Air pollution and early deaths in the United States. Part II: Attribution of PM_2.5 exposure to emissions species, time, location and sector

Combustion emissions constitute the largest source of anthropogenic emissions in the US, and lead to the degradation of air quality and human health. In Part I we computed the population fine particulate matter (PM_2.5) exposure and number of early deaths caused by emissions from six major sectors: electric power generation, industry, commercial and residential activities, road transportation, marine transportation and rail transportation. In Part II we attribute exposure and early deaths to sectors, emissions species, time of emission, and location of emission. We apply a long-term adjoint sensitivity analysis and calculate the four dimensional sensitivities (time and space) of PM_2.5 exposure with respect to each emissions species. Epidemiological evidence is used to relate increased population exposure to premature mortalities. This is the first regional application of the adjoint sensitivity analysis method to characterize long-term air pollution exposure. (A global scale application has been undertaken related to intercontinental pollution.) We find that for the electric power generation sector 75% of the attributable PM_2.5 exposure is due to SO₂ emissions, and 80% of the annual impacts are attributed to emissions from April to September. In the road transportation sector, 29% of PM_2.5 exposure is due to NO_x emissions and 33% is from ammonia (NH₃), which is a result of emissions after-treatment technologies. We estimate that the benefit of reducing NH₃ emissions from road transportation is ~20 times that of NO_x per unit mass. 75% of the road transportation ammonia impacts occur during the months October to March. We publicly release the sensitivity matrices computed, noting their potential use as a rapid air quality policy assessment tool.