AP Syllabus focus:
‘Volatile organic compounds (VOCs) like gasoline and formaldehyde readily evaporate at room temperature; trees also release VOCs naturally.’Volatile organic compounds are a diverse group of carbon-based chemicals that easily enter the air. In AP Environmental Science, VOCs matter because many common human products emit them, and many plants naturally produce them.
What VOCs are (and why they “readily evaporate”)
VOCs are defined by their tendency to vaporise at typical outdoor/indoor temperatures, which is why substances associated with fuels and chemical products can become air pollutants without combustion.
Volatile organic compound (VOC): An organic (carbon-containing) chemical that readily evaporates at room temperature, increasing its concentration in air.
VOCs vary widely in reactivity and toxicity; APES commonly emphasises their sources and their role as precursors that can participate in atmospheric chemistry.
Human (anthropogenic) VOC sources
Human VOC emissions are strongly linked to petroleum use and manufactured chemicals because many are liquids that evaporate during storage, use, or transfer.
Gasoline and other fuel-related VOCs
Gasoline is a major VOC source because it is formulated to be volatile, so it produces vapours whenever it is exposed to air.
Fuel handling and distribution
Refuelling vehicles (vapours displaced from tanks)
Evaporation from fuel storage tanks and containers
Leaks from fuel lines and fittings
Transportation-related evaporation
“Diurnal” emissions: fuel evaporation as temperatures rise during the day
Evaporation from parked vehicles, especially in warm conditions
Because these emissions can occur without visible smoke, gasoline VOCs are a key example of “silent” air pollution from everyday activity.
Formaldehyde and chemical product emissions
Formaldehyde is a VOC associated with many building and household materials, making it especially relevant to human exposure where people spend most of their time.
Building materials and furnishings
Pressed-wood products (e.g., particleboard, plywood) and some adhesives/resins
Some insulation, flooring products, and finishes
Consumer and commercial products
Certain cleaners, disinfectants, and coatings
Some textiles and treated materials
A central idea is off-gassing: VOCs slowly leave materials over time, so indoor sources can persist even when windows are closed and no combustion is occurring.
Other common human VOC categories (source-focused)
While gasoline and formaldehyde are highlighted examples, students should also recognise broad source types:
Solvents (paints, thinners, degreasers)
Aerosol products (sprays where propellants and ingredients can volatilise)
Industrial processes (chemical manufacturing, petroleum refining, dry cleaning)
Natural (biogenic) VOC sources
The specification emphasises that trees also release VOCs naturally. Plants produce VOCs for normal biological functions and ecological interactions.
Why trees emit VOCs
Trees and other vegetation can emit VOCs as part of:
Metabolic processes (by-products of growth and photosynthesis-related activity)
Chemical signalling (communication and interactions with insects and other plants)
Stress responses (heat, drought, physical damage, or herbivory)
Natural VOC emissions can be substantial in forested regions, meaning VOC pollution is not exclusively urban or industrial.
Patterns in natural VOC emissions
Natural VOC release is not constant; it commonly changes with environmental conditions.
Temperature dependence: warmer conditions generally increase VOC volatilisation and biological emission rates.
This figure plots normalized isoprene emission against temperature, showing a strong temperature-driven rise and a peak at very warm conditions. It visually supports the APES idea that hotter days (and heat stress) can substantially boost biogenic VOC emissions, especially in high-emitting tree species. Source
Seasonality: emissions often rise during the growing season when leaves are active.
Species differences: some tree species emit more VOCs than others, so vegetation type matters.
Connecting “sources” to air-quality relevance (without memorising extra chemistry)
For APES purposes, it is enough to connect sources to outcomes at a high level: VOCs in air can contribute to secondary pollutants when they react in the atmosphere, and some VOCs (including formaldehyde) can be directly harmful at elevated concentrations.
This diagram contrasts ‘good’ stratospheric ozone with ‘bad’ surface-level ozone associated with smog. In AP Environmental Science terms, it’s a conceptual anchor for why VOCs matter: VOCs act as precursors that help drive atmospheric reactions leading to unhealthy surface ozone under sunlight. Source
The key learning target here is recognising that both everyday human activities (gasoline use, building materials) and natural vegetation (trees) are important VOC sources.
FAQ
VOCs are often reported as $ppb$ (parts per billion) or $\mu g,m^{-3}$.
$ppb$ expresses a mixing ratio in air, while $\mu g,m^{-3}$ expresses mass of VOC per volume of air.
Emission rates vary with leaf chemistry and ecological strategy.
Some species invest more in chemical signalling/defence compounds
Leaf structure and resin content can increase VOC release potential
Some materials act as long-term reservoirs.
Formaldehyde can be slowly released from resins/adhesives, and emission can continue for months or years, especially with warmer temperatures and limited ventilation.
SVOCs (semi-volatile organic compounds) evaporate less readily than VOCs at room temperature.
They may partition between air, dust, and surfaces more strongly, affecting where they accumulate and how people are exposed indoors.
“Low-VOC” usually means the product meets a regulatory or certification threshold, not that it is VOC-free.
Different programmes set different limits, and some products can still emit VOCs during curing or drying.
Practice Questions
State one human source and one natural source of VOCs. (2 marks)
Human source correctly stated (e.g., gasoline vapours during refuelling / formaldehyde off-gassing from building materials) (1)
Natural source correctly stated (e.g., trees/vegetation releasing VOCs) (1)
Explain why VOCs such as gasoline and formaldehyde can be significant contributors to air pollution, and describe two factors that can increase VOC emissions from both human and natural sources. (6 marks)
Gasoline is volatile and evaporates readily at room temperature, producing vapours without burning (1)
Formaldehyde is a VOC that can off-gas from materials/products into air (1)
Link to “air pollution” via increased airborne concentration/exposure from evaporation/off-gassing (1)
Factor 1 described: higher temperature increases evaporation/volatilisation and can increase plant emissions (1)
Factor 2 described: increased handling/use (e.g., refuelling, solvent use) or increased biological activity/growing season for vegetation (1)
Clear reference to both human and natural sources across the explanation (1)
