The sky's the limit

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Atmospheric scientists are taking to the skies in the quest for ever more reliable and up-to-the-minute data, Cath O'Driscoll reports.

Atmospheric scientists are taking to the skies in the quest for ever more reliable and up-to-the-minute data, Cath O’Driscoll reports.

The back of an aeroplane is not the easiest place to do analyses, admits Ally Lewis of the University of York. Not only do your instruments have to contend with the roll and pitch of the aircraft but there’s also the problem of power - typically restricted to a few hundred watts, as opposed to the kilowatts normally available on the ground. What’s more, if, like Lewis, you’re really serious about measuring the dynamics and chemistry of air then, above all, your equipment has to be fast. Very fast. Because as soon as you detect an interesting result, you need to turn the aeroplane in that direction and go after it. ’It’s all about being in the right place at the right time,’ Lewis says.

The best days to take to the air are wet, windy ones when there’s a better chance of seeing an interesting change in atmospheric composition, he adds: ’Imagine a great thick cloud and lots of rain coming over. As the cloud sweeps over a city, all the organics and inorganics get caught up in it; you get a front coming over and then clean air after it because most of the gunk gets sucked up into a frontal system and is spewed out at the top into the free troposphere, so surface level pollution can affect altitudes of 4, 5 or 6 km above ground’. For atmospheric scientists like Lewis, the excitement lies in knowing where all of this ’gunk’ comes from, how long it took to travel up into clouds and how it gets processed into other, sometimes nastier, substances on route.

When Chemistry World caught up with Lewis and his former colleagues Mike Pilling and Dwayne Heard at Leeds University last year, all three were eagerly anticipating the lease of a new aeroplane: a BAE 146 (BAE Systems) that is much more heavily instrumented than the old Met Office Hercules C130 that researchers had been used to flying in until then. ’The main advantage is that it will be equipped as a real laboratory,’ Pilling told Chemistry World. ’The plane is obtained on JIF [Joint Infrastructure Fund] funding; it’s leased so we don’t actually own the aircraft but we’ve had four new engines and new fuel tanks put in and it’s been kitted out as a laboratory. There’s even a website for it [www.faam.nerc.ac.uk/public/aircraft.html]’.

In its first major chemistry ’campaign’ later this year - involving researchers from six or seven different universities - the plane will be based around the Azores in the middle of the Atlantic Ocean. Two other planes, a Nasa DC8 as well as a P3 plane from NOAA (the US National Oceanographic and Atmospheric Administration), will start by flying out across the Atlantic from the US, and the plan is for the BAE 146 to pick up the same air mass they’ve been tailing in the mid-Atlantic.

The air should continue it’s journey from there across the Atlantic, where French and German aircraft, will pick it up and follow it on the last leg to Europe. ’It is a very big, very ambitious project with lots of different objectives, but the main thing is really to see how chemical pollutants get transformed as they get transported across the Atlantic,’ Lewis says.

Later this year the pair will be taking part in an intensive campaign in Antarctica, where scientists from around the UK will be gathered to engage in a comprehensive survey of atmospheric conditions in the continent. During the campaign, researchers will be stationed mainly at the new Clean Air Sector Laboratory, at Halley Base, where they will be looking especially closely at levels of NO_x. Interest in NO_x levels in the continent follows the recent suggestion that NO_x (a mixture of nitrogen oxides) may be being trapped - and periodically released - by snow.

Heard explains the significance of this: ’If you think of the size of Antarctica, if NO _x is being released over the whole of the continent, then that’s a lot of NO_x. It doesn’t go very far vertically; it’s trapped in quite a thin [boundary] layer of air, 30-50 m above the ground, so this enhanced chemistry probably occurs over a very shallow layer across Antarctica. But it could produce a lot of things, it could be transported away in the form of organic nitrates. basically not much is known, hence our study’.

Another significant observation, Heard continues, is that higher levels of hydroxyl radicals were found than was expected, owing to the increased amount of NO _x coming from the snow. Hydroxy radical concentration is a measure of the oxidising capacity of the atmosphere, and hence controls the lifetime of many trace gases. In fact, in summer months the oxidising capacity at the South Pole (averaged over 24 hours of sunlight) is similar to that in the Tropics.

Increasingly, Lewis and Heard are spending ’a significant amount of time in polluted regions, trying to understand more complex chemistry where there are significant emissions’. According to Lewis, ’surprisingly little is known about the composition of polluted air. Nerc [the Natural Environment Research Council, which funds much of atmospheric science research in the UK] has never been very interested in polluted air; it has always seen this to be a Defra [the UK Government Department for Environment, Food & Rural Affairs] responsibility, but it has begun to appreciate that there is research to be done. We can’t know more about the detailed chemistry of the atmosphere in Antarctica than we do about the atmosphere in London’.

A study by Lewis and his group of the air quality in Melbourne, Australia, a few years ago, highlights some of the problems encountered in an urban location. Armed with his own specially constructed GC equipment, Lewis and the team were able to detect parts per trillion (ppt) to parts per billion (ppb) concentrations of up to 500 volatile organic compounds (VOCs) - an unprecedented number seen in gas phase air at that time.

More worryingly perhaps, roughly a fifth of the VOCs that they detected are ’volatile aromatics’, highly reactive species known to be involved not only in ozone formation but also in producing so-called secondary organic aerosols, which are thought to be harmful to human health. ’Altogether, urban air might contain as many as a thousand compounds, ranging in concentration from a few tens of ppt up to a few ppb, so overall there might be 0.1 or 0.5 parts per million (ppm) of organics,’ Lewis observes.

Melbourne is not alone in having such high numbers of carbon-containing pollutants. The situation here in the UK is ’the same or even worse’, Lewis says: ’The main problem in urban areas is petrochemicals from transport sources, and petrochemicals are fabulously complicated. The Melbourne experiment was the first time we’d tried to get an overview of what was there, because although petrochemicals are a massive source of urban air pollution and take part in these global chemical cycles, people hadn’t looked at just what was being contributed’.

The starting point for most atmospheric oxidation schemes is the hydroxyl radical. Most should ultimately lead to carbon dioxide and water vapour, but there are less welcome by-products as well. Ozone, particulates and NO₂ are all known for their harmful effects on human health, and are responsible for the hazy photochemical smog associated with places like Los Angeles. Heard had the opportunity to see for himself this relationship between OH and ozone during the solar eclipse of 11 August 1999, without the complicating effects of sunset and sunrise that otherwise accompany nightfall.

Most hydroxyl radicals are synthesised photochemically; not surprisingly, the level plummeted in the absence of sunlight. What Heard and the team were more surprised to see was that the level of ozone also fell significantly, to about 60 per cent of its original value, but after a short lag of about five minutes or so. This was evidence of ’a clear and direct link between OH - the initiator of oxidation processes - and ozone, which is formed as a secondary pollutant’.

But OH does occasionally pull some surprises. During fieldwork in Birmingham in mid-winter a couple of years ago, Heard and the team expected to find significantly lower levels of hydroxyl radical than are found in mid-summer. In fact, the amount of OH was only 50 per cent less. ’To begin with, we thought we must have measured this wrong, because it was such a surprise to us. But then modellers started to consider all the chemistry in this complex urban environment and found that there were several reactions that didn’t need sunlight to produce OH, and that a lot of OH is formed as a result of VOCs being broken down by sunlight at longer wavelengths, which are still there in winter. And even things like ozone and unsaturated hydrocarbons could give OH.’

Tagging organic compounds onto aerosols is significant, because it changes their properties, which in turn may make them more or less predisposed to forming clouds. It also has important implications for climate change and the toxicology of particulate pollution, Lewis elaborates.

Lewis and Heard are already on the case. Last summer, along with a handful of other atmospheric researchers from around the UK, the pair were stationed just outside the M25 near Chelmsford in Essex, in the first stage of a campaign called Torch (Tropospheric organic chemistry experiment). ’What we are looking at is how organic compounds transform in the atmosphere - seeing when they are emitted and where they end up. Do they end up as CO ₂ and H ₂O or do they end up condensing onto particles? So this is an experiment that’s half gas phase and half particle phase, and this is really the first time that anyone has tried to do an experiment that has these two phases: gas and particle, actually interlocked.’ Interestingly, the timing of the Heard’s campaign coincided with the most polluted period the UK has seen for many years, with levels of ozone reaching 150 ppb - the highest for more than 20 years - and temperatures soaring past 35?C.

Acknowledgements

Cath O’Driscoll

Fast work

To cope with the exacting demands of working at altitude, Ally Lewis, York University, has resorted to building his own GC equipment known as ORAC (Organics by real-time airborne chromatography).

Using ORAC, Lewis can analyse some of the key carbon-containing compounds in an air sample (for example benzene and toluene) in about 4 minutes, more than 10 times faster than with standard GC-MS equipment in the laboratory. Pairs of compounds such as benzene and toluene are useful because they carry information on the history of an air mass. At source, there is four times the concentration of toluene to benzene present, but since toluene reacts significantly more quickly with OH, after four or five days the ratio reduces to around 1:1. This helps Lewis and the team to ’age’ the air mass or to estimate the amount of OH encountered on route.

To construct ORAC, Lewis had to eke out the dimensions of the standard GC column to less than 0.2 mm diameter and about 30 m in length. In this way, the column can be heated and cooled between temperatures of 35 ?C and 150 ?C in just a couple of minutes, using only a few hundred watts of power. So that this process operates continuously, sample gas flowing into ORAC passes through a series of three traps: while one trap is being loaded with sample, the second contains air that is being analysed and the third is being cleaned and cooled before further use.

ORAC is typically installed in place of passenger seats in an aircraft, and can detect individual species in air at levels as low as 3-5 ppt. To ensure that this information is fed swiftly back to the scientist controlling the flight (and then subsequently to the pilot), ORAC uses commercial software to integrate and quantitate peaks of interest. Lewis sees this information on the aircraft’s computer as a constantly updating web page.

All of the detailed analytical work is done back on the ground, using air collected during the flight in a series of automated collection canisters arranged in the aircraft hold. Each canister holds between one and five litres of air, depending on altitude and pressure. With the aircraft travelling at typical speeds of about 180 ms^-1 (just over 400 mph), each canister collects air over a distance of about 10 km. However, conventional GC-MS separations can fail to resolve up to two-thirds of the carbon mass that is present in urban air due to the large number of possible isomers.

For really detailed analyses of samples collected from urban locations, Lewis has another tool in his armoury: two-dimensional or orthogonal GC coupled with mass spectrometry. With this technique, he is able to produce an astonishing 200 mass spectra per second; conventional GC-MS apparatus would take just one spectrum in the same time.

Taking the air

Dwayne Heard made the first measurements of tropospheric hydroxyl levels by a UK research group in the summer of 1996 at Mace Head on the west coast of Ireland. Air sampled here is some of the cleanest in the northern hemisphere, because the prevailing westerly winds do not cross land for five days or more before reaching these shores. To carry out the experiment, Heard used his own ’portable’ laboratory: a ten-ton shipping container housing a laser system and considerable amounts of electronic equipment, including a computer.

Aptly known as the ’HOxBOx’, the basis of all this gadgetry is a technique called FAGE (fluorescence assay by gas expansion), which detects OH by a characteristic fluorescence fingerprint. A pump draws air into a detection chamber where it is irradiated at low pressure with a UV laser beam to produce excited OH radicals. When the laser light is switched off, the radical lose excess energy as weak fluorescence, which the computer records as a ’fingerprint’. Depending on local conditions, Heard found that the concentration of OH at midday in Mace Head was roughly 5 x 10⁶ molecules per cm³ (0.02 ppt). This sounds like a lot, until you consider that OH radicals make up less than 1 part in 10¹³ of the air that we breathe, and each lasts for less than a second.

Heard’s original FAGE instrument, which used copper vapour lasers and dye lasers, took up half his HOxBOx; it was 3 m long and weighed several hundred kilograms. While it worked well on the ground, this equipment was hardly suitable for bringing aboard aircraft. The new model, which includes a solid-state YAG-pumped Ti sapphire laser, is much smaller and will ’sit happily on the rack in the new aircraft,’ Heard hopes. What’s more, it consumes significantly less power and can be rapidly switched on via a conventional AC source. The airborne FAGE will be tested in the Azores this summer.

This new solid-state laser can also cover different wavelengths more easily, simply by changing the orientation of a non-linear crystal. This should make it useful for studying other molecules such as NO (226 nm) and IO (445 nm). The latter is generated following photolysis of iodides produced from, for example, seaweeds and is thought to play a major role in aerosol formation in coastal regions. Heard has detected these molecules with his laser system in early field tests and hopes that one day fast measurements can provide a check on those made using other methods.

Another interesting development from Heard’s laboratory is a new laser instrument that will not only measure the concentration of hydroxyl radicals but also record their lifetime in the atmosphere. OH radicals are inherently unstable and last for as little as a tenth of a second, depending on air quality. In heavily polluted air, at least half of the hydrocarbons present may not be detected directly, so measuring hydroxyl radical lifetimes can give a fair indication of what may have been missed. This instrument was used for the first time last year, and very short lifetimes of OH were measured. Whether this agrees with model calculations remains to be seen.

Laboratory in the sky

In addition to ORAC and FAGE, the FAAM BAE 146 will host a range of other equipment from various universities:

AMS: aerosol mass spectrometer (UMIST). Used to measure the chemical properties of aerosols smaller than 1 mm.
AVAPS or Dropsonde (Facility instrument): a device at the rear of the aircraft that allows small probes known as ’sondes’ to be dropped out of the rear of the aircraft. As they fall through the atmosphere, each sonde sends back information on, eg temperature, humidity and O₃ concentration.
BBR: broad band radiometers (Leicester). These radiometers are used to measure the intensity of specific wavelengths of light important to the initiation and propagation of atmospheric chemistry, in particular O₃ and NO₂ photolysis.
CO analyser (Facility instrument): highly sensitive and fast measurement using vacuum fluorescence. Very useful in-flight since CO is a conserved tracer of pollution, and is the main tool used to determine quickly an air masses properties, which in turn is used to steer or adjust the aircraft track.
Core chemistry (Facility instrument): range of commercial pollution instruments used to measure key species such as ozone, NO_x and SO₂.
NIR-TDLAS: near infrared tuneable diode laser (Cambridge/NPL). A flexible, compact, absorption instrument that can measure a range of atmospheric constituents at high temporal resolution. The instrument uses different laser wavelengths for each compound it detects.
NO_xy: four channel nitrogen oxides detector (UEA). Measures not only NO and NO₂, but also nitric acid and the sum of all NO containing species known as NOy. A key instrument for atmospheric chemistry flights since the nitrogen species are at the heart of cycles controlling O₃ concentrations.
PERCA: peroxy radical chemical amplifier (UEA/Leicester). This instrument is used to measure the sum of HO₂ and organic peroxy radicals known collectively as RO₂. The instrument mimics the chemistry in the atmosphere but on a faster timescale, by adding NO and CO at ppm levels to the sample gas.
PTR-MS: proton transfer mass spectrometer (UEA). Direct inlet mass spectrometer that measures volatile organic compounds in the atmosphere. The protonation of analyte organics with H₃O⁺ leads to molecular organic ions which are subsequently resolved using a quadrupole mass spectrometer. Particularly effective for fast measurements of aromatic and carbonyl compounds.
WAS: whole air sampling system (York/UEA). A large array of stainless-steel sampling containers stored in the hold of the aircraft which collect air for analysis on the ground.