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.

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.

Transit flight and Pico observatory fly-by

So the science team meeting was really good; as well as finding out a lot of important information about the BORTAS-B campaign this summer we saw some interesting data from the BORTAS-A campaign last summer. I will try and put together a series of posts over the next couple of weeks about what people had to say. The thing that I think was quite impressive was the number of people and organisations that were going to be involved this summer. So my first post concerns one data source which i think could prove quite valuable and which allows me to post some pretty pictures!


Obviously the aircraft has to get to Halifax somehow, and since the instruments are all onboard it seems sensible to make the most of the opportunity. This means that planning the transit flight is quite important as it gives us the opportunity to make some measurements further out in the Atlantic Ocean than will be possible from our base in Halifax. It has been decided that our route will probably look like this:

So we will fly from Cranfield, where the aircraft is based in the UK, to Porto in Portugal then to Pico in the Azores, St John's in Newfoundland and finally to Halifax in Nova Scotia. That might seem like a lot of stops but it gives us more flexibility in terms of doing maneuvers. This is important for the science as we will want to be able to look at changes in concentration at different heights and hopefully we will get lucky and intercept a forest fire plume. Even if we don't manage to see any forest fire emissions another brilliant opportunity is presented by the stop in Pico. There is an atmospheric observatory perched near the top of a dormant volcano on Pico Island which makes measurements of atmospheric composition. Details of the observatory are given here.

Pico Mountain showing the site of the Pico Mountain International Chemistry Observatory.

Measurements at this observatory no longer run continuously but they will be started in May 2011 and will run throughout the BORTAS-B campaign. The gas phase species that will be measured are carbon monoxide, ozone and non-methane hydrocarbons. The black carbon content and aerosol size will also be measured along with a number of meteorological parameters. Before landing at Pico Airport the aircraft will perform a circuit around the mountain and a height profile to see the changes in the composition of the air and physical parameters at different altitudes. This data can then be compared to the data from the observatory both providing context for the observatory measurements and allowing the two sets of instruments to be compared. 

 

 The instruments at the Pico Mountain International Chemistry Observatory.

Hopefully during the campaign the circulation of air will be such that air masses that we measure with the aircraft will travel on to be measured at the Pico observatory giving more information about how the plumes change with age. It is just a shame that I won't get the opportunity to go up and see the observatory (although I'm not sure how much I'd enjoy the steep trek up there) and the amazing views from the site.

The view from the observatory.

Thanks to Detlev Helmig from the Institute of Arctic and Alpine Research at the University of Colorado for pictures of the Pico Mountain Observatory.

Science Team Meeting

So we have the BORTAS science team meeting on Monday, Tuesday and Wednesday next week. It is taking place at the University of Edinburgh and will include discussions relevant to BORTAS-B, the campaign taking place this summer, and BORTAS-A, the campaign that went ahead last summer. The agenda is available at http://www.geos.ed.ac.uk/research/eochem/bortas/meetings/bortas-april-2011.html and anyone interested in watching online can contact Stephan Matthiesen whose details are on the above page.

There will be talks about the new instruments that have recently flown on the UK atmospheric research aircraft and whose performance is important for the BORTAS flights, and also talks about new instruments that will potentially be making their first real science flights during BORTAS. Presentations about the measurements that will be made across Canada and at the Pico Mountain Atmospheric Observatory on Pico Island in the Azores will also be made. And of course we will be deciding on a logo for the campaign so anyone involved who wants to submit a design better get designing fast. If you're coming I'll see you there, and if not then watch this space for an update on all the exciting news I'm expecting from the science team!

A Post for International Women's Day

So today is the hundreth international women's day. I was alerted to this by many different forums asking for suggestions of influential women in science, which got me thinking...I realised that I couldn't even name an influential female in the field of atmospheric science never mind tell you what their contribution was. I knew people whose work I had come across, whose research or attitude had impressed or inspired me but I wanted someone that would be widely accepted as an exceptional scientist...and if there was such a person shouldn't I have heard of them? So I asked around the people in my office to see if they could name any. Thankfully they seemed better educated than me and gave me two names to research, Susan Solomon and Julia Slingo.

Allegedly, Susan Solomon's interest in science began when watching The Undersea World of Jacques Cousteau however it wasnt biology she went on to study but atmospheric chemistry. She completed her PhD in 1981 at the University of California at Berkeley and then joined the National Oceanic and Atmospheric Administration (NOAA). The work that she is most famous for was her contribution to studies of the Antarctic ozone hole. She theorised that polar stratospheric clouds would provide solid surfaces on which the reactions of CFCs which destroy ozone could take place. The presence of a surface on which atoms can be immobilised greatly increases the rate at which these ozone destroying reactions can take place.

Polar stratospheric clouds

In August 1986 Solomon led an expedition to McMurdo Base, Antarctica to study the formation of the ozone hole. Not satisfied with just one trip she led another expedition in 1987 and from the two gathered enough measurements to show that chlorine dioxide was present at much greater concentrations than predicted, the first physical evidence pointing to chlorine chemistry as the cause of the ozone hole. Since then she has continued research at NOAA and was a co-chair of the Science Working Group I of the Intergovernmental Panel on Climate Change (IPCC). She has gained many awards for her work including the international Blue Planet Prize in 2004 and a share in the 2007 Nobel Peace Prize, she has even had an Antarctic glacier named after her.

The icebreaker USCGC Glacier approaching Winter Quarters Bay at McMurdo station, Antarctica 

Professor Julia Slingo is also an important figure in atmospheric science. She is now the Met Office Chief Scientist and before that she was the Director of Climate Research in NERC's National Centre for Atmospheric Science. Her background is in climate modelling and her research focuses on tropical climate variability and its influence and response to climate change. In 2006 she founded the Walker Institute for Climate System Research at the University of Reading. She was involved with the IPCC fourth assessment on climate change and in 2008 became the first woman President of the Royal Meteorological Society. Also in 2008 she received an OBE for her outstanding contribution to climate science.