The use of stable oxygen and nitrogen isotopic signatures to reveal variations in the nitrate formation pathways and sources in different seasons and regions in China.

Nitrate (NO3-) is one of the most important inorganic ions in fine particulate (PM2.5) and drives regional haze formation; however, the NO3- sources and formation mechanisms in different seasons and regions are still debated. Here, PM2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO3- (δ18O-NO3- and δ15N-NO3-), estimated the δ18O-HNO3 values produced by different oxidation pathways, and quantified the NO3- formation pathways based on the isotope mass-balance equation. Our results showed that the δ18O-NO3- values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ18O-HNO3 values arising from the OH radical pathway (POH, 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ18O-NO3- values are mainly influenced by POH, which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO3 produced by POH and reduced the amount of HNO3 produced by PN2O5, which produced low δ18O-NO3- values. Increased air pollution emissions decreased the contribution from POH and increased the contribution from N2O5 and NO3 pathways (PN2O5+NO3). The δ15N-NO3- values of PM2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NOx emissions with positive isotopic values led to high δ15N-NO3- values in northern China and during cold months. A higher fNO2 (fNO2 = NO2/(NO + NO2), temperature, and contribution of POH produced lower N isotope fractionation between NOx and δ15N-NO3-, which was found to further decrease the δ15N-NO3- values in southwestern China and during warm months.

Isotopic source analysis of nitrogen-containing aerosol: A study of PM2.5 in Guiyang (SW, China).

The source of fine particulate matter (PM2.5) has been a longstanding subject of debate, the nitrogen-15 isotope (δ15N) has been used to identify the major sources of atmospheric nitrogen. In this study, PM2.5 samples (n = 361) were collected from September 2017 to August 2018 in the urban area of Guiyang (SW, China), to investigate the chemical composition and potential sources of PM2.5. The results showed an average PM2.5 of 33.0 µg m-3 ± 20.0 µg m-3. The concentration of PM2.5 was higher in Winter, lower in Summer. The major water resolved inorganic ions (WSIIs) were Ca2+, NH4+, Na+, SO42-, NO3-, Cl-. Nitrogen-containing aerosols (i.e., NO3- and NH4+) suddenly strengthened during the winter, when NO3- became the dominant contributor. Over the sampling period, the molar ratio of NH4+/(NO3- + 2 × SO42-) ranged from 0.1 to 0.9, thus indicating the full fixation of NH4+ by existing NO3- and SO42- in PM2.5. The annual value of NOR was 0.1 while rised to 0.5 in Winter. The variations of NOR (Nitrogen oxidation ratio) (0.1-0.5) values suggest that the secondary formation of NO3- occurred every season and was most influential during the winter. The total particulate nitrogen (TN) δ15N value of PM2.5 ranged from -5.9‰ to 25.3‰ over the year with annual mean of +11.8‰ ± 4.7‰, whereas it was between -5.9‰ and 14.3‰ during the winter with mean of 7.0‰ ± 3.8‰. A Bayesian isotope mixing model (Stable Isotope Analysis in R; SIAR) was applied to analyze the nitrogen sources. The modeling results showed that 29%, 21%, and 40% of TN in PM2.5 during the winter in Guiyang was due to nitrogen-emissions from coal combustion, vehicle exhausts, and biomass burning, respectively. Our results demonstrate that biomass burning was the main contributor to PM during the winter, 80% of the air mass comes from rural areas of Guizhou border, this transport process can increase the risk of particulate pollution in Guiyang.

Oxidation and sources of atmospheric NOx during winter in Beijing based on δ18O-δ15N space of particulate nitrate.

The determination of both stable nitrogen (δ15N-NO3-) and stable oxygen (δ18O-NO3-) isotopic signatures of nitrate in PM2.5 has shown potential for an approach of assessing the sources and oxidation pathways of atmospheric NOx (NO+NO2). In the present study, daily PM2.5 samples were collected in the megacity of Beijing, China during the winter of 2017-2018, and this new approach was used to reveal the origin and oxidation pathways of atmospheric NOx. Specifically, the potential of field δ15N-NO3- signatures for determining the NOx oxidation chemistry was explored. Positive correlations between δ18O-NO3- and δ15N-NO3- were observed (with R2 between 0.51 and 0.66, p < 0.01), and the underlying environmental significance was discussed. The results showed that the pathway-specific contributions to NO3- formation were approximately 45.3% from the OH pathway, 46.5% from N2O5 hydrolysis, and 8.2% from the NO3+HC channel based on the δ18O-δ15N space of NO3-. The overall nitrogen isotopic fractionation factor (εN) from NOx to NO3- on a daily scale, under winter conditions, was approximately +16.1‰±1.8‰ (consistent with previous reports). Two independent approaches were used to simulate the daily and monthly ambient NOx mixtures (δ15N-NOx), respectively. Results indicated that the monthly mean values of δ15N-NOx compared well based on the two approaches, with values of -5.5‰ ± 2.6‰, -2.7‰ ± 1.9‰, and -3.2‰ ± 2.2‰ for November, December, and January (2017-2018), respectively. The uncertainty was in the order of 5%, 5‰ and 5.2‰ for the pathway-specific contributions, the εN, and δ15N-NOx, respectively. Results also indicated that vehicular exhaust was the key contributor to the wintertime atmospheric NOx in Beijing (2017-2018). Our advanced isotopic perspective will support the future assessment of the origin and oxidation of urban atmospheric NOx.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate.

Nitrate (NO3-) has become recognized as the most important water-soluble ion in fine particulate (PM2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO3-concentration, δ18O-NO3- and δ15N-NO3- values in PM2.5 were analyzed. NO3- concentrations increased as PM2.5 levels increased during both polluted and non-polluted days over the entire year. δ18O-NO3- values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO2 + OH (POH) in warm months to the N2O5 + H2O (PN2O5) and NO3 + VOCs (PNO3) pathways in cold months. Especially during cold months, δ18O-NO3- values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM2.5 increased from ∼25 to >100 µg/m3, but when PM2.5 > 100 µg/m3, there were relatively small variations in δ18O-NO3-. These results suggested that nitrate formation pathways changed from POH to PN2O5 and PNO3 pathways when PM2.5 < 100 µg/m3, but that PN2O5 and PNO3 dominated nitrate production when PM2.5 > 100 µg/m3. Higher δ15N-NO3- values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NOx emission sources and nitrogen isotopic fractionation among NOx and NO3-. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM2.5 levels.

Methylmercury biomagnification in aquatic food webs of Poyang Lake, China: Insights from amino acid signatures.

As the dominant mercury species in fish, methylmercury (MeHg) biomagnifies during its trophic transfer through aquatic food webs. MeHg is known to bind to cysteine, forming the complex of MeHg-cysteine. However, relationship between MeHg and cysteine in large-scale food webs has not been explored and contrasted with MeHg biomagnification models. Here, we quantified the compound-specific nitrogen isotopic analysis of amino acids (CSIA-AA), MeHg, and amino acid composition in aquatic organisms of Poyang Lake, the largest freshwater lake in China. The trophic positions (TPAA) of organisms ranged from 1.0 ± 0.1-3.7 ± 0.2 based on CSIA-AA approach. The trophic magnification factor (TMF) of MeHg, derived from the regression slope of Log-transformed MeHg in organisms upon their TPAA for the entire food web was 9.5 ± 0.5. Significantly positive regression between MeHg and cysteine (R2 = 0.64, p < 0.01) was documented, suggesting MeHg-cysteine complex may potentially play a critical role in the bioaccumulation of MeHg. Furthermore, TMFs of MeHg calculated with and without cysteine normalization compared well (7.7-8.7) when excluding primary producers. Our results implied that MeHg may biomagnify as the complex of MeHg-cysteine and contribute to our understanding of MeHg trophic transfer at the molecular level.

Response of fine aerosol nitrate chemistry to Clean Air Action in winter Beijing: Insights from the oxygen isotope signatures.

The implementation of Clean Air Actions improved air quality in Beijing, and the mass loadings of PM2.5 and numerous gaseous precursors decreased considerably. However, concentrations of particulate nitrate (p-NO3) changed slightly in wintertime aerosol and its mass fraction increased from 11.3% in 2013 to 29.8% in 2018. Therefore, understanding the characteristics of nitrate chemistry is imperative for controlling the aerosol pollution in the context of emission reductions in China. To this end, the properties of aerosol, chemistry of nitrate during the past winter seasons of Beijing (2013-2017) were investigated. Results indicated that nitrate remained entirely in the particle phase. The weak response of p-NO3- to NOx emission reduction (~17.9%) was attributed to the enhanced gaseous HNO3 formation. The oxygen isotopic signatures of p-NO3- (δ18O-NO3-) in winter 2017 of Beijing (75.0 ± 12.6‰) was significantly lower than that in winter 2014 (82.6 ± 12.7‰), but comparable with that in Puding (77.3 ± 4.6‰, background site of Southwest China) and Nanning (72.9 ± 4.9‰, urban site in South China) of winter 2017. We inferred an increased photochemistry activity in the formation of winter p-NO3- in Beijing recently. The enhanced photochemistry was mainly due to the increased photolysis of HONO, a major source of atmospheric hydroxyl radical (OH) in polluted urban environment. The considerable generated HONO was attributed to the photolysis of p-NO3- according to δ18O isotopic evidence. The present study implied that the response of aerosol species to emission reductions and their feedbacks associated with the atmospheric oxidants and aerosol properties were complex and needed further investigations.

Rayleigh based concept to track NOx emission sources in urban areas of China.

Despite the implementation of some effective measures to control emissions of nitrogen oxides (NOx = NO + NO2) in recent years, the ambient NOx concentration in urban cities of China remains high. Therefore, a quantitative understanding of NOx emission sources is critical to developing effective mediation policies. In the present study, the dual isotopic compositions of nitrate (p-NO3-) in fine-particle aerosol (PM2.5) collected daily at a regional scale (Beijing-Tianjin-Shijiazhuang) were measured to better constrain the NOx emission sources. The specific focus was on a typical haze episode that occurred simultaneously in the three urban cities (October 22-29, 2017). It was found that the nitrogen isotopic values of nitrate in PM2.5 (hereafter as δ15N-NO3-) ranged widely from -3.1‰ to + 11.4‰, with a mean value of 3.5 ± 3.7‰. Furthermore, a negative relationship between the δ15N-NO3- values and the corresponded p-NO3- concentrations during the haze period was observed. This implied the preferential formation of 15N-enriched NO3- into a fine-particle aerosol. Taking a different approach to previous publications, the Rayleigh fractionation model was used to characterize the initial isotopic signatures of ambient NOx. After accounting for the δ15N difference between p-NO3- and the source NOx, the estimated initial δ15N-NOx ranged from -20‰ to 0‰, which indicated a cosniderable contribution of non-fossil fuel emissions. The individual contributions of potential sources were further quantified using the Bayesian mixing model, revealing that NOx from coal or natural gas combustion, vehicle exhausts, biomass burning, and the microbial activity contributed 17.9 ± 11.4%, 29.4 ± 19.6%, 29.1 ± 18.9% and 23.5 ± 12.7% to NO3- in PM2.5, respectively. These results highlighted that tightening controls of gaseous NOx emissions from non-fossil sources may represent an opportunity to mitigate PM2.5 pollution in urban cities of China.

Fossil fuel-related emissions were the major source of NH3 pollution in urban cities of northern China in the autumn of 2017.

As the most important gas-phase alkaline species, atmospheric ammonia (NH3) contributes considerably to the formation and development of fine-mode particles (PM2.5), which affect air quality and environmental health. Recent satellite-based observations suggest that the North China Plain is the largest agricultural NH3 emission source in China. However, our isotopic approach shows that the surface NH3 in the intraregional urban environment of Beijing-Tianjin-Shijiazhuang is contributed primarily by combustion-related processes (i.e., coal combustion, NH3 slip, and vehicle exhaust). Specifically, the Batch fractionation model was used to describe the partitioning of gaseous NH3 into particles and to trace the near-ground atmospheric NH3 sources. With the development of haze pollution, the dynamics of δ15N-NH4+ were generally consistent with the fractionation model. The simulated initial δ15N-NH3 values ranged from -22.6‰ to -2.1‰, suggesting the dominance of combustion-related sources for urban NH3. These emission sources contributed significantly (92% on hazy days and 67% on clean days) to the total ambient NH3 in urban cities, as indicated by a Bayesian mixing model. Based on the Batch fractionation model, we concluded the following: 1) δ15N-NH4+ can be used to model the evolution of fine-mode aerosols and 2) combustion-related sources dominate the near-ground atmospheric NH3 in urban cities. These findings highlight the need for regulatory controls on gaseous NH3 emissions transported from local and surrounding industrial sources.

Comparison of four methods for spatial interpolation of estimated atmospheric nitrogen deposition in South China.

Spatial interpolation methods have been applied in many environmental research studies. However, it is still a controversial issue to select an appropriate interpolation method for the conversion of discrete sampling sites into continuous maps. This study aimed at selecting an optimal interpolation method to analyze the spatial pattern of atmospheric N deposition in South China. N deposition was calculated by 259 moss sample data. Four spatial interpolation methods, including inverse distance weighting (IDW), radial basis function (RBF), ordinary kriging (OK), and universal kriging (UK), were utilized for modeling the spatial distribution of N deposition. It is the first time that these methods were applied to analyze N deposition in South China. Validation method was used to evaluate the interpolation precision of the various methods, and the cross-validation method was used to evaluate their interpolation accuracy. Comparison of predicted values with measured values indicated that OK was the optimal method for analyzing the spatial distribution of N deposition in this study; it had the highest precision (mean error (ME) = -0.059, root-mean-square error (RMSE) = 5.240, mean relative error (MRE) = 0.129, mean absolute error (MAE) = 4.007) and the lowest uncertainties (standard deviation (SD) = 5.47, coefficient of variation (CV) = 0.15). RBF produced similar results as good as OK, while the worst performed interpolation method was UK. By using the OK method for analyzing N deposition, this work revealed systematic temporal and spatial variations in atmospheric N deposition in South China.

A reliable compound-specific nitrogen isotope analysis of amino acids by GC-C-IRMS following derivatisation into N-pivaloyl-iso-propyl (NPIP)esters for high-resolution food webs estimation.

The signatures of natural stable nitrogen isotopic composition (δ(15)N) of individual amino acid (AA) have been confirmed to be a potentially effective tool for elucidating nitrogen cycling and trophic position of various organisms in food webs. In the present study, a two-stage derivatisation approach of esterification followed by acylation was evaluated. The biological samples underwent acid hydrolysis and the released individual AA was derivatived into corresponding N-pivaloyl-isopropyl (NPIP) esters for nitrogen isotopic analysis in gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Usually, 13 individual AA derivatives were separated with fine baseline resolution based on a nonpolar gas chromatography column (DB-5ms). The minimum sample amount required under the presented conditions is larger than 20ngN on column in order to accurately determine the δ(15)N values. The δ(15)N values determined by GC-C-IRMS with a precision of better than 1‰, were within 1‰ after empirical correction compared to the corresponding measured by element analysis (EA)-IRMS. Bland-Altman plot showed highly consistency of the δ(15)N values determined by the two measurement techniques. Cation-exchange chromatography was applied to remove interfering fraction from the extracts of plant and animal samples and without nitrogen isotope fractionation during the treatment procedure. Moreover, this approach was carried out to estimate the trophic level of various natural organisms in a natural lake environment. Results highly proved that the trophic level estimated via the presented AA method well reflected the actual food web structure in natural environments.

[N% and S% in Leaves of Vascular Plants Cinnamomum camphora and Pinus massoniana Lamb. for Indicating the Spatial Variation of Atmospheric Nitrogen and Sulfur Deposition].

N and S contents in Cinnamomum camphora leaves, Pinus massoniana Lamb. leaves, epilithic Haplocladium microphyllum(Hedw.) Broth. and rhizosphere soil collected along 3 directions from urban area to rural area at Guiyang city in a total of 296 samples were investigated systematically. The level of atmospheric N deposition and atmospheric SO2 concentrations at each sampling site were calculated according to the quantitative relationship between moss N content and atmospheric N deposition and the quantitative relationship between moss S content and atmospheric SO2 concentration. Leaves N content in Cinnamomum camphora(1.01%-2.37%) and Pinus massoniana Lamb.(0.99%-2.42%)showed significant decrease from urban area with the highest atmospheric nitrogen deposition to suburb, while slightly higher leaves N reemerged at rural area beyond 24 km, suggesting increased N deposition occurred in rural area. Leaves S content in Cinnamomum camphora(0.16%-0.43%) and Pinus massoniana Lamb.(0.18%-0.32%) showed significant decrease from urban area to suburb, the highest level at urban was mainly contributed by the high sulfur released from the production and living of urban areas into the atmosphere, and the lowest level occurred at rural area(30-36km). No significant difference was seen for soil N and S contents. The relationships between the estimated input of atmospheric N deposition and the leaves N content of Cinnamomum camphora and Pinus massoniana Lamb. at the sites investigated were found to be significant linear correlations, and the relationships between the estimated atmospheric SO2 concentration and the leaves S content of Cinnamomum camphora and Pinus massoniana Lamb. were also significant linear correlations(P<0.05). The results indicated that N and S contents in Cinnamomum camphora and Pinus massoniana Lamb. leaves can be used to show the spatial variation of atmospheric N and S deposition.

[Sulfur isotopic signatures in leaves of Pinus massoniana Lamb. and source apportionment].

This study analyzed the inorganic sulfur (SSO4) and total sulfur (ST) content as well as the isotopic signatures (delta34SSO4 and 834ST) in leaves of Pinus massoniana lamb. collected from Guizhou and Yunnan areas. The results indicated that the SSO4 and ST content in leaves at Guiyang areas was significantly higher than that at Yunnan areas, and the content of inorganic sulfur in the leaves was found to be directly related to the concentration of ambient sulfur dioxide, but no correlation was seen between the ST content and the ambient sulfur dioxide, showing the SSO4 content in leaves was more reliable to reflect the ambient sulfur input. The average value of delta34SSO4 in leaves at Guiyang areas ( -7. 22%o) was significantly lower than that at Yunnan areas(3. 85 per thousand) , which was related to the fact that the sulfur isotopic composition of coal at Guiyang areas is lower than that at Yunnan areas. The SSO4 and ST content in leaves around Kunming steel and Qujing power plant was inversely proportional to the distance from the factories, while around Kunming steel plant the value of 83SSO4 in leaves became more negative when the distance became larger while around Qujing power plant the value of 834Sso4 became more positive when the distance became larger, indicating that the SSO4 content and delta34SSO4, in leaves around Kunming steel and Qujing power plant were controlled by coal sources of atmospheric sulfur deposition.