Comprehensive Study Notes on Atmospheric Aerosols and Particulate Matter

Composition of Air and Heterogeneous Mixtures

Air is not a pure substance but a complex mixture containing various types of particles. These particles enter the atmosphere through various processes, primarily the evaporation of liquids and the release of emissions from industrial activities. The primary liquid responsible for many atmospheric phenomena is water ($H_2O$), which enters the air via evaporation and boiling, as well as through industrial conduits such as pipes in factories and heating plants.

Mixtures within the air are classified based on their state of matter and composition. The most common form is the gaseous mixture, which constitutes the bulk of the atmosphere. This mixture consists of gases such as Nitrogen ($N_2$), Oxygen ($O_2$), Argon ($Ar$), and Carbon Dioxide ($CO_2$).

When liquid or solid particles are dispersed in a gas, they form heterogeneous mixtures known as aerosols. Aerosols are broadly categorized into two types: mist (fog) and smoke (dým). These are characterized by the presence of suspended particles that do not chemically bond with the air but remain dispersed within it.

Mist, Fog, and Humid Air

Mist and fog are liquid-based aerosols frequently found in specific environments such as cities, forests, industrial zones, and areas surrounding rivers or marshlands. Humid air is specifically defined by the presence of water in two forms: water vapor (gaseous state) and very small liquid droplets. In bathrooms, near water bodies, or in industrial settings, this humidity is highly concentrated.

The formation of mist occurs through two primary mechanisms. First, it can form through the condensation of water vapor as the air cools. Second, it can form through the evaporation of a liquid or boiling, which breaks the liquid down into tiny droplets. These droplets vary significantly in size, ranging from a few nanometers ($nm$) to several millimeters ($mm$). Due to the physical property of surface tension ($\sigma$), these droplets naturally assume a spherical shape.

The movement of these droplets in the atmosphere is governed by gravity and air currents (flow). Smaller droplets are particularly notable for their ability to remain suspended in the air for extended periods.

Solid Particles and Particulate Matter (PM)

Solid particles in the air consist of various substances including soot, dust, pollen, sand, and salt crystals. The atmosphere also carries microorganisms. A critical category of solid pollutants is known as Particulate Matter, abbreviated as PM. These particles are classified by their aerodynamic diameter, which determines how deeply they can penetrate the human body and how long they remain in the environment.

PM10 particles have a diameter of up to $10\,\mu m$. These particles are generally trapped in the nose and the upper respiratory tract. PM2.5 particles, with a diameter of up to $2.5\,\mu m$, are far more dangerous as they can penetrate deep into the lungs, reaching the pulmonary alveoli (air sacs). From there, they can even enter the bloodstream. Exposure to PM2.5 is a significant cause of heart disease and asthma. PM1 particles are the smallest, with a diameter of $1\,\mu m$ or less. These are the most dangerous because they penetrate the deepest into the organs and tissues of the organism.

In heavily polluted or industrial areas, the concentration of these particles can reach several million per $cm^3$. Large clouds of dust can travel vast distances. Furthermore, these PM particles can act as carriers for other toxic substances, exacerbating their impact on health and leading to various chronic respiratory diseases.

Atmospheric Phenomena and Environmental Impact

Clouds are a prominent example of aerosols in the nature. They are essentially massive collections of water droplets or, depending on the temperature, small ice crystals suspended in the atmosphere. The composition of a cloud is heavily influenced by the ambient temperature.

Particles in the air have significant environmental implications. One positive interaction is the role of rain in air purification; as rain falls, it captures and carries suspended particles to the ground, effectively "washing" the atmosphere. Conversely, high concentrations of particles contribute to poor visibility, increased dustiness, and are often the result of heavy transport, industrial production, and local heating systems.

Questions & Discussion

1) What does the abbreviation PM stand for? PM stands for Particulate Matter, which refers to tiny solid or liquid particles suspended in the gas (air).

2) How do we call liquid aerosols? Liquid aerosols are generally referred to as mist or fog (mlha).

3) What belongs to the category of aerosols? Aerosols include heterogeneous mixtures such as mist, fog, and smoke (dým).

4) What is air composed of? Air is a mixture of gases, primarily Nitrogen ($N_2$), Oxygen ($O_2$), Argon ($Ar$), and Carbon Dioxide ($CO_2$), along with water vapor and various suspended particles.

5) What are the sources of particles in the air? Sources include industrial pipes, heating plants, transport/traffic, dust, pollen, soot, and the evaporation of liquids like water.