A new marine biogenic emission: methane sulfonamide (MSAM), DMS and DMSO2 measured in air over the Arabian Sea

We present the first ambient measurements of a new marine emission methane sulfonamide (MSAM), along with dimethyl sulfide (DMS) and dimethyl sulfone (DMSO2) over the Arabian Sea. Two shipborne transects (W→ E, E→W) were made during the AQABA (Air Quality and Climate Change in the Arabian Basin) measurement campaign. DMS mixing ratios were in the range 0.3–0.5 ppb during the first traverse of the Arabian Sea (first leg) and 0.1 to 0.3 ppb in the second leg. In the first leg DMSO2 was always below 0.04 ppb and MSAM was close to the limit of detection. During the second leg DMSO2 5 was between 0.04–0.12 ppb and MSAM was mostly in the range 0.02–0.05 ppb with maximum values of 0.06 ppb. An analysis of HYSPLIT back trajectories combined with calculations of the exposure of these trajectories to chlorophyll a content in the water revealed that most MSAM originates from the Somalia upwelling region, known for its high biological activity. This new marine emission is of particular interest as it contains both sulfur and nitrogen, making it potentially relevant to marine nutrient cycling and particle formation. 10


DMS
. DMS oxidation scheme focusing on the trace gases discussed (Barnes et al., 2006). The bottom part of the figure illustrates the principle of the Somalia upwelling. Wind blowing along the coast displaces surface water and leads to upwelling of cold nutrient rich water which can support a phytoplankton bloom (Kämpf and Chapman, 2016).

Sampling
A 5.5 m high (above deck level) high volume-flow inlet (HUFI) (diameter 15 cm) was used to draw ambient air down to the containers at a flow rate of 10 m 3 /min. The HUFI was situated between the four containers on the foredeck so that when the ship headed into the wind no interference from the vessel's smokestack or indoor ventilation were measured. From the center of the HUFI, air was drawn continuously at a rate of ca. 5 standard liter per minute (slpm) (first leg) or 3 slpm (second leg) into 5 an air-conditioned laboratory container via an insulated FEP (fluorinated ethylene propylene) tube (1/2" = 1.27 cm o.d., length ca. 10 m). The tube was heated to 50-60°C to avoid condensation inside the air-conditioned container. To prevent sampling of sea spray and particles, a weekly changed PTFE (polytetrafluorethylene) filter was installed in the inlet line before it entered the container. This inlet system was employed for the measurements of VOCs and total OH reactivity (Pfannerstill et al., 2019) pinene. Clean synthetic air was measured every three hours for ten minutes to determine the instrument background. The time resolution of the measurement was 1 minute and the mass range extended to 450 amu. Mass resolution (full width half maximum) was ca. 3500 during the first leg and > 4500 during the second leg at mass 96 amu.
The total uncertainty of the DMS measurement was < 30% (main sources of uncertainty: standard gas mixture 5%, flow meter 1%, calibration ≈ 10%), and the precision < 5 %. DMSO 2 and MSAM were not present in the calibration gas. Calculation of 5 the mixing ratio was therefore conducted based on theory and more specifically on the rate coefficients for proton transfer (Su and Chesnavich, 1982;Chesnavich et al., 1980), the knowledge of transmission factors, amount of H 3 O + ions and parameters of the drift region (Lindinger et al., 1998). Applying this method results in a greater uncertainty than for compounds included in the calibration gas mixture of approximately 50%. Due to the fact that we do not know the inlet transmission for these two substances, we conservatively estimate an uncertainty of up to a factor of 2 for MSAM and DMSO 2 .

HYSPLIT back trajectories
Air mass back trajectories were calculated to investigate the origin of air masses encountered. The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT, version 4, 2014), a hybrid between a Lagrangian and an Eulerian model for tracing small imaginary air parcels forward or back in time (Draxler and Hess, 1998), was used to derive back trajectories from a start height of 200 m above sea level, going 216 hours back in time on an hourly grid beginning at the ship position.

Dimethyl sulfide (DMS)
Measurements of DMS (m/z 63.0263) during AQABA showed elevated mixing ratios when the vessel traversed the Arabian Sea during both legs (brown shaded region in Fig. 3 a). During the first leg over the Arabian Sea ( Fig. 3 b), DMS mixing ratios were generally in the range of 0.3-0.5 ppbv, with occasional peaks of 0.8 ppbv. During the second leg ( Fig. 3 c), the DMS 20 mixing ratios over the Arabian Sea were significantly lower in the range of 0.1-0.3 ppbv, again with elevated peaks of short duration (around 2 h).

Dimethyl sulfone (DMSO 2 )
Dimethyl sulfone (DMSO 2 ) is an oxidation product of DMS by the OH radical (Arsene et al., 2001;Barnes et al., 2006). It was measured by the PTR-ToF-MS at m/z 95.0161. A thorough investigation of other plausible mass formulas, which would yield 25 a m/z value inside the error margins due to the mass resolution, gave no plausible alternative molecular structure. Additionally, the head space of the pure compound (TCI Deutschland GmbH, purity > 99 %) was sampled yielding a peak at the same position as found in ambient air. Therefore we assigned this mass to DMSO 2 . Measurements of DMSO 2 in the Arabian Sea region showed elevated levels between 0.04-0.12 ppb during the second leg (Fig. 3 c)) but more modest levels (< 0.04 ppb) in leg 1 (Fig. 3 b)). To our knowledge, there have been no measurements of DMSO 2 performed in this region previously. At m/z 96.0144, a signal was observed which displayed a strong correlation with DMSO 2 (Pearson correlation coefficient: r around 0.8) over the Arabian sea during the second leg (see Fig. 4). This mass corresponded to methane sulfonamide (MSAM), which has a similar structure to DMSO 2 , the difference being that one methyl group is replaced by an amine group (see Fig.   1 for the chemical structures of the molecules). This molecule has not previously been measured in ambient air. To confirm 5 the assignment of mass m/z 96.0144 to MSAM, the head-space of the pure substance MSAM (Alfa Aesar, purity > 98 %) was sampled by the PTR-ToF-MS. The analysis of the pure compound MSAM by PTR-ToF-MS matched the mass found in ambient air. No other plausible molecular structures could be found for this mass within the error margins due to the mass resolution.
Based on the correlation of mass m/z 96.0144 to DMSO 2 in ambient data, the mass spectral match to the pure compound, and the absence of alternative structures at that exact mass we identify the measured signal as MSAM. In order to test whether each. The resulting mixing ratios measured ranged from 0.65 ppb (lowest concentration and lowest flow rate) to 130 ppb (highest concentration and highest flow rate). During the Arabian Sea section of the second leg, values of up to 0.06 ppb were measured, but mostly it was found in the range of 0.02-0.05 ppb. In the first leg, MSAM was occasionally detected in the 15 Arabian Sea, but concentrations were generally close to the limit of detection (LOD) which was 5 ppt (3×standard deviation of background).

Discussion
Here we discuss DMS, DMSO 2 and MSAM measurements in air from a rarely sampled region, the Arabian Sea. First we discuss the difference in DMS abundance between the two legs. Secondly we evaluate the source regions of these trace gases 20 based on knowledge of their atmospheric lifetimes and chlorophyll exposure. Then finally we address the implications of these measurements to marine boundary layer chemistry.

Atmospheric lifetimes of DMS, DMSO 2 and MSAM
The lifetime of DMS in the atmosphere with respect to the primary oxidant OH is around 1.3 days (reaction rate = 7.8 × 10 −12 cm 3 molec −1 s −1 , [OH] = 1.1 × 10 6 molec/cm 3 (Albu et al., 2006)). In some regions of the marine boundary layer, BrO may also contribute to the oxidation of DMS leading to shorter DMS lifetimes (Breider et al., 2010;Khan et al., 2016;Barnes et al., 2006). The high and variable levels of DMS encountered during the Arabian Sea crossing suggest that DMS mixing . As MSAM has a long lifetime with respect to reaction with OH, we must also consider 10 its physical removal by deposition to the ocean surface. We therefore carried out experiments to determine the Henry's law constant for MSAM (details see Sect. S2) and found it to be in the range 3.3×10 4 M atm −1 -6.5×10 5 M atm −1 . DMSO 2 has a similarly large Henry's law constant > 5 × 10 4 M atm −1 (Bruyn et al., 1994). The exchange flux between the gas and aqueous phase can be described phenomenologically to derive an estimate of the effective lifetimes (Schwartz, 1992;Yang et al., 2014).
Because of the high Henry's law constant for both MSAM and DMSO 2 , to a good approximation their lifetime is dependent on

Chlorophyll exposure of HYSPLIT back trajectories
MSAM and DMSO 2 were close to the LOD during the first leg, despite the fact that DMS mixing ratios were even higher than during the second leg. This observation excludes a simple relationship between the emissions of DMS and DMSO 2 /MSAM. DMSO 2 is known to be an oxidation product of DMS and is therefore linked to marine biogenic activity (Barnes et al., 2006). 25 We hypothesize that the newly detected trace gas MSAM is also linked to marine biogenic activity. This is based on the observation that MSAM displays the highest values when influenced mainly by remote marine air without recent contact with land, it correlates well with DMSO 2 (see Fig. 4 (c)) and is similar in chemical structure to DMS and DMSO 2 . A good indicator for marine biogenic activity is phytoplankton. Phytoplankton in the water can be detected from space via the chlorophyll a pigment used for photosynthesis. Satellite images of regional chlorophyll can be exploited to investigate emission areas 30 of marine biogenic VOCs. In the following sections we will investigate, with the help of HYSPLIT back trajectories and chlorophyll a water content, where the source of MSAM and DMSO 2 is located.

Chlorophyll a water content
We used data from the satellite MODIS-aqua (Jackson et al., 2019). In Fig. 6 a-d) the chlorophyll a concentration averaged over 8 days is plotted for the time periods relevant for our measurements. During the first leg (Fig. 6 a, b)

Back-trajectory-chlorophyll analysis
Air-masses arriving at the ship which have traveled over marine areas with high biological activity (meaning high phytoplankton content) will likely contain higher levels of marine emissions. To investigate the provenance of air-masses sampled at the ship in relation to the chlorophyll distributions shown above, HYSPLIT back-trajectories were calculated (9 days back) for every full hour of the cruise. A weighting factor was applied to emphasize regions closer to the ship. In order to determine 5 quantitatively to what extent the air sampled had passed over areas of high phytoplankton content (indexed with chlorophyll a) we summed up the chlorophyll a content detected by satellite in the water for each trajectory. Only time points when the trajectory was within the marine boundary layer, as calculated from the HYSPLIT model, were considered. The weighting factor (w) was applied to this calculation by multiplying the chlorophyll a water content by: w = 1 1+p * t , where p is the weighting parameter and t is the number of hours before arrival at the ship's location. This was to account for the fact that marine emis-10 sions from phytoplankton closer to the ship will have a bigger impact on the measured mixing ratios as they will undergo less oxidation and dispersion. Several weighting factors were applied in order to determine the impact of the various parameters on the results. The weighting parameters p ranged from p = 0.02 to p = 1 and for exponential weighting factors in the form w exp = p t , p was varied from p = 0.8 to p = 0.99 (details see Sect. S3). Varying these parameters did not affect the conclusions drawn.

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The results of these calculations are displayed in Fig. 7. The graphs show the total chlorophyll exposure and the exposure of chlorophyll specifically from the region of the Somalia upwelling (see Fig. 6 the region in the black rectangle). In the first leg ( Fig. 7 (a)), when both DMSO 2 and MSAM mixing ratios were low, air reaching the ship did not cross chlorophyll a rich waters in the previous 1 to 2 days. This is the case for the total exposure as well as for the exposure to chlorophyll in the Somalia upwelling region.

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However, during the second leg ( Fig. 7 (b)), when DMSO 2 and MSAM levels were high, the air measured had traveled over extensive chlorophyll a rich waters. In general, the exposure in the Somalia upwelling region always constituted the majority of the total exposure, except for one peak in the beginning (August 13 th from 12:00 till 19:00) where chlorophyll patches closer to the ship constituted roughly half of the total chlorophyll exposure. From these calculations we can conclude that the occurrence of DMSO 2 and MSAM is related to marine emissions in the Somalia upwelling region. MSAM and DMSO 2 25 mixing ratios started to increase around midday of August 12 th but the chlorophyll exposure only started to increase around 6:00 on August 13 th (Fig. 7 (b)). The reason for this being that trajectories arriving on August 12 th had seen low chlorophyll a water content, as displayed in Fig. 6 (c), where no phytoplankton bloom was present in the Somalia upwelling region resulting in low chlorophyll exposure. This bloom developed afterward as seen in Fig. 6 (d) but might have started already on August 12 th and escaped detection by MODIS-aqua. An inspection of daily data from MODIS-aqua revealed that parts of the 30 Somalia upwelling were in a blind spot of the satellite on August 12 th . The chlorophyll exposure sharply fell to zero at the beginning of the 15 th of August, roughly 8 h before DMSO 2 and MSAM values start to decline as well (Fig. 7 (b)). In this case the calculated HYSPLIT back-trajectories no longer pass over the Somalia upwelling but cross Somalia before arriving at the ship. Our measurements thus indicate that we were impacted by the Somalian upwelling region for longer than calculated  . Weighted amount of chlorophyll a trajectories crossed over before arrival at the ship for leg 1 (a) and leg 2 (b). The total chlorophyll a exposure (yellow line) and the chlorophyll a exposure originating from the Somalia upwelling region (black line) is plotted. The corresponding y-axis for the chlorophyll a exposure for both graphs (a,b) is displayed on the right side. Measured ambient mixing ratios in ppb for DMSO2 and MSAM are plotted in red and blue with the corresponding y-axis on the left side. from the trajectories. This is not unexpected as meteorological data for this region are sparse, and the trajectories therefore correspondingly uncertain.

DMSO 2 , DMSO, MSIA and MSA
We observed DMSO 2 mixing ratios during the second leg between 0.04 and 0.12 ppb, which is high compared to previous measurements of 0.2-11 ppt (Berresheim et al., 1998;D. Davis et al., 1998) made in Antarctica. As mentioned in the introduc-5 tion, DMSO 2 is known to be formed from oxidation of DMS with OH, BrO or NO 3 (see Fig. 1). However laboratory studies indicate that OH oxidation of DMS, via the intermediate DMSO, forms mainly methane sulfinic acid (MSIA) and not DMSO 2 (Barnes et al., 2006;Kukui et al., 2003;Hoffmann et al., 2016). BrO oxidation of DMSO generating DMSO 2 will be negligible for concentrations of 2 ppt for BrO, which have been proposed to be ubiquitous in the marine troposphere (Read et al., 2008;Platt and Hönninger, 2003), due to the slow reaction compared to the reaction with OH. NO 3 oxidation of DMSO was found 10 to only yield DMSO 2 (Falbe-Hansen et al., 2000), but during the night, due to the lack of OH and BrO producing DMSO, DMSO 2 generation will be hindered.
With the data presented here it is not possible to decide which of the above mentioned mechanisms is responsible for the observed DMSO 2 . We did not detect DMSO, MSIA or methane sulfonic acid (MSA). (Hoffmann et al., 2016). The reaction rate 15 of DMSO with OH is 15 times faster than that of DMS with OH, making it a potentially important sink in the remote marine atmosphere (Barnes et al., 2006). A model study of the sulfur cycle in the global marine atmosphere suggested values of around 10 ppt for DMSO in the region of the Arabian Sea . This is below the limit of detection (LOD) of around 15 ppt for DMSO in our instrument and probably the reason why we do not observe it in this dataset. Measurements of DMSO made on Amsterdam Island ranged from 0.36 to 11.6 ppt (Sciare et al., 2000).

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MSIA has a very high reaction rate of 9 × 10 −11 cm 3 molec −1 s −1 with OH radicals (Burkholder et al., 2015;Kukui et al., 2003;Hoffmann et al., 2016). In the model study mentioned above, this leads to concentrations over the Arabian Sea of around 2 ppt which again is below the LOD for MSIA with our instrument (around 20 ppt) .
MSA, on the other hand, is predicted to be around 20-40 ppt over the Arabian Sea (gas phase and aqueous phase combined), which is above its LOD, but almost all of it will be in the aqueous phase Hoffmann et al., 2016). In the gas 25 phase, the maximum MSA values reported to date are below 1 ppt, which is far too low to be measured with our setup (LOD around 15 ppt) (Eisele and Tanner, 1993;Chen et al., 2016;Berresheim, 2002).

MSAM
We are not aware of a possible formation pathway for MSAM in the gas phase, therefore we consider it rather unlikely that it is formed via DMS gas phase oxidation. A microbial formation from DMS or DMS products in the water of the highly biological 30 active upwelling region and subsequent emission into the atmosphere seems plausible (see Fig. 1). To our knowledge, no measurements of MSAM have been reported in the atmosphere so far and thus no information about the potential role it could play there is available. SO 2 , which oxidizes to sulfuric acid (H 2 SO 4 ), is an oxidation product of MSAM (Berasategui et al., 2009)) its role as an acid in these reactions is probably limited.
Studies indicate, that the dominant driving force in new particle formation and growth are the hydrogen bonds formed between common atmospheric nucleation precursors (Xie et al., 2017;Cheng et al., 2017;Li et al., 2018). The newly found trace gas MSAM is very intriguing because it contains a sulfonamide group, which is a sulfonyl group connected to an amine group. The sulfonyl and the amine group both support hydrogen bonding, giving MSAM a high hydrogen-bonding capacity, potentially 10 enabling nucleation.
Because of the comparable lifetimes of MSAM and DMS, we can estimate the relative emission of MSAM to DMS from the ratio of the mixing ratios of ([MSAM]/[DMS]). We only included ratios observed in the afternoons of 14 th and 15 th of August, when the ship was in closest proximity to the Somalia upwelling. The afternoon was chosen to ensure that OH, the primary oxidant of DMS, was present. We derived ratios ranging from 0.1 to 0.27, meaning that emissions of MSAM over the Somalia 15 upwelling can be almost a third of the DMS emissions. Therefore, MSAM could play an important role in particle formation and/or growth over and downwind of upwelling regions. To verify these possibilities, further experiments regarding particle growth and formation with MSAM need to be performed.

Conclusions
During the AQABA campaign we made the first measurements of MSAM in ambient air. Back-trajectories-chlorophyll analy-20 ses suggest that it is a marine biogenic emission from the highly productive upwelling region off Somalia. During the first leg of the AQABA campaign the ship encountered mostly biogenic emissions from sources located along the ship route when crossing the Arabian Sea. The enhanced DMS values observed there could be attributed to seasonally enhanced DMS fluxes and small local phytoplankton blooms visible from space along the Arabian Sea transect. No oxidation products or other organosulfur compounds were detected in substantial amounts from the local emissions. In contrast, during the second leg not only DMS 25 but also DMSO 2 and MSAM were measured. DMSO 2 , like MSAM, was shown to originate from the Somalia upwelling region. DMSO 2 mixing ratios of up to 0.12 ppb were measured during the second leg, which is quite substantial considering that previous studies indicate it to be a minor or negligible product in DMS gas phase oxidation. MSAM is a molecule which, to our knowledge, was never reported in the atmosphere. We detected it in concentrations up to 0.06 ppb during the second leg in the Arabian Sea. Emissions of MSAM over the Somalia upwelling can reach close to a third of the DMS emissions.

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A marine emission containing a nitrogen atom is somewhat surprising since under most circumstances primary productivity in the ocean is nitrogen limited. The emission of a nitrogen containing compound may be related to the abundance of reactive nitrogen provided by the upwelling. Emissions of reactive nitrogen containing species have been previously measured from on the recent advances on precursor characterization and atmospheric cluster composition in connection with atmospheric new particle formation, Annual review of physical chemistry, 65, 21-37, https://doi.org/10.1146/annurev-physchem-040412-110014, 2014.