Topic 7.4 Notes – Atmospheric CO₂ and Particulates

What Atmospheric CO₂ and Particulates Are

Atmospheric CO₂ is a gas. It is a normal part of the carbon cycle, which means carbon moves among the atmosphere, living things, soils, water, and rocks. A source releases carbon, and a sink absorbs more than it releases.

Carbon cycle diagram

Particulate matter is totally different. It is made of tiny solid particles or liquid droplets suspended in air.

A few distinctions matter a lot:

• Natural vs anthropogenic depends on the process that produced it, not the substance itself.
  • CO₂ from respiration is natural.
  • CO₂ from burning fossil fuels is anthropogenic.
• One event can release both.
  • Volcanic eruptions release CO₂ gas and ash particles.
  • Lightning-caused wildfires release CO₂, smoke, and ash.
• Atmospheric concentration is not the same as emission rate.
  • A place can measure high pollution because of transport, poor mixing, or slow removal, even if the source is far away.

Natural Sources of Atmospheric CO₂

The main natural CO₂ sources you need here are respiration, decomposition, and volcanic activity.

Respiration

Cellular respiration happens in plants, animals, fungi, protists, and many microorganisms. In aerobic respiration, organisms use oxygen to break down organic molecules and release CO₂, water, and energy.

$$\text{organic molecules} + O_2 \rightarrow CO_2 + H_2O + \text{energy}$$

A common mistake is forgetting that plants also respire. They photosynthesize and respire. Whether an ecosystem is a net source or sink depends on the balance over time.

Decomposition

Decomposers, especially bacteria and fungi, break down dead organisms and wastes. Much of the CO₂ released during decomposition comes from the respiration of those decomposers.

• Aerobic decomposition releases CO₂.
• Anaerobic decomposition can release CO₂ and CH₄, especially in wetlands, waterlogged soils, and sediments.
• Warm, moist, oxygen-rich conditions usually speed decomposition.

Volcanic activity

CO₂ dissolved in magma escapes as pressure drops during eruptions or degassing. This carbon comes from geologic reservoirs, not recent biomass. Also, keep this straight on tests:

• Volcanic CO₂ = gas
• Volcanic ash = particulate matter

Other natural CO₂ sources

• Naturally ignited wildfires
• Surface waters and oceans releasing dissolved CO₂
• Soils releasing CO₂ from roots and microbes

Where Natural Particulates Come From

Particulate matter includes dust, ash, smoke, sea salt, pollen, spores, and liquid aerosols.

Size matters:

• PM₁₀ = particles $10$ micrometers or less
• PM₂.₅ = particles $2.5$ micrometers or less

Smaller particles stay suspended longer and travel farther. The size comparison below gives you a quick sense of how tiny PM₁₀ and PM₂.₅ are relative to a human hair and fine sand. Rain can remove particles from air.

Volcanic particles

Volcanoes release ash, pulverized rock, and droplets directly. They can also produce secondary particulates when sulfur dioxide forms sulfate aerosols.

• Primary PM = emitted directly
• Secondary PM = forms in the atmosphere

Windblown mineral dust

Strong winds lift dust from deserts, dry lake beds, beaches, and exposed soils. This is more likely when soils are dry and vegetation is sparse.

• Illustrative example: Saharan dust crossing the Atlantic Ocean

Wildfire smoke and ash

Naturally ignited wildfires release ash, soot, and fine smoke particles. The classification depends on what started the fire.

Sea spray

Breaking waves and bursting bubbles eject seawater droplets. These can stay liquid or evaporate and leave sea-salt particles behind.

Biological particles

Illustrative examples:

• Pollen grains
• Fungal spores

These often peak seasonally. Dry, windy weather helps dispersal, and rain temporarily removes them.

How These Sources Affect Air Measurements

Measured air concentration depends on more than emissions. It also depends on:

• transport by wind
• dispersion and atmospheric mixing
• topography
• precipitation
• particle size

That is why pollution can be measured far from its source, especially fine particles. Also, natural does not mean harmless, and the existence of natural emissions does not mean recent atmospheric changes are entirely natural.

How Scientists Measure CO₂ and Particulates

Measuring particulate matter

A common method is gravimetric sampling. Air is pulled through a pre-weighed filter, and the filter is weighed again.

$$\text{PM concentration} = \frac{\text{final filter mass} - \text{initial filter mass}}{\text{volume of air sampled}}$$

Good design includes:

• size-selective inlets for PM₁₀ or PM₂.₅
• consistent flow rate and sampling time
• careful filter handling
• consistent sampling height
• blank filters to detect handling or humidity effects

Scientists may also use particle composition, wind direction, air-mass trajectories, and upwind vs downwind comparisons to identify sources.

Measuring CO₂ release from respiration or decomposition

A chamber is placed over soil or decomposing material, and scientists track CO₂ concentration over time.

• Upward slope on a CO₂ vs time graph = net CO₂ release
• Steeper slope = greater release rate

Strong design features include replicates, controls, calibrated sensors, equal sampling periods, and tracking temperature and soil moisture.