Extreme Value Theory View of High Emitter Diesel Vehicles in the US from 1991 to 2021.

Extreme value theory (EVT) is a statistical method to characterize the tails of distributions, such as high emitter vehicle emissions. The present work addresses the United States (US) diesel fleet. EVT confirms that high emitters contribute disproportionately to adverse air quality. It further reveals that high emitter emissions have responded differently to progressively tighter emissions standards compared to fleet averages. Carbon monoxide (CO) and hydrocarbons (HC) high emitter emissions fell in concert with the fleet average after widespread implementation of the diesel oxidation catalyst to meet US environmental protection agency (EPA) Tier 1 emissions standards, but remained unchanged after the introduction of Tier 2 and 3 standards, even as fleet averages continued to decline. Whereas fleet average diesel nitric oxide (NO) emissions saw a steep decline following EPA's 2010 diesel engine NOx standard, the mean 99th percentile emissions showed no impact. These results, combined with our previous gasoline vehicle high emitter study, demonstrate a growing disparity between fleet average and high emitter emissions. Addressing this disparity may present a more productive route to improved air quality than continued tightening of new vehicle emissions standards.