Mapping gaseous amines, ammonia, and their particulate counterparts in marine atmospheres of China’s marginal seas: Part 1-Differentiating marine emission from continental transport

To study sea-derived gaseous amines, ammonia, and primary particulate aminium ions in the marine atmospheres 10 of China’s marginal seas, an onboard URG-9000D Ambient Ion Monitor-Ion chromatography (AIM-IC, Thermo Fisher) was set up on the front deck of the R/V Dongfanghong 3 to semi-continuously measure the spatiotemporal variations in the concentrations of atmospheric trimethylamine (TMAgas), dimethylamine (DMAgas), and ammonia (NH3gas) along with their particulate matter (PM2.5) counterparts. In this study, we differentiated marine emissions of the gas species originating from continental transport using data obtained from December 9 to 22, 2019 during the cruise over the Yellow and Bohai Seas, 15 facilitated by additional measurements collected at a coastal site near the Yellow Sea during summer 2019. The data obtained during the cruise and the coastal site demonstrated that the observed TMAgas and protonated trimethylamine (TMAH+) in PM2.5 over the Yellow and Bohai Seas overwhelmingly originated from marine sources. During the cruise, there was no significant correlation (P>0.05) between the simultaneously measured TMAH+ and TMAgas concentrations. Additionally, the concentrations of TMAH+ in the marine atmosphere varied around 0.28±0.18 μg m-3 (average ± standard 20 deviation), with several episodic hourly average values exceeding 1 μg m-3, which were approximately one order of magnitude larger than those of TMAgas (approximately 0.031±0.009 μg m-3). Moreover, there was a significant negative correlation (P<0.01) between the concentrations of TMAH+ and NH4+ in PM2.5 during the cruise. Therefore, the observed TMAH+ in PM2.5 was overwhelmingly derived from primary sea-spray aerosols. Using the TMAgas and TMAH+ in PM2.5 as tracers for sea-derived basic gases and sea-spray particulate aminium ions, the values of non-sea-derived DMAgas and NH3gas, 25 https://doi.org/10.5194/acp-2021-258 Preprint. Discussion started: 12 May 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction
Gaseous amines and their particulate counterparts are important reduced nitrogen compounds in the marine atmosphere 35 (Facchini et al., 2008;Müller et al., 2009;Hu et al., 2015;Hu et al., 2018;van Pinxteren et al., 2015; NH4 + , DMAH + , and TMAH + in the injection solution were 0.001, 0.008, and 0.001 mg/L, respectively. The ICS-1100 was calibrated onboard prior to the commencement of regular measurement collection, and the second calibration was conducted when the vessel was anchored at the port. The AIM-IC analysis was not affected by ambient water vapor as the device 100 directly measured the ions. More detailed information regarding AIM-IC analysis is provided in the studies of Teng et al. (2017) and Xie et al., (2018). It should be noted that strong K + contamination unexpectedly occurred occasionally and then disappeared during different campaigns. When contamination occurred, DMAH + and TMAH + were undetectable due to the increased baseline at the corresponding residence time in the ion chromatograph; as such, some PM2.5 DMAH + and TMAH + concentration data were unavailable in Fig 2. However, the concentrations of gaseous amines were still correctly detected 105 with a low baseline at the residence. The K + contamination remains under investigation. An automatic weather system that provides real-time meteorological data is available on the R/V Dongfanghong-3. The heading wind was corrected to determine the true wind speed and direction. The surface seawater temperature was not measured during this cruise campaign, and typically has a delay of a few hours when compared to the ambient air temperature (Deng et al., 2014). The NH4 + and aminium ion concentrations in the surface seawater were also not measured 110 as the analytical methods are still hindered by high sea-salt ion contents.
On August 1-9 and September 12 to October 1, 2019, the AIM-IC was set up at a coastal site in Qingdao (36.34°N,120.67°E) to collect routine measurements (Fig 2). The summer measurement data were obtained three to four months before the winter cruise campaign. The sampling site was located in a new high-technology zone near the Yellow Sea, with the shortest distance from the sea being approximately 1 km in the south. The AIM-IC was housed in a research lab on the fifth story of a 115 building, approximately 16 m above ground-level. The sampling probe extended out of the window and was directly connected to the ambient air. Typically, higher biogenic emissions of reduced nitrogen compounds over the continents are expected in the summer than the winter due to the temperature effect (Yu et al., 2016;Teng et al., 2017).

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Before analyzing the basic gases and their counterparts in the marine atmosphere, we first presented their continental https://doi.org/10.5194/acp-2021-258 Preprint. Discussion started: 12 May 2021 c Author(s) 2021. CC BY 4.0 License.
concentrations at the coastal site facing the Yellow Sea, as these observations provide important evidence to facilitate the analysis of the contributors to these species in the marine atmosphere. Figures 2a & b show that the TMAgas and TMAH + concentrations in PM2.5 always approached the detection limit, varying at approximately 0.002±0.001 μg m -3 (average ± standard deviation), regardless of the presence of offshore or onshore winds. The DMAgas and DMAH + concentrations varied 125 at 0.017±0.023 and 0.017±0.012 μg m -3 , respectively, which were approximately one order of magnitude larger than those of TMAgas and TMAH + . This suggests that the TMAgas and TMAH + concentrations in the upwind continental and coastal atmospheres were extremely low. However, this was not the case-five to ten years ago. For example, the concentrations of the two aminium ions were comparable in atmospheric particles collected at two other coastal sites located approximately 20 km from the study area (Yu et al., 2016;Xie et al., 2018). The cause of this change is beyond the scope of this study, but 130 may be the large decrease in manure application, based on our recent survey in the Qingdao area.
The DMAgas and DMAH + in PM2.5 concentrations with offshore winds were substantially higher than those with onshore winds, suggesting that their continental emissions and related secondary sources were stronger. Moreover, the concentrations of DMAgas and DMAH + were moderately correlated with those of NH3gas and NH4 + , i.e.,

Spatiotemporal variations in the concentrations of basic gases over the seas
Throughout Campaign A, the TMAgas concentrations varied at approximately 0.031±0.009 μg m -3 (Fig 1a-c), with three peaks occurring at 4-to 5-d intervals (gray shadowing in Fig. 1c). Peaks 1 and 2 were generally associated with offshore winds, while Peak 3 was mostly associated with onshore winds (Fig. 1b). The peaks lasted from tens to dozens of hours and 140 were not caused by onboard dew evaporation at sunrise. For example, the highest value (0.060 μg m -3 ) occurred at 23:00 on December 16. The observed concentrations were one order of magnitude higher than those measured in the coastal atmosphere during the summer. The values were also significantly higher than those of DMAgas (P<0.01), which varied at approximately 0.006±0.006 μg m -3 (Fig 1d). The comparison results strongly indicated that the TMAgas observed during Campaign A was largely derived from marine sources. The same conclusion could be drawn by analyzing the three peaks of 145 https://doi.org/10.5194/acp-2021-258 Preprint. Discussion started: 12 May 2021 c Author(s) 2021. CC BY 4.0 License.
TMAgas and its temporal variations during the anchoring port period. For example, during Peak 1 (Fig. 1a), the concentrations of TMAgas increased by approximately 100% from 20:00 on December 9 to 11:00 on December 10 with a decrease in the SO4 2concentration of approximately 30% (from 23 to 17 µg m -3 ; Fig 1b). Moreover, the peaks in the TMAgas concentrations corresponded to troughs in the SO4 2concentrations during Peak 3, as shown in Figs 1c & d. The self-vessel emissions of SO4 2in PM2.5 were negligible due to the use of low-sulfur diesel, which will be discussed later. The increased 150 SO4 2concentrations of PM2.5 may be a good indicator of continental transport, and vice versa.
Unlike TMAgas, continental transport likely acted as an important contributor to the DMAgas and NH3gas observed in the marine atmosphere, particularly during Peak 1, when higher SO4 2concentrations were observed in PM2.5 (Figs 1c-e). The DMAgas and NH3gas concentrations were negatively correlated with those of TMAgas during Peak 1, suggesting that most of the DMAgas and NH3gas were likely derived from continental transport, rather than marine sources. During Peak 2, increased 155 TMAgas, DMAgas, and NH3gas concentrations were observed concurrently with increasing SO4 2concentrations, suggesting that both the marine emissions and continental transport may contribute to the observed DMAgas and NH3gas at the same moment. During the port-anchoring period on 20-22 December, the DMAgas and NH3gas concentrations varied slightly, and were moderate and low, respectively. However, the TMAgas concentrations continuously increased by over 100% as the ambient temperature increased (Figs 1c and f). Additionally, the SO4 2concentrations of PM2.5 varied greatly and followed a 160 bell-shaped pattern during the port-anchoring period.
Additionally, the NH3gas concentrations varied at approximately 0.53  0.53 μg m -3 from December 9 to 22. The variation narrowed to approximately 0.24  0.07 μg m -3 during the port-anchoring period on December 19-22. When the data during Campaign A were used for analysis, the NH3gas concentrations were significantly correlated with those of DMAgas; i.e., [DMAgas] = 9.310 -3  [NH3gas] (R 2 =0.35, P<0.01). However, there was no correlation between the NH3gas and TMAgas 165 concentrations. As the TMAH + concentrations were approximately two orders of magnitude higher than the observations at the coastal site during summer 2019, the observed TMAH + were likely largely derived from marine sources. The TMAH + concentrations followed a spatiotemporal pattern that was clearly different from that of DMAH + and NH4 + , while the latter two ions exhibited a similar spatiotemporal pattern during most of the periods throughout Campaign A (Figs 3a-c). A significant 180 negative correlation (P<0.01) was obtained between the concentrations of TMAH + and NH4 + in PM2.5 (not shown). The spatiotemporal pattern of the TMAH + concentration was also greatly different to those of SO4 2- (Fig. 1d) and SO2 (Fig. 3b).

Spatiotemporal variations in the aminium and NH4 + ion concentrations of PM2.5 over the seas
For example, the extremely strong TMAH + peaks occurred concurrently with low SO4 2-, NH4 + , and SO2 concentrations, while accompanying with high concentrations of Na + under high wind speeds as indicators of sea spray aerosols (Feng et al., 2017).
Moreover, the TMAH + concentrations were approximately one order of magnitude larger than those of TMAgas, and no 185 significant correlation was observed between them (P>0.05). This suggests that the observed TMAH + may not be derived from the neutralization reactions of TMAgas with acids in the marine atmosphere, and may have been derived from primary sea-spray organic aerosols (Ault et al., 2013;Prather et al., 2013;Quinn et al., 2015;Hu et al., 2018;Dall'Osto et al., 2019).
The DMAH + concentrations varied at approximately 0.065±0.068 μg m -3 on December 9-22; however, they varied at approximately 0.10 ±0.04 μg m -3 during the port-anchoring period. The 25th percentile value of DMAH + during Campaign A 190 was 0.021 μg m -3 , suggesting a low background concentration in the marine area. The DMAH + concentrations were significantly correlated with those of NH4 + (R 2 =0.71, P<0.01; data not shown). When the data obtained at 03:00-05:59 and 14:00-16:59 on December 18 (strong peaks of TMAH + with a simultaneous increase in DMAH + ) were removed for correlation, the R 2 value improved to 0.78. Unlike the TMAH + , the observed DMAH + may have been partially derived from The NH4 + concentrations of PM2.5 varied greatly at approximately 4.7±7.2 g m -3 during Campaign A (Fig. 3c). However, the 25 th percentile values were as low as 0.21 g m -3 , suggesting low marine background values. The 50 th percentile value was also only 1.2 g m -3 , which was much smaller than the average value due to the presence of strong peaks in the NH4 + 200 concentrations. The increased NH4 + concentrations associated with NO3and SO4 2during Campaign A were likely due to long-range transport from the upwind continents.

Effects of temperature on the observed basic gases in the marine atmosphere
As mentioned above, the observed TMAgas likely originated from marine sources. We plotted the concentrations of TMAgas 205 against the ambient air temperature (T) in Fig. 4a, which generally increased with increasing T. We further separated the Under Scenario 2, including all others, measurements of the DMAH + in the surface seawater were required to confirm whether the seas were the net sources or sinks of DMAgas. created a large triangular zone that likely reflected the different ratios of DMAgas/TMAgas in primary marine emissions. We assumed that any data beyond the purple-red dashed line reflected the contribution of non-sea-derived DMAgas, which should be attributed to continental transport. Therefore, we assumed that the non-sea-derived DMAgas (DMAgas # ) concentrations  Fig. 5c. Based on the triangular zone in Fig. 5a

Estimation of non-sea-spray particulate DMAH + in the marine atmosphere
We plotted the concentrations of DMAH + against those of TMAH + in PM2.5 (Fig. 6a) using the data obtained from 15:00 on December 16 to 01:00 on December 19 ( [DMAH + ]PM2.5 =0.13  [TMAH + ]PM2.5, R 2 =0.91, P<0.01). We assumed that the nonsea-primarily derived DMAH + concentrations in PM2.5, marked as DMAH +# , were equal to the observed DMAH + values minus the predicted values using the regression equation. The calculated DMAH +# values are shown in Fig. 6b. The 255 DMAH +# concentrations varied at approximately 0.0420.070 μg m -3 throughout Campaign A, during which the calculated average DMAH +# accounted for 65% of the observed average. Additionally, the calculated DMAH +# values accounted for over 80% of the observed values in 26% of the Campaign-A period. Again, the decomposition of TMAH + to DMAH + may have occurred in surface seawater and/or the marine atmosphere, to an extent, and the estimated DMAH +# should be considered as the upper limit. Note that the NH4 + and TMAH + concentrations were negatively correlated during Campaign A, 260 and no primary particulate NH4 + from sea-spray aerosols could be identified.

Formation and chemical conversion of aminium ions in the transported and self-vessel SO2 plumes
When the sea-spray particulate DMAH + was deducted, the increased concentrations of DMAH +# were generally associated with increased SO4 2and SO2 concentrations. Combining this with the moderate correlation between DMAH +# and NH4 + , it can be inferred that the DMAH +# likely originated from concurrent secondary formation with NH4 + . However, we separated 270 the air pollutant plumes into two groups. Group 1 represented an increase in SO4 2and NH4 + together with SO2, while Group 2 represented an increase in SO2 without increases in SO4 2and NH4 + . Group 1 likely reflected the transport of aged air pollutant plumes from the continents, while Group 2 may reflect self-vessel SO2 plumes. As shown in Figs 6b and 3b-c, the concentrations of DMAH +# and NH4 + in the self-vessel SO2 plumes did not increase in the intervals between Peaks 1 and 2, and between Peaks 2 and 3. Therefore, no fresh formation of DMAH +# and NH4 + in the self-vessel emissions was detected.

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However, the concentrations of TMAH + decreased in some self-vessel SO2 plumes. The TMAH + concentrations were approximately one order of magnitude higher than those of TMAgas in the marine atmosphere. Assuming that the decreased TMAH + was released from PM2.5 to the gas phase, a simultaneous large spike in TMAgas should be observed. However, this was not the case, as shown in Fig 1c. The decreased TMAH + may persist in the PM2.5, but could not be detected by AIM-IC.

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In continental China upwind of the Yellow Sea, the TMAgas and TMAH + concentrations in PM2.5 were extremely low (0.002±0.001 µg m -3 ), close to the detection limit of the AIM-IC. Taking the observations as a reference, the largely increased TMAgas (0.031±0.009 μg m -3 ) and particulate TMAH + (0.28±0.18 μg m -3 ) concentrations in the marine atmosphere were attributed to marine emissions. Therefore, TMAgas and particulate TMAH + can be used as unique tracers to quantify the marine emissions of DMAgas, NH3gas, and particulate DMAH + , as well as the long-range transport from upwind continental 285 China.
Through comprehensive comparison and correlation analyses, the high concentrations of TMAH + in PM2.5 observed over the Yellow and Bohai Seas, with episodic average hourly exceeding over 1 μg m -3 , were inferred to originate from strong primary sea-spray aerosol emissions. Moreover, the TMAgas concentrations generally increased with increasing ambient temperature and sea surface wind speeds, suggesting that the observed TMAgas was likely released from the surface seawater. However, the TMAgas concentrations were substantially lower than those of particulate TMAH + , and were not significantly correlated. Although different mechanisms of the release of TMAgas and particulate TMAH + from the seas have been reported in the literature, the lack of a significant correlation between them was surprising and is explored in the companion study.
The DMAgas and NH3gas concentrations varied at approximately 0.006±0.006 and 0.53±0.53 μg m -3 during Campaign A, in 295 which at least 16% and 34 % of the observational values were derived from continental transport, respectively. The seaderived DMAgas and NH3gas were likely released with TMAgas as they peaked simultaneously. The DMAH + concentrations of PM2.5 varied at approximately 0.065±0.068 μg m -3 during Campaign A, 65% of which was derived from continental transport.
Our analysis results did not support the occurrence of the photolysis of marine organic nitrogen to generate NH3gas in the 300 marine atmosphere during winter as there was no correlation between the sea-derived NH3gas and particulate TMAH + concentrations. Additionally, Peaks 2 and 3 of NH3gas persisted for dozens of hours under strong winds and were unlikely to be derived from seabird emissions. Alternatively, a good exponent correlation was observed between the observed NH3gas concentrations and T during the period lacking continental air pollutant transport, suggesting that the observed NH3gas was released from seawater. NH3 emissions via seabirds were unlikely to be an important contributor to the observed NH3gas in 305 the marine atmosphere during winter, although this may not have been the case during other seasons.
Additionally, no formation of particulate NH4 + and DMAH + in the self-vessel SO2 plume was observed in the marine atmosphere. However, the particulate TMAH + concentration clearly decreased in the self-vessel SO2 plume without a simultaneous increase in the TMAgas concentrations. Undetectable chemical conversion of particulate TMAH + by AIM-IC likely occurred and requires further investigation.

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Data availability. The data of this paper are available upon request (contact: Xiaohong Yao, xhyao@ouc.edu.cn).