Aging of Atmospheric Organic Particulate Matter: Insights from Remote Ambient Measurements and Laboratory Experiments

LEA HILDEBRANDT (1), Evangelia Kostenidou (2), Gabriella J. Engelhart Farnham (1), Valentin A. Lanz (3), Claudia Mohr (3), Peter F. DeCarlo (3), Andre S. H. Prevot (3), Urs Baltensperger (3), Nikos Mihalopoulos (4), Neil M. Donahue (1), Spyros N. Pandis (1,2)

(1) Carnegie Mellon University, Pittsburgh, USA (2) University of Patras, Patra, Greece (3) Paul Scherrer Institut, Villigen, Switzerland (4) University of Crete, Heraklion, Greece

     Abstract Number: 412
     Last modified: May 12, 2010

Abstract
The chemical transformation (aging) of organic aerosol is highly complex and currently poorly understood. Aging changes the chemical composition (e.g. oxygen content) and physical properties (e.g. vapor pressure, hygroscopicity) of organic aerosol and thereby its effects on human health and the climate. Thus, understanding the aging of organic aerosol is crucial to developing policy actions aimed at reducing atmospheric organic aerosol and its adverse effects.

We present results from remote ambient measurements and from laboratory aging experiments using secondary organic aerosol (SOA) formed from traditional anthropogenic and biogenic precursors. The experiments show that the concentration and degree of oxidation of organic aerosol change with aging. The extent of these effects depends on organic-aerosol type, as well as on experimental (or ambient) conditions. In general, studying the aging of organic aerosol in the laboratory is challenging due to the limited time scale of laboratory experiments. Ambient measurements at a remote location on the island of Crete, Greece have allowed us to study highly aged organic aerosol in the summer and more moderately aged organic aerosol in the winter. We show that the highly aged organic aerosol has very similar characteristics (degree of oxidation, volatility), regardless of its original source. In the winter, when photo-oxidative conditions are milder, the organic aerosol is less oxidized and its characteristics are more variable than in the summer. Overall, the variability between different organic aerosol types decreases significantly with aging. Thus, the age of organic aerosol may be as important as the aerosol source in understanding aerosol concentrations and characteristics. While atmospheric transformation of organic PM is a dynamic process, it appears to converge to a highly oxidized organic aerosol. This implies that the degree of oxidation of organic aerosol can be used to map its atmospheric evolution in air-quality models.