Air pollution and early deaths in the United States : attribution of PM₂.₅ exposure to emissions species, time, location and sector

Abstract

Combustion emissions constitute the largest source of anthropogenic emissions in the US. They lead to the degradation of air quality and human health, by contributing to the formation of fine particulate matter (PM2 .5 ), which is harmful to human health. Previous work computed the population PM2 .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 the present work we go beyond aggregate sectors and now attribute exposure and early deaths to sectors, emissions species, time of emission, and location of emission. This enables determination of the emissions reductions that would have the greatest benefit by sectors, species, time and location. We apply a long-term adjoint sensitivity analysis with population exposure to PM2 .5 in the contiguous US as the objective function, and calculate the four dimensional sensitivities (time and space) of PM2 .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 PM2 .5 exposure is due to SO2 emissions, and 80% of the annual impacts are attributed to emissions from April to September. This suggests that burning of low sulfur coal has greatest benefit in the summer. In the road transportation sector, 29% of PM2 .5 exposure is due to NO, emissions and 33% 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 NOx per unit mass. 75% of the road transportation ammonia impacts occur during the months October to March. We rank the states based on their contribution to the overall combustion emissions-attributable PM2 .5 exposure in the US, and calculate that California contributes 12%, Pennsylvania 7% and Ohio 5.8%. We publicly release the sensitivity matrices computed, noting their potential use as a rapid air quality policy assessment tool.