Wildfires drive interannual variability of organic carbon aerosol in the western US in summer

Forest wildfire area burned in the western U. S. has increased in recent decades resulting in a substantial organic carbon ( OC) source with large interannual variability. We derive OC emissions from wildfires using data for area burned for 1980 - 2004 and ecosystem specific fuel loadings. For the period 1989 - 2004 we analyze OC observations in the western U. S. from the IMPROVE network and use a global chemical transport model to simulate OC concentrations. Modeled and observed OC concentrations are highly correlated when we use interannually varying fire emissions ( R-2 = 0.88); the correlation is smaller with climatological emissions ( R-2 = 0.4). We estimate that the observed increase in wildfire activity after the mid 1980s has caused mean OC concentrations in summer over the western U. S. to increase by 30% relative to 1970 - 1984. In the coming decades, climate change will likely cause further increases in wildfires resulting in increased OC concentrations with implications for health and visibility.