Ambient Air Monitoring System

The AP-370 series is a HORIBA’s measuring unit of Continuous Ambient Air Quality Monitoring System (CAAQMS). Continuous Ambient Air quality monitoring system are custom designed best to meet each application unique demands. This system is meant for measuring the concentration of air pollutants at various times instance of the day. The Concentration of air pollutants (such as S02, NOX, CO, O3, THC, etc). These stations monitor air pollution and send the data to remote server for archiving data and analysis. Mobile CAAQMS can be published via the internet to easy public access to raise awareness on current air pollution levels.

Conventional technology uses an optical chopper to obtain modulation signals. Instead, the APMA-370 uses solenoid valve cross flow modulation. Fixed amounts of the sample gas and the reference gas are injected alternately into the measurement cell. With the cross flow-modulation method, if the same gas is used for both the sample gas and the reference gas (e.g., zero gas could be used for both), no modulation signal will be generated. This has the great advantage that, in principle, when analyzing minute amounts of gas there is no generation of zero-drift. An additional advantage is that the elimination of rotary sectors precludes the need for optical adjustment. These features assure greatly improved stability over long periods of measurement. A further improvement is that in the front chamber of the detector, the measurable components, including interference components, are detected; in the rear chamber, only interference components are detected. By means of subtraction processing, the actual signal obtained is one that has very little interference.

The UV fluorescence method operates on the principle that when the SO2 molecules contained in the sample gas are excited by ultraviolet radiation they emit a characteristic fluorescence in the range of 220-420 nm. This fluorescence is measured and the SO2 concentration is obtained from changes in the intensity of the fluorescence. The reactive mechanism is (1) SO2+hν1→SO2* (2) SO2*→SO2+hν2 (3) SO2*→SO+(O) (4) SO2*+M→SO2+M Here, (1) shows the excited state of the SO2 molecules that have absorbed the amount of energy hν1 by ultraviolet radiation. (2) shows the amount of energy, hν2 emitted by the excited molecules as they return to the ground state. (3) shows the decomposition by the light emitted from the excited molecules. (4) shows the quenching, i.e., the energy lost by the excited molecules colliding with other molecules. The APSA-370 uses an Xe lamp as the light source, and the fluorescent chamber design minimizes scattered light. The optical system has been carefully designed with low background light, making it possible to take measurements with a highly stable zero point. In addition, a reference detector monitors any fluctuation in the intensity of the light source. This allows the unit to calibrate itself automatically for sensitivity, resulting in greater span stability.

Cross flow modulation type, reduced pressure chemiluminescence (CLD). The chemiluminescence method uses the reaction of NO with O3 NO+O3→NO2*+O2 NO2+NO2+hν A portion of the NO2 generated as the result of this reaction becomes NO2*. As these excited molecules return to the ground state, chemiluminescence is generated in the range of 600 nm to 3,000 nm. The light intensity is in proportion to the concentration of NO molecules and by measuring it we obtain the NO concentration of the sample. A DE oxidation converter changes the NO2 to NO, which is measured. In other words, the NO2 concentration can be obtained by the difference between (1) the NOx concentration measured when the sample gas is directed through a converter and (2) the NO concentration measured when the gas is not run through the converter.

Flame ionization detection method (FID) with selective-combustion. The flame ionization detection method (FID) ― used in combination with the selective-combustion system ― utilizes the ionization that occurs as the result of the high-temperature energy from combustion at the tip of the burner jet when organic carbon compounds are introduced into the hydrogen flame. The hydrogen flame is located between two electrodes. When an electrical voltage is applied across these electrodes a minute ion current proportional to the hydrocarbon concentration is produced. This current is monitored by a low leakage amplifier, giving a voltage readout for THC. To measure CH4 the sample gas is passed through the selective catalytic combustion unit (the NMHC cutter), which oxidizes NMHC without oxidizing CH4. This is shown as A below. B represents the THC concentration measured without passing the gas through the NMHC cutter. Thus B- A will give the concentration of NMHC. The final concentration value is calculated using a relative-sensitivity correction coefficient, k, as shown below.

CH4 Concentration A

NMHC Concentration k (B – A)

THC Concentration A + k (B – A)

Cross flow modulation type, Non dispersive ultra-violet absorption method (NDUV) The ultra-violet absorption method works on the principle that ozone absorbs ultra-violet rays in the area of 254 nm. Measurements are taken from continuous, alternate injections of the sample gas and the reference gas into the measurement cell, controlled by a long-life solenoid valve. The cross flow modulation method is characteristically zero drift free. A comparative calculation circuit automatically compensates for all fluctuations in the mercury vapour light source and in the detector. This means that, in principle, the APOA-370 makes it possible to carry out zero-span drift free, continuous measurements. In addition, HORIBA’S unique deozonizer for the comparison gas line is unaffected by interference elements or moisture retention, prolonged, stable measurement is possible.