Estimating Temporal Trends in Biogenically Formed Secondary Organic Aerosols Resulting From Reduction in Atmospheric Aerosol Water Content Across the Continental United States

William Malm1, Bret A. Schichtel2 and Jennifer L. Hand3

Particulate organic carbon (POM) aerosols contribute significantly to nitrogen deposition.  POM is about 30-40% of PM2.5 in the Eastern United States (U.S.) and more than 50% in parts of the Western U.S.  Therefore understanding the fate and origin of the PM2.5 POM aerosol is essential to mitigating its contribution to atmospheric deposition, visibility, health, and climate forcing.  To some degree apportionment of POM in the form of secondary organic aerosols (SOA) to an emission source is an ill-defined problem in that volatile organic carbon (VOC) gases may have their origin in biogenic emissions while the oxidation of that VOC and formation of SOA particles may be dependent of anthropogenic emissions from a variety of sources.  A recent analysis has suggested that atmospheric aerosol water, in the form of water absorbed by hygroscopic aerosols, such as sulfates and nitrates, is the dominant pathway for the formation of SOA in parts of the Eastern United States.  Therefore, as aerosols, such as sulfates, and associated aerosol water decrease over time it is expected that a concurrent decrease in SOA should occur.

In this analysis trends in POM over time, collected over a 16 year time period in the IMPROVE monitoring program, are explored.  It is shown that particulate organic mass (POM) is decreasing in almost all parts the U.S. and in all seasons of the year.  The decrease in POM over time has its origins in changing emissions from wild and prescribed fire and reductions in mobile and industrial sources and possible in reductions in atmospheric aerosol water.  An approximation of temporal trends in biogenically derived SOA across the U.S. is developed by accounting for the fraction of POM that is biogenic and addressing how anthropogenic POM has changed over time and across the U.S.  The analysis suggests that in much of the Eastern U.S. a significant portion of the decrease in summer-time POM aerosol is associated with fire and vegetative emissions.  A concurrent reduction in sulfate aerosol is consistent with the hypothesis that reducing aerosol water results in reductions in biogenically derived SOA.


1Colorado State University,
2National Park Service, Air Resource Division,
3Colorado State University,