## 13.6 Methods to Measure Air Quality 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. ### 1. Measuring Particulate Matter (PM) Particulate matter (PM) refers to a mixture of solid particles and liquid droplets suspended in the air. These particles are categorized by their size. The most common measurements are for: * **PM10:** Inhalable particles with a diameter of 10 micrometers ($10\,\mu m$) or smaller. * **PM2.5:** Fine inhalable particles with a diameter of 2.5 micrometers ($2.5\,\mu m$) or smaller. Several instruments, known as particulate matter monitors, are used to detect and quantify these particles. #### a. Beta Attenuation Monitors (BAM) * **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. #### b. Gravimetric Method * **Principle:** This is a direct physical measurement of the mass of collected particles. * **Mechanism:** 1. Air is pumped through a pre-weighed filter for a specific period. 2. Particulate matter is collected on the filter. 3. The filter is weighed again after sampling. 4. The difference in mass indicates the concentration of PM in the volume of air sampled. #### c. Optical Particle Counters * **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. ### 2. Measuring Gaseous Pollutants (e.g., Carbon Monoxide, CO) Various gas sensors and analysers are employed to detect specific gaseous pollutants in the air. Carbon monoxide (CO) is a common example. For more information on air pollutants, see . #### a. Electrochemical Sensors * **Application:** Commonly used in personal safety devices and for monitoring indoor air quality. * **Mechanism:** These sensors operate based on a chemical reaction. 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. #### b. Infrared (IR) Absorption Spectroscopy * **Principle:** This method is based on the fact that CO molecules absorb infrared light at a specific wavelength. * **Mechanism:** An IR light source emits radiation through an air sample. A detector on the other side measures the amount of light that passes through. The amount of IR light absorbed by the sample is used to calculate the concentration of CO molecules. #### c. 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 carbon monoxide. They provide a simple, visible indication that CO is present but are less precise for determining exact concentrations. #### d. Specialized Techniques for Other Gases For other pollutants like $NO_x$ and $SO_2$, specific analytical techniques are required to ensure accuracy at trace levels. ### 3. Concentration Units and Air Quality Index Pollutant concentrations are often expressed in parts per million (ppm) or parts per billion (ppb).