What percent of air pollution is caused by cars?

The following lists the United States' motor vehicle emissions standards for carbon monoxide (CO), nitrogen oxides (NOx), and non-methane hydrocarbons (NMHC) in grams per mile (GPM) (Masters and Ela, 2008).

Vehicle type: CO, NOx, NMHC, 3.4, 0.4, 0.7, 0.7, 0.4, 0.4, 0.5, Light duty trucks: 4.4, 5.0, 1.1, 1.0, 0.7, 0.5, 0.5, Moderate duty trucks: 5.0, 1.1, 1.1, 0.9, 0.5, Motorcycles: 19.3 (NOx+HC)

To the best of my knowledge, there are 260 million motor vehicles in the United States (out of 320 million people). However, these figures are subject to change based on various factors such as the type of city, the total number of miles driven per day, driving habits (e.g., sudden acceleration, sudden stop, etc.), and so on. In certain instances, there are problematic vehicles that emit higher emissions per mile.

There is no magic number when it comes to the percentage of air pollution caused by traffic versus stationary sources (space heating, power plants, industrial sites, etc.). The problem of air pollution in many urban centers is caused by emissions of carbon monoxide, nitrogen oxides, and various volatile organic compounds that are emitted from stationary sources (power plants, for instance) and motor vehicles. Photochemical smog is most closely associated with motor vehicles.

Citation: Masters, G.M. and Ela, W.P. (2008) Pearson International Edition, New Jersey, USA. Introduction to Environmental Engineering and Science, Third Edition.

The percentage of air pollution caused by cars varies depending on factors such as location, population density, and transportation infrastructure. In urban areas with heavy traffic congestion, vehicles can contribute a significant proportion of air pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter (PM). Estimates suggest that in some cities, cars and other forms of transportation can account for up to 50% or more of total air pollution emissions. However, the exact percentage may fluctuate and is influenced by factors such as the presence of industrial facilities, power plants, and natural sources of pollution.