Primary air pollutants are emitted directly from sources such as vehicles, power plants, and industrial facilities. Particulate matter is a primary air pollutant because it is emitted directly from these sources.

Secondary air pollutants are formed in the atmosphere from reactions between primary air pollutants. Ozone is a secondary air pollutant because it is formed from reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight.

Electrostatic precipitators use an electrical charge to remove particulate matter from the air. The particles are attracted to the charged plates and then collected.

Flue gas desulfurization (FGD) systems use a chemical reaction to remove sulfur dioxide from the air. The most common FGD system is the wet scrubber, which uses a liquid to absorb the sulfur dioxide.

Selective catalytic reduction (SCR) systems use a catalyst to convert nitrogen oxides into nitrogen and water. SCR systems are typically used on power plants and industrial facilities.

All of the options listed are best practices for reducing air pollution from vehicles. Using public transportation and driving less reduces the number of vehicles on the road, which reduces air pollution. Keeping your vehicle well-maintained ensures that it is operating efficiently and producing less emissions. Using alternative fuels, such as electric or hybrid vehicles, also reduces air pollution.

All of the options listed are best practices for reducing air pollution from power plants. Using renewable energy sources, such as solar and wind power, reduces the amount of fossil fuels that are burned, which reduces air pollution. Installing air pollution control technologies, such as electrostatic precipitators and FGD systems, removes pollutants from the air before they are released into the atmosphere. Improving energy efficiency reduces the amount of fuel that is needed to generate electricity, which also reduces air pollution.

All of the options listed are best practices for reducing air pollution from industrial facilities. Using cleaner production processes reduces the amount of pollutants that are generated in the first place. Installing air pollution control technologies, such as electrostatic precipitators and FGD systems, removes pollutants from the air before they are released into the atmosphere. Improving energy efficiency reduces the amount of fuel that is needed to operate the facility, which also reduces air pollution.

All of the options listed are government policies that can be used to reduce air pollution. Emission standards set limits on the amount of pollutants that can be emitted from a source. Cap-and-trade programs limit the total amount of pollutants that can be emitted by a group of sources. Renewable portfolio standards require utilities to generate a certain percentage of their electricity from renewable energy sources.

All of the options listed are international agreements that aim to reduce air pollution. The Kyoto Protocol is an international agreement that aims to reduce greenhouse gas emissions. The Paris Agreement is an international agreement that aims to limit global warming to well below 2 degrees Celsius. The Montreal Protocol is an international agreement that aims to protect the ozone layer by phasing out the production and consumption of ozone-depleting substances.

All of the options listed are non-governmental organizations that work to reduce air pollution. Greenpeace is an international environmental organization that works to protect the environment and promote peace. The Sierra Club is an American environmental organization that works to protect the environment and promote sustainable development. The World Wildlife Fund is an international conservation organization that works to protect wildlife and their habitats.

All of the options listed are websites that provide information about air pollution. AirNow is a website that provides real-time air quality data for the United States. The EPA Air Quality Index is a website that provides air quality data for the United States. The World Air Quality Index is a website that provides air quality data for cities around the world.

All of the options listed are mobile apps that provide information about air pollution. AirVisual is a mobile app that provides real-time air quality data for cities around the world. BreezoMeter is a mobile app that provides air quality data and forecasts for cities around the world. Plume Labs is a mobile app that provides air quality data and forecasts for cities around the world.

All of the options listed are social media campaigns that aim to raise awareness about air pollution. #CleanAirNow is a social media campaign that aims to raise awareness about air pollution and promote clean air solutions. #AirPollutionAwareness is a social media campaign that aims to raise awareness about the health effects of air pollution. #BreatheClean is a social media campaign that aims to raise awareness about the importance of clean air.

All of the options listed are citizen science projects that collect data on air pollution. PurpleAir is a citizen science project that collects data on air quality from sensors that are installed in homes and businesses. AirCasting is a citizen science project that collects data on air quality from sensors that are attached to smartphones. Sensor Community is a citizen science project that collects data on air quality from sensors that are installed in public spaces.