2024 Cambridge IGCSE Environmental Management (0680) Practice Paper with Answers

(a)
1. Set up measuring tapes at right angles to mark out a coordinate grid of \(50\text{ m} \times 50\text{ m}\).
2. Use a random number generator to select coordinates to avoid bias.
3. Place the quadrat at each generated coordinate pair.
4. Count and record the number of individual wildflowers inside each quadrat.
5. Calculate the average (mean) number of wildflowers per quadrat and multiply by the total number of quadrats that would fit in the entire \(2500\text{ m}^2\) area to estimate the total population.

(b)
1. A larger sample size is more representative of the entire area, which minimizes the effect of anomalous patches or uneven distribution.
2. It increases the reliability and accuracy of the population estimate.

(c)
Any three from:
- Overgrazing by livestock, which damages vegetation.
- Land clearance for agriculture or building developments.
- Introduction of non-native invasive plant or animal species.
- Uncontrolled or frequent deliberate burning/wildfires.

(a) [Max 4 marks]
- grid established using measuring tapes / coordinates [1]
- use of random number generator to select coordinates [1]
- placing the quadrat at the selected coordinates [1]
- counting the number of wildflowers in each quadrat [1]
- calculate mean per quadrat and scale up to the total area of \(2500\text{ m}^2\) [1]

(b) [Max 2 marks]
- more representative of the whole area / reduces effect of anomalies [1]
- increases accuracy / reliability of the estimate [1]

(c) [Max 3 marks]
- overgrazing [1]
- land clearance / habitat destruction [1]
- introduction of invasive alien species [1]
- uncontrolled burning / wildfires [1]

(a) (i)
- A transect is used because there is an environmental gradient or a systematic change in conditions from the sea inland.
- Random sampling would not show the progressive pattern of succession or zonation of plants across the dunes.

(a) (ii)
1. Lay out a long tape measure in a straight line starting from the high-tide mark on the shoreline and extending inland.
2. Place a quadrat at regular, pre-determined intervals along the tape measure (e.g., every 5 metres).
3. Estimate the percentage cover or record the abundance of each plant species inside each quadrat.
4. Measure and record the exact distance from the shoreline for each quadrat placement.

(b)
Any three from:
- Soil salinity
- Soil pH
- Soil moisture / water content
- Wind speed
- Light intensity
- Soil or air temperature

(a) (i) [Max 2 marks]
- used because there is an environmental gradient / systematic change in conditions [1]
- random sampling would not show the zone-by-zone pattern / change over distance [1]

(a) (ii) [Max 4 marks]
- lay out a tape measure from the shoreline inland (at right angles to the shore) [1]
- place quadrats at regular / systematic intervals along the tape [1]
- estimate percentage cover / record species presence or abundance [1]
- record the distance from the shore for each quadrat [1]

(b) [Max 3 marks]
- soil salinity [1]
- soil pH [1]
- soil moisture [1]
- wind speed [1]
- light intensity [1]
- soil/air temperature [1]

(a) (i)
- The container must be buried so that its top rim is completely level with the soil surface.
- A raised lid or cover should be placed over the trap to shield it from rainfall and to hide it from predators.

(a) (ii)
- To prevent the captured invertebrates from escaping the trap.
- To prevent predators (like birds or larger insects) from entering and eating the trapped organisms.
- To prevent trapped animals from dying due to starvation or drowning if rainwater accumulates.

(b) (i)
1. Capture a sample of woodland beetles, count them (\(N_1\)), and mark each with a non-toxic, durable paint or ink.
2. Release the marked beetles back into the woodland and allow them sufficient time to mix back into the wild population.
3. Capture a second sample after a set period of time, count the total number caught (\(N_2\)), and count how many of these are marked (recaptures, \(R\)). Calculate population size using: \((N_1 \times N_2) / R\).

(b) (ii)
Any two from:
- No significant births, deaths, immigration, or emigration during the study period.
- The marking method does not affect the survival of the beetles (e.g., does not make them more visible to predators).
- The marked individuals distribute themselves randomly back into the population.
- The marks do not wear off or disappear.

(a) (i) [Max 2 marks]
- container rim level with soil surface [1]
- raised cover/lid to protect from rain / predators [1]
- drainage holes in bottom [1]

(a) (ii) [Max 2 marks]
- prevent animals escaping [1]
- prevent predators eating captured animals [1]
- prevent mortality from dehydration/starvation/drowning [1]

(b) (i) [Max 3 marks]
- capture first sample, count (\(N_1\)), and mark with non-toxic substance [1]
- release and allow time to mix [1]
- collect second sample, count total (\(N_2\)) and recaptured marked individuals (\(R\)) / use Lincoln Index formula [1]

(b) (ii) [Max 2 marks]
- no births / deaths / migration [1]
- marks do not affect survival / predation rates [1]
- marked individuals mix randomly [1]
- marks do not rub off [1]

(a) (i)
- The layer closest to the surface is the troposphere.
- In this layer, temperature decreases as altitude increases.

(a) (ii)
- Stratosphere.

(b)
- The natural greenhouse effect traps infrared radiation / heat within the atmosphere.
- This keeps the Earth's average global temperature warm enough (approximately \(15\,^{\circ}\text{C}\)) to support liquid water and sustain living organisms, preventing extreme cold.

(c)
- Intensive livestock farming (specifically cattle and sheep rearing) produces massive amounts of methane through enteric fermentation (digestive processes).
- Wet rice cultivation in flooded paddy fields creates anaerobic soil conditions where methanogenic bacteria decompose organic matter and release methane.
- Organic waste decaying under anaerobic conditions in municipal landfill sites produces landfill gas (mostly methane).
- Extraction, processing, and transport of fossil fuels (such as coal mining and leaks from natural gas pipelines).

(a) (i) [Max 2 marks]
- troposphere [1]
- temperature decreases with altitude [1]

(a) (ii) [Max 1 mark]
- stratosphere [1]

(b) [Max 2 marks]
- traps heat / infrared radiation in the atmosphere [1]
- maintains a temperature warm enough to sustain life / liquid water [1]

(c) [Max 4 marks]
- intensive livestock / cattle farming releases methane from digestive systems [1]
- flooded rice paddies create anaerobic conditions / microbial decay [1]
- decay of organic waste in landfill sites under anaerobic conditions [1]
- venting / leaks from natural gas pipelines / fossil fuel extraction [1]

(a) (i)
- Sonar allows commercial fishing vessels to locate large shoals of fish with high precision.
- This drastically reduces searching time and increases catching efficiency, leaving very few adult fish to reproduce and rebuild populations.

(a) (ii)
- Definition: By-catch refers to non-target marine species (such as turtles, dolphins, sharks, or juvenile fish) that are caught unintentionally during commercial fishing operations.
- Consequences: It causes severe declines in vulnerable or endangered species that have slow reproductive rates.
- It also disrupts marine food webs by removing key predators or prey species from the ecosystem.

(b)
- Quotas / Catch limits: Placing strict legal limits on the total mass of each fish species that can be harvested per season.
- Net mesh regulations: Enforcing minimum mesh sizes so that juvenile fish can escape, allowing them to reach breeding age.
- Marine Protected Areas (MPAs) / No-take zones: Establishing restricted ocean zones where all fishing is banned to allow populations to regenerate.
- Closed seasons: Banning fishing during critical breeding times of the year to protect spawning adult fish.

(a) (i) [Max 2 marks]
- precise location of fish shoals / schools [1]
- decreases search time / increases catching efficiency (leaving fewer fish to reproduce) [1]

(a) (ii) [Max 3 marks]
- definition: non-target species caught unintentionally [1]
- leads to decline of endangered/vulnerable species (e.g. turtles, dolphins) [1]
- disrupts food webs / trophic balance [1]

(b) [Max 4 marks]
- quotas/catch limits restrict the total tonnage harvested [1]
- minimum net mesh sizes allow juvenile fish to escape [1]
- marine reserves / no-take zones protect habitats and allow populations to recover [1]
- closed seasons during spawning periods [1]
- pole-and-line fishing limits by-catch [1]

(a) (i)
- Sulfur dioxide (\(\text{SO}_2\))
- Nitrogen oxides (\(\text{NO}_x\))

(a) (ii)
- Acid rain leaches essential mineral nutrients (like calcium and magnesium) from the soil, depriving plants of what they need to grow.
- It mobilizes toxic aluminium ions in the soil, which damages plant roots and restricts water absorption.
- It damages leaves directly, destroying the waxy cuticle and reducing a plant's ability to photosynthesise.

(b) (i)
- Exhaust gases containing sulfur dioxide are directed through a scrubber unit.
- An alkaline slurry containing crushed limestone (calcium carbonate) or lime (calcium oxide) is sprayed into the gas stream.
- The acidic sulfur dioxide reacts chemically with the calcium compounds to form calcium sulfite/sulfate (gypsum), which is collected as a solid residue, cleaning the gas.

(b) (ii)
- Switching from coal to low-sulfur coal or alternative energy sources (like solar, wind, or natural gas).
- Installing catalytic converters in road vehicles to reduce nitrogen oxide emissions.
- Implementing energy conservation measures to reduce the overall demand for electricity generated by burning coal.

(a) (i) [Max 2 marks]
- sulfur dioxide [1]
- nitrogen oxides [1]

(a) (ii) [Max 2 marks]
- leaches nutrients (calcium/magnesium) from soil [1]
- releases toxic aluminium ions that damage roots [1]
- damages leaf cuticles / reduces photosynthesis [1]

(b) (i) [Max 3 marks]
- waste gases passed through a scrubber [1]
- limestone/lime slurry sprayed into the gases [1]
- sulfur dioxide reacts with calcium compounds to form gypsum / solid waste [1]

(b) (ii) [Max 2 marks]
- use low-sulfur coal [1]
- transition to renewable energy sources (wind/solar) [1]
- catalytic converters on vehicles to reduce \(\text{NO}_x\) [1]

(a) (i)
- High sea temperatures stress the coral polyps.
- This stress causes the corals to expel the microscopic, symbiotic algae (zooxanthellae) living in their tissues, which deprives them of food and turns them white.

(a) (ii)
- Fertilizers containing high amounts of nitrates and phosphates wash into the sea.
- This nutrient enrichment triggers rapid algal blooms over the reef.
- The dense algae layer blocks out sunlight and smothers corals, preventing photosynthesis and killing the reef.
- Suspended sediment in agricultural run-off can also physically settle on and block light from the corals.

(b)
- MPAs restrict or ban destructive fishing practices (like dynamite fishing or bottom trawling) that physically smash reefs.
- They regulate tourism, reducing physical damage from boat anchors and recreational divers.
- They allow herbivorous fish populations (which graze on algae) to recover, maintaining the natural ecological balance on the reef.
- They provide a safe breeding ground, helping coral larvae disperse and colonize neighboring degraded areas.

(a) (i) [Max 2 marks]
- increased ocean temperatures cause stress to coral polyps [1]
- corals expel symbiotic algae (zooxanthellae), turning white / losing food source [1]

(a) (ii) [Max 3 marks]
- run-off contains nitrates/phosphates from fertilizers [1]
- causes algal blooms / rapid growth of algae [1]
- algae smother corals / block sunlight [1]
- sediment blocks light / smothers polyps [1]

(b) [Max 4 marks]
- bans/limits destructive fishing methods [1]
- regulates anchoring / tourism to prevent physical damage [1]
- protects herbivorous fish that control algae growth [1]
- creates a secure habitat for breeding and larval spillover [1]

(a) (i)
- Selective logging involves cutting down only specific, mature trees of high value, leaving the rest of the forest stand intact.
- Clear-cutting involves the complete removal of all trees in a given area at the same time.

(a) (ii)
- Selective logging preserves the forest canopy, which protects soil from being eroded by wind and heavy rain.
- It maintains the complex structural habitats, preserving forest biodiversity.
- It leaves parent trees in place, allowing natural regeneration of the forest over time.

(b)
- Tree roots bind the soil, preventing wind and water erosion of fertile topsoil, which sustains crop yields.
- Decomposing leaf litter from the trees adds organic matter and nutrients back into the soil, reducing the need for chemical fertilizers.
- Tree canopies act as windbreaks and provide partial shade, reducing soil moisture evaporation.
- The trees offer habitats, nesting sites, and food sources for wildlife, including birds, beneficial insects, and pollinators, which increases biodiversity.

(a) (i) [Max 2 marks]
- selective logging cuts only specific, mature trees [1]
- clear-cutting removes all trees in an area [1]

(a) (ii) [Max 3 marks]
- maintains forest canopy / prevents soil erosion [1]
- preserves habitats and wildlife niches [1]
- allows natural seed dispersal / regeneration [1]
- reduces carbon release [1]

(b) [Max 4 marks]
- roots bind soil / reduce erosion to preserve fertile topsoil [1]
- leaf litter decomposes to return nutrients to soil [1]
- trees act as windbreaks / reduce soil moisture loss [1]
- provides habitat/food for pollinators and wild species [1]
- structural diversity increases overall biodiversity [1]

(a)
- Both the TAC and actual catch showed an overall downward trend from 2018 to 2022.
- TAC decreased steadily from 120 thousand tonnes in 2018 to 80 thousand tonnes in 2021, staying constant into 2022.
- Actual catch exceeded the TAC in three of the five years (2019, 2020, 2021) but was below the TAC in 2018 and 2022.

(b) (i)
- Change in TAC = \(120 - 80 = 40\) thousand tonnes.
- Percentage decrease = \(\frac{40}{120} \times 100 = 33.33\%\).
- Answer: 33.3% (or 33%).

(b) (ii)
- In 2019, actual catch (115) exceeded TAC (110) by 5 thousand tonnes.
- In 2020, it exceeded by 3 thousand tonnes.
- In 2021, it exceeded by 1 thousand tonnes.
- Greates excess year: 2019.
- Excess amount: 5 thousand tonnes (or 5000 tonnes).

(c)
- A minimum mesh size ensures that the holes in the fishing nets are large enough for young/juvenile fish to escape.
- This allows them to grow, reach reproductive age, and spawn to maintain the population size of the fishery.

(a) Max 3 marks:
- Both TAC and actual catch decreased overall [1]
- TAC fell from 120 to 80 thousand tonnes / leveled off at 80 thousand tonnes [1]
- Mention of actual catch exceeding TAC in 2019, 2020, or 2021 / actual catch falling below TAC in 2018 or 2022 [1]
- Comparative data quote with years [1]

(b) (i) Max 2 marks:
- Correct working shown: \(\frac{120 - 80}{120} \times 100\) [1]
- Correct calculation: 33.3% / 33.33% / 33% [1]

(b) (ii) Max 2 marks:
- Identifies 2019 [1]
- Calculates excess of 5 thousand tonnes / 5000 tonnes [1]

(c) Max 2 marks:
- Allows young / small / juvenile fish to escape / pass through [1]
- Gives them a chance to mature / reproduce / maintain breeding stock [1]

(a) (i) Purpose: To collect ground-dwelling insects / active invertebrates. Setup: Dig a small hole in the ground and place a cup/container inside so the rim is level with the soil surface. Cover with a raised lid/slate to protect against rain/predators. Place multiple traps across a grid pattern (random or systematic) to ensure a representative sample.
(a) (ii) Biotic: Presence of predators near the trap, density of vegetation surrounding the trap. Abiotic: Temperature, rainfall/humidity, soil moisture.

(b) (i) \(\frac{80}{90} \times 100 = 88.88...\%\) which rounds to \(88.9\%\) (or \(89\%\)).
(b) (ii) Richness: Plot B has a higher species richness (5 species present) compared to Plot A (4 species present). Evenness: Plot B has a much higher species evenness (the counts are relatively equal, ranging from 15 to 25) compared to Plot A, which is heavily dominated by one species (Beetles, 80 out of 90).

(c) Intercropping creates multiple canopy layers and varied food sources/microhabitats, supporting different species. Organic farming avoids synthetic pesticides and herbicides, reducing non-target mortality of insects and weeds, allowing food webs to remain intact.

(d) 1. Provides natural predators (like spiders/beetles) to control agricultural pests, reducing crop damage.
2. Ensures a stable population of pollinators, improving fertilization and yield of crops.

(a)(i) Max [3 marks]:
- 1 mark for stating the purpose is to capture active ground-dwelling invertebrates.
- 1 mark for describing the trap insertion (rim level with soil surface).
- 1 mark for describing representative sampling (e.g., using a grid, replication, random placement).

(a)(ii) [2 marks]:
- 1 mark for a valid biotic factor (e.g., predation, food availability).
- 1 mark for a valid abiotic factor (e.g., temperature, rainfall).

(b)(i) [2 marks]:
- 1 mark for correct working: \(\frac{80}{90} \times 100\).
- 1 mark for correct answer: \(88.9\%\) (accept \(88.89\%\) or \(89\%\)).

(b)(ii) Max [3 marks]:
- 1 mark for stating Plot B has higher richness (5 vs 4).
- 1 mark for stating Plot B has higher evenness.
- 1 mark for supporting data citation (e.g., Plot A is dominated by beetles at 80/90, whereas Plot B's lowest count is 15).

(c) Max [4 marks]:
- 1 mark for intercropping providing diverse niches/shelter.
- 1 mark for organic farming avoiding chemical pesticides.
- 1 mark for organic farming preserving soil organic matter and food webs.
- 1 mark for intercropping creating complex food chains.

(d) [2 marks]:
- 1 mark for pollination services.
- 1 mark for biological pest control (predation of pests).

(a) (i) Decrease in cover = \(1200 - 520 = 680\) hectares.
Percentage decrease = \(\frac{680}{1200} \times 100 = 56.66...\%\) which rounds to \(56.7\%\).
(a) (ii) There is a positive correlation between forest cover and the number of groups. As the bamboo forest cover decreases from 1200 to 520 hectares, the number of vervet groups steadily decreases from 45 to 15.

(b) Habitat fragmentation splits larger populations into smaller, isolated groups. This restricts movement, making it difficult for individuals to find mates, leading to inbreeding depression and loss of genetic diversity. Smaller patches also increase the 'edge effect', exposing them to predators and human encroachment, and make them more vulnerable to localized natural disasters.

(c) 1. Agricultural expansion / clearing land for crops.
2. Logging for timber, fuelwood, or bamboo harvesting for construction.
3. Infrastructure development, such as roads and settlements.

(d) Wildlife corridors connect isolated forest fragments, allowing gene flow, reducing inbreeding, and helping animals migrate to find food. Community education programs raise awareness about the ecological value of biodiversity, helping to reduce illegal poaching, encourage sustainable bamboo harvesting, and foster community-led forest protection initiatives.

(a)(i) [2 marks]:
- 1 mark for correct working: \(\frac{1200 - 520}{1200} \times 100\) or \(\frac{680}{1200} \times 100\).
- 1 mark for correct calculation: \(56.7\%\) (accept \(56.67\%\) or \(57\%\)).

(a)(ii) [2 marks]:
- 1 mark for identifying positive relationship / direct correlation.
- 1 mark for quoting data from both variables to illustrate (e.g., when forest cover drops to 520, groups fall to 15).

(b) Max [4 marks]:
- 1 mark for isolation of breeding populations / inbreeding.
- 1 mark for restriction of movement to find food / resource scarcity.
- 1 mark for increased susceptibility to diseases / environmental changes.
- 1 mark for increased edge effects / vulnerability to hunting or predators.

(c) [3 marks]:
- 1 mark for agricultural land clearance.
- 1 mark for extraction of bamboo for building materials / fuelwood.
- 1 mark for road building / urbanization.

(d) Max [5 marks]:
- Corridors (Max 3): 1 mark for linking patches, 1 mark for facilitating gene flow / genetic variation, 1 mark for migration routes.
- Education (Max 2): 1 mark for reducing conflict/poaching, 1 mark for encouraging sustainable alternative livelihoods / community policing.

(a) (i) Increase = \(414.2 - 338.8 = 75.4\) ppm.
(a) (ii) Both average atmospheric \(CO_2\) concentration and global temperature anomaly show an overall upward/increasing trend over the 40-year period.

(b) Incoming solar radiation reaches the Earth's atmosphere mainly as shortwave radiation (light). The Earth's surface absorbs this energy and re-emits it as longwave radiation (infrared/heat). Greenhouse gases in the atmosphere absorb this outgoing longwave radiation and re-radiate it in all directions, including back to the Earth's surface, trapping heat and increasing temperatures.

(c) Gas 1: Methane (\(CH_4\)). Source: Rice cultivation, livestock farming (enteric fermentation), or landfill decay.
Gas 2: Nitrous oxide (\(N_2O\)). Source: Agricultural soil management (use of synthetic nitrogen fertilizers), or industrial processes.

(d) Reforestation acts as a carbon sink because growing trees absorb carbon dioxide from the atmosphere during photosynthesis and store it as biomass (organic carbon). Another geoengineering strategy is Carbon Capture and Storage (CCS), which captures carbon dioxide emissions directly from industrial sources like power stations, compresses it, and injects it deep underground into geological formations for permanent storage.

(a)(i) [2 marks]:
- 1 mark for correct subtraction: \(414.2 - 338.8\).
- 1 mark for correct value: \(75.4\) ppm.

(a)(ii) [2 marks]:
- 1 mark for identifying that both increase overall.
- 1 mark for noting any detail (e.g., temperature fluctuates slightly in 2000 but the long-term trend is upward).

(b) Max [4 marks]:
- 1 mark for shortwave radiation from the sun passing through the atmosphere.
- 1 mark for the Earth absorbing shortwave and re-emitting energy as longwave radiation.
- 1 mark for greenhouse gases absorbing outgoing longwave radiation.
- 1 mark for re-emitting heat back to Earth, raising global temperatures.

(c) [4 marks]:
- 1 mark for each greenhouse gas (accept Methane, Nitrous oxide, CFCs, Water vapor) [Max 2].
- 1 mark for a corresponding correct anthropogenic source for each gas [Max 2].

(d) Max [4 marks]:
- 1 mark for stating that trees absorb \(CO_2\) during photosynthesis.
- 1 mark for stating that carbon is locked in tree biomass/wood.
- 1 mark for identifying another strategy (e.g., Carbon Capture and Storage (CCS), ocean fertilization, or biochar).
- 1 mark for explaining how that chosen strategy works.

(a) (i) Decrease in biomass = \(180 - 45 = 135\) thousand tonnes.
Percentage decrease = \(\frac{135}{180} \times 100 = 75\%\).
(a) (ii) The TAC was reduced because the breeding stock biomass had declined dramatically (from 180 to 45 thousand tonnes). This reduction was necessary to prevent complete collapse of the fishery and allow the remaining population to reproduce.

(b) Maximum Sustainable Yield (MSY) is the maximum level at which a natural resource can be routinely harvested without long-term depletion. Harvesting above MSY removes individuals faster than the population can reproduce, depleting the breeding stock and leading to rapid population decline and eventual fishery collapse.

(c) 1. Mesh size regulations: Setting a minimum net mesh size allows smaller, juvenile fish to escape, ensuring they reach reproductive age and can spawn before being harvested.
2. No-take marine reserves: Designated areas where all fishing is banned protect critical habitats, spawning grounds, and nurseries, allowing fish populations to recover and eventually spill over into surrounding fishable waters.
3. Fishing quotas: Limiting the total mass of fish that can be landed by each vessel or country prevents unlimited harvesting and maintains the extraction rate at or below sustainable levels (MSY).

(d) 1. Monitoring large areas of open ocean is logistically difficult and expensive.
2. Illegal, unreported, and unregulated (IUU) fishing practices / bypass of quotas by discarding dead non-target fish (bycatch) at sea.

(a)(i) [2 marks]:
- 1 mark for correct working: \(\frac{180 - 45}{180} \times 100\) or \(\frac{135}{180} \times 100\).
- 1 mark for correct calculation: \(75\%\) (must state decrease or use minus sign, or write percentage change is \(-75\%\)).

(a)(ii) [2 marks]:
- 1 mark for recognizing the rapid decline in breeding stock biomass.
- 1 mark for explaining that a lower TAC allows the population to recover / prevents overfishing.

(b) Max [4 marks]:
- 1 mark for definition: MSY is the largest catch that can be taken over an indefinite period.
- 1 mark for linking MSY to recruitment / birth rates balancing mortality rates.
- 1 mark for explaining that harvesting above MSY depletes the breeding population.
- 1 mark for stating that recruitment falls, leading to stock collapse.

(c) [6 marks]:
- 2 marks for Mesh size: 1 mark for letting juveniles escape, 1 mark for allowing them to reproduce at least once.
- 2 marks for No-take zones: 1 mark for protecting breeding grounds/habitats, 1 mark for the 'spillover effect' enhancing surrounding fisheries.
- 2 marks for Quotas: 1 mark for capping total catch, 1 mark for keeping catches within safe biological limits.

(d) [2 marks]:
- 1 mark for difficulties in patrolling vast ocean zones / high costs.
- 1 mark for issues with bycatch, discarding, or misreporting of landings.

(a) (i) Primary pollutants: Nitrogen oxides (\(NO_x\)) and Volatile Organic Compounds (VOCs). Main source: Combustion of fossil fuels in motor vehicles / industrial emissions.
(a) (ii) Sunlight: Photochemical smog is formed through chemical reactions triggered by solar UV radiation (photochemical reactions) acting on \(NO_x\) and VOCs. Temperature inversion: A layer of warm air traps cooler air below it near the ground, preventing the upward dispersion of pollutants, leading to the accumulation of high concentrations of smog in urban basins.

(b) 1. Irritates eyes, nose, and throat in humans.
2. Exacerbates respiratory diseases such as asthma and bronchitis.
3. Damages plant leaves / enters stomata, reducing photosynthesis and crop yields.

(c) (i) Mean Before = \(\frac{48 + 56 + 62}{3} = \frac{166}{3} = 55.33...\) ppb (or \(55.3\) ppb).
(c) (ii) Mean After = \(\frac{32 + 35 + 41}{3} = \frac{108}{3} = 36.0\) ppb.
Decrease = \(55.33 - 36.0 = 19.33\) ppb.
Percentage decrease = \(\frac{19.33}{55.33} \times 100 = 34.94...\%\) which rounds to \(35\%\).

(d) Catalytic converters contain precious metal catalysts that facilitate chemical reactions to convert harmful gases. They reduce nitrogen oxides (\(NO_x\)) back into harmless nitrogen gas (\(N_2\)), and oxidize unburnt hydrocarbons (VOCs) and carbon monoxide (\(CO\)) into carbon dioxide (\(CO_2\)) and water vapour (\(H_2O\)), thus reducing primary smog precursors.

(a)(i) [3 marks]:
- 1 mark for Nitrogen oxides (\(NO_x\)).
- 1 mark for Volatile Organic Compounds (VOCs).
- 1 mark for identifying vehicle exhausts / fossil fuel combustion as the main source.

(a)(ii) Max [4 marks]:
- 1 mark for stating UV light initiates/drives the chemical reaction between precursors.
- 1 mark for defining temperature inversion (warm air layer aloft trapping cold air beneath).
- 1 mark for explaining that warm air is less dense and stops vertical air mixing.
- 1 mark for stating that pollutants are trapped close to ground level, leading to high concentration build-up.

(b) [3 marks]:
- 1 mark for human health respiratory effect (e.g., asthma, chest pain, coughing).
- 1 mark for eye/sensory irritation.
- 1 mark for agricultural impact (reduced photosynthesis, damage to leaf tissue/necrosis, lower yields).

(c)(i) [2 marks]:
- 1 mark for correct working: \(\frac{48 + 56 + 62}{3}\).
- 1 mark for correct answer: \(55.3\) ppb (accept \(55.33\) or \(55\)).

(c)(ii) [2 marks]:
- 1 mark for calculating the new mean (\(36.0\) ppb) and showing the difference relative to the original mean: \(\frac{55.33 - 36.0}{55.33} \times 100\).
- 1 mark for correct calculation rounded to nearest whole number: \(35\%\).

(d) [2 marks]:
- 1 mark for stating that catalytic converters convert nitrogen oxides (\(NO_x\)) to nitrogen (\(N_2\)).
- 1 mark for stating that they convert unburnt hydrocarbons/VOCs/CO to water and carbon dioxide.