Day 13
Around mid-morning, we made it back to our eddy #2 and the three
drifters we had previously deployed. At first glance, it looked like two
of the drifters had gotten tangled together, despite being deployed 1km apart.
We went ahead and did a normal and trace metal CTD cast.
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| Preparing the trace metal CTD for deployment |
The weather outlook for the next day or so is not that great.
We are expecting rain, which is good for the project, but the winds are
supposed to pick up as well. Already we can tell the seas are beginning
to get a bit rough. Since day 15 will be our last science day, and might
still be rough, we decided to pick up two of the drifters now.
We went for the two that were tangled together, but they ended up
not being tangled, just floating side by side. After battling some large
waves, during which almost everyone on the back deck got soaked, we managed to
pull in both drifters.
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| Pulling the drifter over the side railing |
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| Rinsing off the drifters with freshwater after retrieving them. |
Then, we traveled back to the last drifter and did a PAR cast as
well as another CTD cast. It was too rough to deploy the towfish, so we
stopped sampling for the day and waited for the rain to arrive.
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| One drifter had a passenger that we returned to the sea |
Day 14
The rain did arrive today, but the seas were very rough. Too
rough, in fact, to deploy any sort of scientific equipment. The danger
is, in rough seas, that the boat rolls quite a bit side to side. It has
to be stationary to deploy the CTD, and that leaves us at the mercy of the
swell. If the swell is too much, the cable holding the CTD will lose
tension as we roll to one side, then snap back tight as we roll to the other.
Too much of a snap and the cable breaks!
It rained pretty good in the morning, but we were unable to
collect any because the salt spray was reaching the height of the bucket, which
is several decks up. The seas calmed down a bit later on - not enough to
deploy any equipment, but we were able to collect rain from a second shower in
the afternoon.
The rest of the day was spent planning out our final science day
tomorrow and discussing packing up and loading up to go back home.
Atmospheric chemistry
If you think about the process we are studying, where rain
deposits nutrients to the surface ocean, causing the phytoplankton to respond,
atmospheric chemistry comes in at the very beginning. We want to know how
the nutrients got in the rain to begin with.
The majority of the atmosphere is made up of nonreactive chemical
compounds, mainly oxygen and nitrogen. We are interested in the ~1% that
is reactive, specifically, how pollutants get into the rainwater. The
main indicator of man-made pollution is nitric oxide, or NO. During the
process of combustion, say when you drive your car, or when our ship is
steaming through the ocean, the heat from the process breaks apart N2 molecules
and produces nitric oxide. There are a few natural sources of nitric
oxide, including lightning and certain soil microbes, but they are episodic or
not significant enough to be important in a marine environment.
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| Setting up instruments on the fly bridge before we departed - Photo courtesy of Doug Martins |
Once nitric oxide is produced, it can react with ozone (O3) in the
atmosphere to form nitrogen dioxide (NO2). If there is sunlight
available, the nitrogen dioxide can split back up into nitric oxide and oxygen,
which goes back to ozone. Essentially, it is a balanced reaction.
Both processes happen at once, keeping the relative levels the same in
the atmosphere. However, since one process requires energy input by
sunlight, the balance levels are different overnight than they are during the
day. So at night, there is a tendency to build up nitrogen dioxide.
Now let's talk about how these molecules get into rainwater.
Nitrogen dioxide is more soluble in water than nitric oxide, which means
it can more easily dissolve and be included in the water. Nitrogen
dioxide can also react with water to form nitric acid, which is super soluble.
Once nitric acid gets into the rain water, it splits apart to form
nitrate, a form that phytoplankton can use. The rest of the process is
straightforward - the nitrate-filled rain falls and supplies nutrients to the
surface ocean.
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| Performing some maintenance on the atmospheric chemistry insturments |
In order to measure this process occurring in the atmosphere, we
measure nitrogen dioxide and nitric oxide (NO2 and NO). The air in
general over the ocean is very clean. Thus, anything we measure is from
air masses than used to be over land and have picked up pollution from sources
there. There has been a slight problem with the ship's smoke plume
interfering with the instruments. About 50% of the time, we end up
measuring air that is contaminated by the plume. The relative ratios of
the two molecules we are measuring are so different in the plume versus in
clean ocean air, that it isn't an issue to identify those points and remove
them from our analysis.
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| Our atmospheric chemist hard at work analyzing data. |
As a side project, we are also measuring nitrogen dioxide and
ozone in the total atmospheric column above us. In part this is to detect
plumes of pollution that are above the bottom atmospheric layer, which our
other instrument would not detect. The other part is to validate
satellite measurements. We can measure the same molecules in the atmosphere via
satellite, but we need observations on the surface to make sure those
calculations are done correctly. This is only the third time this
instrument has been used over water (instead of land), and the first time it is
out on the open ocean (instead of coastal zones).
Eventually, the measurements we make will be analyzed to help us
learn more about the process of depositing nutrients from rain. They will
help increase the accuracy of satellite measurements of the atmosphere, and
will also be used to help validate an atmospheric model.
Thanks for reading! As you know, we are getting to the end
of our cruise. The next post will be the last of the daily updates, and
I'll also talk about what lies ahead for us after this phase of the project
finishes.
More posts in this series:
Upcoming Cruise
Cruise Delays
Upcoming Cruise, Part II
Update #1
Update #2
Update #3
Update #4
Update #5
Update #6 Cruise Delays
Upcoming Cruise, Part II
Update #1
Update #2
Update #3
Update #4
Update #5
Update #7
Update #9
