Topic 7.4: Atmospheric Carbon Dioxide and Particulate Matter

Objective: To describe the natural sources of carbon dioxide and particulate matter in the atmosphere.
Skill Development: Describing measures, procedures, or variables used in scientific experiments (specifically for Practice FRQ 7.4).
General Natural Sources: * Lightning Strikes: A primary natural source of nitrogen oxides ( $NO_x$ ). The high energy from lightning converts atmospheric nitrogen ( $N_2$ ), which makes up $78\%$ of the atmosphere, into nitrogen dioxide ( $NO_2$ ) or nitric oxide ( $NO$ ). * Forest Fires: These are major sources of multiple pollutants, including: * Carbon monoxide ( $CO$ ). * Particulate matter (PM). * Nitrogen oxides ( $NO_x$ ). * Carbon dioxide ( $CO_2$ ). * Water vapor: Although a greenhouse gas and part of the natural hydrological cycle, it is not typically targeted for regulation like anthropogenic greenhouse gases. * Plants (Biological Sources): * Plants emit Volatile Organic Compounds (VOCs), particularly coniferous trees. * Terpenes: A specific class of VOCs emitted by plants, often identified by a strong, pungent pine odor. * Natural Photochemical Smog: The VOCs emitted by trees in areas like the Smoky Mountains National Park contribute to natural smog formation. * Volcanoes: These are significant sources of several gases and particles: * Sulfur dioxide ( $SO_2$ ) - volcanoes are one of the primary natural sources. * Particulate matter (ash). * Carbon monoxide ( $CO$ ). * Nitrogen oxides ( $NO_x$ ).

Respiration: All living organisms, including plants, release $CO_2$ into the atmosphere through cellular respiration. While essential to the carbon cycle, it is generally not characterized as a major contributor to rising atmospheric levels in the context of human-driven climate change on FRQs.
Aerobic Decomposition: * Process: The breakdown of organic matter (dead trees, animal waste, carcasses) by microbes and decomposers such as bacteria, fungi, or worms in the presence of oxygen. * Chemical Outcome: Because oxygen ( $O_2$ ) is available during aerobic respiration, the carbon in the organic material bonds with oxygen to form and release carbon dioxide ( $CO_2$ ).

Anaerobic Decomposition: * Environment: Occurs in conditions where oxygen levels are very low or non-existent, such as bogs, marshes, wetlands, or melting permafrost. * Chemical Outcome: Instead of producing $CO_2$ , the carbon bonds with hydrogen to produce methane ( $CH_4$ ). * Memory Aid: Aerobic decomposition requires oxygen to produce carbon dioxide; anaerobic decomposition lacks oxygen, so carbon bonds with hydrogen to form methane.

Terminology: The terms "particulate matter," "particles," and "particulates" are interchangeable. In AP Environmental Science, the abbreviation "PM" is standard.
Natural Sources of PM: * Sea Salt: From the ocean. * Pollen: Released by plants. * Forest Fires: Releasing ash and soot. * Dust/Soil: Picked up and carried by wind.
Haze: A term used to describe low visibility specifically due to high concentrations of particulate matter in the atmosphere. This is distinct from photochemical smog, which involves chemical reactions between sunlight, $NO_x$ , and VOCs.

PM10 (Particulate Matter $10$ ): * Size: Particles with a diameter of $10\,\mu m$ (micrometers or microns) or smaller. * Examples: Droplets of dust, pollen. * Health Risk: These particles are too small to be filtered by the body's natural defenses, such as nose hairs or the cilia in the trachea. They can travel into the respiratory tract and reach the bronchioles or lungs, causing irritation, inflammation, and breathing difficulties.
PM2.5 (Particulate Matter $2.5$ ): * Size: Particles with a diameter of $2.5\,\mu m$ or smaller. * Health Risk: Significant concern due to their ability to penetrate even deeper into the lungs than PM10. They can embed in lung tissue and significantly degrade lung function. * Long-term Effects: Exposure is linked to chronic bronchitis (permanent inflammation of the bronchioles), heart problems, and an increased risk of lung cancer.
Visual Scale: A human hair spans approximately $50$ to $70\,\mu m$ in width, illustrating how small and invasive PM10 and PM2.5 are.

Experimental Design: * Background: Students are testing how road construction impacts atmospheric PM levels. * Setup: Vaseline is spread on the bottom of $6$ petri dishes to trap particulates. * Distances: Dishes are placed at $50$ , $100$ , and $200$ yards from two different sites. * Groups: * Experimental Group: Dishes placed at the road construction site (where old road is being torn up and new road is being laid using heavy machinery like bulldozers/steamrollers). * Control Group: Dishes placed at an existing active road (normal traffic, but no construction).
Questions and Discussion: * Identify the control group: The group of three petri dishes placed at $50$ , $100$ , and $200$ yards from the existing local road that is not under construction. * Identify the likely dependent variable: The amount of particulate matter collected on the petri dishes. * Describe the measurement procedure: Students could measure the dependent variable by using a microscope to count the number of particles trapped in the Vaseline within a specific area of the dish (e.g., a square centimeter), or by using a mass balance to measure the weight of the dish before and after exposure to find the mass of the accumulated particulates.