The article (2008) published in PNAS looks at the life-cycle climate-change and health effects of greenhouse gas (GHG) and fine particulate matter (PM2.5) emissions from gasoline, corn ethanol, and cellulosic ethanol. According to the article, the researchers found out that “for each billion ethanol-equivalent gallons of fuel produced and combusted in the US, the combined climate-change and health costs are $469 million for gasoline, $472–952 million for corn ethanol depending on biorefinery heat source (natural gas, corn stover, or coal) and technology, but only $123–208 million for cellulosic ethanol depending on feedstock (prairie biomass, Miscanthus, corn stover, or switchgrass).” This shows that cellulosic ethanol can reduce PM2.5 emissions and hence offer health benefits than other fuels. The researchers looked at the two categories of emission: GHG and fine particulate matter that looks at global climate change and human health respectively. These emissions are emitted at all stages throughout the process of gasoline and ethanol production and combustion such as feedstock production, transportation, extraction, refinery and fuel combustion.
Three methods of producing ethanol from corn (using natural gas, coal, or corn stover for process heat at biorefineries) and four methods of producing cellulosic ethanol (from corn stover, switchgrass, diverse prairie biomass, or Miscanthus) were considered in this study. Life-cycle emissions was calculated by using the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model, which tracks long-lived GHG and most PM2.5 related emissions. A separate analysis was conducted to estimate the life-cycle NH3 emissions related to PM2.5 formation that was not tracked by GREET. They found out that cellulosic ethanol produced from perennial plants grown on former Conservation Reserve Program (CRP) lands, higher biomass yields (Miscanthus vs. switchgrass) and lower nitrogen fertilizer inputs (diverse prairie biomass vs. switchgrass) resulted in lower GHG emissions, lower than that emitted by corn-based ethanol or gasoline.
Life-cycle GHG emissions from corn ethanol depended on biorefinery heat source, assumptions about technology, and land-use change which determined whether it was lower than gasoline or not. Before the GHG emissions from land-use change were included, corn-based ethanol from natural gas had lower GHG emissions than gasoline. However, when GHG emissions from converting CRP grassland to corn cultivation was included, GHG emission from corn-based ethanol was much higher than that from gasoline. Corn ethanol has the highest PM2.5 regardless of how it is refined while cellulosic ethanol has the lowest PM2.5 . In this way, the article gives an in-depth idea of how GHG emission can be calculated and how the emissions emitted from the fuels varies based on the different factors considered.