Emissions standards are easy for politicians. They just consult some people whose opinion they happen to like and pick a number. 

Then it is up for science and engineering to make it possible, whatever the cost. 

In 2012, California approved standards to reduce emissions from passenger cars to 3 milligrams, or a millionth of an ounce, per mile over the 2017-2021 automobile model years. Part of the justification politicians used was a gross miscalculation by the California Air Resources Board, which overstated some emissions by 340% to make the problem look urgent.

Particulate matter in emissions is associated with health problems, such as asthma and other lung conditions, and California has also gone after strange, inconsequential sources of particulate matter like hamburgers (see Is A Hamburger Really Worse For Pollution Than A Diesel Truck?).

The U.S. Environmental Protection Agency wants to use the same target and also expects it to remain consistent for 150,000 miles of a vehicle's lifetime. Car manufacturers have been putting more cars with GDI technology, which boosts fuel efficiency by injecting gasoline straight into a vehicle's combustion chamber, on the market but no one had looked at the emissions picture over the lifetime of these fuel-efficient vehicles to see if gasoline direct injection (GDI) technology cars can also meet the new particulate matter standards.

A group recently tested the particle emissions from two GDI vehicles. They found in these examples that the emissions hovered near or below the limit set by the new California and EPA standards over a lifetime of 150,000 miles.

Citation: M. Matti Maricq, Joseph J. Szente, Jack Adams, Paul Tennison, and Todd Rumpsa, 'Influence of Mileage Accumulation on the Particle Mass and Number Emissions of Two Gasoline Direct Injection Vehicles', Environ. Sci. Technol., 2013, 47 (20), pp 11890–11896 September 16, 2013  DOI: 10.1021/es402686z