Hygroscopic growth and cloud condensation nuclei activity of fresh and chemically-aged biomass-pyrolyzed organic aerosol

air quality;aerosol research;particulate matter;cloud condensation nuclei;hygroscopicity

Shah, Rishabh Urvesh ; Bond ; Tami C. ; Rood ; Mark J.

关键词: air quality;    aerosol research;    particulate matter;    cloud condensation nuclei;    hygroscopicity;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/88130/SHAH-THESIS-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Biomass burning is one of the prominent contributors of organic aerosols and cloud condensation nuclei (CCN) in the atmosphere. Aerosol-cloud interactions contribute to uncertainties in estimates of climate forcing, not only because of the complexities in their initial size and chemical composition, but also because of transformations (aging) they undergo in the atmosphere upon exposure to reactive species (e.g., NH3 and O3). This study presents results of bench-scale experiments on biomass pyrolysis organic carbon (OC) particles to determine its hygroscopic growth at sub-saturated relative humidities (RH) as well as CCN activity and droplet sizes at super-saturated humidity. This thesis investigates changes in these properties upon controlled, atmospherically-relevant exposures of NH3, O3 and RH. Measurements of hygroscopic growth and CCN activity are analyzed using κ-Köhler theory to calculate representative hygroscopicity parameters, κGF and κCCN, respectively (c.f. Petters and Kreidenweis 2007). Discrepancies as large as factors of three between κGF and κCCN suggest that approximating the surface tension of solution droplets to that of pure water, as assumed in κ- Köhler theory, overestimates the CCN activity of these complex organic particles. A possible evidence of the presence of surfactants is the formation of more than one hygroscopic activation modes in size-resolved CCN activation curves. While no change in κGF is observed after a 9 day- equivalent of atmospheric NH3-aging, a 72% increase in κCCN of OC particles suggests the presence of organic acidic groups in sufficient amount to influence the overall hygroscopic behavior of the particles. Chemical aging with O3 has no measurable impact on the κGF and κCCN of OC particles. My results suggest the possibility that oxidation of gas-phase volatile organic compounds in the aerosol by O3 causes them to condense as films on pre-existing particles. An evidence of such film formation is the reduced diameters of droplets exiting a cloud chamber, wherein the CCN have been exposed to a controlled supersaturation ratio for a fixed amount of time. We find that the possible participation of water taken up during aging at controlled relative humidity conditions does not affect hygroscopicity of OC particles. The results in this thesis are consistent with previously published results of effects of chemical aging with NH3 and O3 on the hygroscopicity of organic particles. Moreover, while previous studies have investigated organic particles of controlled initial composition, the results presented here apply to biomass pyrolysis OC particles. This thesis aids in understanding the important chemical aging mechanisms that organic particles emitted from pyrolysis of biomass could undergo, leading to their possibly increased hygroscopicity in the atmosphere.