Measuring street level pollutants that affect human health is the focus.  The sensor selection is guided by looking at the risks imposed by various pollutants.   

Health risks, at the street level, prioritized

• Particulate matter (PM)
• Ozone - VOCs+NOx+Sun => Ozone
• Nitrogen dioxide (NO2) - stress to lungs
• Volatile Organic Compounds (VOCs) - promotes ground level ozone, “smog”
• Nitrogen oxides (NOx) - promotes ground level ozone, “smog”
• Carbon monoxide (CO) - toxic to humans.  Not so concerning for street level
• Sulfur dioxide (SO2) - more an industrial pollutant

Gasses such as CO2 are not an immediate risk to human health.

Sources:

https://www.ucsusa.org/clean-vehicles/vehicles-air-pollution-and-human-health/cars-trucks-air-pollution#.W9ku5ntKimx

Based on the health risks, 3 pollutants will be measured

• Particulate matter (PM)
• Ozone
• Nitrogen dioxide (NO2) 

The criteria for sensor selection are:

1. Demonstrated effectiveness
2. Form factor
3. integration with microcontroller
4. Cost

Particulate matter

A Plantower 5003 unit is the pick to measure particulates.  The Plantower lineup of particulate sensors are used in many sensor units.  A tear-down of the unit shows that the design is effective and robust for particulate sensing.  The cost of the unit is around USD $15.  Measurement readings are read from the device with a UART interface.

https://aqicn.org/sensor/pms5003-7003/
https://ourairquality.org/

Ozone, Nitrogen dioxide

The Array of Things project in Chicago is the inspiration for selecting the remaining two gas sensors. 

https://arrayofthings.github.io/

The Array of Things node is built using gas sensors manufactured by SPEC Sensors.  The Ozone and NO2 sensors cost around USD $60 each.  Measurement readings are output as a 0-3V analog signal.

https://www.spec-sensors.com/array-of-things/