Submicron atmospheric aerosols have a highly uncertain effect on climate (IPCC, 2007), and they adversely affect human health (Dockery et al., 1993; Davidson et al., 2005; Pope and Dockery, 2006). Organic aerosol globally comprises a significant fraction (20-90%) of the submicron particle mass (PM1) (Kanakidou et al., 2005; Zhang et al., 2007), but its formation and evolution in the atmosphere remain poorly understood (Jimenez et al., 2009). Organic aerosol is highly complex: it is composed of thousands of species (Goldstein and Galbally, 2007), many of them unidentified, and has a myriad sources – both anthropogenic and biogenic, particle-phase and gas-phase (Hallquist et al., 2009). 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 can then re-condense onto particles (Robinson et al., 2007). This “aging” of organic aerosol results in elevated organic PM concentrations far away from sources, contributing to the regional nature of the PM1 problem. Atmospheric aging changes the chemical composition (e.g. oxygen content) and physical properties (e.g. oxidative potential, vapor pressure, hygroscopicity) of organic aerosol and thereby changes its effects on climate and human health. Thus, understanding the aging of organic aerosol is crucial to developing policy actions aimed at reducing PM1 and its adverse effects.
We discuss results from laboratory aging experiments using secondary organic aerosol (SOA) formed from traditional anthropogenic and biogenic precursors such as toluene and α-pinene. 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.
We also present results from ambient measurement campaigns conducted at a remote site on the island of Crete, Greece in the summer of 2008 and winter of 2009. The ambient measurements show that highly aged organic aerosol has very similar characteristics (degree of oxidation, volatility), regardless of its original source. Overall, 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 aerosol concentrations and characteristics. While atmospheric processing 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 chemical transport models.
Finally, we present results from our chemical transport model PMCAMx-2008, which uses a volatility basis set framework to track the chemical evolution of organic aerosol. The model incorporates several improvements over previous versions, including a chemical aging mechanism to simulate the oxidation chemistry altering organic aerosol compounds. The model performs well in predicting measured organic aerosol concentrations and approximate oxidative states in urban and rural environments, and in different seasons. Previous versions of the model are unable to recreate the observed degree of oxidation of the organic aerosol. Thus, including the effects of atmospheric aging in chemical transport models is crucial to accurately represent atmospheric organic aerosol concentrations and characteristics, and hence to evaluate policy options.
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