instruction: What is the largest pollutant?
response: Carbon dioxide (CO2) - a greenhouse gas emitted by burning fossil fuels, deforestation, and other human activities that contribute to climate change.
  
  Particulate matter - tiny particles of pollutants such as dust, smoke, and soot that can be inhaled and cause respiratory problems.
  
  Nitrogen oxides (NOx) - gases emitted by vehicle exhausts and industrial processes that contribute to air pollution and can also react with other pollutants to form ozone.
  
  Sulfur dioxide (SO2) - a gas emitted by burning fossil fuels and industrial processes that can cause respiratory problems and acid rain.
  
  Methane (CH4) - a potent greenhouse gas emitted by agriculture, waste management, and natural gas production.
  
  Chlorofluorocarbons (CFCs) - chemicals used in refrigeration, air conditioning, and industrial processes that contribute to the depletion of the ozone layer.