Impact of an accidental explosion in Tianjin Port on enhanced atmospheric nitrogen deposition over the Bohai Sea inferred from aerosol nitrate dual isotopes

ABSTRACT In recent years, emergent pollutant’s accidents have occurred frequently in China, causing serious harm to the ecological environment. In this study, the impact of an accidental fire and explosion at Tianjin Port in 2015 on the atmosphere over the Bohai Sea was explored. Results showed sharp increases in the concentrations of several important components of fine particulate matter (e.g. NO3 −, SO4 2−, NH4 +, organic carbon, elemental carbon) over Beihuangcheng Island after the explosion. Among them, NO3 − was most affected (about 10 days), with a maximum concentration of 16.45 μg m−3. The δ15N-NO3 − ranged from −1.58‰ to +8.74‰, with an average of +2.79‰ ± 3.32‰. Influenced by the explosion, δ15N-NO3 − decreased significantly, which was in accordance with the industrial processes of explosives. The δ18O-NO3 − varied between +49.40‰ and +69.52‰, and showed a marked increase (+66.62‰ ± 3.92‰) in the explosion-affected period. Using Monte Carlo simulation, the •OH pathway for NO3 − formation was 51.79% ± 10.94% at that time — much lower than in the regular period. The elevated dry deposition of NO3 − caused by the explosion was 266.08 μmol N m−2 d−1 over the Bohai Sea — again, much higher than in the regular period. With the dry nitrogen deposition of NH4 + (42.41 μmol N m−2 d−1), the total nitrogen deposition increased by 308.49 μmol N m−2 d−1, leading to severe ecological risk. Through the inverse computation of the dry deposition flux of NO3 −, the affected area over the Bohai Sea was less than 1.42 × 104 km2, which is about 20% of the total area. Graphical Abstract


Introduction
Environmental pollution events mainly include air pollution, water pollution, soil pollution, and other emergent pollution events or radiation pollution events (Fu, Wang, and Yan 2016). Since China's period of 'reform and opening up', total production and the scale of industrial enterprises in the country have expanded greatly. As such, the longterm accumulated environmental risks have evolved and intensified, and China has entered a period of high incidence of environmental pollution events (Huang and Zhang 2015). In recent years, all kinds of environmental pollution events have begun to occur more frequently (Text S1), causing huge economic and environmental losses. Among them, air pollution emergencies have a wider range of influences, and their secondary pollutants are more complicated. They are thus of relatively greater concern to the public and media compared with other types of pollution events (Chen, Tang, and Zhao 2015).
Tianjin Port is located at the mouth of the Haihe Riverthe intersection of the Beijing-Tianjin-Hebei urban agglomeration and the Bohai economic circle. Its annual throughput amounts to about five hundred million tons, making it the largest port in the north of China ). At about 2330 LST (local standard time) 12 August 2015, a dangerous goods warehouse belonging to Ruihai International Logistics Co. situated in the Dongjiang Bonded Port Area of Tianjin Binhai New Area exploded. The explosion can be divided into two phases: (1) a first explosion occurred at 2334, for which the local magnitude (ML) was about 2.3equivalent to 3 tons of TNT; and (2) a second explosion occurred after 30 s, for which the ML was about 2.9equivalent to 21 tons of TNT. The explosion led to 165 deaths, 8 people missing, and more than 800 people who were injured (Yu et al. 2016). The explosives included a large amount of NO 3 − , such as NH 4 NO 3 and KNO 3 (http:// news.china.com/zh_cn/focus/tjgbz/), which burned for more than 15 h and generated a large amount of polluting gas and atmospheric particulates. According to the forward air trajectories at that time, the nitrogen-containing pollutants were mostly transmitted to the Bohai Sea, but there remains a lack of understanding regarding the impacts over the region.
In this study, the impacts of this accidental fire and explosion at Tianjin Port (hereafter referred to simply as 'Tianjin explosion') on the atmosphere over Bohai Sea were explored based on fine particulate (PM 2.5 ) data recorded on Beihuangcheng (BH) Island, which is located in the middle of the Bohai Strait. The objectives were to: (1) determine the affected time of Tianjin explosion on the Bohai Sea, based on the variation in the concentrations of PM 2.5 and its species; (2) confirm the effects of Tianjin explosion through nitrogen/oxygen (δ 15 N-NO 3 − /δ 18 O-NO 3 − ) isotope analysis; and (3) assess the elevated dry nitrogen deposition caused by the explosion and estimate roughly the affected area over the Bohai Sea. To the best of our knowledge, this is the first attempt at interpreting the impact of Tianjin explosion on the atmosphere over the Bohai Sea, and using isotopic analysis of NO 3 − in an explosion-affected period is also a first. The results should prove helpful in understanding the ecological risks of environmental pollution events. Forward trajectories starting at 2300 LST 12 August 2015 at the Tianjin explosion point were generated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, which is a complete system for computing simple air parcel trajectories in complex dispersion simulations (Bressi et al. 2014). It is available on the National Oceanic and Atmospheric Administration's Air Resource Laboratory website (www.arl.noaa.gov/ready/hys plit4.html). As indicated by Jiang, Ji, and Teng (2017), the pollutants produced in the explosion were mainly raised to 1-2 km into the upper air. Therefore, the model was adopted to generate 72-h forward trajectories encountered

Results and discussion
3.1 Component characteristics of PM 2.5 before, during, and after the explosion , NH 4 + , OC, and EC) were observed beginning at the time of Tianjin explosion, while other species, such as Cl − , Na + , K + , and Mg 2+ , did not change significantly. This indicates that the explosion inflicted a marked impact on secondary particulate components and carbonaceous species in the Bohai region. For better revealing the impacts of the explosion, the sampling period was divided into three phases: (1) before the explosion (named BE, 31 July-9 August); (2) during the explosion (DE, 12 August-21 August); and after the explosion (AE, 24 August-30 August). The average concentrations of PM 2.5 , NO 3 − , SO 4 2− , and NH 4 + were 61.11 ± 10.06 μg m −3 , 14.04 ± 2.34 μg m −3 , 9.33 ± 0.81 μg m −3 , and 6.42 ± 3.64 μg m −3 , respectively, in the DE periodmuch higher than those in the BE period (23.01 ± 10.02 μg m −3 , 3.02 ± 2.52 μg m −3 , 4.90 ± 2.16 μg m −3 , 2.35 ± 1.48 μg m −3 ) and AE period (23.41 ± 10.24 μg m −3 , 2.52 ± 1.18 μg m −3 , 4.32 ± 1.63 μg m −3 , 2.46 ± 1.02 μg m −3 ). Comparatively, the concentrations of these secondary constituents in the BE and AE phases were not much different (p > 0.05), reflecting the basic feature of this season, while the high concentrations in the DE period implied a considerable influence of the explosion on BH Island. Different from the continuous tendency of high concentration for secondary constitutes, carbonaceous species, such as OC and EC, only peaked on 12 August, and then declined rapidly. In addition, the concentration of Ca 2+ , which had been stable, increased on 21 August accompanied by a decrease in NH 4 + concentration.
Based on the Geostationary Ocean Color Imager (Jiang, Ji, and Teng 2017), the explosive gas mass had transported to and passed BH Island in about 24 h. The time was exactly consistent with the time of the increase and decrease for OC and EC concentrations. This may indicate that the high contents of OC and EC were mostly primary products of Tianjin explosion (Liu et al. 2014). However, judging from the variation of secondary components, we speculate that the impact of Tianjin explosion on the Bohai Sea region could have lasted about 10 days. In the BE and AE periods, the concentration of NO 3 − was mostly lower than that of SO 4 2− , but the trend reversed in the DE period, suggesting that the effect of Tianjin explosion on NO 3 − was the most significant in terms of single components in PM 2.5 . The highest concentration of NO 3 − was 16.45 μg m −3 in the DE periodabout 25 times the lowest value in the BE and AE periods; and taking the average value of the BE and AE stages (2.81 μg m −3 ) as the background value of the Bohai Sea, Tianjin explosion caused the concentration of NO 3 − on BH island to rise by 13.64 μg m −3 . As reported, the explosives mainly included NaCN (700 tons), NH 4 NO 3 (800 tons), KNO 3 (500 tons) (Jiang, Ji, and Teng 2017), and possible chemical reactions occurred via the following chemical equations: Assuming that the above chemical reactions occurred completely, we can infer that the amount of NO x emission from the explosion was approximately 1600 tons.
The NO x would have been discharged into the atmosphere with more diffusivity, longer duration, and a more extensive diffusion area compared with solid pollutants, which might explain the high concentration of NO 3 − observed after 12 Augustthe time of the explosion and the transmission to BH Island of explosive gas masses. Although there are no precursors of NH 4 + in the explosion, the high concentrations might be attributable to the promotion of NO x . NO 3 − formed due to the explosion would have increased the acidity of the atmosphere and greatly promoted the conversion of gaseous NH 3 to particulate NH 4 + (Pan et al. 2016). We can speculate that the NH 3 content in the Bohai region was very low at that time, although there are no direct observational data. However, it can be inferred by the increase in Ca 2+ concentration and decrease in NH 4 + on 21 August, as the high acidity gas would have continued to acidify the Ca material to balance the acidity in the case of insufficient NH 3 in the atmosphere (Xiao et al. 2017).

Nitrogen and oxygen isotope characteristics impacted by the explosion
In order to further confirm the impact of Tianjin explosion on the atmosphere over the Bohai Sea, we carried out δ 15 N and δ 18 O analysis for NO 3 − , which was most severely affected by the explosion. During the observation period, δ 15 N-NO 3 − ranged from −1.58‰ to +8.74‰, with an average of +2.79‰ ± 3.32‰ (Figure 2). In the DE period, the mean δ 15 N-NO 3 − was +0.06‰ ± 1.19‰, which was significantly lower than that of the BE period (+4.63‰ ± 3.05‰) and the AE period (+3.99‰ ± 3.90‰). Similar to the concentration tendency, there was no significant change between the BE period and AE period, indicating the local characteristics of the Bohai Sea in August (about +4‰), while the decrease of δ 15 N-NO 3 − in the DE period indicated that the high concentration of NO 3 − was caused by the explosion.
Generally, atmospheric NO 3 − is mainly derived from the conversion of NO x , of which δ 15 N of different anthropogenic and natural sources usually varies over a large range (Elliott et al. 2019). For example, NO x from coal combustion owns a higher δ 15 N value, while δ 15 N from microbial processes is more negative (Hastings, Sigman, and Lipschultz 2003). From the source apportionment result in our previous research based on an improved Bayesian model, the sources of NO x in August on BH Island were coal combustion, biomass burning, mobile sources, and microbial processes, with contributions of 32.66% ± 6.67%, 31.84% ± 3.65%, 22.16% ± 4.87%, and 13.34% ± 4.61%, respectively (Zong et al. 2017). However, this method is not suitable for the source apportionment of NO x during the DE period. As mentioned above, NO x mostly originated from the explosion reaction of NaCN, NH 4 NO 3 , and KNO 3 , which were all industrial products with δ 15 N characteristics significantly different from those of conventional emission sources. According to the reaction principles (S 3 -S 6 ), the nitrogen in the explosives was primarily from the N 2 in the atmosphere. Although a certain degree of isotope fractionation would have occurred during the transformation (Walters, Simonini, and Michalski 2016) and +69.52‰, with a mean of +61.18‰ ± 6.15‰, which was well within the broad range of values previously reported (Fang et al. 2011). Generally, the oxygen atoms of atmospheric NO x are rapidly exchanged with O 3 in the NO/NO 2 cycle (S 7 -S 9 ), and then NO 2 is translated to HNO 3 based on the •OH pathway (S 10 ) or O 3 pathway (S 11 -S 13 ). Thus, the δ 18 O-NO 3 − value is determined by its generation pathways, and can be adopted to explore the conversion process of NO x to NO 3 − in the atmosphere. In the DE period, δ 18 O-NO 3 − showed a significant increase (+66.62‰ ± 3.92‰), which was higher than those in the other two stages (BE: +61.65‰ ± 2.24‰; AE: +53.29‰ ± 3.38‰). This indicated that the explosion had a certain influence on the δ 18 O-NO 3 − , and impacted on the conversion pathway from NO x to NO 3 − . For exploring this effect, we adopted the assumption that 2/3 of oxygen atoms of NO 3 − may have derived from O 3 and 1/3 from •OH in the •OH generation pathway, and 5/6 of oxygen atoms were from O 3 and 1/6 from •OH in the O 3 pathway (Hastings, Sigman, and Lipschultz 2003), and then assessed the respective contributions of the two-generation pathways via a Monte Carlo simulation (Text S3) (Goulden et al. 1996). The results showed that the average contributions for the •OH generation pathway were 76.29% ± 7.10%, 51.79% ± 10.94%, and 91.76% ± 2.48% in the BE period, DE period, and AE period, respectively. Obviously, the conversion ratio of the O 3 pathway increased significantly in the DE period, which may have been due to the massive generation of O 3 as a byproduct during the chemical processes in Tianjin explosion. Besides, the increase of PM 2.5 mass (61.11 ± 10.06 μg m −3 ) could also have facilitated the O 3 pathway through the N 2 O 5 heterogeneous chemistry (Qu et al. 2019).

Elevated dry nitrogen deposition triggered by the explosion
Generally, an increase in the nitrogen concentration (e.g. NO 3 − , NH 4 + ) in the atmosphere will inevitably elevate the amount of nitrogen deposition (Duce et al. 1991).
Here, we evaluated the characteristics of atmospheric dry deposition over the Bohai Sea during the explosion. Adopting dry deposition velocities used for PM 2.5 samples of 0.017 m s −1 for NO 3 − and 0.0022 m s −1 for NH 4 + (Nakamura, Matsumoto, and Uematsu 2005) Figure 3) and 42.41 μmol N m −2 d −1 , respectively; the total nitrogen deposition increased by 308.49 μmol N m −2 d −1 . This indicated that a great amount of nitrogen had entered the Bohai Sea region and may have caused serious damage to its ecosystem because marine areas are usually nitrogen-limited (Jickells et al. 2005). For example, an increasing nitrogen input has changed the N:Si ratio in the Yellow Sea, contributing to shifts in phytoplankton assemblages from diatoms to nondiatoms, as well as harmful algal blooms of nondiatoms (Liu and Glibert 2018); and enhanced atmospheric deposition of inorganic nitrogen in the Bohai Sea region caused a 56.5% increase in the phytoplankton biomass on average (Shou et al. 2018). Specifically, NO 3 − was directly produced by the explosion, while NH 4 + was generated indirectly by the high concentration of NO x . The high deposition of NH 4 + may have been due to the change from gaseous NH 3 to particulate NH 4 with a more intense trend of deposition (Park et al. 2019). Correspondingly, the deposition would have decreased after the event, but the total NH 4 + deposition was elevated. Therefore, it can be seen that attention should be paid to the direct emissions as well as the byproducts in the event of an emergency, especially for atmospheric incidents with complex reaction mechanisms. Bohai is a semi-enclosed sea with an area of 7 × 10 4 km 2 . Based on the deposition volume and time of influence (about 10 days indicated by the DE period), the dry deposition flux of N-NO 3 − induced by Tianjin explosion was estimated at 2607 tons, which significantly exceeded the amount of N-NO x produced (about 527 tons). Such a large difference may have been caused by the total area of the Bohai Sea adopted in the above calculation. Assuming that all the NO 3 − generated uniformly and deposited into the Bohai Sea, the area affected by the explosion in the Bohai Sea can be reverse-estimated. The re-estimated area was approximately 1.42 × 10 4 km 2 , which is about 20% of the Bohai sea. This was also in accordance with the forward trajectories, because most of the air flow was mainly transmitted to the middle of the Bohai Sea (Figure 1). The actual impact area, of course, should be smaller than the computation, because the NO 3 − produced could not enter into the Bohai Sea completely. Chung and Kim (2015) proposed that the smoke plumes of Tianjin explosion could have transported to the Korean Peninsula, based on a variety of satellite remote sensing data.

Conclusion
The concentrations of PM 2.5 , NO 3 − , SO 4 2− , and NH 4 + were 61.11 ± 10.06 μg m −3 , 14.04 ± 2.34 μg m −3 , 9.33 ± 0.81 μg m −3 , and 6.42 ± 3.64 μg m −3 , respectively, in the DE periodmuch higher than those in the BE and AE period. This confirms the effect of Tianjin explosion on the Bohai Sea region, and the effect could have lasted about 10 days, as inferred by the duration of the DE period. Compared with some alkali ions, secondary particulate components and carbonaceous species were more affected. Specifically, NO 3 − was most affected in PM 2.5 , with an increase of 13.64 μg m −3 . The mean δ 15 N-NO 3 − in the DE period was +0.06‰ ± 1.19‰, which was similar to the isotopic value of N 2 in the atmosphere. Based on the industrial processes of explosives, the observed δ 15 N-NO 3 − proved that the high concentration of NO 3 − in the DE period mainly came from Tianjin explosion. The δ 18 O-NO 3 − on BH Island varied between +49.40‰ and +69.52‰ with a mean of +61.18‰ ± 6.15‰, and it showed a significant increase (+66.62‰ ± 3.92‰) in the DE period. Using Monte Carlo simulation, the •OH generation pathway of NO 3 − was 51.79% ± 10.94% at that timemuch lower than in the BE and AE period, indicating an increase in the conversion ratio of the O 3 pathway for NO 3 − formation. The elevated dry nitrogen deposition of NO 3 − and NH 4 + caused by Tianjin explosion was 266.08 and 42.41 μmol N m −2 d −1 , respectively, in the Bohai Sea. Through inverse computation of the dry deposition flux of NO 3 − , the affected area of the Bohai Sea was less than 1.42 × 10 4 km 2 . It is undeniable that there are some uncertainties in the calculation, but the results of this paper truly reflect the impact of Tianjin explosion on the Bohai Sea, which will inevitably help with its ecological recovery in the future.