(70a) Biogenic Organic Aerosol: Aging Under Various Light Conditions and Oxidant Concentrations

Fine particulate matter (PM) affects visibility and cloud formation, scattering and/or absorbing light, and ultimately affecting climate (IPCC, 2007). These small particles also impact human health, causing damage to respiratory and cardiovascular systems (Pope et al., 2002). Atmospheric PM consists of many inorganic species (such as sulfate, nitrate, sea salt, and dust), which have been relatively well constrained. However, between 20-90% of the total PM is organic, consisting of thousands of compounds, and this complex mixture is not well understood (Kanakidou et al., 2005; Zhang et al., 2007). In addition, most studies have focused on only the first few hours of organic aerosol (OA) formation. PM can remain in the atmosphere for ~1 week, therefore we need to understand further chemical processing of these compounds, which is termed “aging”.

An additional layer of complexity is added when we consider the volatile organic compounds (VOCs) in the atmosphere. This gas phase is in equilibrium with the OA; therefore, gas-phase reactions in turn affect the particle phase for both initial OA formation and aging. This work ages the OA system formed from alpha-pinene ozonolysis. alpha-Pinene is a biogenic compound, emitted from pine trees, and a model for many biogenic systems with ring compounds containing an endocyclic double bond. Because of the equilibrium between all gas and aerosol species, the presence of biogenic aerosol may influence the formation of anthropogenic aerosol and vice versa. Biogenic VOC emissions can be up to 10 times higher than anthropogenic VOCs; therefore it is essential that we isolate the biogenic system to understand its atmospheric processing. This aging is done by gas-phase reactions with the OH radical, the most powerful atmospheric oxidant. We then look for an affect on the particle phase, as our concerns for human health and climate lie with PM concentrations.

Experiments were performed at the Carnegie Mellon Smog Chamber, a 12 m³ Teflon reactor. OH aging is performed via photolysis of either HOOH or HONO using UV lights or in the dark via tetra-methyl ethylene ozonolysis. In this way we can isolate the features of both light and dark aging scenarios. In addition, the HONO source produces higher concentrations of OH, allowing us to probe the OH concentration variable. Results show that this alpha-pinene OA system is very photochemically rich and dynamic. In the dark, OA concentrations grow, while under the UV lights the aerosol evaporates. This evaporation can be combated by an order of magnitude increase in OH concentrations with the HONO source. This complex mechanism and equilibrium partitioning must continue to be explored as it may impact other, chemically similar systems. Finally this aging can be included in chemical transport models, which can aid policy makers in decisions regarding emissions reductions.

References:

IPCC: Forth Assessment Report: Climate Change, 2007.

Kanakidou, M., Seinfeld, J. H., Pandis, S. N., Barnes, I., Dentener, F. J., Facchini, M. C., Van Dingenen, R., Ervens, B., Nenes, A., Nielsen, C. J., Swietlicki, E., Putaud, J. P., Balkanski, Y., Fuzzi, S., Horth, J., Moortgat, G. K., Winterhalter, R., Myhre, C. E. L., Tsigaridis, K., Vignati, E., Stephanou, E. G., and Wilson, J.: Organic aerosol and global climate modelling: a review, Atmospheric Chemistry and Physics, 5, 1053-1123, 2005.

Pope, C. A., Burnett, R. T., Thun, M. J., Calle, E. E., Krewski, D., Ito, K., and Thurston, G. D.: Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution, JAMA-J. Am. Med. Assoc., 287, 1132-1141, 2002.

Zhang, Q., Jimenez, J. L., Canagaratna, M. R., Allan, J. D., Coe, H., Ulbrich, I., Alfarra, M. R., Takami, A., Middlebrook, A. M., Sun, Y. L., Dzepina, K., Dunlea, E., Docherty, K., DeCarlo, P. F., Salcedo, D., Onasch, T., Jayne, J. T., Miyoshi, T., Shimono, A., Hatakeyama, S., Takegawa, N., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer, S., Demerjian, K., Williams, P., Bower, K., Bahreini, R., Cottrell, L., Griffin, R. J., Rautiainen, J., Sun, J. Y., Zhang, Y. M., and Worsnop, D. R.: Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes, Geophys. Res. Lett., 34, 6, L1380110.1029/2007gl029979, 2007.