Analysis of air.

1. Analysis of air. Pollutants in the air arise from both natural (biogenic) and human (anthropogenic) sources. Burning of fossil fuels in power stations, automobile exhausts, forest fires and incineration of waste to name but a few sources. But further action of UV radiation from the sun on these pollutants create ozone and organic species such as peroxyacetylnitrate (PAN), and reaction with water in clouds produces acidic gases.

2. Sources and some types of air pollution. Concentrations of analytes in air samples. Common analytes observed in air samples. Analysis of air. Water (H20) (all 3 phases!!) Carbon dioxide (CO2) & Carbon monoxide (CO) Sulphur dioxide (SO2) Nitrogen oxides (NOx) & Ozone (O3) Methane (CH4) & Volatile Organic Compounds (VOC) On average an adult inhales 13,000 litres of air every day and over 1 trillion particles Do not forget that air sampling also includes the collection of aerosols which exist in solid and liquid form particles….LATER

3. Analysis of air pollution exists in many forms Testing air in working environments (occupational). Outdoor tests of air surrounding industrial sites or landfill. Background air at remote sites Stratospheric air sampling by aircraft/balloons. Combined with the boundless motion of air and the sheer size volume the atmosphere possesses, we have a very complex pollution picture. • One thing it is important to realise when analysing gaseous samples compared to liquid or solid analysis:Analytes in air are usually at very low concentrations. • For example; pollutant X comprises one part per million (ppm) of a water sample, one litre of the water (1 kg) therefore contains 1 mg of X. • One litre of air however 1ppm corresponds to only 0.002 mg. • This is due to the low density of air, but means that samples contain very little analyte.

4. Analysis of such low amounts of pollutant requires either very sensitive instrumentation (lasers), very large volumes of air sample (PAH sampling), or pre-concentration of the analyte into a smaller volume of air (VOC sampling). If air samples are to be taken from the field for analysis back in the lab, samples of 2 litres or more are commonly required with concentration of analytes carried out prior to analysis in the lab. • ‘Field’ instruments are portable and sensitive enough to measure certain analytes in situ. • Samples may be collected in bags or deactivated stainless steel canisters. • Diffusive samplers, simple to deploy. Results either by simple colour change (chemioptic) or lab analysis (NO2, VOC)

5. Targeting Pollutants - What are we looking for?? There are some 200 airborne species for which concentrations are regulated, all of which are considered pollutants. Oxides of nitrogen, carbon and sulphur are present in the atmosphere as combustion products of fossil fuels and car exhausts, but some also come from natural processes. These emissions are NOT inert and will react in the atmosphere often producing more harmful products

6. Lightning, volcanoes and photochemistry Lightning contributes to the presence of nitrogen oxides along with the life cycle of microorganisms living in the ocean and soil • Volcanic action is a significant source of the gaseous sulphur in the atmosphere CO2 is produced by most living things CH4 is also released by a large number of species • Finally reaction of naturally occurring & combustion compounds with sunlight leads to the formation of SECONDARY pollutants e.g. ozone, PAN, formaldehyde

7. Measuring Water in the Atmosphere. There are a wide range of methods to monitor ambient water. Water essentially drives the energy balance in the atmosphere through evaporation and condensation processes. Water can be present in all 3 normal phase (ice, liquid and gas) Relatively high concentrations but VERY variable. Wide range of units:- (dewpoint, relative humidity, % water, ppm) There are a wide range of methods to monitor ambient water. The amount of water in the atmosphere is critical in computer models AND will affect many measurements so simultaneous measurements is often required.

8. Chilled Mirror Hygrometer. DEWPOINT MEASURMENT The dewpoint is the temperature at which gaseous water begins to condense out. At 100% humidity dewpoint = ambient temperature Mirror Thermocouple Cooler • Light is directed at a cooled mirror • Clear mirror surface, Lightin = Lightout • If mirror is BELOW the dewpoint, condensation forms • Actively controlling the cooler at the dewpoint • Temperature recorded by thermocouple if the DEWPOINT

9. Carbon Dioxide (CO2) by IR. White cell (10-20 metre path length) The filter modulates the light produced and the difference between the light detected on the photomultiplier is proportional to the gas concentration in the cell.

10. Infra-Red Detection for Atmospheric Gases. • The absorption of infrared light at a specific wavelength (e.g. 4.67 nm for CO) is the standard method for continuous analysis in modern instrumentation. • Not good for low concentrations (Urban CO only!). • IR is used for a number of different gases, e.g. H2O, CO2, CO. • Also known as Gas Filter Correlation. • By using the target gas in the reference cell, this is a very specific technique. A white cell typically of path length 10-20 m is used in conjunction with a gas filter wheel. Beer-Lambert!

11. Carbon Monoxide (CO). • Carbon monoxide is highly toxic toward humans in very low concentrations, using the IR absorption of CO, instruments solely built to monitor its presence run constantly in high risk environments. • Very important environmental measurement. • Domestic: incomplete combustion of gas - poorly maintained central heating systems. • Direct emission from traffic. • Excellent tracer for pollution. • Photochemical product of hydrocarbon oxidation. • Many methods for determining ambient levels, (NDIR, VUV, GC, EC)

12. CO by VUV Resonance Fluorescence. The detection limit of the non dispersive continuous stream IR analyser for CO is 0.1 ppm at best. In background air carbon monoxide levels often below this so another method has recently been developed; resonance fluorescence. Such instruments have a detection limit of <10 ppb and are fast 10 Hz, this makes them particularly good for aircraft measurements. • Resonance fluorescence of CO in the vacuum UV has been used for a highly sensitive and rapidly responding instrument. • The excitation light comes from a continuous wave lamp. • The fluorescence in the wavelength range (160-190 nm) is detected by a VUV photomultiplier followed by a fast counter. • The discharge of the CO resonance lamp is imaged into the fluorescence chamber by means of two CaF2 lenses. • The optical filter is continuously flushed with clean N2 which is necessary to avoid absorption by oxygen and from impurities, in particular CO.

13. Alternative Analytical Techniques for CO. A chromatographic method exists which relies upon the reduction of CO in the presence of hydrogen to methane over a catalytic bed, typically nickel and subsequent detection using a flame ionisation detector, however this method suffers from considerably lower sampling rate (~10 mins). This method is frequently used for long term measurements. An alternative method utilises an electrochemical technique in which CO is oxidised to CO2 using a platinum catalyst in an acidic aqueous solution and the subsequently produced current flow is measured. In-line filters are required since there are a number of interferrants to this method. A response time of 25 seconds and detection limit of 1 ppm has been reported for such an instrument, recently a miniaturised device suitable for mounting on ‘street furniture’ has recently been developed.

14. Sulphur Dioxide (SO2). • To detect SO2 a flame photometric detector (FPD) can be used. • The FPD is used to continuously measure sulphur compounds in ambient air. Light is emitted in the near UV region when the sulphur compounds are thermally decomposed in a hydrogen flame. This technique can be applied to GC if the air contains a number of sulphur containing compounds.

15. SO2 by Fluorescence. More common however is to use the fluorescence technique, SO2 absorbs UV light and the produce a fluorescence which is easily detected with a photomultiplier tube (PMT). The PMT detects the UV light emission from the decaying SO2 molecules. Hydrocarbons must be removed first since some do absorb the same wavelength UV light and this can result in errors. This is the most common method. It is the standard air quality method of measuring Sulphur dioxide in the atmosphere (and stack/flue gases) No gases required (calibration)

16. Nitrogen Oxides (NOx). • NOx (NO and NO2) formed mainly as a result of transport emissions. • Until relatively recently ambient concentrations of nitrogen dioxide were determined using a alkaline bubbler, and then converted into an easily detectable azo dye, detection limits as low as 0.01 ppbv were obtained using this technique • Nowadays the most common method of detect is ‘chemiluminescence’. This is a form of emission spectroscopy where the observed species gives out UV/Visible radiation when taking part in a chemical reaction.

17. NO by Chemiluminescence. NO + O3 NO2* + O2 NO2*  NO2 + h The NO2* species is simply NO2 in an excited (electronic) state which in time relaxes to emit photons of light. The radiation has a range of wavelengths but is strongest centered around 1200 nm in the near infrared region. The light intensity varies linearly as predicted by the Beer-Lambert relationship which can be expressed in the form: Ln (IT/I0) = -cl IT = intensity of light passed through cell IO =Intensity of light passed through ‘clean’ cell  = Extinction coefficient l = Path length c = concentration of monitored species

18. Instrument Layout. The technique ONLY measures NO so it converts NO2 into NO by reducing it over a hot metal surface: 3NO2 + Mo 3NO + MoO3 (315C) Switching between the convertor and direct gives [NOx] and [NO] NO2 concentration by subtraction.

19. Ozone (O3). Ozone is relatively abundant in the troposphere (0 - 400 ppbV) There are no specific wet chemical methods for the analysis of ozone, although a broad oxidant method using the oxidation of potassium iodide is available (interferences from alkyl nitrates, hydrogen peroxide and even, to a limited extent nitrogen dioxide). It is the strong absorption of ozone in the 254 nm region in the stratosphere that protects life lower in the atmosphere. First reported use of absorption spectroscopy was in 1912, modern instruments generally use ultraviolet photometry at 253.7 nm for direct determination of atmospheric ozone. Light is produced by a mercury vapour lamp, the majority of the light emitted is at the 254 nm wavelength, it is this wavelength that there is a strong absorption. Some hydrocarbons particularly aromatics can also absorb BUT only when present in high concentrations (chemical plants, biomass burning plumes).

20. Continuous stream of sample air passes to the sample cell where it is exposed to UV @ 254 nm. Ozone absorbs a portion of the photons the remainder being detected by a photosensitive cell. An in-line ozone scrubber is used to alternate produce a zero measurement, the difference in absorption is determined and thus a value for the concentration of ozone. Instrument Schematic. Using the Beer-Lambert relationship we have the basis of the calculation which leads to the ozone concentration. Alternative Ozone Methods. Ozone also reacts with a number of compounds chemiluminescently, this is a fast reaction and hence very good for fast measurements (10 Hz). There are 2 main types of chemiluminescent instruments. Nitric oxide (NO) - identical principal to the NOx boxes already mentioned. Ethene (ethylene) this is a lightweight hydrocarbon (C2H4).

21. Air Sampling for Lab Analysis. As well as the continuous analysis of the atmosphere for harmful contaminants, much environmental analysis of air requires more sensitive instrumentation which may observe many compounds at the same time, such as in chromatography. In many situations it is not possible to have such sensitive instruments in the field so air samples must be collected and transported to the lab for later analysis. This is either done by collecting a whole air sample in a suitable vessel or collecting the desired analytes on a trap for later analysis. Passive Sampling. Another simple alternative for ambient determinations of many atmospheric compounds is by passive sampling or diffusive monitoring. The target compound is ‘captured’ on an adsorbent or converted into a stable compound which can then be measured. NO2 is very commonly monitored in this way. Small tubes which can easily be deployed to locations where the use of an analyser is impractical such as on poles by traffic junctions. The tubes are typically deployed for several days. They can be used as an array which offers low temporal but high spatial information. The construction of an archive of such measurements could provide a unique insight into the urban and suburban troposphere where costs preclude the use of modern analysers.

22. Methane (CH4) LOAD INJECT Sample loop Sample loop Sample in Sample in • Methane is the simplest hydrocarbon and probably the easiest to measure (1800 ppbV). • A sample loop is filled with air and injected directly on the capillary column. • Detection usually done by FID.

23. Volatile Organic Compounds (VOC). • VOC’s in the atmosphere consist of vapor phase compounds from sources such as industrial leaks, solvent evaporation, incomplete combustion and the burning of waste products. • These compounds generally have low boiling points but less volatile compounds of boiling points up to 250 0C are also classed as VOC’s because of relative vapor pressures. • Atmospheric VOC concentrations are in the low ppb region so large samples or pre-concentration of analytes is usually required. • Trapping analytes from an air mass on an adsorbent trap is a common method for volatile organic compound (VOC) analysis.

24. Pre-Concentration of VOC using an Adsorbent trap LOAD INJECT Sample in Sample in Air is passed through the trap for a given time at a known flow rate (e.g.; 100 ml/min for 10 minutes gives a sample of one litre). Trap is heated (>200oC) and the valve is turned allowing helium to transfer the analytes onto the capillary column.

25. In the lab, collected compounds are desorbed from the trap through heating, in most cases these can then be analysed directly. • Analysis of VOCs is normally conducted using GC. • Many adsorbent materials are available and should be carefully chosen to collect only the compounds required and eliminate the presence of interfering substances such as water. • It is very important to know that ALL the analytes through the trap are held by the adsorbent and that ALL are removed on desorption. • It is therefore very important that we know the breakthrough limits of the trap, and make sure these are not exceeded. • If we are collecting several compounds from a sample of air, it is a possibility that those weakly adsorbed will ‘break-through’ the trap before enough sample has been taken. In this situation we can use a ‘layered sorbent trap’: Glass wool plugs Acquisition Desorption Carbotrap C (least volatile) Carbotrap B Charcoal (most volatile) By using a trap containing adsorbents of increasing strengths it can be ensured that all compounds are retained. • MUST desorb by flowing gas in the opposite direction.

26. Easily adsorbed compounds are trapped at the front by weak sorbent. Less easily adsorbed compounds move through trap to be trapped by increasingly strong sorbents. • Desorbing the trap in the opposite direction prevents the easily adsorbed analytes from having to pass through the strong sorbents. Carbon molecular sieve and graphitised sorbents are used for small weakly adsorbed and larger readily adsorbed hydrocarbons respectively. Tenax is a more general adsorbent collecting organic compounds with BP <200 0C.

27. Though traps collect many compounds from the atmosphere, sometimes we require that all components in the sample air make it back to the lab. Sampling with canisters. • Canisters are now commonly used in the field to collect ‘whole air samples’. • Not any container can be used to collect the air, it’s surface must very clean so it does not affect the sample composition. • Stainless steel canisters are coated inside deactivated silica, or undergo a process of electrolyte polishing.