Title: The Impact of Organic Aerosol Volatility on Particle Microphysics and Global Climate
Description: Atmospheric aerosols are tiny particles suspended in the atmosphere. They affect global air quality, public health and climate (Boucher et al., 2013; Myhre et al., 2013; Seinfeld and Pandis, 2016), thus playing a key role in the Earth system. However, due to the complexity of aerosol processes and climate change feedbacks, our understanding of aerosols in a changing world is still limited (Boucher et al., 2013). To understand the impact of organic aerosol volatility on particle microphysics and global climate, I developed a new aerosol microphysics scheme, MATRIX-VBS, and its evaluation and application are presented in this dissertation. MATRIX-VBS couples the volatility-basis set (VBS, Donahue et al., 2006) framework with the aerosol microphysical scheme MATRIX (Multiconfiguration Aerosol TRacker of mIXing state, Bauer et al., 2008) that resolves aerosol mass and number concentrations, size, and mixing state. With the inclusion of organic partitioning and photochemical aging of semi-volatile organic aerosols, aerosols are able to grow via organic condensation, a process previously not available in the original model MATRIX, where organic aerosols were treated as nonvolatile.

Both MATRIX and MATRIX-VBS can be used as stand-alone box models or within a global model. After the development of MATRIX-VBS in the box model framework, both model’s simulations were performed and assessed on the box and global scales. On the box model scale, idealized experiments were designed to simulate different environments, clean, polluted, urban, and rural. I investigated the evolution of organic aerosol mass concentration and volatility distribution among gas and aerosol phases, and results show that semi-volatile primary organic aerosols evaporate almost completely in the intermediate-volatility range and stay in the particle phase in the low volatility range. I also concluded that the volatility distribution of organics relies on emission, oxidation, and temperature, and the inclusion of organic aerosol volatility changes aerosol mixing state. Comparing against parallel simulations with the original model MATRIX, which treats organic aerosols as nonvolatile, I assessed the effect of gas-particle partitioning and photochemical aging of semi-volatile organics on particle growth, composition, size distribution and mixing state.

Results also show that the new model produces different mixing states, increased number concentrations and decreased aerosol sizes for organic-containing aerosol populations. Monte-Carlo type experiments were performed and they offered a more in-depth look at the impact of organic aerosol volatility on activated number concentration, which is the number concentration of aerosols that are activated but has not yet formed into a cloud droplet. By testing multiple parameters such as aerosol composition, mass concentration and number concentration, as well as particle size, I examined the impact of partitioning organic aerosols on activated aerosol number concentration. I found that the new model MATRIX-VBS produces fewer activated particles compared to the original model MATRIX, except in environments with low cloud updrafts, in clean regions at above freezing temperatures, and in polluted areas at high temperature (310K) and extremely low humidity conditions. I concluded that such change is caused by the differences in aerosol number concentration and size between the two models, which would determine how many particles could activate.

On the global scale, MATRIX-VBS was implemented in the NASA GISS ModelE Earth systems model. I assessed and evaluated the new model by comparing aerosol mass and number concentrations, activated cloud number concentration, and AOD against output from the original MATRIX model. Further, I evaluate the two models against observations of organic aerosol mass concentration from the aircraft campaign ATom (Atmospheric Tomography Mission), and aerosol optical depth from ground measurement stations from AERONET (Aerosol Robotic Network) as well as satellite retrievals from MODIS (MODerate resolution Imaging Spectroradiometer) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations). Results show that organics in MATRIX-VBS experience more distant long-range transport, and their mass concentration increase aloft and decrease at the surface as compared to those in MATRIX. There are still underestimations in the vertical profiles of mass concentration in both models, especially in the high latitudes in the Northern Hemisphere and South Pacific Ocean basin, possibly due to the application of universal distribution of mass-based emission factors among different volatilities that perhaps is not realistic in all climate zones, thus affecting organic aerosol lifetime and transport.

Just as the box model results, there are more particles and generally more activated ones (except for rare cases such as the highly polluted Eastern China) in MATRIX-VBS than in MATRIX. As for AOD comparisons, MATRIX-VBS have generally lower AOD than MATRIX, which can be due to smaller aerosols and different aerosol composition in the new model, which is also underestimating biomass burning in the Amazon and Congo basins. Compared to satellite retrievals from MODIS and ground measurements from AERONET, both models overestimate aerosol optical depth over anthropogenic polluted regions and biomass regions such as central Africa. Overall, both models also underestimate AOD as compared to AERONET in the winter (DJF), whereas they generally overestimate or estimate it well in other seasons. Even though during its initial evaluation, MATRIX-VBS does not seem to have improved from MATRIX on the global scale in representing the real world, it made the first key step in improving our understanding of organic aerosols on the process level.

Changes in mass, number concentration, size distribution, and mixing state (composition) have great implications and impact on climate. Further studies are needed in examining and improving factors linked to the new representation of semi-volatiles in an aerosol microphysics model, including but not limited to the treatment of mass-based emission factor distribution among different organic volatilities and the size distribution of tiny organic particles that have evaporated but not completely. Challenges in evaluations of organic aerosol against measurements remain in that remote regions of significant interest lack available measurements, and additional field campaigns will be important for us to better understand real world conditions and shed light on model performance.