The online monitoring technique for NOx is based on the Chemiluminiscence of which of the following molecules in excited state?
Nitrogen dioxide
Understanding NOx Monitoring through Chemiluminescence
Nitrogen oxides (NOx), primarily consisting of nitric oxide (NO) and nitrogen dioxide (NO2), are significant air pollutants. Monitoring their concentrations is crucial for air quality assessment and control. One widely used online monitoring technique for NOx is based on the principle of chemiluminescence.
Chemiluminescence is the emission of light as a result of a chemical reaction. In the context of NOx monitoring, a specific reaction is utilized to produce light, the intensity of which is proportional to the concentration of the target gas.
Principle of Chemiluminescence for NOx Measurement
The online monitoring technique for NOx, specifically for measuring nitric oxide (NO), relies on its reaction with ozone (O3). This reaction produces nitrogen dioxide (NO2) in an excited state (\(NO_2^*\)).
The chemical reaction is:
\(NO + O_3 \rightarrow NO_2^* + O_2\)
The molecule in the excited state here is nitrogen dioxide (\(NO_2^*\)). This excited molecule is unstable and quickly returns to its ground state (\(NO_2\)), releasing energy in the form of light (photons). This emitted light is chemiluminescence.
The decay from the excited state to the ground state is represented as:
\(NO_2^* \rightarrow NO_2 + h\nu\)
where \(h\nu\) represents the emitted photon (light).
The intensity of the light emitted by the excited nitrogen dioxide (\(NO_2^*\)) is directly proportional to the concentration of nitric oxide (NO) in the sample gas, provided that ozone is present in excess.
Measuring Total NOx and Nitrogen Dioxide
Chemiluminescence primarily measures NO concentration. To measure the total NOx (NO + NO2), the sample gas is first passed through a thermal or catalytic converter. This converter converts any nitrogen dioxide (NO2) present in the sample into nitric oxide (NO):
\(NO_2 \xrightarrow{Converter} NO\)
After passing through the converter, the sample contains the original NO plus the converted NO from NO2. This mixture is then reacted with ozone, and the total chemiluminescence signal corresponds to the total NOx concentration. By subtracting the initial NO concentration (measured without the converter) from the total NOx concentration (measured with the converter), the concentration of nitrogen dioxide (NO2) can be determined.
Analyzing the Options for NOx Chemiluminescence
Let's consider the given options in the context of the chemiluminescence reaction used for NOx monitoring:
- Option 1: Ozone (\(O_3\)): Ozone is a crucial reactant in the chemiluminescence reaction with NO. However, it is the reactant that causes the excitation of another molecule; ozone itself does not become the excited species emitting light in this specific NOx monitoring technique.
- Option 2: Nitrogen dioxide (\(NO_2\)): As explained above, nitrogen dioxide is formed in an excited state (\(NO_2^*\)) from the reaction between NO and O3. The subsequent decay of this excited nitrogen dioxide (\(NO_2^*\)) is what produces the chemiluminescent light detected for NOx monitoring.
- Option 3: Dioxygen (\(O_2\)): Dioxygen is a stable product of the reaction between NO and O3. It does not get excited or emit light in the chemiluminescence process used for NOx monitoring.
- Option 4: Nitric oxide (\(NO\)): Nitric oxide is the primary target pollutant measured by this technique. It is a reactant with ozone, leading to the formation of excited nitrogen dioxide. Nitric oxide itself is not the molecule that emits light in the excited state during this reaction.
Based on the fundamental chemical reaction and the principle of chemiluminescence used in this monitoring technique, the molecule in the excited state responsible for the light emission is nitrogen dioxide (\(NO_2^*\)).
Revision Table: Key Aspects of NOx Chemiluminescence
| Component | Role in Chemiluminescence |
|---|---|
| Nitric Oxide (NO) | Analyte (what is measured), Reacts with O3 |
| Ozone (O3) | Reactant, Oxidizes NO |
| Excited Nitrogen Dioxide (NO2*) | Product in excited state, Emits light (Chemiluminescence) |
| Ground State Nitrogen Dioxide (NO2) | Stable product after light emission |
| Dioxygen (O2) | Stable product of reaction |
| Photomultiplier Tube (PMT) | Detects emitted light intensity |
Additional Information on NOx Monitoring Techniques
NOx is a collective term referring to nitric oxide (NO) and nitrogen dioxide (NO2). These gases are primarily produced from the combustion of fossil fuels.
Apart from chemiluminescence, other techniques used for monitoring NOx include:
- Differential Optical Absorption Spectroscopy (DOAS)
- Infrared Spectroscopy
- Electrochemical Sensors (often used in portable monitors)
- Passive Sampling Tubes
Chemiluminescence offers high sensitivity and specificity for continuous, real-time monitoring of NOx in ambient air and stack emissions.
Important Questions from Environmental Pollution and Control
A tributary flowing at a rate of 4 m3/s converges into a river flowing at a rate of 8.0 m3/s. The concentration of a pollutant 'X' at the upstream of the tributary before convergence was 12 mg/L and that of the river was 30 mg/L. If the pollutant X is completely mixed in the downstream, what would be its concentration?
Consider Gaussian-point source dispersion model with effective stack height H and dispersion coefficient σz = \(\frac{H}{\sqrt2}\). If the effective stack height H increases by 10%, the downwind ground-level concentration of pollutants would
In the two lists given below, List I provides the list of Pollutants, while List II indicates Health Effects. Match the two lists and choose the correct answer from the code given below :
List - I
(Pollutant)
List - II
(Health Effect)
(a) SO2 (i) Nerve and blood effects (b) O 3 (ii) Bronchitis
(c) CO
(iii) Asphyxiant (d) Pb
(iv) Pulmonary injury A 500 MW coal based power station is operating at an efficiency of 30%. If the coal has 1% of sulphur content and 1 tonne of coal produces 8000 kWh energy, how much SO2 will be emitted daily by the plant?
Name the low heat thermal process that destroys the pathogen in biomedical waste by heating that occurs inside the waste material.
