School of Civil and Environmental Engineering
Ph.D. Thesis Defense Announcement
Understanding the formation of secondary organic aerosol and the fates of organic nitrates formed from monoterpene oxidation
By: Masayuki Takeuchi
Advisor:
Dr. Nga Lee (Sally) Ng (CEE, ChBE, EAS)
Committee Members:
Dr. Armistead G. Russell (CEE), Dr. L. Gregory Huey (EAS), Dr. Rodney J. Weber (EAS), Dr. Pengfei Liu (EAS)
Date and Time: July 10th, 2023 at 10:00 AM
Location: ES&T L1116; https://gatech.zoom.us/j/6983273142
Secondary organic aerosol (SOA) contributes to a substantial fraction of fine particular matter (PM2.5) that imposes adverse health effects, large uncertainty in radiative forcing, and poor visibility. Monoterpenes represent about 15% of the annual emissions of biogenic volatile organic compounds (VOC), and their oxidation is considered to contribute to global SOA budget substantially. In the presence of NOx, oxidation of monoterpenes also produces organic nitrates. The formation of organic nitrates sequester NOx from the atmosphere, directly suppressing the ozone production near source regions. To what extent organic nitrates affect atmospheric oxidation capacity largely depends on their fates; whether NOx is released back or permanently removed. Chemistry about the monoterpene SOA formation and the fates of monoterpene organic nitrates are not well understood, calling for the need of fundamental laboratory studies.
This dissertation presents the original studies investigating hydroxyl (OH) and nitrate (NO3) radical oxidation of monoterpenes (i.e., α-pinene, β-pinene, and limonene). First, the formation and properties of SOA formed from NO3 radical oxidation of α-pinene and limonene were studied. Oxidizing mixtures of monoterpenes simultaneously led to different formation and properties of SOA compared to when oxidizing each monoterpene separately. 50% more α-pinene SOA was formed while limonene SOA formation was reduced by 20%. New oxidation products were also observed, suggesting potentially important interactions of VOC and their oxidation products. Second, the SOA formation mechanism of OH radical oxidation of α-pinene in the presence of NOx was investigated. By integrating the laboratory and simulation studies, multi-generational autoxidation chemistry was found to be important to explain a substantial fraction of SOA formed in laboratory chamber experiments. Third, chemical composition and hydrolysis of particulate organic nitrates formed from OH and NO3 radical oxidation of α-pinene and β-pinene were investigated. Hydrolysis lifetime was found to be short (<30 min) for all systems explored, significantly shorter than previous chamber studies (i.e., 3–6 h) but consistent with bulk solution measurement studies (i.e., 0.02–8.8 h). The discrepancy stemmed from the choice of proxy used to estimate the hydrolysis lifetime. The measured hydrolyzable fractions (FH) were ~30% and ~10% for OH and NO3 radical oxidation, respectively. Next, photolysis of gaseous organic nitrates formed from OH and NO3 radical oxidation of α-pinene and β-pinene were investigated. While not all gaseous organic nitrates underwent photolysis, some showed a rapid photolysis decay (~10-4 s-1) possibly due to the conjugation of carbonyl and nitrooxy groups as well as long carbon backbone. Gas-phase photolysis may be comparably important to particle-phase hydrolysis, pointing out the role of monoterpene organic nitrates as a NOx sink at a lesser degree but as a temporary NOx reservoir (or NOx source in regions away from the actual emission sources) at a higher extent.