Aging of Organic Particulate Matter and Implications to Population Exposure: Evidence from Ambient Measurements and Laboratory Experiments

LEA HILDEBRANDT (1), Kaytlin Henry (1), Evangelia Kostenidou (2), Gabriella J. Engelhart (1), 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, USA (2) University of Patras, Greece (3) Paul Scherrer Institut, Switzerland (4) University of Crete, Greece

     Abstract Number: 250
     Last modified: November 9, 2009

Abstract
Organic aerosol globally comprises a significant fraction (20-90%) of the submicron particle mass (PM1). Therefore, understanding the concentrations and properties of organic aerosol is crucial to reducing exposure to PM1 and the associated health effects. Organic aerosol is highly complex: it is composed of thousands of species, many of them unidentified, and has a myriad of sources – both anthropogenic and biogenic, particle-phase and gas-phase. The concentrations and properties of organic aerosol are governed by dynamic processes: organic aerosol components can evaporate, react further in the atmosphere and/or are transported, and then re-condense onto particles. This “aging” of organic aerosol results in elevated organic PM concentrations far away from sources, contributing to widespread exposure to PM1. Atmospheric aging changes what we breathe: It results in more oxidized organic aerosol, which has a higher oxidative potential and is therefore more harmful to human health than fresher, less oxidized organic aerosol. Therefore, we need to understand the aging of organic aerosol in order to decrease population exposure and develop policy actions to mitigate adverse human health effects.
Laboratory studies and ambient measurements allow us to shed light on organic aerosol aging and the resulting changes in ambient aerosol concentrations and characteristics. We report results from laboratory experiments using secondary organic aerosol (SOA) formed from traditional anthropogenic and biogenic precursors (toluene and alpha-pinene). The experiments show that the concentration and degree of oxidation of organic aerosol increase with aging, but the extent of these effects depends on organic aerosol type, as well as on experimental (or ambient) conditions. We also report results from three ambient measurement campaigns conducted in different locations (urban and remote) and seasons (summer and winter). The Finokalia Aerosol Measurement Experiments (FAMEs) took place at a carefully-selected, remote site on the island of Crete, Greece in the summer for 2008 and winter of 2009. A third ambient measurement campaign took place in Paris, France, in the summer of 2009. The measurements show that fresh organic aerosol measured closer to the source has different characteristic than aged organic aerosol which was processed in the atmosphere before it was measured. Furthermore, highly aged organic aerosol has very similar characteristics (degree of oxidation, volatility), regardless of its original source. The variability between different organic aerosol types decreases significantly with aging. Thus, the age of organic aerosol may be just as important as the aerosol source in understanding population exposure and human health effects.
While atmospheric processing of organic PM is a dynamic process, it appears to converge to a highly oxidized organic aerosol. This implies that the oxidation state of organic aerosol can be used to map its atmospheric evolution in air-quality models. Including the effects of atmospheric aging in air-quality models is crucial to accurately represent atmospheric organic aerosol concentrations and characteristics and, therefore, to assess population exposure and policy options.