(4ac) Atmospheric Organic Particulate Matter: Measurements, Models and Mitigation

Submicron atmospheric particles adversely affect human health, and they have a highly uncertain effect on climate. Organic aerosol globally comprises a significant fraction (20-90%) of the submicron particle mass (PM1). While the formation of inorganic aerosol is relatively well understood, organic aerosol is not. In contrast to inorganic aerosol, which is mostly composed of a few well-characterized components such as sulfates, nitrates and ammonium, organic aerosol is composed of thousands of species, many of them unidentified, and has a myriad sources ? both anthropogenic and biogenic, particle-phase and gas-phase. Even though much progress has been made in recent years, these studies have revealed that atmospheric organic aerosol is much more complex than originally thought, complicating the analysis of policy options aimed at reducing organic aerosol concentrations.

The goal of my research is to improve our understanding of the processes governing the properties and concentrations of organic aerosol in the atmosphere through a series of laboratory experiments and ambient measurements. The findings from these studies are implemented in our three-dimensional chemical transport model PMCAMx, and the model is then tested against observations. Finally, the improved model can be used to evaluate different emission control options for reducing organic aerosol concentrations and their adverse effects.

My experimental work focuses on investigating the initial formation, mixing and aging of organic aerosol. Laboratory experiments using environmental chambers are complicated by issues such as losses of particles and vapors to the chamber walls. Even though much work remains to be done to truly understand wall losses, we have treated these issues thoroughly (Pierce et al., 2008; Hildebrandt et al., 2009), and we are continuing to work on them. In the aerosol formation experiments, we found that the aerosol mass yields from anthropogenic organic aerosol precursors such as toluene are much higher than previously reported values (Hildebrandt et al., 2009). We suggested new model parameters based on our studies, and their use is significantly improving the ability of chemical transport models to reproduce observations.

Most chemical transport models such as PMCAMx use a pseudo-ideal mixing assumption for modeling organic aerosol. This is a critical assumption which greatly affects modeled organic aerosol concentrations, but the assumption has not been confirmed experimentally. A main experimental challenge is that organic aerosol from different sources exhibits very similar mass spectra in an aerosol mass spectrometer (AMS), especially after the aerosol has undergone further reactions. The AMS from Aerodyne, Inc. is currently the most powerful and the most widely used instrument to obtain online information on aerosol composition at high time resolution. In order to be able to distinguish and track organic aerosol from different sources using an AMS, I conducted novel experiments using a High Resolution Time-of-Flight AMS (HR-ToF-AMS) and isotopically labeled compounds. Analyzing these experiments included identifying all isotopically labeled organic fragments based on their exact mass, and subsequently separating the labeled and non-labeled organic fragments using mathematical methods such as positive matrix factorization and chemical mass balance. I was able to confirm that anthropogenic and biogenic secondary organic aerosol components mix ideally. In the future, we will use this experimental approach to also probe the ideal mixing assumption between primary and secondary organic aerosol components.

Chemical transformation (aging) of organic aerosol is highly complex and currently poorly understood. In aging experiments, we have found that the concentration and degree of oxidation of organic aerosol change with aging, but that 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 in a remote location have allowed us to study highly aged organic aerosol. We have found that highly aged organic aerosol has very similar characteristics (oxidative state, vapor pressure), regardless of its original source (Hildebrandt et al., 2010a; Lee et al., 2010; Pikridas et al., 2010). In the winter when oxidative conditions are milder, the organic aerosol is less oxidized and its characteristics are more variable than under the harsher oxidative conditions in the summer (Hildebrandt et al., 2010b). Overall, the variability between different organic aerosol types decreases significantly with aging. Thus, the photochemical age of organic aerosol may be just as important as the aerosol source in understanding organic aerosol concentrations and characteristics.

A complete understanding of the formation and evolution of atmospheric organic aerosol is still far away. Nevertheless, our studies have allowed us to develop new parameterizations for our chemical transport model PMCAMx, and to improve existing ones. The updated PMCAMx performs well in predicting measured organic aerosol concentrations and approximate oxidative states in urban and rural environments, and in different seasons. It represents organic aerosol processes and concentrations more accurately than any other available model, making it suitable for evaluating different policy options aimed at reducing atmospheric aerosol concentrations.

References:

Hildebrandt, L., et al. (2009), High formation of secondary organic aerosol from the photo-oxdiation of toluene, Atmospheric Chemistry and Physics, 9, 693-733.

Hildebrandt, L., et al. (2010a), Aged organic aerosol in the Eastern Mediterranean: The Finokalia Aerosol Measurement Experiment ? 2008, Atmospheric Chemistry and Physics, 10, accepted.

Hildebrandt, L., et al. (2010b), Formation of low-volatility oxygenated organic aerosol in the atmosphere: Insights from the Finokalia Aerosol Measurement Experiments -2008/2009, Geophysical Research Letters, in preparation.

Lee, B.-H., et al. (2010), Volatility of organic aerosol sampled during FAME-2008, Atmospheric Chemistry and Physics Discussions, in preparation.

Pierce, J. R., et al. (2008), Constraining particle evolution from wall losses, coagulation, and condensation-evaporation in smog-chamber experiments: optimal estimation based on size distribution measurements, Aerosol Science and Technology, 42, 1001-1015 .

Pikridas, M., et al. (2010), The Finokalia Aerosol Measurement Experiments - 2008 (FAME-08): An Overview, Atmospheric Chemistry and Physics Discussions, 10, 6641?6679.