Part of the Radiation Budget, pulled from the IPCC Fourth Assessment Report. Source: Kiehl and Trenberth (1997).

Carbon dioxide has a carbon cycle much like water has the water cycle. CO2 is emitted and absorbed in numerous ways. Whether it’s a chemical reaction in the atmosphere that changes molecules of CO2 into something else, the combustion of a fossil fuel producing CO2, or the oceans taking in CO2 from the air and depositing it deep under the surface, they all play roles as sources and sinks of atmospheric carbon dioxide.

Figure from ESRL Carbon Cycle Greenhouse Gases Group. The data comes from the Carbon Cycle Network that includes NOAA Baseline Observatories (like the South Pole Observatory) as well as cooperative programs around the world.

The figure above graphically depicts the carbon cycle on an annual basis. The red strip indicates the data that has been gathered over the last 10 years from the South Pole. The waves in the graph are from the seasonal uptake of carbon dioxide by plant life. For example, in the summertime when trees have their leaves, and plant life is not dormant, you see a large dip in CO2 values especially in the northern hemisphere because plants take in CO2 for photosynthesis. Also notice the difference in variation (waviness) of CO2 between the Northern Hemisphere and Southern Hemisphere. This is due to the very large forests in the N. Hemisphere compared to the S. Hemisphere, and the fact that the N. Hemisphere is more populated. Aside from the annual variation, notice the steady rise of CO2.

In previous posts, we’ve looked at some of the instruments at the Atmospheric Research Observatory (ARO) that measure things like solar radiation. At the South Pole, carbon dioxide has been measured for just over 50 years now and there are a few different methods of obtaining a CO2 value.

One of them is using our Portable Sampling Unit (PSU) that looks like a suitcase containing a pump inside. Using the PSU, we walk out into the Clean Air Sector (CAS), turn on the unit, and pump air into glass flasks which are then shipped back to the Earth Systems Research Laboratory in Boulder, CO for analyzing. (I shot a video of the process which can be seen here.)

The second is hooking the same type of glass flask that we use in the PSU to the Through Analyzer. What this does is bottle up a sample using the same sample lines as our third method uses a Non-Dispersive Infrared Detector (NIDR).

The Portable Sampling Unit (PSU). Flask samples with this unit are done once a week. On the first and fifteenth of the month, they are done in conjunction with sampling from the Through Analyzer.

Upper right: The Li-COR Non-Dispersive Infrared Analyzer. Lower Left: A display graphing the voltages recorded over the past four hours. The spikes are the calibration gases measured once every hour.

The NIDR uses an infrared source which is a heated filament that emits infrared radiation around the same wavelength that carbon dioxide likes to absorb (usually around 4.26 µm). This energy travels through two absorption cells, one of which is containing a sample of air from outside, and another that is containing a reference gas from a compressed gas cylinder. A mechanical chopper wheel then alternates between the sample and reference gas measuring the difference in the amount of absorption between the two. Using the difference of the two cells helps negate the problem of changes in temperature and pressure. Changes in pressure and temperature change the density of the sample which would skew the amount of carbon dioxide molecules in the measurement. This is a very useful machine that requires very little maintenance and gives us measurements continuously 24 hours a day, 7 days a week (there is about 15 min of each hour that it measures accurate known amounts of carbon dioxide in other gas cylinders for calibration).

On the right are the three calibration gases and the reference gas.

Here is a comparison of when the winds are in the Clean Air Sector (CAS), and when they are blowing station air towards the Atmospheric Research Observatory (ARO). The this graph is when the winds were from the direction of the station and the graph that follows is when the winds are in the CAS.

As you can see we easily pick up local carbon dioxide when the winds are blowing from the station. Winds at ARO are within the CAS probably greater than 90 or 95 percent of the time which is why it is such a good place to get long term continuous measurements of CO2.

Part of the Radiation Budget, pulled from the IPCC Fourth Assessment Report. Source: Kiehl and Trenberth (1997).

Aerosols have different properties depending on what they are made of. Some of those properties that are important are roughness, color, and size. These properties affect what happens to solar radiation as it reacts with the particle such as whether it will scatter or absorb. For example, a particle of black carbon (left over after burning of a fossil fuel let’s say) is going to be much more effective at absorbing solar radiation than a piece of salt that has a much lighter color as well as a shinier surface (shiner, brighter surfaces reflect radiation better). It is also important how they are distributed spatially around the globe and how long they stay in the atmosphere. To think more about the spatial distribution, at the South Pole, we have no vegetation, dirt, salt water, or large sources of combustion anywhere close to us (aside from our own station which is downwind from where we sample). We have extremely low concentrations of aerosols in the air here compared to a sand desert or near a volcanic eruption. The time that they spend in the atmosphere can depend on many things. If it is a large particle, it may settle back to the surface quicker. If it rains, the aerosol may get collected by the raindrops and land back on the surface. Depending on the hygroscopic (the ability for a surface to become wetted or have water stick to it) properties of the aerosol, water can also condense on them to make cloud droplets.

Speaking of cloud droplets, aerosols can indirectly affect solar radiation by being an ingredient for clouds to form. Water likes particular sizes and types of aerosols to condense upon. The name this particular type are Cloud Condensation Nuclei. If there are no Cloud Condensation Nuclei present, the water has nothing to condense onto and there will be no cloud. The size and type of aerosol affect the physical properties of the cloud as well. Therefore a change of aerosols in a region can change the type of cloud, thus changing its radiative properties. This dependency on Condensation Nuclei brings up yet another complicated variable that can affect the radiation budget.

So that explains a little bit why we are so interested about the “dust” in the air. Aerosols have a significant influence on climate processes. Now let’s take a look at the instruments that deal with aerosols at the South Pole.

The Condensation Nucleus Counter (CNC) does basically what its name says. It uses butyl alcohol to create a cloud by cooling the flow of the air through the instrument. Inside is a particle counter that counts how many droplets there are.

The Nephelometer measures the radiation scattering ability of the aerosols. By running air through the instrument and shining a light through the air, we can detect how much of that light is getting scattered with a photomultiplier tube (PMT). There are filters in front of the PMT in order to detect several specific wavelengths of light. If we know the output of light initially, and subtract what is detected, then we know how much light is scattered by the aerosols.

The Aethalometer is an instrument that we use to measure radiation absorbing aerosols. For this instrument, air is passed through a filter where the aerosols will deposit onto. The filter is illuminated by a lamp and there are two photocells that sense the light. One is a reference sensor on a spot of the filter with no aerosols, and the other is sensing where the aerosols where deposited. The difference between these 2 sensors is the amount of light absorbed.

Here I am using the Pollack which is an older instrument that is used to compare to the CNC. It creates a cloud by depressurizing a chamber which causes the air to cool and form a cloud. There is a PMT that detects light and I watch an ammeter to see how much the current goes down when the cloud forms.

Not pictured is the Water-Based Condensation Particle Counter which is very similar to the Condensation Nuclei Counter that is shown above. The laser inside broke during the summer and we were not able to send it to the lab, get it fixed, and get it back to the Pole before station closing.

So that is how we measure aerosol concentrations and their scattering/absorbing properties at the South Pole. The next process that we measure at the South Pole will be the Carbon Cycle and Greenhouse Gas (CCGG) group. We’ll take a look at the CO2 analyzer and the role that greenhouse gases play in the climate.

The Final Flight: February 14th, at 2:30am New Zealand time.

We have just barely over two weeks until the sun sets and temperatures are already starting to drop quickly. The day of station closing, temperatures were around -40F. Today it is the coldest since I’ve been here at -63F, and tomorrow it’s suppose to bottom out at almost -70F. It’s amazing how quickly it drops when that sun gets low. The cold temperatures also make everyday things difficult to deal with. We had an emergency response drill today that took place outside and I volunteer on the fire team. You have to be really conscious about your gear because the SCBA (Self Contained Breathing Apparatus) hoses start to freeze and can crack easily. A fire fighter isn’t much good without a working SCBA. Frostbite is a big concern as well. The fire gear gloves and boots are not insulated for cold and do a very poor job of keeping your fingers and toes warm.

South Pole Station from ARO.

As for the science here at ARO (Atmospheric Research Observatory), not too much has changed. I’m still coming out here every day to check to make sure everything us running as it should be and taking air samples in flasks every week. One thing that is starting to change is our ability to do Dobson observations. The Dobson Spectrophotometer is an instrument that uses sunlight to measure total column ozone in the atmosphere. When the sun is this low on the horizon, there is a lot of stray refracted light that affects the measurements and can give us bad results. You may ask, “How do you take measurements in the winter?” Well this is done by using the reflected sunlight off of the moon. So we are able to take sporadic observations to coincide with our balloon flights through the winter. The solar radiation instruments on the roof will be coming down soon after sunset as well, which will be a small project for us. Here is a brief description of the solar radiation measurements we have at ARO and why we are measuring it.

Incoming solar radiation is the backbone of what drives our climate. Changes in the amount of radiation reaching the earth from the sun can be the difference between being in an ice age or not. It is important for us to know how much radiation is a) reaching the surface, b) what type of radiation it is (wavelength), and c) how much is bouncing back off the surface. This is what’s called the “Radiation Budget” in its most basic form. The “Radiation Budget” involves many other processes but the pictures and descriptions below show how we break down the “Radiation Budget” into its basic components at ARO.

The Solar Tracking NIP (Normal Incidence Pyroheliometer)

The NIP tracks the sun in all 360 degrees. It measures direct incoming solar radiation of specific wavelengths.

Diffuse Pyranometer

The diffuse pyranometer blocks out the incoming direct solar radiation and measures any radiation that is getting reflected and refracted from substances in the atmosphere (or any radiation taking an indirect path to the surface).

Pyranometers

These pyranometers detect all incoming solar radiation both direct and indirect. The two outer ones have filters on them to divide it up into shortwave (UV) and longwave (infrared) radiation.

Albedo Instruments

The “Albedo Rack” is basically exactly the same as the pyranometers except that they are turned upside down. They then measure the amount of solar radiation that is reflected off of the earth’s surface. Roughness and color play a role in Albedo meaning that a smooth surface is going to reflect more than a rough surface, and a white surface is going to reflect more than a black surface.. Therefore, it is important not to disturb the snow under these instruments because we want the natural state of the surface. In addition to reflected radiation, it monitors infrared radiation emitted by the earth.

A more complex version of the “Radiation Budget” or “Energy Balance” pulled from the IPCC Fourth Assessment Report.

As you can see, in the above figure, there is a lot that really goes into the “Radiation Budget” and it is a very complex system. When the solar energy comes into the atmosphere, it can take a variety of paths. It can get interrupted by clouds, gases, aerosols and other substances. Two of these processes in the system we observe at ARO as well such as Aerosols, and Greenhouse Gases which I will talk about in a later post.

Hopefully this explains a little bit what’s behind the solar radiation observations that we take at ARO. The South Pole and Mauna Loa have the longest continuous running solar radiation observations of this kind. It’s extremely important that we understand what happens to solar radiation as it passes through the atmosphere and hits the earth’s surface if we want to gain a good understanding of how earth’s climate works. It is even more important as we try to predict future climates.