Good air quality is essential for human health and the environment. Air quality is an important contributor to quality of life, and plays a role in the livability of our towns, cities and environment. Ambient air quality is determined by the types and amounts of pollutants emitted into the atmosphere, and the processes associated with their transport, transformation, mixing, and removal from the atmosphere. Many different pollutants exist in our atmosphere, including gases (e.g. carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, and volatile organic compounds (VOCs) such as benzene and formaldehyde) and particulate matter (PM, including particulate matter less than 10 microns in size [PM10] and particulate matter less than 2.5 microns in size [PM2.5]). These pollutants can interact in multiple ways to produce new pollutants, and there are several removal processes. Weather conditions have a significant influence on air quality. For example, calm winds can lead to the build-up of pollution levels in an urban environment. Temperature and sunlight have a strong influence on the chemical transformation of pollutants in the atmosphere.