With reference to Figure 3, calculate the difference between the highest concentration and lowest concentration of tropospheric ozone.

State two factors necessary for the chemical formation of ozone in the troposphere.

Outline why a high concentration of ozone in the troposphere is a direct problem for humans, while in the stratosphere it is a benefit to humans.

Suggest possible reasons for the overall trends of tropospheric ozone levels in Figure 3.

(peak of 175 – lowest point of 73 =) 102 (ppb);

Accept 72 – 75 as lowest point, ie 100–103 (ppb).

Units and working are NOT required for the 1 mark.

[1]

sunlight/UV light;
NOx/oxygen (atoms/ free radicals/molecules)/hydrocarbons/VOCs;

Note: Only credit necessary reactants for ozone formation as shown above. Do not credit sources of these active pollutants (eg fossil fuel use, organic solvents, pesticides etc).

[2]

in the troposphere it causes respiratory illnesses / eye/nose/throat irritations / heart failure;

in the stratosphere it prevents entry of UV that is harmful to humans/can cause mutations/skin cancer/tissue damage/cataracts/crop damage; [1 max]

Note: Do not credit responses that simply suggest global warming/climate change as an impact of tropospheric ozone without referring to some direct impact of those phenomena on humans.

Note: Accept “ozone in stratosphere protects humans from UV” WTTE … (use of “protects humans” implies UV is harmful to humans).

[2]

in first few years, increase due to:

• increase in population / car ownership/use / fossil fuel use;
• increased industrialisation / oil industry;
• lack of political awareness/funding to address the issue;

from around 1991 onwards, a decline may be due to:

• improvements in technology, eg energy efficiency/hybrid cars/catalytic converters/scrubbers;
• stricter monitoring/regulations on air quality control for industries / car-owners;
• switch to cleaner-burning/alternative energy sources;
• green initiatives/education campaigns/lobbying of politicians to promote, eg public transport/green roofs/car-pooling/registration plate limits etc;

Award [3 max] if only discussing decline or only discussing increase or failing to specifically identify either decline or increase.

[4 max]

Most were able to extract and calculate data with sufficient accuracy.

Many were able to suggest two factors though a good number forgot sunlight/UV. There were also some confused responses suggesting CFCs/refrigerants.

Mostly correct, with occasional confusion of stratospheric/tropospheric ozone.

There were a good number of candidates mistakenly addressing stratospheric ozone issues, i.e. CFCs/ODSs/Montreal Protocol etc. Those that were on the right track frequently scored 2 or 3 of the available marks, though rarely all 4.