Characterizing the Role of pH in the Formation of Isoprene-Derived Secondary Organic Aerosol in Continental and Marine Environments
Creators: name: Cooke, Madeline
Publisher: name: My University
Year: 2023
Subjects: subject: Isoprene-Derived Secondary Organic Aerosol
subject: Air Pollution
subject: Atmospheric Chemistry
subject: Secondary Organic Aerosol
subject: Aerosol Particles
subject: Chemistry
subject: Science

description: Atmospheric aerosol particles impact climate by altering radiative forcing and health by increasing risk for illness after exposure. Atmospheric particles are categorized as primary aerosol, emitted directly into the atmosphere, and secondary aerosol, formed through reactive uptake of gases to existing particles. Secondary organic aerosol (SOA) accounts for over 50% of organic aerosol mass globally, but regulating SOA is a challenge as many chemical processes remain unclear. Isoprene, the volatile organic compound with the second highest emission globally, can undergo oxidation with hydroxyl radicals to form lower-volatility isoprene epoxydiols (IEPOX). IEPOX undergoes reactive uptake onto sulfate particles to form isoprene-SOA (iSOA) through the nucleophilic addition of sulfate or water, forming methyltetrol sulfates (MTS) and 2-methyltetrols (2-MT), respectively. Tropospheric aerosol particles are quite acidic (pH = 0 – 3) and exist with high ionic strengths; knowledge gaps remain regarding iSOA formation under these conditions. High ionic strength conditions alter chemical parameters, such as the acid/dissociation constant (pKa). We modeled the shift in the sulfate/bisulfate pKa and demonstrated a shift to ~1 with increasing ionic strength. Parameterizing the pKa under high ionic strengths is important given the ubiquity of atmospheric sulfate and its reactivity in important atmospheric processes. Models assume that iSOA increases with increasing [H+]; however, iSOA formation has not been systematically explored under highly acidic conditions. We conducted chamber experiments on iSOA formation from the reactive uptake of IEPOX onto ammonium sulfate (AS) particles (pH = 0 – 3) and characterized aerosol formation and resulting product distribution. We observed maximum aerosol formation at the sulfate/bisulfate pKa, below which the lower nucleophilicity of bisulfate decreases iSOA formation. Below the pKa, the product distribution shifts forming more 2-MT than MTS. Additionally, we explored the mechanism of MTS formation (SN1 vs. SN2) and found that the SN1 mechanism predominates at atmospherically relevant pH. These results provide parameters necessary to improve models accounting for the pH-dependence of iSOA formation. Studies on iSOA have traditionally focused on reactive uptake of IEPOX onto AS mixed with H2SO4 to replicate urban aerosol pH. In contrast, for marine environments, sodium is the dominant cation, and sodium sulfate (SS) has long been observed to form from sea spray aerosol. We demonstrated that substantial iSOA, including MTS, are formed on acidic SS particles. Additionally, we found that enhanced iSOA formation occurs on pH 1 SS particles compared to pH 1 AS. Our results suggest that iSOA formed on acidic SS is potentially an important, but underappreciated, source of iSOA and that organosulfates in marine regions could modify SOA-budgets. Urban regions often contain highly acidic aerosol, and it is unclear how this impacts iSOA formation in these environments. Aerosol mixing state and iSOA formation were characterized in field samples from Mexico City. Aerosol internally mixed with sulfate represented 25-41% of aerosol and a significant mass of both 2-MT and MTS was quantified, suggesting that iSOA is an important aerosol class in this region. Through these studies, we gain a better understanding of the formation of iSOA in urban regions, which will improve predictions of iSOA formation in environments with highly acidic aerosol. Overall, this dissertation improves understanding of iSOA formation under atmospherically relevant conditions. These results can improve predictions of iSOA formation in continental, marine, and urban environments, and assist in mitigating the effects of iSOA on climate and public health.