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Atmospheric Chemistry

A break-up in the relationship between organic carbon in sea spray and chlorophyll-a concentrations

Article: P.K. Quinn, T.S. Bates, K.S. Schulz, D.J. Coffman, A.A. Frossard, L.M., Russel, W.C. Keene, D.J. Kieber. (2014) Contribution of sea surface carbon pool to organic matter enrichment in sea spray aerosol. Nature Geoscience. Vol. 7, 228-232. DOI:10.1038/NGEO2092

Background Information 

Marine organic matter is a complex mixture containing pieces of phytoplankton, zooplankton, fish and all other living organisms. This oceanic organic matter is actually important for cloud formation and other atmospheric processes. So how does organic matter in seawater end up in the air we breathe? The answer is waves! When waves break, whether on shore or in the middle of the ocean, the air bubbles it creates causes organic matter to be ejected up in the air as tiny drops of water called aerosols.

 

This process, called organic matter scavenging, is important to understand the movement of organic aerosols and the formation of clouds. Organic matter can act as cloud condensation nuclei (CCN), which is how individual water droplets actually form and become a cloud.

 

Marine scientists have created a proxy to measure this process from the seats of their homes. Before this study, scientists assumed that the amount of organic matter ejected into the air would be linked to how much phytoplankton was in the water. This proxy is appealing since phytoplankton concentrations can be measured using satellite calculations of chlorophyll-a (which phytoplankton have to allow them to photosynthesize).

 

While using satellites to figure out how much organic matter is ejected into the air is easy, cheap, and gives global coverage, it has a few uncertainties. Mainly, this method has not been tested on aerosols from the open ocean (which is most of the global ocean!). In this study, Quinn et al., (2014) wanted to go out to the ocean and measure the actual relationship between chlorophyll-a concentrations and the organic matter found in sea spray aerosols. A summer 2012 cruise took them to two regions in the North Atlantic to test the relationship between chlorophyll-a and organic matter in sea spray.

The Approach

During the August of 2012, Quinn et al., went on a cruise to test the concentrations of phytoplankton and organic sea spray aerosols in a productive area (high concentrations of phytoplankton) and a low-chlorophyll area (Figure 1). The study was called the Western Atlantic Climate Study (WACS). The first Station this study went to was within George’s Bank, a shelf area located about 170 nautical miles east of Boston and is known to be one of the most productive regions in the world. The chlorophyll-a concentrations ranged from 4 to 14 µg/L (very high!). The second Station was located in the Sargasso Sea, which is in the North Atlantic Subtropical Gyre and is known to be an area with low productivity; the average chlorophyll-a concentration in this region was 0.03 µg/L. In addition to their observations, the scientists in this study also used data from the CalNex cruise, which occurred off the coast of California in May 2010.

March_pohl_2

Figure 1: CalNex (a) and WACS (b) cruise tracks superimposed on maps of satellite-derived chlorophyll-a concentrations using the Aqua Modis satellite.

 

Organic carbon in seawater can be measured as particulate organic carbon (POC) or dissolved organic carbon (DOC). Scientists determine the concentrations of POC in seawater by measuring the amount of organic matter large enough to be stopped by a filter. The DOC is measured by the amount of organic carbon that can pass through a that same filter. Quinn et al. measured POC, DOC, and chlorophyll-a concentrations continuously throughout the cruise. An instrument called the Sea Sweep was used onboard to simulate the creation of sea spray aerosols. The Sea Sweep instrument gets filled with ambient (local) seawater and uses air bubbles to simulate breaking waves that create aerosols. Those created-aerosols were then collected and measured. Lastly, a CCN (cloud condensation nuclei) counter was used to measure the amount of particles in the aerosols that could act as cloud-forming nuclei.

The Findings

The concentration of POC was at least an order of magnitude lower at the Sargasso Sea than in George’s Bank. However, the concentration of DOC was almost the same between the two sites. This is interpreted as there being more phytoplankton and productivity at George’s Bank, but the pool of dissolved organic matter (made from pieces of living things) was very close and not as variable regardless of productivity. The POC and chlorophyll-a concentrations were highly correlated at the study sites, however, the DOC and chlorophyll-a had no correlation at all.

 

The organic enrichment factor, or how much organic carbon was in the sea spray aerosols compared to the surface water, was calculated at each station. This factor determines how important chlorophyll-a and POC are in the organic matter that is ejected into the atmosphere. At both George’s Bank and the Sargasso Sea, the organic matter enrichment factor had no relationship with chlorophyll-a. Interestingly, the enrichment factor was actually quite high in the Sargasso Sea even though there was very low concentration of both POC and chlorophyll-a. This is important since it suggests that satellite chlorophyll-concentrations may not be that great of a proxy for sea spray aerosols after all!

 

Similarly, the authors also found that the ratio of organic carbon aerosols (in the air by sea spray ejection) to sea salt was mostly the same between the high (George’s Bank) and low (Sargasso Sea) chlorophyll-a stations. They also discovered that the organic aerosols between the two sites had almost the same behavior when heated; only about 15% of the aerosols volatilized which means that the organic sea spray contribution was semi-volatile despite the chlorophyll-a concentration. The CCN activity was also determined to be the same between all the stations, regardless of chlorophyll-concentrations (Figure 2). In other words, the potential for cloud formation from the organic sea spray aerosols was the same in areas with high and low levels of productivity.

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Figure 2: The cloud condensation nuclei (CCN) activity at WACS stations 1 (George’s Bank) and 2 (Sargasso Sea) at different degrees of supersaturation.

Significance

This study is the first to measure the organic carbon concentration of sea spray aerosols and surface water simultaneously. The composition of freshly ejected organic aerosols and CCN activity was the same, regardless of the chlorophyll-a concentration. This ultimately means that the biologically productivity of a region, as measured by the chlorophyll-a, has no correlation to the amount of organic matter found in sea spray.

 

Quinn et al. concluded that 1) the organic matter ejected by waves is fairly constant in the regions measured by this study and had no relationship with productivity and 2) satellite measurements are a poor proxy for determining the relationship between organic matter in seawater and the overlying air. This is a major shortcoming on our understanding of organic matter in aerosols and CCN activity for cloud formation in the open ocean!

 

Discussion

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