the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2025
Formation of marine atmospheric organic aerosols associated with the spring phytoplankton bloom after sea ice retreat in the Sea of Okhotsk
Abstract. The Sea of Okhotsk is one of the most biologically productive regions, where primary production during spring phytoplankton blooms after sea ice melting/retreat has the potential to contribute to the sea-to-air emission flux of atmospheric organic aerosols (OAs). To elucidate the effect of oceanic biological activity during blooms on the formation process of OAs, aerosol samples and surface seawater were collected during the bloom period of April 2021. Organic matter (OM) was the dominant component of submicrometer aerosols during both the bloom (53±16 %) and bloom-decay periods (44±12 %), with OM being highly water-soluble during the bloom. Stable carbon isotope ratios of aerosol organic carbon (OC) showed that 73–82 % of the observed aerosols were of marine origin. Relations between water-soluble OC (WSOC) and molecular tracers suggested that the majority of WSOC of marine origin was affected by secondary formation from precursors such as α-pinene and DMS-relevant compounds instead of primary emissions of sea spray aerosols. The amounts of water-soluble organic nitrogen (WSON) in aerosol and dissolved organic nitrogen (DON) in seawater during the bloom were larger than those during bloom-decay period, suggesting the preferential formation of N-containing water-soluble OAs of marine origin during the bloom. The increase in the amount of DON during the bloom was likely associated with the predominant diatoms, Thalassiosira spp. and Fragilariopsis spp. This study highlights the significant contribution of the secondary formation of marine biogenic OAs with increased N-containing components during the bloom after sea ice melting/retreat in the subarctic ocean.
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RC1: 'Comment on egusphere-2025-2689', Anonymous Referee #1, 10 Jul 2025
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The organic component of aerosol is an important and uncertain aspect of aerosol composition, particularly in the marine atmosphere. Given the importance of aerosols in atmospheric chemistry and climate developing a better understand of this aerosol organic matter is valuable and this paper is a useful contribution to this goal.
This is a thorough and interesting study of aerosols during the spring bloom period in the Sea of Okhotsk. The chemical characterisation particularly of the aerosol is comprehensive, sophisticated. and well described. Overall I believe the paper is well worth publication but I do have some suggestions for modifications prior to final publication.
Firstly I believe it would be useful to include some further descriptions of the conditions at the time of sampling.
- There is talk of ice algae and I’m not clear whether thee was a lot of ice at the time of sampling or not. This is potentially important because an ice cap can allow a build up of quite high concentrations of marine biogenic gases which are then released rapidly as the ice breaks up.
- What were the wind conditions like? – this is relevant to ice break up, seawater mixing and bloom development and to seaspray emissions.
- The apparently very low contribution of terrestrial derived atmospheric aerosol organic matter leads to a question of where the air came from during the sampling period?, so including some air-parcel back trajectories would be useful.
Throughout the discussion the authors should be clear which size of aerosol particles they are discussing. I became confused at several points. Gas phase emissions from seawater will form fine mode particles, while ejection of seawater itself will produce coarse mode particles. Some of the correlations such as in Figure 5 are not really useful given these differences.
Section 3.2 is a bit misleading. As the authors correctly note at the end of this section (line 271-2) the tracer species they use represent only a tiny fraction of the WSOM and so the origin of this material is still essentially unknown, although the correlations to MSA and 3MBTCA are intriguing. I would suggest reorganising this section to avoid any misunderstandings over what can and cannot be said about the sources of the WSOM.
I was also a little confused by the logic of the argument in sections 3.3 and 3.4. The DOC and DON in seawater is overwhelmingly of high molecular weight and long lived. The observed relationships of DOC and DON in seawater (Fig 9) reflect the fact that they are probably actually bonded together in the same complex organic matter and the variations in concentrations in both compounds may reflect changes in production and consumption, or alternatively may reflect physical mixing of water masses. The correlations of DOC and DON in the aerosols look less convincing in Figure 9, and this correlation too could also represent mixing of air masses. Given its molecular weight, the direct emissions of seawater DOC and DON into the atmosphere will be via bubble bursting type processes and hence associated with coarse mode aerosol, as with sodium. This process cannot therefore explain the fine mode WSOM or the relationships of WSOM to MSA and other gaseous marine biogenic emissions reported here. All the data I have seen published suggests that marine amine emissions are very small, particularly in comparison to say ammonia emissions. Hence the emission of gaseous organic compounds from seawater into the atmosphere does not seem to be able to explain aerosol DON, although it could arise from marine biogenic gas emissions of other non-nitrogenous compounds with nitrogen being subsequently incorporated during aerosol formation. So I find the authors observations valuable and interesting, I am not sure they do provide a clear explanation of the formation mechanism for the aerosol WSON as implied particularly in the abstract. I would suggest that the logic of the argument in sections 3.3 and 3.4 might therefore be clarified.
Citation: https://doi.org/10.5194/egusphere-2025-2689-RC1
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