1、INTERNATIONAL STANDARD IS0 10313 First edition 1993-04-I 5 Ambient air - Determination of the mass concentration of ozone - Chemiluminescence method Air ambiant - D therefore, the time during which the ambient air remains in the sampling line must be sufficiently short to keep this effect to a minim
2、um. NOTE 1 It has been reported by some researchers that, at about 80 % relative humidity and 22 “C, the responses for some commercially available chemiluminescence analysers were about IO % higher than that for dry air. However, comparisons of ambient ozone measurements using com- mercially availab
3、le chemiluminescence and UV-photometric anaiysers showed no significant differences. This suggests that, in practice, any errors associated with calibrations us- ing dry air are compensated by other effects. 2 Definition For the purposes of this International Standard, the following definition appli
4、es. 2.1 transfer standard: An apparatus (transportable) with associated operational procedures that is capable of accurately measuring or reproducing ozone con- centration standards which are quantitatively related to the authoritative primary standard. 3 Principle The air sample is drawn continuous
5、ly, at a constant flow rate, through a particle filter before it enters the chemiluminescence analyser. It then flows into a re action chamber where it is mixed with an excess flow of ethylene. Ozone and ethylene react instan- taneously to produce light in the visible region with a maximum at a wave
6、length of about 400 nm. The emitted light intensity is proportional to the concen- tration of ozone in the air sample and is measured by a photomultiplier tube. The resulting voltage is ampli- fied, displayed, and calibrated in terms of the ambient ozone concentration. 4 Reagents and materials 4.1 S
7、ampling line The sampling line shall be made of material that is inert to ozone, such as glass or fluorocarbon polymer, and it shall be as short as possible to keep the time the air sample remains there to a minimum. Any am- bient nitrogen(ll) oxide present in the air sample will react with some of
8、the ozone during the time it re- mains in the sampling line. This decay of ozone is a complicated function of the mass concentration ratio, p(03)/p(NO), and the difference in mass concen- trations, (0) - p(N0). Calculations have shown that, if the residence time is less than 0,5 s, the decay in init
9、ial ozone will be less than 1 % for most ambient ozone and nitrogenIll) oxide concentrations encoun- tered. Hence, it is recommended that the sampling 1 IS0 10313:1993(E) line length and pumping speed be chosen such that the time the air remains within the sampling line is less than O,5 s. However,
10、in situations of routine monitoring, a time of up to 5 s is allowed. Any sampling line or manifold preceding the filter shall be clean. Proper precautions shall be taken to prevent condensation inside the sampling line, for example, by heating the line. 4.2 Particle filter The filter and its support
11、 shall be made of material inert to ozone, such as fluorocarbon polymer, and it shall remove all particles likely to alter the perform- ance of the analyser. It shall be changed on a regular basis, depending on the concentrations of ambient particles at the sampling site. This is necessary be- cause
12、 excessive accumulation of particles on the filter can cause loss of ozone from the air sample and an excessive pressure drop across the filter. NOTES 2 Frequently, a filter pore size of 5 pm is used. 3 Generally, new filters need some time to be conditioned by the ambient atmosphere. As a result, m
13、easured mass concentrations of ozone have been obsenred to decrease temporarily by 5 % to 10 % for periods of 5 min to 15 min immediately following filter changes. 4.4 Zero air, required in the anatyser calibration procedure. It shall be free of ozone, nitrogen oxides and any other interfering subst
14、ance that csn cause an undesired positive or negative response in either the ultraviolet photometer or the chemiluminescence analyser. The response of the ozone chemiluminescence ana- lyser is affected by the oxygen content in the gas sample. Therefore, if synthetic air is used, the oxygen content s
15、hall be within f 2 46 of the normal atmos- pheric concentration of 20,9 %. NOTE 4 Details on a scrubbing system for making zero air from ambient air may be found in I and t2 in annex B. 5 Apparatus 5.1 Chsmllumlnescence analyser, consisting of sample and ethylene flow inlet systems, a constant tempe
16、rature reaction chamber coupled to a photo- multiplier detector and accompanying signal process- ing electronics. A flowmeter, if it is not an integral part of the analyser, shall be available for verifying that the flow meets the specifications of the instrument. A schematic diagram of a typical sy
17、stem is shown in figure 1. 4.3 Ethylene, of minimum purity 995 %. WARNING - Ethylene has a lower exploslvs llmlt of 27,5 l/mR Any excess shall be vented outdoon or, tf necessary, chemically removed, especially at sites where hydrocarbons are being measured. Observe standard safety precautions when u
18、rlng this flammable gas. 59 Apparatus for callbratlon, an ozone calibration system shall be used. (See the simplified schematic diagram in figure 2.) NOTE 5 An equivalent system would ba one that usas only the air line with the ozone generator to bring zero and ozonizad air to the manifold, hence el
19、iminating the need for the four-ports two-way valve. IS0 10313:1993(E) Flow controller II Flowmefer I Temperature controller II I I II Quartz A I h Sample inlet - =f window - II M - I, ._ ,:-I Reaction chamber Particulate filter Thermo-electric cooler Flow I- controller r . Flowmeter - CX”cIU5T Figu
20、re 1 - Schematic diagram of a chemiluminescence ozone analyser 3 IS0 10313:1993(E) under callbratlon Ultraviolet photometer r”“-“-,.-”“1 I optics i I Absorptlon cell 0 I I Low pressure ; mercury vapwr ; Lamp I I ! I I Figure 2 - Schematlc dlagram of a typlcal ultmvlolet photometric callbrstlon syste
21、m 5.2.1 Ultravlolet photometar, for primary cali- bration only consisting of a low pressure mercury discharge lamp, a single or dual absorption cell, and detection and signal processing electronics. It shall be meticulously maintained. To prevent ozone generation within the absorption cell, a high-s
22、itica glass window (or equivalent) shall be inserted between the mercury lamp and the absorption cell to remove the 185 nm Hg line but transmit the 253,7 nm Hg meas- urement line. The detector shall be capable of meas- uring the transmittance through the absorption cell at a wavelength of 253,7 nm,
23、with less than 0,5 % of the radiation detected being from other wavelengths (vacuum photodiodes with a caesium telluride sensitization meet this requirement). The length of the light path through the absorption cell shall be known to an accuracy better than 0,5 %, and the cell and associated plumbin
24、g shall be designed to mini- mize surface loss of ozone. Provision shall be made for measuring the temperature and pressure of the gas in the absorption cell (see 5.2.8 and 5.2.91. See the simplified schematic diagram of a photometer system in figure 2. 52.2 Ozons generator, which generates stable o
25、zone concentrations that, in turn, are measured by the calibrated photometer (5.2.1). The ozone genera- tor shall be capable of producing steady ozone con- centrations in the range of measurement, at the required flow rate, and throughout the period of cali- bration. If a variable ozone generator is
26、 not available, the calibration system shall include a means of suit- ably diluting the ozone with additional zero air (see 4.4). In such a case, a mixing chamber shall be in- stalled before the output manifold. WARNING - Ozone Is a toxle gas wlth a threshold IlmIt value In alr of 0,1 ml/m% and any
27、excess should be vsntud Into an activated charcoal scrubber or outdoors (well away from any sam- pllng Intake). 5.2.3 Ahflow controllers, capable of maintaining steady airflows throughout the period of calibration. 4 IS0 10313:1993(E) 5.2.4 Airflow meters, of any type that is suitable for measuring
28、the required airflows. NOTE 6 The actual airflows are not used in the calcu- lations but are used only for achieving the nominal concen- trations required in the calibration procedure. 5.2.5 Air pump, used to draw the required sample flow through the absorption cell. 5.2.6 Output manifold, made of m
29、aterials inert to ozone, such as glass or fluorocarbon polymer. It shall be of sufficient diameter and be vented to ensure equal atmospheric pressure inside and outside the manifold. The vent shall be located so as to prevent intrusion of ambient air. 5.2.7 Four-ports two-way valve, with interior su
30、r- faces made of a material inert to ozone, such as a fluorocarbon polymer. When a calibration system such as that shown in fig- ure2 is used, a four-ports two-way valve or some equivalent system of valves shall be used for switch- ing the manifold flow between zero air and the ozone test atmosphere
31、. 5.2.8 Temperature indicator, capable of measuring temperature with an accuracy of 0,l “C. 5.2.9 Pressure indicator, capable of measuring pressure with an accuracy of f 0,l kPa. 6 Procedure 6.1 Operation of chemiluminescence analyser Install the instrument in a suitable location. Follow the manufac
32、turers operating instructions for the specific analyser in use to set the various parameters, includ- ing sample and ethylene flow rates. Check to ensure that the manufacturers performance characteristics are met or exceeded. If necessary, the location shall also be temperature controlled, so as to
33、minimize the effect of the temperature dependence of the instru- ment. (See annex A for typical performance specifi- cations.) Introduce the air sample into the instrument and record the ozone concentration by means of a suitable recording device (for example, chart recorder, electronic data acquisi
34、tion system, etc.). NOTE 7 During continuous operation of the instrument, it is recommended that a multipoint calibration be carried out every 3 months. In addition, frequent operational checks on the instrument zero and span should be made, usually once a week. 6.2 Calibration of the chemiluminesce
35、nce analyser 6.2.1 Principle A schematic diagram of an ultraviolet photometric calibration system is shown in figure2. Various ozone concentrations (in air) are generated and accurately measured with a primary calibration ultraviolet photometer. These ozone calibration atmospheres are simultaneously
36、 sampled by the chemiluminescence analyser via a common manifold. The analyser re sponses are plotted against the ozone concentrations measured by the ultraviolet photometer. Alternatively, a calibration method using a transfer standard can be calibrated against the ultraviolet photometer and used t
37、o calibrate the chemiluminescence analyser at the sampling location. NOTE 8 See 2 in annex B for details on the principles of operation of the ultraviolet photometer. 6.2.2 Calibration procedure using the primary UV standard During the calibration procedure, the chemi- luminescence ozone analyser sh
38、all be operated at its correct flow rate and temperature. The calibration in- cludes measurements using zero air (see 4.4) and at least four ozone concentrations (using the ozone generator described in 5.2.2) which shall be reason- ably spaced to cover the range being measured. For all measurements,
39、 the input flow to the manifold shall exceed, by at least IO%, the total flow required by the instruments attached to this manifold, with the ex- cess appropriately vented at atmospheric pressure. Carry out the following steps in the detailed calibration procedure. - - - - Assemble the apparatus as
40、shown in figure 2. Introduce zero air into the manifold. Set the zero control of the ozone analyser being calibrated to indicate a reading of zero, and set the transmittance of the photometer at 100 %. Adjust the output of the ozone generator to gen- erate the most concentrated ozone atmosphere requ
41、ired. Turn the valve to introduce the ozone atmosphere into the manifold. - Note the transmittance on the photometer at 254 nm, and also the temperature and pressure in the photometric cell. - Set the span on the ozone analyser being cali- brated to the ozone concentration calculated using equation (1) (see 7.1).
