Tuesday, 31 May 2011

Ozone Sonde Launches

I have mentioned before on Twitter that we have some brilliant collaborations going on which will help us understand the data we collect during BORTAS this summer. Many of these have already started with scientists from a number of universities and institutes collecting data during the BORTAS-A period last year. In earlier posts I talked about the Pico Mountain Observatory and the lidar at Dalhousie University but another useful and exciting activity that will be going on this summer, and in fact goes on all year round, is the launching of ozone sondes across Canada by the Environment Canada Experimental Studies Unit (ARQX).

The first question might be 'what is an ozone sonde?'....good question. Sonde is the French word for probe and so it makes sense that this is essentially a probe to measure ozone. They are lightweight instruments that can be carried by a helium filled balloon (not a party balloon, these ones are slightly bigger and more robust). The iodide redox reaction (shown below) is used to allow ozone to be detected.

2KI + O3 + H2O -> I2 + O2 + 2KOH

This reaction is used to produce an electrical signal which is proportional to the ozone concentration. This can be done in a number of ways each of which is explained at http://www.atmosp.physics.utoronto.ca/SPARC/SPARCReport1/1.08_O3sondes/1.08_O3sondes.html. The sonde also includes meteorological instruments to measure pressure, temperature and humidity. The balloon carrying the ozone sonde can travel upwards as far as 35 km before the balloon bursts. This happens because as pressure decreases higher up in the atmosphere the helium inside the balloon expands into the lower pressure surroundings. The balloon can not expand indefinitely and so eventually it bursts.

 On the left is the inside of an ozone sonde showing the two solutions used to create the electric current. The other two pictures show ozone sonde launches with the middle one using a special balloon which can get to higher altitudes (it looks very dramatic!).

ARQX launch ozone sondes weekly from ten locations across Canada. During BORTAS they will also launch daily sondes from Yarmouth, Sable Island, Goose Bay, Egbert and Bratt's Lake. There is also the possibility of additional launches if a plume is predicted to pass over one of the launch sites. The sites are all shown on the map below. Ozone sonde launch sites are identified by the symbol that resembles a balloon carrying an object (an ozone sonde of course). Also depicted on this map are sites that operate lidar instruments similar to that operated at Dalhousie University and sites which have Brewer spectrophotometers. These instruments measure total ozone and UV radiation, to find out more have a look at the Environment Canada pages at http://exp-studies.tor.ec.gc.ca/e/ozone/ozone.htm.

Network of ozone sonde launch sites, lidar locations and sites with Brewer spectrophotometers.

David Tarasick from ARQX kindly provided me with some plots from data collected during the BORTAS-A period last year. On numerous occasions the vertical profiles from ozone sonde flights show regions of elevated ozone. An example of this is shown below.

Left hand plot shows the vertical profiles taken by the ozone sonde at Edmonton with ozone mixing ratio in black. The area of high ozone is circled in red. The right hand plot shows where the air at Edmonton had come from overlayed on the fire counts for the previous day.

These plots show that the air sampled is likely to have been impacted by the fires north east of Edmonton and this could be the cause of the elevated ozone. Another example is shown below from the Goose Bay site. This shows not only the data from the ozone sonde but also aerosol measurements from the lidar and the carbon monoxide (CO) forecast carried out by scientists at Edinburgh University. All plots indicate something is happening between 4 and 8 km with CO and ozone mixing ratios and aerosol backscatter ratio and cross section all being elevated.This supports the suggestion that the elevated ozone is a result of biomass burning activity. 

The top left plot shows the ozone sonde profiles, top right is the predicted CO from the GEOS-5 model, bottom left is the aerosol backscatter cross section and bottom right is the aerosol backscatter ratio.

From this data we can say that elevated ozone is observed at ozone sonde stations downwind of large boreal forest fires. Back trajectories suggest that the sampled air passed over a region of burning. This elevated ozone may be the result of reactions involving nitrogen oxides, CO and hydrocarbons which are present in biomass burning plumes. The presence of aerosol layers at similar altitudes would support the suggestion that burning plumes have influenced the composition of the air mass. A possibility that can not yet be ruled out is that the elevated ozone is from air that has travelled into the lower atmosphere from the stratosphere where ozone concentrations are significantly higher. This is supported by the relative humidity profiles which show dry air at the altitudes where ozone is elevated but the coincidence of plume interception (suggested by the lidar aerosol data and trajectories) and downwards mixing of stratospheric air is unexplained. Hopefully measurements this summer will add some pieces to this puzzle and help us understand the processes going on in and around aging forest fire plumes.

Thanks to David Tarasick for pictures, data plots and information.

Friday, 27 May 2011

Satellite Measurements of Trace Species

Post written by Keith Tereszchuk.

One of the objectives of BORTAS will be to compare the in-situ aircraft measurements made during the flight campaign with remote sensing data provided by satellites. Space-borne observation is used extensively in many facets of monitoring of the Earth’s surface and atmosphere including weather forecasting, air quality measurements, ozone levels (UV Index), volcano emissions, ocean currents, sea/lake roughness (wave height), desertification assessment and numerous other such environmental management studies.  These are just a few examples of the importance of satellite monitoring and how the information they provide us directly impacts our day-to-day lives. The identification and characterization of biomass burning plumes and their effect on atmospheric chemistry is yet another area of study conducted using satellite remote sensing.


BORTAS will be using data from the Atmospheric Chemistry Experiment (ACE) on-board the Canadian satellite SCISAT-1, which uses a high-resolution Fourier transform spectrometer (ACE-FTS) for remote sensing of the limb (see picture) of the Earth's atmosphere down to 3-km above the Earth’s surface (just above the tropospheric boundary layer). The ACE-FTS has wide spectral coverage in the infrared region of the electromagnetic spectrum scanning a contiguous region from 750 to 4400 cm-1 and currently offers data retrievals for 38 molecular species as well as their isotopologues, over a dozen of which are known biomass burning marker species, e.g. CO, HCN, HCOOH, H2CO, C2H6, C2H2, CH3OH, HNO3, CH4.

View of the limb of the Earth at sunset

Unlike nadir instruments, which look directly down towards the surface of the Earth, ACE peers through the limb of the atmosphere recording sequential absorption spectra using the sun as an emission source. Each sunrise and sunset of the satellite is called an occultation and they provide concentration profile information of each molecular species with respect to altitude.  Much like ogres and onions, ACE can be used to separate the atmosphere into distinct layers with a spatial resolution of 1 km. On average, 20 measurements are made on a daily basis.

The Atmosphere: It has layers

ACE occultation of the Earth’s Atmosphere

               ACE provides widespread global coverage and during the BORTAS campaign, it will make a total of five measurements that will be within the 500 nautical mile range of the FAAM aircraft which will be based in Halifax.  These measurements will be used to compliment the data recorded during the aircraft flights to further understand the chemical evolution of molecular species emitted by biomass burning. 

Positions of the predicted ACE occultations, shown by circular ACE logos. The red circles show the distance from Halifax where the aircraft will be based.

              Ideally we would like to be able to identify the sources of the plumes that will be measured during the campaign to study molecular evolution within them, and satellites can help us do that too. We will be using data from the MODIS Terra instrument, which is a spectroradiometer that records surface temperatures of the Earth including thermal anomalies such as actively burning fires, to identify potential source regions. When the location of fires are known, we can determine the plume sources by calculating air-mass trajectories using a program called HYSPLIT, which is a Lagrangian particle dispersion/trajectory model based on satellite climatologies. These climatologies provide information on air-mass flow at a particular time at any point in the atmosphere across the entire globe.  Using the ACE profiles of the biomass burning marker species, we can determine the altitude that corresponds to the highest concentrations of these molecules and use this altitude as the injection point for backtracking the air-mass flow to its source using HYSPLIT. In addition, we can calculate forward trajectories from identified sources. If these trajectories coincide both spatially and temporally, we can confidently confirm the source of the plume measured.

HYSPLIT trajectories. Backtracking from an ACE measurement made over Hudson Bay and four forward trajectories made from known biomass burning events over northern Saskatchewan and the Northwest Territories (July 2008).

              Bringing together all the aforementioned information, we will be able to characterize biomass burning plumes to further our understanding of the overall impact of biomass burning on atmospheric chemistry.

Wednesday, 25 May 2011

BORTAS logo!

During the science team meeting in April there was a logo competition. The best design won the honour of featuring on presentations, badges, stickers, mugs and anything else the project team decide they want to get printed. There were 3 entries, all of which had their individual merits but the winner, a very professional looking logo designed by Keith Tereszchuk, is shown below.

This logo will now be displayed permanently at the right hand side of this blog. Keep a look out for the exciting BORTAS merchadise that will hopefully become available in the next few months (I could do with a new coffee mug!). Different size versions of the logo are available at http://www.geos.ed.ac.uk/research/eochem/bortas/logos.html.

Tuesday, 17 May 2011

LIF Instrument Performance

So in the post on the 11th October I told you about my first experience of research flights with the newly installed laser induced fluorescence instrument (LIF). This instrument can measure NO2 and with a specially designed inlet also the sum of peroxynitrates and alkylnitrates. In theory the system can also measure nitric acid (HNO3) but because nitric acid is a 'sticky' molecule it is more difficult to measure and so we will be interested to see the comparison between the LIF HNO3 measurements and those made by the CIMS technique described in the previous post. The instrument draws in air from outside the aircraft and fires a laser pulse through the sample at 532 nm. Any NO2 in the air absorbs this radiation and becomes excited. The excited NO2 then fluoresces, giving off radiation and returning back to the ground state. The radiation can be detected and is proportional to the concentration of NO2 in the sample. Using calibration data we can then calculate the NO2 concentration in the air.

For the RONOCO flights we were very interested to see how the LIF system would compare to the existing chemiluminescence system (operated by FAAM) which measures NO and NO2. The NO2 measurement on this instrument is obtained by passing air through a photolytic converter and then into the chemiluminescence analyser. The photolytic converter contains 2 arrays of blue light emitting diodes (LEDs) which break the NO2 up into NO and an oxygen atom. The NO produced is then passed into the analyser where it is reacted with ozone (O3) to produce excited state NO2 and oxygen. This excited state NO2 emits radiation in a chemiluminescent reaction and the amount of radiation is proportional to the NO that was in the sample. By turning off the LEDs the NO2 is not converted to NO and the signal is due to any NO that was originally in the air sample when it entered the system. If we subtract this signal from that obtained with the LEDs on we get the signal that was due just to the converted NO2. Complicated, but I hope that makes sense!

So to the data.....the plot below shows the flight track for flight B534 off the east coast of the UK. The height of the track shows the level of NO demonstrating that we were sampling both inside and out of a plume of pollution coming off the coast.

The flight track for B534. The height of the track indicates the concentration of NO, a tracer for pollution.

This second plot shows the LIF NO2 data on the same axis as the chemiluminescence NO2 data. You can see that we measured a range of concentrations from below 1 ppbv up to 30 ppbv. The time series traces seem to follow each other well but a better test is to do a correlation plot which is shown on the lower of the two axis. This shows that the relationship between the two measurements is linear and the equation y = 1.1x - 0.22 shows that despite a slight offset (-0.22) changes in the measurements are almost equal (the gradient is close to 1). The R2 value of 0.995 shows a good correlation between the two with few outlying points. All in all a very encouraging comparison! Now we must await the results from a more thorough comparison carried out using these two instruments plus a broadband cavity enhanced absorption spectrometer (operated by University of Cambridge) which will hopefully show that within the instrument errors these two instruments measure the same.

Top axis: Time series of NO2 measurements from the LIF analyser in black and the chemiluminescence analyser in red. Bottom axis: Correlation plot of LIF NO2 against chemiluminescence NO2 giving the equation of the fitted line (black) and the R2 value.

Tuesday, 3 May 2011

CIMS instrument confirmed for BORTAS

CIMS stands for Chemical Ionisation Mass Spectrometry and this technique has been shown to be brilliantly versatile with the potential to measure a vast array of important atmospheric species. The system that will be used on the BORTAS flights (operated by the University of Manchester) this summer uses polonium-210, one of the 33 radioactive isotopes of polonium. The polonium-210 emits alpha particles which can knock electrons out of the nitrogen molecules that are present. These 'free' electrons are then captured by neutral molecules which become negatively charged. In this case it is the reagent gas CH3I (methyl iodide) that captures the electron and dissociates giving negatively charged iodine atoms (I-). This I- then reacts with the species we want to detect and transfers the negative charge as shown below or attaches to the other species forming a negatively charged adduct such as HCN.I-.

Because our compounds of interest are now negatively charged they can be separated according to their mass to charge ratio and then detected. Chemical ionisation is used because it is gentle giving little fragmentation (where the ion splits up creating many more ions with different mass to charge ratios which complicate the spectrum). I- is used because it is fairly selective; it is unreactive with most compounds present in the atmosphere but reacts with the compounds we want to detect. So this set up will allow us to detect the sum of NO3 and N2O5 (they rapidly interconvert), nitric acid, formic acid, propanoic acid, butanoic acid and HCN. HCN has been shown to be an important tracer for biomass burning (eg. see http://bit.ly/laf97J) so this measurement particularly is useful for us.

The CIMS instrument on board the UK Atmospheric Research Aircraft

Like the LIF instrument (see blog posts from 11th October 2010 and 11th June 2010), the CIMS was flown on the RONOCO flights in August and September last year and January this year. It was during these flights that a correlation between the NO3 and N2O5 measured by broadband cavity enhanced absorption spectroscopy (BBCEAS, operated by the University of Cambridge) and a peak in the CIMS spectrum was noticed. Then through a comparison exercise carried out at the Facility for Airborne Atmospheric Measurements the University of Leicester's NO3/N2O5 calibration unit was used to confirm that the suspected NO3 peak was indeed due to the sum of NO3 and N2O5.

 The top plot shows the time series from the CIMS suspected NO3 peak and the BBCEAS NO3 and the bottom plot shows the strength of the correlation between the two.

At the beginning of the post I mentioned the versatility of the CIMS technique so as an example here is a list of the species I haven't mentioned that can be measured using different reagent ions: SO2, HO2NO2, PAN, PPN, MPAN, HOOH, CH3C(O)OOH, HCl, ClONO2, NH3, DMSO, C5H8, HO2 HO2+RO2, H2SO4, amines, various volatile organic compounds.

If you want to learn more about the CIMS or what the guys at University of Manchester do see this page and pages linked from it. Thanks to Jennifer Muller from the University of Manchester for the information on CIMS and the results from the RONOCO campaign.