This section outlines the common scientific methods used to measure the concentration of pollutants in the air, focusing on particulate matter and specific gaseous pollutants like carbon monoxide.
Particulate matter (PM) refers to a mixture of solid particles and liquid droplets suspended in the air. These particles are categorized by their size:
- PM10: Inhalable particles with a diameter of 10μm or smaller.
- PM2.5: Fine inhalable particles with a diameter of 2.5μm or smaller. These are more hazardous as they penetrate deep into the lungs and bloodstream.
Several instruments, known as particulate matter monitors, are used to detect and quantify these particles.
- Principle: These monitors measure PM by analyzing the attenuation (weakening) of beta radiation as it passes through a filter where particles have been collected.
- Mechanism: The degree to which the beta radiation is blocked is directly proportional to the mass concentration of the particulate matter on the filter.
- Principle: This is a direct physical measurement of the mass of collected particles.
- Mechanism:
- Air is pumped through a pre-weighed filter for a specific period.
- Particulate matter is collected on the filter.
- The filter is weighed again after sampling.
- The difference in mass indicates the concentration of PM in the volume of air sampled.
- Principle: These instruments use the principles of light scattering or laser diffraction to count and size individual particles.
- Mechanism: Air is passed through a chamber with a light source (often a laser). When a particle passes through the light beam, it scatters the light. A detector measures the intensity and pattern of the scattered light, which is related to the particle's size and allows for determining the particle concentration.
Various gas sensors and analysers are employed to detect specific gaseous pollutants in the air. For more information on air pollutants, see 13.1 AIR POLLUTION→(/chemistry-11/13-environmental-chemistry-air/13-1-air-pollution).
Electrochemical Sensors
- Application: Commonly used in personal safety devices and for monitoring indoor air quality.
- Mechanism: When CO comes into contact with an electrolyte at an electrode, it generates a measurable electrical current. The magnitude of this current is proportional to the concentration of CO.
Non-Dispersive Infrared (NDIR) Spectroscopy
- Principle: CO molecules absorb infrared light at a specific wavelength (≈4.67μm).
- Mechanism: An IR light source emits radiation through an air sample. A detector measures the amount of light that passes through. The amount of IR light absorbed is used to calculate the concentration of CO. This is the standard continuous monitoring method for CO.
Chemical Colour Change Detectors
- Application: Used for qualitative assessments rather than precise quantitative measurements.
- Mechanism: These detectors contain a chemical that changes colour in the presence of CO. They provide a simple, visible indication that CO is present but are less precise for determining exact concentrations.
- Principle: Based on the light-emitting reaction between nitric oxide (NO) and ozone (O3).
- Mechanism: The sample air reacts with O3; the intensity of light produced is proportional to the NO concentration. NO2 is first converted to NO before measurement.
- Principle: SO2 molecules absorb UV radiation and re-emit it as fluorescence at a longer wavelength.
- Mechanism: The intensity of the fluorescent emission is proportional to the SO2 concentration. This is the standard reference method for ambient SO2 monitoring.
Pollutant concentrations are often expressed in parts per million (ppm) or parts per billion (ppb):
1 ppm=1000 ppb
The Air Quality Index (AQI) is a numerical scale used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. AQI values of 0–50 indicate good air quality, while higher values indicate increasing health risk.