass is vertically uniform in winter. Based on summer observation on Yellow Sea, the mass weighted average δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ was −3.2‰ and 0.9‰, respectively. In total, the mass weighted average δ¹⁵N-DIN was −0.7‰, which is lower to that by fixation N₂ (0.6‰; Yan et al., 2019) and is even lower than the average δ¹⁵N-NO₃⁻ of the Yellow Sea water (7.0‰; Wu et al., 2019a). Thus, atmospheric low δ¹⁵N-DIN may regulate ¹⁵N on new nitrogen inputs to thermocline, resulting decreasing δ¹⁵N-NO₃⁻ under thermocline.

To quantitatively illustrate the relative contribution from atmospheric Nr and N₂ fixation to thermocline, we follow a similar approach adopted in Bermuda and Dongsha (Knapp et al., 2010; Yang et al., 2014) to estimate the fraction (f) of lowering nitrate δ¹⁵N of the Yellow Sea thermocline induced by atmospheric DIN deposition. The f is written as f = (Δδ¹⁵N₁×F₁)/(Δδ¹⁵N₂×F₂+Δδ¹⁵N₁×F₁)), of which Δδ¹⁵N₁ is the difference of thermocline δ¹⁵N-NO₃⁻ (7‰) and aerosol δ¹⁵N-DIN (−0.7‰), F₁ is aerosol DIN deposition flux (0.7 μmol N·m⁻³·h⁻¹), Δδ¹⁵N₂ is the difference of thermocline δ¹⁵N-NO₃⁻ (7‰) and δ¹⁵N newly fixed N₂ (−1‰, Karl et al., 1997), and F₂ is flux of fixed N₂ of thermocline (132 nmol N·m⁻³·h⁻¹). This yields a value for f up to 83%, which is higher than those values at Bermuda and Dongsha (Knapp et al., 2010; Yang et al., 2014). Also, the influence of atmospheric active nitrogen deposition to lower δ¹⁵N decreases with the increase of thermocline depth; the remainder of the ¹⁵N depletion may result from the remineralization of N₂ fixation inputs. Note that a better perspective on the impact of the atmosphere on the nitrogen cycle in the Yellow Sea should consider atmospheric DON flux and its isotopic composition.

Conclusions

One-year collection of TSP aerosols was conducted on the Qianliyan Island in 2018 for the measurements of inorganic nitrogen species (NO₃⁻ and NH₄⁺) and their isotopic ratios (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃^−,and δ¹⁵N-NH₄⁺). At the Qianliyan Island, the average NO₃⁻ and NH₄⁺ concentrations were 2.49 ± 2.12 and 3.33 ± 2.68 μg·m⁻³; the average δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃^−,and δ¹⁵N-NH₄⁺ were 2.4‰ ± 5.7‰, 78.7‰ ± 8.0‰, and −2.6‰ ± 6.3‰, respectively.

Using the Bayesian mixing model, nitrate formation pathways and inorganic nitrogen source apportionment were obtained. At the Qianliyan Island, the major nitrate formation pathways in aerosols were •OH oxidation and N₂O₅ hydrolysis paths with a transition of the main pathway of nitrate formation from the •OH oxidation in summer to the O₃ oxidation in winter. Source apportion results make clear that terrigenous source and marine source accounted for about 80% and 20%, respectively, in Qianliyan inorganic nitrogen aerosols, of which the most dominant source was coal combustion (29% ± 7%). Additionally, three seasonal source characteristics of inorganic nitrogen isotopes were captured, which were the highest proportion of coal burning in winter, the highest portion of marine source in summer, and the lowest proportion of fertilizer in winter.

At the Qianliyan Island, aerosol δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ were obviously higher in winter and lower in summer; on the contrary, aerosol δ¹⁵N-NH₄⁺ was slightly higher in summer and slightly lower in winter. Based on current cognition, aerosol nitrate δ¹⁵N-NO₃⁻ are coaffected by source δ¹⁵N-NO_x and ¹⁵N isotope fractionation during the conversion of NO_x oxidation to nitrate, including temperature and formation pathway. The seasonal difference in nitrogen sources was expected to be the best explanation for the difference of Qianliyan aerosol nitrate δ¹⁵N-NO₃⁻ between summer and winter, because the influence of temperature was too small and nitrogen was conservative during oxidation. Based on current cognition, aerosol nitrate δ¹⁸O-NO₃⁻ are coaffected by sources, including δ¹⁸O-NO_x and δ¹⁸O-H₂O, and ¹⁵N isotope fractionation during the conversion of NO_x oxidation to nitrate, including temperature and formation pathway. The seasonal difference in nitrate formation paths was considered to be the best explanation for the difference of Qianliyan aerosol nitrate δ¹⁸O-NO₃⁻ between summer and winter, because the influence of temperature and δ¹⁸O-H₂O was too small, and δ¹⁸O-NO_x was modified during oxidation. Based on current cognition, aerosol ammonium δ¹⁵N-NH₄⁺ is coaffected by source NH₃ and ¹⁵N isotope fractionation during the conversion of NH₃ oxidation to ammonium. The seasonal difference in nitrogen sources was expected to be the best explanation for the difference of Qianliyan aerosol ammonium δ¹⁵N-NH₄⁺ between summer and winter, because the correlation of temperature and δ¹⁵N-NH₄⁺ was insignificant, the seasonal trend of the equilibrium fractionation coefficients between NH₃ and NH₄⁺ was opposite to the seasonal trend of δ¹⁵N-NH₄⁺ of Qianliyan aerosols, and N was conserved during the non-oxidation reaction. Coal combustion contributed the most to the difference of Qianliyan aerosol nitrate δ¹⁵N-NO₃⁻ and ammonium δ¹⁵N-NH₄⁺ between summer and winter, which were due to the maximum change of its proportion in summer and winter and its ¹⁵N isotope values.

The DIN deposition flux of Qianliyan aerosol was estimated to be 3.4 nmol N·m⁻²·s⁻¹, which induced less than 1% to primary production. As aerosol inorganic nitrogen and N₂ fixation were considered as two sources of nitrate in the Yellow Sea thermocline in summer, aerosol inorganic nitrogen contributed some 80% of δ¹⁵N-NO₃⁻ depression of the summer Yellow Sea thermocline.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

KZ: assisting sample collections, laboratory analysis, data analysis, and running software. SL: funding, conceptualization, and data analysis. NW: culturing denitrifying bacteria and data analysis. WX: running isotope ratio mass spectrometry and data analysis. All authors contributed to the article and approved the submitted version.

Funding

This work was supported by the Natural Sciences Foundation of China (U1806211, 42176040) and the Taishan Scholars Program of Shandong Province.

Acknowledgments

The authors are very grateful to the staff of the North Sea Branch of the State Oceanic Administration for their assistance in collecting aerosols. The authors once again thank the funds for affording the observations and experiments. This study is a contribution to the IMBeR/FEC-CMWG Program.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2022.993160/full#supplementary-material

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