Ozone, continued measuring

It is important to separate conceptually, and in practice, the calibration process from the monitoring process. Photochemical oxidants consisting primarily of ozone were firrt continuously measured in southern California by measuring the color change of potassium iodide solutions brought into contact with the ambient air. This measurement continues to yield valid photochemical-oxidant data in California. However, it has yielded questionable data at ambient air monitoring sites elsewhere in... 

Multiple-stage ozonization seems to be more effective than single-stage ozonization, both followed by biodegradation, for DOC ehmination in treatment of reservoir waters and secondary effluents of a domestic wastewater treatment plant . The fate of O3 in water ozonization consists of a fast reaction with the DOC and a slow first-order decay of unreacted O3. A method for optimization of a two-stage water ozonization process is based on control with a FIA unit, where the ozone concentration is continuously measured by oxidation of indigotrisulfonate(8) . The various fractions of DOC (in the ppm range) may react with the traces of bromide (sub-ppm) found in natural waters, as this anion... 

Continuous Measurement Methods for Trace Cases and Aerosols. Ozone. Three basic types of ozone instruments have been used in aircraft the ultraviolet photometric method and two chemiluminescent techniques measuring, respectively, light emitted from the reaction of 03 with ethylene and light emitted from the reaction of 03 with NO. Ultraviolet absorption photometry is one of the preferred methods for measuring 03 from aircraft because of the stability and reliability of commercially available instruments. The method is specific for 03 provided there are no immediate... 

Since 1992 the two Italian stations of Rome, urban site (latitude 41.9° N, longitude 12.5° E, altitude 60 m), and Ispra, semi-rural site (latitude 45.8° N, longitude 8.6° E, altitude 240 m), collect regular continuous measurements of spectral UV (290-325 nm) irradiance by means of Brewer Spectrophotometry. The measured data are compared with the output of the STAR model (System for Transfer of Atmospheric Radiation) [1], STAR is a multiple scattering radiative transfer model which considers all atmospheric factors modulating UV radiation at ground (ozone, aerosol, clouds, pollutants, albedo, pressure, temperature, humidity) [2], The model involves combination of a radiative transfer code, an initialisation procedure and an integration scheme. 

Record low levels of ozone were measured in 2000 in the Arctic with increasing evidence that these levels are likely to continue for at least the next 20 years. ... 

Commercial equipment. The Welsbach Corp., Ozone Process Division, offers Laboratory Ozonators which produce ozone concentrations up to 4% by weight in air and up to 8% by weight in oxygen. Accessory units are as follows. An ozone concentration meter based on the principle of the thermal conductivity cell continuously measures ozone concentration in the carrier gas. A strip-chart recorder is designed specifically for use with the ozone meter. A portable air dryer is a self-contained unit consisting of a compressor and dryer on a common portable base no attendance is needed. 

The impact of the observed ozone decline on UV-B radiation at the Earth s surface is one of the key factors which motivated long-term measurements of UVR worldwide, in order to investigate if there is an associated increase in solar UV-B. However, cloudiness has a stronger modulating impact on UVR reaching the Earth s surface than ozone. Therefore statistical analyses have shown that several decades of continuous measurements with broadband detectors under all conditions of cloudiness are necessary to identify the actual changes due to ozone [51]. 

Within the CEC funded project MOZAIC ( Measurements of Ozone by Airbus In-Service Aircraft ), continuous measurements of water vapor (and ozone) in the upper troposphere are being made during commercial airbus flights. At 40° - 60° N latitude over the North Atlantic, contents of water vapor were found to range between 0.07 and 0.12 g/kg depending on the seasonal large-scale circulation and jet stream pattern [107]. 

Furthermore, based on specific color reactions, indicator papers for the continuous measurement of ammonia, bromine, chlorine, hydrocyanic acid, hydrofluoric acid, ozone, phosgene, sulfur dioxide, and hydrogen sulfide are commercially available using photometric evaluation. Because of the often extremely intense coloration, detection hmits as low as the lower ppb range can be achieved by this method. Table 6.34 gives an overview of the range of determination and the detec-... 

Takeuchi K, Kutsuna S, and Ibusuki T (1990) Continuous measurement of ozone in air by chemiluminescence using indigo-5,5 -disulphonate. Analytica Chimica Acta 230 183-187. 

In Mexico City, several air quality parameters are measured continuously by an Automated Monitoring Network operated by the Under Secretariat of Ecology. Carbon monoxide, particulate matter, sulfur dioxide, nitrogen oxide, and ozone are the contaminants exceeding Air Quality Standards. Emissions produced by 2.7 million vehicles and 35,000 commercial and industrial outfits are not easily dispersed in a Valley located at 2240 m and surrounded by two mountain chains which hinder air circulation. An Integral Program, recently established to alleviate pollution, is briefly described. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

Since 1968, total-oxidant concentrations have been measured continuously with a Mast ozone meter (calibrated by the California Air Resources Board method) from May through September at Rim Forest-Sky Forest.- The fall, winter, and early spring months have generally been omitted until recently, because synoptic patterns are usually not conducive to oxidant accumulation and transport. For example, average maximal hourly oxidant concentrations ffom October through April 1973 and 1974 stayed below 0.10 ppm those for April were 0.10-0.15 ppm. The main data-collection period coincides with the growing season and thus permits a reasonable estimate of the total annual dose of oxidant air pollution received by vegetation. 

All areas in the United States presently meet the current (1971) NO2 NAAQS, with annual NO2 concentrations measured at area-wide monitors well below the level of the standard (53 ppb). Annual average ambient NO2 concentrations, as measured at area-wide monitors, have decreased by more than 40% since 1980. Currently, the annual average NO2 concentrations range from approximately 10 to 20 ppb [1], However, many areas in the U.S. do not meet the ozone NAAQS and is why NO continues to be a pollutant of concern. 

All of these approaches, as well as others such as the MLS (Microwave Limb Sounder) that have not been used for sufficient lengths of time to give long-term trends, have some uncertainties associated with them, as is the case for all analytical techniques (e.g., see Harris et al., 1997). For example, regular calibration is particularly difficult with satellite-based measurements and care must be taken to ensure that a continuous degradation in the instrument sensitivity, for example, is not interpreted as a trend in total ozone. 

Electrochemical (amperometric) techniques provide the possibility for in situ, continuous and automated measurements of ozone in the liquid. The membrane electrode usually consists of a gold cathode, a silver anode, an electrolyte (e. g. AgBr, K2S04 or KBr) and a teflon membrane. Several companies offer such electrodes in different configurations. The application range and accuracy differs depending on the kind of electrode. 

Continuous analytical methods (amperometric and UV-absorption methods) are advantageous. However, sometimes only discontinuous methods (titrimetric and some photometric methods) are available due to expense. In such cases it is important to measure immediately after sampling to avoid the decay of ozone and in the case of liquid ozone to avoid degassing. Discontinuous photometric methods requiring the addition of chemicals to the sample can be converted to a continuous method by combination with flow injection analysis (FIA). This analytical technique requires instrumentation and is not easy to handle. 

The continuous-flow nonsteady state measurements can be made after the reactor has reached steady state, which usually takes at least 3 to 5 times the hydraulic retention time under constant conditions. Then an appropriate amount of the compound to be oxidized (e. g. Na2S03) is injected into the reactor. An immediate decrease in the liquid ozone concentration to c, 0 mg L-1 indicates that the concentration is correct. Enough sulfite has to be added to keep cL = 0 for at least one minute so that it is uniformly dispersed throughout the whole reactor. Thus a bit more than one mole of sodium sulfite per mole ozone dissolved is necessary. The subsequent increase in cL is recorded by a computer or a strip chart. The data are evaluated according to equation 3-24, the slope from the linear regression is - (2/,/Vj + KLa(03)). 

Another aspect to be considered is that ozone also decays during the experiments. To obtain a constant ozone concentration in a system where ozone decay occurs, the reactor can be operated semi-batch. Gaseous ozone is continuously sparged to the reactor and after the ozone concentration reaches steady-state, the investigated compound is injected into the reactor. Another possibility is to measure the ozone decay rate independently and take this into account in the calculations. 

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