7.4 - Atmospheric CO2 and Particulates

Learning Objectives

I. Natural Sources of Air Pollution

• Lightning strikes: The energy from a lightning strike can convert atmospheric $\text{N}_2$ gas, which makes up 78% of the atmosphere, into nitrogen oxides (NOx).

• Forest fires: A major source of many air pollutants, including carbon monoxide, particulate matter, NOx, and carbon dioxide. Forest fires also release a lot of water vapor, which is a greenhouse gas.

• Plants (VOCs): Many volatile organic compounds in the atmosphere are naturally emitted by plants, especially coniferous trees.
• One class of VOCs commonly emitted by plants is terpenes.
• The Great Smoky Mountains get their name from a natural blue haze. The trees release VOCs that react in the air to form tiny particles, which scatter blue light.

• Volcanoes: Volcanoes are major sources of sulfur dioxide ($\text{SO}_2$) in the atmosphere. They also release particulate matter (ash), carbon monoxide, and nitrogen oxides.

II. Natural Sources of Carbon Dioxide and Methane

• Respiration: All living things, including plants, release carbon dioxide into the atmosphere through cellular respiration ($\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$). It's one of the biggest natural sources of $\text{CO}_2$, but photosynthesis takes in about the same amount each year, so respiration alone doesn't make $\text{CO}_2$ levels keep rising. That's why FRQs about rising $\text{CO}_2$ focus on human sources like burning fossil fuels.

• Aerobic decomposition: The breakdown of organic matter, like a dead tree or animal body, by decomposers (bacteria, fungi, worms) in the presence of oxygen. This process releases carbon dioxide.

• Anaerobic decomposition: The breakdown of organic matter by decomposers in conditions with very low or no oxygen.
• Examples include bogs, marshes, wetlands, or puddles caused by melting permafrost.
• When oxygen is low, some of the released carbon bonds with hydrogen instead of oxygen, which produces methane ($\text{CH}_4$) along with carbon dioxide.

• Volcanic eruptions: magma contains dissolved $\text{CO}_2$ that escapes during eruptions and from vents between eruptions. It sounds like a lot, but all the world's volcanoes together release only about 0.13 to 0.44 billion metric tons of $\text{CO}_2$ a year (USGS estimate). Burning fossil fuels released about 38 billion metric tons in 2025 (Global Carbon Project estimate), at least 80 times more.

III. Particulate Matter (PM) and Human Health

• Natural PM examples:
• Wildfire smoke: on June 7, 2023, smoke from wildfires in Canada pushed New York City's AQI to 484 (hazardous), mostly from PM2.5, and turned the sky orange.
• Dust storms: every summer, winds carry dust from the Sahara Desert across the Atlantic Ocean to the Caribbean and the southeastern U.S.
• Volcanic ash and sulfate particles: Mount Pinatubo in the Philippines (1991) put about 15 million tons of $\text{SO}_2$ into the stratosphere, where it formed tiny sulfuric acid droplets that reflected sunlight and cooled Earth by about 1°F (0.6°C) over the next 15 months.
• Sea spray and pollen: breaking waves fling tiny salt crystals into the air, and plants release pollen, especially in spring.

• PM10: Particulates that are 10 micrometers or smaller.
• These are concerning because they’re small enough to get past the hairs in your nose and settle in the larger airways of your lungs.
• They can irritate and inflame the respiratory tract, making it harder to breathe.

• PM2.5: Particulates that are 2.5 micrometers or smaller.
• These are especially concerning because they can make it deeper into the lungs.
• Increased exposure to PM2.5 has been linked to increased chronic bronchitis (long-term inflammation of the bronchi, the main airways into the lungs), heart problems, and an increased risk of lung cancer.

Big picture: Air pollution is often associated with human activity, but natural processes like volcanoes (releasing $\text{SO}_2$ and particulates), lightning strikes (creating NOx), and forest fires (releasing CO and $\text{CO}_2$) are fundamental sources of atmospheric pollutants. The breakdown of organic matter also produces greenhouse gases: aerobic decomposition creates carbon dioxide, while anaerobic decomposition in low oxygen wetlands creates potent methane. The smallest particulates, PM2.5, are the most dangerous because they penetrate deepest into the lungs, increasing the risk of chronic bronchitis and lung cancer.