How do renewable energy and energy efficiency technologies reduce air emissions?

How do renewable energy and energy efficiency technologies reduce air emissions?
Efficient use of electricity reduces the need for electricity generation and associated emissions. Some forms of renewable
electricity generation have zero emissions during generation, such as wind and photovoltaics. Others, such as biomass
and geothermal electricity generation, have lower emissions. Advanced vehicles and alternative fuels can reduce
emissions. Electric and fuel cell vehicles have zero emissions during operation; and alternative fuels such as natural gas,
ethanol, and biodiesel can reduce emissions during vehicle use and during the fuel cycle. Efficient vehicles use less oil
and reduce fuel-cycle emissions.