Common Outdoor Air Pollutants & Sources

In British Columbia, air pollutants of special concern are fine particulate matter (PM2.5), ground-level ozone (O3), nitrogen dioxide (NO2) and sulphur dioxide (SO2). These pollutants are monitored by the Ministry of Environment from stations located across the province.
The British Columbia State of the Air, State of the Air Report 2015 report presents data on each of the following pollutants (BC Lung Association, 2015).
Why do scientific studies and literature focus on particulate matter that is less than 2.5 micrometers in diameter (PM2.5) as a benchmark?
Make a list of the air polluting impacts of the automobile. Which of these are easiest to deal with by improving existing fuel use technology, and which require changes in fuel use?