Particulate Matter monitoring – PM2.5

What is PM_2.5?

“PM”, Particulate Matter, is not a single pollutant, but refers to a complex mixture of solids and aerosols of varying shape, size, and chemical composition and may contain many chemical species like organic compounds, inorganic ions, metallic compounds, elementary carbon, etc. These atmospheric particles are defined by their diameter for air quality regulatory purposes.

The fine inhalable particles that are less than or equal to 2.5 micrometres in diameter are collectively known as PM_2.5. They are more than 100 times thinner than a human hair (too small to be detectable by the human eye) making them more harmful than larger particles as they can penetrate deeper into the human respiratory tract when inhaled. 

PM_2.5 in the atmosphere consists of toxic organic compounds and heavy metals released from various sources. Once released, they stay in the air for a long time i.e. days or weeks and can travel hundreds of miles. 

Sources

Particulate matter PM_2.5 are either directly emitted from a source (primary PM) or are formed through the chemical reactions of gases such as oxides of sulfur (SO_x), nitrogen oxides (NO_x), organic compounds, etc. in the atmosphere (secondary PM). The natural sources of particulate matter are wind-blown dust from open land, pollen, spores, mold, dirt, soil erosion, and forest fires.  Anthropogenic sources of PM_2.5 include:

• Burning fossil fuels such as gasoline, oil, diesel or wood, etc. 
• Waste burning, agricultural burning, etc.
• Emission from motor vehicle exhaust
• Emission from energy supply and industrial combustion processes
• Metal and steel production, specifically smelting and processing of metals

Permissible levels of PM_2.5

The breakpoints concentrations describing the quality of air based on the PM_2.5 concentrations for different countries are given below. In India, the daily average PM_2.5 levels of up to 60 μg/m³ are considered satisfactory. The Air quality guidelines by the World Health Organization suggests a 24-hr mean of 25 μg/m³.

Breakpoints of PM_2.5 (μg/m³)

India (24-hr)	US (24-hr)	China (24-hr)	EU (8-hr)
AQI Category	Break point concentration	AQI Category	Break point concentration	AQI Category	Break point concentration	AQI Category	Break point concentration
Good	30	Good	35	Excellent	15	Very low	10
Satisfactory	60	Moderate	75	Good	35	Low	20
Moderately polluted	90	Unhealthy for sensitive	115	Lightly Polluted	65	Medium	30
Poor	150	Unhealthy	150	Moderately Polluted	150	High	60
Very Poor	250	Very Unhealthy	250	Heavily Polluted	250	Very high	60+
Severe	250+	Hazardous	250+	Severely Polluted	250+

Health & Environmental Impact of PM_2.5

Health Impact

The potential of the particulate matter to affect human health depends directly on the size of particulate matter. The particles with < 10μm diameter generally pass through the nose and throat and enter the lung. PM_2.5 particles are so small that they tend to travel deep into the lungs through the respiratory tract and deposit on the surface of the deeper parts of the lung inducing tissue damage, and lung inflammation.

When inhaled for short periods, PM_2.5 (i.e. up to 24-hours) causes irritation to the eyes, nose, throat, and upper respiratory tracts, coughing, sneezing, and shortness of breath. It aggravates already present respiratory diseases such as asthma and can result in premature mortality, acute and chronic bronchitis, other respiratory symptoms, etc. 

Exposure to PM_2.5 pollution, for a long period (i.e. months or years), can cause permanent respiratory problems such as asthma, chronic bronchitis, and heart disease and cause reduced lung function growth in children. It can also lead to premature death, particularly for people suffering from chronic lung or heart diseases. Children are the most vulnerable. Increases in particulate matter levels, specifically PM_2.5 has been linked to an increase in deaths due to viruses such as novel coronavirus disease (COVID-19). Harvard University, in one of its studies, has claimed that an increase in the PM_2.5 values by 1 µg/m3, corresponded to an 8% increase in the COVID-19 deaths.

Environmental impact

The major environmental impacts of PM_2.5 pollution are visibility reduction and smog. Particulate matter, specifically PM_2.5 alters the absorption and scattering of light in the atmosphere affecting visibility. Additionally, the settling of the air-borne particles on plants, soil, and water ecosystems have harmful effects on them. The metal and organic compounds reduce plant growth and yield while the deposition of PM into water bodies affects its quality and clarity. 

Atmospheric particles affect the heating and the cooling of the atmosphere. Some components of PM such as black carbon promote climate warming while some components such as sulfates and nitrates have cooling properties. PM_2.5 particles are also the major precursors of smog.

Possible corrective measures 

The primary step is PM_2.5 monitoring to identify the areas with high particulate levels or areas where air quality does not meet the PM_2.5 national standards. In addition to this, the following corrective measures can be taken:

• Limit outdoor activities and close windows and vents during times of high air pollution.
• Avoid keeping your vehicle idle for long. 
• Avoid areas with traffic congestions, construction activities, or unpaved roads. Breathing such polluted air daily also leads to health effects in the long run. 
• Eliminate the use of the fireplace, wood stove, gas-powered lawn, or garden equipment. 
• Also, eliminate open burning of leaves, trash or other materials

Measurement methods

Different working principles for particulate matter monitoring in the ambient environment are Gravimetric, TOEM, Beta Attenuation (BAM), and Laser scattering. 

High-volume Gravimetric Method – The PM_2.5 monitors based on the gravimetric principle takes in the ambient air for 24 hours at a constant flow rate through a size-selective inlet that only allows particulate matter with an aerodynamic diameter of 2.5µm or less to pass through. The particulate matter is collected onto a pre-weighed filter conditioned at constant temperature and humidity conditions. After the 24-hour sampling period, the filter is conditioned again under the same temperature-humidity conditions and reweighted. Therefore, the difference in the weight corresponds to the mass collected on the filter, which along with known flow-rate, sampling period, and the total volume of the air sampled is used to calculate the PM_2.5 concentration. 

Tapered Element Oscillating Microbalance (TOEM) – It is a proprietary system for particulate matter monitoring. The PM_2.5 monitor based on TOEM determines the PM_2.5 concentration by continuously weighing the particulates deposited on the filter attached to a hollow tapered element that oscillates in an applied electric field. The oscillating frequency decreases with the accumulation of particulates on the filters. Thermal mass flow controllers constantly control and measure the flowrate of the PM_2.5 monitor which along with the mass concentration, temperature, and other factors is used to continuously calculate the PM_2.5 concentrations. 

Beta Attenuation Monitor (BAM) – The PM_2.5 monitoring based on the BAM principle measures the particle mass density using beta radiation attenuation. The particulates in the ambient air drawn into the PM_2.5 monitor are deposited on a paper-band filter and exposed to beta rays (i.e. electrons with energies in the 0.01 to 0.1 MeV range) which get attenuated as a function of the particulate mass. The beta count reduces with an increase in the PM_2.5 mass and is recorded by the detector and converted to concentration.

Laser Scattering – The PM_2.5 monitor based on the physical principle of light scattering, also known as optical particle counter (OPC), measures dust particles illuminated by laser light at a 90° angle. The light scattered from each particle is collected at approximately 90° by a mirror and detected by a photo-diode. This signal is then fed into a multi-channel size classifier where a pulse height analyzer is used to classify each pulse that is proportional to the particle size. The counts in the channel corresponding to PM2.5 converts to the concentration of PM_2.5.

Among all the above principles of PM_2.5 monitoring, PM monitors based on laser scattering are typically found to be preferred for ambient air monitoring as they yield quick and accurate measurements and are inexpensive in comparison with the others.

Oizom’s working principle for PM_2.5 monitoring

Oizom’s DUSTROID is an online particulate monitoring system that measures the concentration of various particulate sizes ranging from 1 micron to 100 microns such as PM_1, PM_2.5, PM₁₀, and PM₁₀₀ in the ambient air. Our PM sensor works on the principle of laser scattering. The active sampling powered sensor-based air quality monitor DUSTROID is deployed across several cities, campuses, universities. They are also used for drawing actionable insights to tackle the rise in ambient PM_2.5 concentrations.

5 reasons why PM_2.5 monitoring is important:

• PM_2.5 is a complex mixture of solid and aerosols with very small size (about 100 times thinner than a human hair) that can easily pass through the nose and throat and penetrate deep into the lungs and cause severe health issues. 
• Particulate matter is emitted into the atmosphere from vehicular exhaust, power plants, combustion processes such as the burning of fossil fuels, waste, etc. They are also formed into the atmosphere from various chemical reactions of other air pollutants such as NO_x and SO_x. 
• When inhaled, it irritates the eyes, nose, throat, and the airways and can also lead to aggravation of respiratory diseases leading to premature mortality, acute and chronic bronchitis, other respiratory symptoms, etc.  
• PM_2.5 monitoring is an efficient way to detect high concentrations of particulate matter and prevent high-level exposures.
• Real-time monitoring of particulate matter PM_2.5 levels helps in calculating air quality index to deliver health advisories as well as formulating an action plan to meet standards.